Everything I used to know (and maybe forgot) about the 7.3L
With the 7.3L five years out of production and still as popular as it is, I find myself remembering and reflecting…
It was, I believe, in late 1993 that I took a trip to Montgomery, Alabama with my shop foreman to a Ford training session where we got our first peek at a 7.3L Power Stroke. Having spent five years of my early professional career maintaining forklifts, pipe loaders, and cranes down in Texas, (I was working on cars and pickups too), I was no stranger to diesel engines. I knew enough about diesel fuel injection to wonder how in the heck an electronic diesel would work. Rob Bartow was the Ford instructor, but the PSD was new enough at that point that he was learning along with us. I learned a lot that day.
One of my first points of interest was the firing order. In my thinking, I had no reason to memorize it – there were no spark plug wires. But as I analyzed the way the cylinders are numbered (1-7 on the passenger side, 2-8 on the driver side) and superimposed normal Ford V8 numbering (1-4 on the passenger side, 5-8 on the driver side) I drew a little diagram following both firing orders and realized that the 7.3L has the same firing order as a 302 gas burner. In my mind, since I already had that one down pat, I could draw on that pattern if I needed to do any analysis that required a knowledge of which cylinder fired when. Even when doing the ‘buzz’ test, the PCM fires the injectors in numerical order, not firing order.
While we were in training, one of the exercises Rob had us perform required the use of the 104 pin breakout box to sample some inputs and manipulate some outputs. In the course of that exercise, I accidentally fed a hard ground through breakout box pin 83 to the ICP regulator, a maneuver that brought control oil pressure above and beyond anything the PCM ever calls for. Sixty five percent is as high as the duty cycle ever goes when the PCM is calling the shots, but I had forced it to 100% with my jumper connection, and as the big rattler rocked, snorted, and blew smoke for a few seconds, Rob pretty much did the same thing the engine was doing. He was ordinarily a mild mannered character, but in those stressful moments, he was red-faced and hopping around wide-eyed. When the jumpered ground was removed and the engine settled down, Rob was still breathing hard.
“I have to drive this thing back to Birmingham! Don’t ever do that again!!”
Hey, what could I say? It was an honest mistake that I never repeated.
In this article, we’ll have another look at the 7.3L and include a few of the things a brave and wrench-smart do-it-yourselfer can handle. We don’t intend to make you a professional technician, because building a professional tech takes more teaching on this end and a lot more gumption on the receiving end than most people have.
The key to a relatively trouble-free experience (and this is a ‘duh’ statement) is diligent maintenance. We expect a lot out of our vehicles and rightly so. But to expect as much as we do from our vehicles without performing regular maintenance and safety checks is downright foolhardy to the point of being dangerous.
Fuel Delivery – The Way It Was
On any diesel engine, the timing of the fuel spray from the injector, along with the volume and mass of the fuel delivered is critical to a diesel’s power curve.
That’s old news.
Older diesels delivered their fuel mechanically, with ingeniously designed pumps delivering thousands of pounds of pressure to calibrated injectors which would “pop” when the pressure pulse rose above a certain level at the injector tip. If you’ve ever tried to see the holes in the tip of an injector where the fuel comes out, you’ll know what a fine spray that is. And while a gas burner lights off its spark plug to fire the mix, a diesel relies on the 2 degrees per pound of compression that naturally accompanies the squeezing of atmosphere. At 450 lbs (more or less) of combustion chamber compression the air will be at nearly 1000°, and the injector generally sprays its fuel into the chamber when the piston is near or slightly after top dead center, depending on load and engine speed. At TDC, the head of the piston is only 0.040 inch away from the head – that’s about the half the thickness of a wooden matchstick, and the combustion chamber winds up being a bowl-shaped cut in the center of the piston crown.
On any engine, some of that compression will blow past the piston rings into the crankcase, but if there’s too much blowby, you’ll see it with the engine running and the oil filler cap off. Get used to what’s normal at that opening and it’ll be easier for you to recognize what’s not.
The job older generation diesels did mechanically is done electronically and hydraulically by the Powerstroke’s fuel delivery system.
