Finding a weak Power Stroke Nozzle

Richard McCuistian

Isolating a malfunctioning injector on an ailing Power Stroke can be challenging, to be sure.  When I was at the Ford dealer, before the $1000 Rotunda cylinder-killing tool became available, I used a Radio Shack project box and built a tool that could be plugged in between the valve cover connectors and the wire harness, and by operating eight toggle switches, we could kill injectors and listen for a change in sound with the idea that we could isolate the weak cylinder by sound like killing cylinders on a gas burner. I sent the plans for my homemade tool to Ford and received a reply that they weren't interested in anything like that.  A year later, Ford tool supplier Rotunda came out with one of their own. Go figure. 

Anyway, the biggest problem with that approach is the fact that a faulty Power Stroke injector might after all be delivering at least some of the fuel it is supposed to inject, and even when the guilty injector is electrically neutralized with the tool (whether it be my tool or somebody else’s), the PCM’s propensity to instantaneously alter the fuel delivery to the other cylinders usually renders the listening ear ineffective.This ain't like crackin' lines on an old diesel to find a misfire, I can tell you that for sure!

Ford’s approach to this problem was to access the Mass Fuel Desired (MFDES) Datastream PID on a tool that can record minimum, current, and maximum numbers and watch for the injector that changes the MFDES PID the least. For example, if killing injectors 1-7 each cause the MFDES to increase but killing injector 8 doesn’t raise the MFDES value, then 8 is the guilty nozzle.  The injectors that are completely dead are fairly easy to find, but locating the partially dead ones can be disgustingly time consuming and uncertain.

My friend Tim Hogan, who was the training director of AASP Houston a few years ago armed himself with an Interro Systems PDA and a low amp probe, and he came up with a better way to isolate faulty injectors.

Setting the scope’s timeframe to 500 microseconds and the range to at least a 70 amp scale with the low amp probe clamped around the injector trigger wire in question, the pattern in the screen shot tells the tale.  The jagged sawtooth part of the pattern represents the jittering signal that machine-guns the oil control poppet off its seat, creating what Tim calls a “Bart Simpson Head” (we’ll call it a BSH for short).  

First generation Power Strokes only have one BSH, as seen in the ‘94 injector waveform illustration. The illustration you see here is of a bad waveform; notice the ragged downslope - it shouldn't be there.

Second generation Power Strokes apply an initial amount of amperage to the injector coil to unseat the poppet, then a lesser amount of current to hold it open for a short period of time, thus the two BSH areas on second generation engines.  Interestingly enough, the second generation PSD waveform looks almost exactly like the Duramax injector waveform, BSH included!  The point is that the down slope right after the BSH should be a straight line as you see indicated in the waveform below. 

The ragged bump (see the area indicated by the yellow arrows on the bad waveforms) indicates an injector that needs replacing. If you don’t replace every injector with this bad bump pattern, the engine still won’t run right. 

The exact reason why a bad injector waveform exhibits this scratchy bump (some look considerably nastier than this) is something of a mystery at this point.

Tim says he has used this method on a lot of different trucks with excellent results. His report is that the scope doesn't lie.