Pressurized Black Gold – an Engine’s Lifeblood
Ed was a very unusual guy I knew in the early 1980’s. He drove a 1974 GMC pickup and he told me one day that it had died and that he had to spend over $400 to get it going again. He had hired it done because quite frankly, Ed wasn’t too wrench smart.
“What was the problem?” I asked.
“Timing chain broke,” he returned. That sounded interesting. How many small block Chevy engines break a timing chain? Usually the chain on an older V8 stretches, gets loose and causes the cam timing to run a few degrees behind, robbing the engine of vacuum and power. The next step is generally a stripping of the nylon teeth from the gear (and an oil pan full of dangerous oil pump-locking nylon shards) but the timing chain generally stays intact. Ed continued. “When the timing chain broke, it bent most of the valves on both heads. This was the first unscheduled maintenance my truck has ever needed.” My mind was still whirling around that broken timing chain issue. I wanted more information, if he had it. He did.
“How many miles did you put on it?”
“I bought it with 2500 miles showing – it was almost brand new. But the speedometer stopped working at 385,000.”
This was getting even more interesting. As we continued our conversation, I discovered Ed’s secret. Remember, I said at the beginning that Ed wasn’t wrench smart? Well, in his own mind, he decided from the beginning that 1500 miles was a long way to go on the same oil, so for the past 400,000+ miles, Ed had changed the oil and filter every 1500 miles. Religiously. Just like clockwork. Think of it. What engine wouldn’t last a long time with a consistent supply of new oil in the crankcase every 1500 miles? Ed didn’t know 1500 miles was too often, and he was also disappointed that his truck should have lasted longer than 400,000 miles – go figure.
I spoke with the guy who does Power Stroke diesel work at the Ford dealer where I worked until the end of 2000, and he was pulling maintenance on a 7.3L that had 600,000 miles on it. According to the owner, that truck had always enjoyed 15 new quarts and a fresh filter every 5,000 miles.
Every sensible vehicle owner knows how important oil changes are, but there are some folks that think they don’t need to do anything under the hood as long as the engine seems to run okay. I’ve actually counseled vehicle owners who voiced that idea.
There’s More to it Than Oil
Granted, frequent oil changes alone are no guarantee that an engine will last. Regular cooling system service (a healthy cooling system should run about 210°F), a good tight air induction system with regular filter replacements, and good quality fuel are all essentials.
Driving habits are just as important. A granny’s-one-owner cream-puff might drive like a dream, look as clean as a pin, even under the hood, but the engine’s innards might be dreadfully sludged if she only drove it a mile or two at the time over several years.
Case in point: I know a guy who bought a 1993 Acura Vigor like that. It looked and drove like brand new, but the first time he drove it to Birmingham, sludge in the oil pan clogged the pump screen, and you don’t have to be a Phi Beta Kappa in engine repair to know how that deal turned out. He had to buy a replacement engine after owning the car only a couple of weeks.
In a word, a traveling salesman’s car, if properly maintained, can rack up some pretty large numbers on the speedo clock if he or she keeps it in the wind, but with evenly spaced pit stops for regular maintenance as prescribed in the owner’s manual. Another case in point: The 1995 Taurus my son now owns has 250,000 miles on it without having ever needed anything except a water pump, but that’s because I maintained it so carefully when it was mine, and Matt has continued the maintenance pattern I started.
Keeping Things Cool and Clean
Lost oil pressure can destroy an engine about as quickly as a rocket propelled grenade, and more than a few engines have welded themselves together after starving for one reason or another. At the college, we worked on a1999 F-150 that was running fine when the lady parked it, but the next morning the engine wouldn’t spin. At the shop, we found it seized, and the rod bearings were literally welded to the crank journals. We replaced that engine with a crate unit from Ford, but I explored the original mill and never did determine what had caused the meltdown. The oil pump was in great shape, nothing was broken, and the relief valve wasn’t stuck either.
But we all know there’s more to the crankcase oil than lubrication. It also cools the parts with its splash and works to suspend solid contaminants so they can be removed by the oil filter. And get this: According to Ford Motor Company, running a 5 quart engine with just 4 quarts of oil can cause those 4 quarts to break down in as little as 1500 miles.
Heart of Steel
The oil pump is basically the only mechanical component in an engine that receives unfiltered oil, and obviously, different engineers choose to drive their pumps in different ways. While camshaft driven oil pumps (with two small intermeshing gears working in a specially machined and ported housing to move the oil) were extremely common (via a connection to the distributor shaft), the driven shaft on that type of pump was essentially located in line with the distributor. These early pumps were generally made of cast iron with steel gears with a pressure relief valve that was a part of the pump.
Since engine oil pumps are positive displacement, they require the relief valve for overpressure protection, but that same valve and spring also determines how much pressure an oil pump is able to shove into the galleries, which are skillfully drilled so as to feed whatever needs lubing and cooling from the crankshaft up.