Between 1994 and mid-model year in 1998, a mechanical fuel pump was mounted in the valley behind the fuel filter with a plunger resting on a camshaft lobe. The pump uses a diaphragm to draw fuel from the tank exactly like the diaphragm pumps used for fuel transfer, but with an important difference. This pump has a piston-type positive displacement
| Once I drew a ticket on a 1996 7.3L that had a hard start concern. I checked the fuel pressure and saw only 20 lbs on the gauge. At that juncture, the driveability work was flowing thick and fast, and so the shop foreman decided to pass the truck over to a line technician, who replaced the fuel pump. Flag time on that job under warranty was about six hours, but a guy with some smarts can swap one out in about 30 minutes if everything goes right. A word of caution here: A bent or collapsed fuel pressure regulator spring in the filter housing can cause exactly the same symptom, so check there first!
Anyway, after the pump was replaced, the fuel pressure was at a comfortable 55 pounds, but now the truck wouldn’t start, and it was producing massive amounts of white smoke with every attempt. Who knows how much life they sucked out of that starter before the shop foreman came to me for advice. (The shop is big and I hadn’t noticed that the truck they were fighting was the same one I had diagnosed.)
“I think it’s flooded,” I told the foreman. He’s no dummy, but he had never considered the fact that a diesel could get too much fuel to start, and my prognosis launched a debate that left him unsatisfied. Unable to convince him, I shrugged, and they spent about another thirty minutes testing the mettle of that grand old Japanese starter, but to no avail. Finally the shop foreman came on bended knee to ask for my help. I settled in behind the wheel of the ailing truck, brought the ICP sensor and the IPR up on the NGS and spun the engine for about ten seconds. The ICP was over 3000 pounds and the IPR was maxed out at 65%.
The PCM’s strategy is such that if high oil pressure is present and the engine doesn’t start right away, adaptive learning ramps the control pressure up in an attempt to force more fuel into the chamber. The 7.3L had been starting hard with low fuel pressure (the PCM has no earthly idea what the fuel pressure is – no input there), and it took really high control pressure to get the engine going. Now, with really good fuel pressure, the PCM was using the same learned numbers , but with very different results. Disconnecting the battery terminals for about five minutes put everything back where it needed to be – the engine fired up immediately with the shop foreman shaking his head in disbelief.
pump as an integral part of the unit, and it takes the 4-6 pounds of pressure from the diaphragm ‘way above that and feeds it to the filter housing where the pressure is regulated by a spring and plunger. The pressure is ordinarily pretty bouncy and I like to see it between 50 and 60 psi. This pressure is pulled all the way from the tank by the diaphragm part of the pump and fed through the filter/separator to the passage in the head to fill the cavity in the injector underneath the intensifier piston.
The irritating task of having to bleed the air out of the diesel fuel system is handled by a couple of small orifices placed at strategic spots, but starting a PSD that has an empty oil rail is another matter entirely. More about that later.
Fuel pressure testing on the early PSD (95-97.5) is handled at the dealership with a special stainless steel Shrader adapter (the test port looks like a regular Ford fuel pressure tap point but the center stem has a different configuration). Checking fuel pressure on the second generation 7.3L PSD involves removing an aggravating 1/8 pipe plug from the fuel pressure port at the front corner of the bank 1 (passenger side) head and/or at the rear corner of the bank 2 head. Some techs try to make an end run around installing a gauge in that annoying head port by making a much easier connection at the fuel filter housing lines, but pressures obtained that way have been known to be troublesome and misleading. One way or another, anybody who does any serious troubleshooting will need to know the real fuel pressure numbers, and it’s definitely easier to check on the first generation system.
Whenever the fuel finally makes it to the Hydraulic Electronic Unit Injectors (HEUI), the PCM-controlled high oil pressure supplied to the rails cast into the cylinder heads is the driving force that the IDM (Injector Driver Module) utilizes to pop the injectors and shove the fuel into the 1000 degree air above the piston.
The PCM monitors various inputs, including Engine Oil Temperature (it couldn’t care less about coolant temp), Injector Control Pressure, Throttle Position, Manifold Pressure, and Cam Position (crank position is inferred, not directly measured), then sends commands to the Injector Driver Module, which provides the 115 DC volt electrical muscle (7-15 amps) required to operate the 2.0 ohm injector solenoids at the necessary speeds, allowing the high pressure oil to act on the head of an intensifier piston. The high voltage and low resistance of the injector coils is necessary because the injectors have to react so quickly – lower voltage and more ohms would render the nozzles less effective, and often does if connections deteriorate anywhere in the injector driver circuits.
Injector & rings
|This isn’t a D-I-Y kind of part, unless you’re uncommonly techie. I have no idea how many of these babies I replaced when I was in the field. The #8 injector is supposed to be a different part number, a fix for what Ford calls a “fuel cackle” noise that was fairly common on some units, but the PSD specialists I know generally put the same part number in all 8 holes. The fuel rail AND the oil rail have to be drained before the injector is removed.
The injector intensifier piston uses Pascal’s Law to multiply the incoming oil pressure by a factor of seven, effectively injecting and atomizing the fuel in the precise quantity and at the precise time the engine needs it.
It’s a no-brainer to realize that a diesel engine doesn’t care how the fuel gets to the tip of the nozzle. The engine responds to air and fuel delivery – period. With no throttle plate, diesels get all the air they want, so unless the filter is clogged or the turbocharger is on the blink, air isn’t an issue, but dirt and dust will be if you try to fix an air inlet tube or air filter housing with duct tape – spend the money for a new part if the filter housing is cracked, and you will probably prevent premature engine failure. These are long life engines when properly maintained, and keeping the incoming air clean is of paramount importance. As far as the fuel goes, don’t use off-road diesel; it’ll cause more problems than the few bucks it saves, and when I was in the service bay I wouldn’t even put a wrench on a PSD that was running that stuff until at least three tanks of the right juice had been consumed. Even then, the injectors sometimes had to be replaced. I know what some of you guys are thinking: They use off road diesel in HEUI-injected Caterpillar equipment, but the engines in those yellow boxes have different tolerances than the more emission friendly PSD does.
Cetane is a clear, colorless liquid with near- perfect combustion qualities, and is rated at 100. Diesel fuels are rated compared to Cetane.
The fuel is pumped into the filter housing, rises to the top of the stand pipe, then is fed back to the fuel pump (lower fitting) through the high pressure, positive displacement part of the fuel pump to the cylinder head fuel rails, where the fuel fills the lower cavity in each injector. Shop manuals call for a Cetane rating of about 40 for Power Stroke engines.
The wrinkle is that clean fuel has to be delivered at the proper time, with the proper pressure, and in the proper amount, and the operator has to be able to control fuel delivery in such a way that the engine responds the way a power plant should, regardless of what the power is being used for. There can be no throttle cable on an electronic diesel.
On the 7.3L, the accelerator pedal sensor (APS) and the idle validation switch (IVS) are both mounted on the accelerator pedal assembly and provide driver input information to the PCM. If these two sensors don’t agree, the engine defaults to an idle-only strategy, which can be a revolting development if it happens somewhere in the great outdoors, so a serious PSD owner might ought to have a spare accelerator pedal assembly (it comes with new sensors) handy just in case. It’s not hard to change the assembly – three bolts, a couple of wire connectors, and about ten minutes of your time, but make sure you do it right.
Fuel filter replacement is pretty simple too, but you need to be sure you do that right too, and practice makes perfect. Look carefully at the instructions that come with the filter (the rubber gasket has to go on a certain way), and have a medium sized Craftsman screwdriver on hand to screw the filter housing cover off unless it has been replaced with one of those different style caps – use what works for you. When you get it uncapped, drain the filter housing by moving the little yellow drain lever next to the housing cover 90 degrees so you can check the condition of the fuel heater and see what’s in the bottom of the housing, but catch the fuel in a pan so as not to make a mess on somebody’s parking lot or put fuel in our ground water.
While inspecting the heater, look for deterioration of the little pucks that hold the heater wire in place – if that hot wire touches the filter housing it’ll blow the fuse that feeds the PCM power relay, and you might perform a pre-emptive first strike by catching it early. If that fuse does blow and you have the confidence to do so, check the fuel heater before you call a tow truck. The heater costs about $100 last time I priced one, but it may be cheaper now. If the fuel filter housing is cracked and leaking (and that happens too), you really ought to go ahead and spend the $500 on a replacement – it wouldn’t be a bad idea to have one of those on hand too, but keep some new worm-type hose clamps in the box with it and be prepared to get your hands dirty.