Bearings shouldn’t have more than 0.0015 clearance on the average, or pressure tends to squirt past the loose bearings back into the pan instead of making its way through the gallery the way it is supposed to. That’s why using green Plastigage (®?) is so important when building an engine – no amount of micrometer measuring can be as accurate as checking actual bearing clearance with a flattened piece of green wax. Even a half-thousandth of an inch too much clearance can cause rumbling main bearings on an otherwise healthy powerplant.
Reinventing the Heart
The demand for lighter weight engine components brought aluminum-housed oil pumps (steel gears remained even until today) that were still distributor driven like the old cast iron units but situated on bottom of the timing cover, and the pressure relief valve on those units was subject to be somewhere else in the oil circuit.
The oil pump gears on those timing cover-mounted distributor-driven pumps are accessible from the outside on these; if the distributor is mounted at an angle in the front of the engine, it’ll generally have an external oil pump on the opposite corner of the timing cover pulling the lube through a tube and screen that leads back to the sump. Ever had to remove the oil pump plate on a late seventies Buick V6 and pack the gears with grease to get it re-primed after an oil change? I have!
In the early eighties, many engine designers began using crankshaft driven gerotor or crescent gear style oil pumps mounted in an aluminum housing behind the balancer, and that trend continued.
A growing number of today’s engines have crankshaft-driven oil pumps of the gerotor or gear-and-crescent design, rather like some of the automatic transmission pumps, and when the pressure goes low for one reason or another, the lifters will generally begin to rattle. The Toyota Camry 2.2L oil pump is a peculiar little 4 tooth gerotor driven by a dedicated cog via the timing belt.
The relief valve on that 2.2 is mounted several inches away from the actual pump in the aluminum front cover. The Ford Power Stroke uses an oil pump like that, and sometimes the relief valve (mounted below the oil pump in the timing cover/oil pump housing) on a 6.0L will get fouled by trash, and it can destroy the pump.
Checking and Tweaking
Years ago as a fleet mechanic, if I knew everything else on an engine was healthy and time was of the essence, I would add a washer to stiffen the oil pump relief valve spring, but only if I could determine that the oil pressure wasn’t leaking past loose bearings and I could tell the spring had weakened and wasn’t allowing the pump to do its best. In a word, the pressure relief valve is as much an oil pressure regulator as anything else. Not only does it relieve high pressure, it creates pressure by providing a calibrated restriction.
To prevent damage due to oil filter clogging, a bypass valve internal to the filter (or built into the filter head) prevents oil starvation, but be careful: Some oil filters actually house the pressure relief valve for the oil pressure system, and if you screw the wrong oil filter on one of those babies, there won’t be any pressure at all in the system, and you just bought an engine. It’s easy for this failure to occur, especially if the oil filter threads, gasket, and whatnot looked close enough on a cursory exam to do the job. I personally know of one oil change outlet has had to buy a Cherokee 4.0L because they used the wrong filter and the Jeep didn’t even make it a mile before the inevitable meltdown.
Overhead cam engines need a pressurized oil feed to the camshaft (fed through the cylinder head gasket), so another possible leak point is present on OHC units of that type. Remember the Dodge Stratus we fixed that had a nasty oil leak from that head gasket passage?
So what are some of the reasons an engine can lose oil pressure?
Try foamy oil (too full or wrong grade), slow idle speed, low oil level, restricted oil filter, oil that is diluted, a bad oil filter bypass valve, or a hole in the pickup tube. Some other fairly obvious mechanical faults would be loose bolts at the oil filter adapter or oil pump, missing or damaged seals, worn out pump, sludge-plugged oil screen (ever replace a set of lifters in a V6 only to have the engine lose oil pressure after you’re done because of sludge migration into the sump?) Missing or incorrectly installed gallery plugs, loose bearings (don’t forget the camshaft!), cracked, broken, or restricted oil galleries, and other such issues.
I’ve heard tell that some mid nineties Cadillac crank-driven oil pumps can lose prime in the time it takes to walk into a convenience store and buy a bag of chips due to a stuck relief valve or a loose harmonic balancer bolt. The fix? Check the bolt first. If it’s tight, then the relief valve might be fouled. If that seems to be the case, dump 10 to 12 quarts of engine oil in the crankcase (on top of the original 7 quarts) to immerse the pump, then start the engine and rev it repeatedly to 3500 rpm to re-prime the pump and break the stuck relief valve loose. Don’t drive it or exceed 3500, or you’ll whip the oil into a foam. When you think you’re done, drain the oil, refill with 7 quarts, and recheck the pressure.
Along that line, my compatriot at the Ford dealership drew a ticket on a late nineties Ranger with a nasty vibration and it turned out that the engine had eight quarts of oil in a five quart crankcase.
Keep it clean, keep it full, and run it warm to keep the odometer rolling. That’s my philosophy. R.W.M.