If the fuel heater needs replacing, use an offset oxygen sensor socket (7/8) to remove the stand pipe and be sure to remember that the pipe has left hand thread! The wire that passes through the filter housing to feed the heater has a rubber insulating grommet that can deteriorate or move around, and if the wire terminal at the grommet touches the housing, you’ll have the same problem – I’ve seen that before too, and you’ll miss it if you don’t look close.
|This is the fuel filter housing with its heater – the water separator will generally have water in it – that’s its job, because water can condense on the upper surface of a fuel tank and its inevitable that some of it will find its way here. One really good Texas tech I know likes to pour this housing full of Sea Foam Motor Treatment after replacing the fuel filter but before screwing the top back on. Sea Foam Motor Treatment purportedly cleans and lubricates the injectors, and I don’t see why it wouldn’t be a good idea.|
Return Head Relief Valve
The small return head on the side of the filter housing has a filter in it that catches solid material (like o-ring particles) coming from the cylinder heads. The small black reservoir was deleted after 1995, but the return filter head (with a built-in non-changeable filter) remained until the birth of the Second Generation Power Stroke, which had a different fuel filter assembly and a simpler fuel delivery system. (below) The fuel pressure is regulated on the return side of the heads by the spring and plunger assembly you see me holding here. If the spring is bent or weak, the fuel pressure will be low, but in most cases, the pump is the culprit.
In spite of the fact that this fuel system is called “Returnless,” the Second Generation Power Stroke fuel system actually does return excess fuel to the tank, but the pressure is regulated before the fuel ever gets to the cylinder heads instead of after it leaves as on the First Generation system. Notice that there is no return from the heads on this diagram. The fuel returns from the filter housing rather than the heads. The fuel pump is controlled by a PCM-controlled relay through an old-fashioned Ford inertial switch and runs for 20 seconds at key on. If no rpm signal is evident after 20 seconds, the PCM shuts the fuel pump down.
The filters you see here are in the tank and are part of the Diesel Thermal Recirculation Module (DTRM), which is a temperature-sensitive mechanical apparatus that mixes return fuel with incoming supply to keep fuel temperature between 50 and 150 degrees Fahrenheit. The filters you see here can cause fuel starvation if they trap enough foreign material. It may happen after a hundred or so miles of highway driving, and the problem may go away after the vehicle sits for awhile, only to return a hundred miles later. The black item in the picture is the “nib” assembly, which sits on the bottom of the tank vacuuming up small particles and globules of water while they’re still at a manageable level. The DTRM is built into the HFCM on 6.0L.
|Make note of this check valve with the oil filter removed – get used to what it normally looks like –if the wafer is gone, replace the filter head.|
Power Stroke’s low pressure oil system is similar to other V8 engines, with a gerotor style pump driven by two flats on the nose of the crankshaft. The pump draws oil in through the large pickup tube from the sump, then sends it through the oil cooler to the filter head, where it is filtered and regulated before being delivered to the oil gallery. The filter head also contains an oil filter bypass valve in case the oil filter becomes too clogged to allow oil flow. An air leak in the pickup tube is rare, but it happens sometimes and can cause driveability problems, because it introduces aeration into the oil system, and the high pressure oil system is negatively affected by that in a big way.
Pay particular attention to the fact that that one short gallery bypasses the rest of the system on startup to quickly deliver oil to the reservoir for cold or initial starts. This is necessary because the reservoir must supply oil to the high pressure pump, where the oil pressure is boosted and controlled by the PCM via the Injection Pressure Regulator (IPR, about $350) before being sent to the injector oil rails.
Without this short circuit oil passage, the high pressure oil pump reservoir’s oil supply would be exhausted before the gallery. If the short circuit check valve (short circuit device) malfunctions and lets the reservoir drain, a hard or no-start condition can result, and the timing cover has to be removed to fix that problem. If, on the other hand, the oil filter bypass valve (you can see it when you remove the oil filter) gets hung or jammed in its open position and lets the oil gallery drain after a soak, the engine will start and run for about fifteen to twenty seconds before reservoir runs out of oil, then you’ll spin it awhile to burp the air out of the gallery and refill the reservoir, then the engine will restart.
Learning what the PIDS look like under normal circumstances is all important – like a treasury guy that studies real money to recognize the counterfeit, you need to know what good readings look like.
PIDS @ Idle in Gear
The high pressure oil system is something you as a DIY person probably shouldn’t fool with, although the most common no-start failure I’ve personally seen has been a stuck IPR, and if you get a P1211 or similar code on your scanner (meaning the PCM called for a pressure it didn’t get), then unplug the IPR with the engine idling – if the plunger isn’t stuck, the engine should stall when you do this.
Once again, Power Stroke’s high oil pressure (purple in the diagram) is controlled by the PCM by way of the ICP regulator, using a 0-65% duty cycle. The higher the duty cycle, the higher the pressure, but the high pressure pump contains an integral relief valve that limits the pressure so that it goes no higher than 3,750 psi.
Feedback information on high oil pressure is provided by the Injector Control Pressure (ICP) sensor, mounted in the front of the driver side cylinder head. The high pressure pump doesn’t produce much volume, so any leakage in the system will either cause poor performance or, more commonly, a no-start. If you suspect that high oil pressure is present but feel you may have a faulty ICP sensor (and they do fail), then disconnect it and see if the engine fires up. When the PCM sees an open ICP sensor, it defaults to a pre-programmed pressure of 725 lbs, which is above the 500 psi threshold below which the PCM will refuse to operate the injectors. Notice the condition of the connector terminals too – those connectors tend to corrode and stuff.
Oh, yeah, before I forget – if you find it necessary to bleed the high pressure oil system for some reason, simply remove the ICP sensor from the oil rail in the driver side head and its counterpart (a blind steel plug) from the corresponding hole in the passenger side head, and watch those two holes while somebody spins the engine. The oil will gently rise to the level of the hole. When the oil rail is full screw the ICP and plug back in place and you should get a quick start.
Don’t ever remove the ICP or blind plug with the engine running, else you could be permanently injured!!
When starting, the IPR duty cycle is usually pretty low (usually around 15%), but the PCM will ratchet that signal higher in degrees if the engine doesn’t start right away. Once again, if IC Pressure remains below 500 psi (it was 363 on the 94 models.), the PCM won’t operate the injectors at all. A high IPR duty cycle with low pressure, (whether spinning or running) indicates a low oil pressure concern that may be caused by a system leak (invisible if it’s under the valve cover), a bad IPR, or a faulty ICP sensor signal.
One item of note is that the second generation 7.3L PSD doesn’t even start operating the injectors until after the engine has been spinning for a few seconds, thus the slightly longer crank times on the newer 7.3L platform. And if you know where to buy ‘em, you can get brand new Caterpillar injectors for about $200 each, and they generally do better than the $300+ Ford reman units.
Turbocharger and Related Systems
|The backpressure control valve can remain closed due to a faulty control solenoid and rob a PSD of its power.
The early Power Stroke turbochargers had no wastegate, but they did have a built-in exhaust backpressure control valve. Interestingly, this butterfly, which is PCM controlled, only operates in ambients of less than 40 degrees Fahrenheit and only at low idle, no-load conditions. Raising the exhaust backpressure on an idling engine warms the coolant faster for better heater and defroster operation.
If, however, this system is rendered inoperative in the open position, it isn’t much of a problem, but in some cases, the oil piston that drives it (by way of a PCM controlled solenoid) can receive oil through a bad solenoid and close the butterfly when it isn’t supposed to. This causes a low power concern, a problem that occurs fairly regularly. As with everything else, get used to what it looks like when it’s at rest, and remember to re-check it – you can see it at the rear of the engine, and on some calibrations it swings shut and then back open on every startup.
Second Generation Power Strokes with Charge Air Coolers (Intercoolers) will have a wastegate, which is also regulated by the PCM through Wastegate Control Solenoid (pictured above).
The glow plugs are PCM controlled through a relay that looks like an old Ford starter relay. With an oil temperature warmer than 131 degrees Fahrenheit (Power Stroke ignores Engine Coolant Temperature), the glow plug relay will not be energized, but the glow plug lamp may come on anyway, since the PCM is programmed to check the bulb. If charging voltage is higher than normal, the PCM will also limit glow plug operation.
Because of the strategy used on pre-Power Stroke glow plug systems, a Ford truck or van wouldn’t start on a cold day if a single glow plug was burned out. The controller will determine that a glow plug is out
of the loop and click the glow plugs at about one short cycle per second. Power Stroke doesn’t operate that way. Even if a glow plug is burned out, the rest of the glow plugs continue to operate.
The glow plugs, like the injectors, are under the valve covers, and get their feed through the same connector shell and wire harness that carries the 115 volts to the injectors. With that in mind, always check the connectors for melting when a running problem is present. Sometimes a glow plug will short out instead of failing open, and when it does, the connector at the valve cover and the small harness for the two injectors and glow plugs in that quadrant will have to be replaced.
Relays like to burn out; the glow plugs pull about 25 amps each on initial burn, dropping back to about 15 amps as they heat up, and you can use an inductive ammeter clamped around the heavy wire leading out to the glow plugs for a quick check. Use the numbers above and do the math – you should see 200 amps and a drop back to 125 or so as the plugs heat up. If for example you see 175 initially and a drop back to near 100 amps, suspect a glow plug and use a test light connected to power to find the guilty glower. Sometimes glow plugs will short out and burn the wire harness at the valve cover, which is nasty and requires replacement of the harness connector (if you can get one) or the harness, not to mention the valve cover gasket, sometimes the small harness under the valve cover, and the glow plug itself. On Second Generation units without a catalyst, a manifold heater is used to reduce white smoke, but it only operates for a short period of time right after startup.
The Power Stroke engine controller is a 104 pin PCM that sells for about $250. The Injector Driver Module (IDM), which is just as important as the PCM to the operation of the engine, runs about $850, and sometimes they have to be replaced to repair a no-start or rough running concern. Pay particular attention to signs of water entry – it seems fairly easy to wet an IDM’s innards and render it null and void.
The MAP basically tells the PCM what the boost pressure is, which is a crucial factor in fuel delivery and timing.
Cruise control is built into the PSD PCM, getting its information from the steering wheel switches, vehicle speed sensor, brake switch, park brake pedal switch, clutch switch (where applicable), and a normally closed brake pressure switch that opens a 12 volt circuit when the brakes are applied. Look for that switch connector cavity to fill with brake fluid – that’s what the Ford recall on this kind of switch is all about. Since the Power Stroke has no throttle linkage, no speed control servo is used. The speed control strategy is programmed into the PCM.
Don’t forget about the Accelerator Pedal Sensor and Idle Validation Switch that are a part of the Accelerator Pedal Assembly we discussed earlier!
The Injector Control Pressure Regulator is a PCM-controlled solenoid and spool valve assembly that is mounted on the high pressure pump. Electromagnetic force applied as the solenoid is energized and it moves its internal spool valve to control high oil pressure feed to the heads. The high oil pressure runs from 500 to as high as 3500 lbs. Feedback from the ICP sensor closes the feedback loop to the PCM in the high oil pressure system.
The exhaust backpressure system has a feedback sensor as well, and it is fed by a long steel tube running from the rear of the passenger side exhaust manifold to a solidly mounted plate near the high pressure oil reservoir. The EBP sensor can short out internally, taking the reference voltage signal to ground, a condition that will disable the engine and kill scan tool communication. Also, the steel tube can stop up with carbon, a condition that will generate Diagnostic Trouble Codes. An old piece of speedometer cable on a drill will usually unclog the tube if that happens. The illustration shows a new EBP sensor plugged into the harness connector and the old sensor still mounted in its hole.
The cam sensor is a major player; the PCM does nothing to start the engine if that signal is absent. The cam sensor signal is read and interpreted by the PCM to determine engine position and speed, particularly marking the positions of cylinders 1 and 4 (first and fifth in the firing order) for sequential injector operation, which means a lot whole lot more on a diesel than it does on a gas burner!
The PCM controls the relay that fires up the Injector Driver Module and communicates with the IDM on three different wires.
The IDM receives two digital control signals from the PCM, namely, the Cylinder ID signal and the Fuel Delivery Control signal. The IDM sends an Electronic Feedback Signal to the PCM, reporting injector circuit problems and other bits of pertinent information the PCM isn’t personally privy to without the IDM’s cooperation.
In closing, if you keep clean air going in, clean oil in the crankcase and the right fuel flowing through the nozzles, you might find yourself shooting for a new record – it’s getting more and more common to see 500,000 mile 7.3L’s running around out there, but it doesn’t happen by accident.
Factoids to follow:
Factoid 2: Specific gravity of a fuel is a measurement of the fuel’s weight as compared to water. The importance of specific gravity to diesel fuel is that it must be heavy enough to achieve adequate spray penetration into the combustion chamber. If the specific gravity is too low, all the fuel will be consumed immediately upon entering the combustion chamber – it should burn all the way through the piston’s power stroke.
Factoid 3: Average Weight Average BTU
Diesel 7.1 lbs (138,000 btu)
Gasoline 6.0 lbs (124,000 btu)
Simple Illustration of the IPR: