Diagnosing Drivabilty Issue

[OmniEquipped]

Hello. I recently bought a 94.5 PSD with 167k miles and 5-speed. On the way home it lost power and died. Diagnostics indicated the computer failed. I installed a reman computer and fixed it, but it slowly developed an issue where the first 50% or so of the throttle pedal was unresponsive. I took it in and had the computer flashed for OBDII protocol and the latest updates. It ran perfect for weeks after that. Now it has developed an intermittent issue where it cuts out under a light load or while trying to maintain a constant speed. When this happens, the cruise control does not work. Clears up under acceleration. Idles smooth, but the miss will manifest when holding high RPM in neutral. There is no smoke, except a little bit at cold start. There is no CEL and no codes present. Live data is all within normal perimeters. Injectors passed buzz test. High pressure oil is good. Fuel pressure is good. I have replaced ICP, IPR, valve cover gaskets and unhooked the fuel heater.

I'm stumped as to where to go from here. I'm suspect of the computer, but the scanner shows everything is normal. I'm thinking maybe injectors, but it doesn't seem to have normal injector symptoms.

 

[greenskeeper]

Check out the throttle position sensor (TPS) and idle validation switch (IVS) at the pedal.

When first pressing the pedal you should hear the IVS switch click closed telling the PCM that the pedal is being depressed and to begin looking at the TPS for throttle input.

My guess is when you get no throttle response, the IVS is sticking and not telling the PCM to come off of idle.

Most of the time this can be cured with a blast of compressed air to free the stuck IVS due to dirt, etc.

Sometimes the pedal has to be removed and the IVS adjusted.

If you have a scanner that can log or display individual parameters you should see the IVS close slightly after the pedal is depressed. The TPS should have a range from roughly 1 volt at idle to 3.5 volts at full throttle (I'd have to look for the exact specifications but thats what I recall)

 

[OmniEquipped]

Thanks for the suggestions...finally got this solved. The IVS checked out good. Turns out there were two issues. The intermittent stutter issue ended up being the TPS. Didn't catch this immediately because the voltages showed to be normal when reading the live data. But we eventually noticed a very minor fluctuation under certain load conditions, even when the throttle itself was at various positions. Stuck a new TPS on it and no more problems. It ran great except the cruise still didn't work. So after testing, metering, probing most all the components in the cruise circuit (aside from the ABS) I found that the "ON" side the ON/OFF switch wouldn't measure anything on my meter. I took the switch out and carefully disassembled it and determined the contact prongs on the ON side were fatigued so I bent them upward slightly. Switch then metered good and the cruise works like a new one.

So two things I learned: The TPS can cause problems beyond the common dead or spotty throttle issues that I've read about. Also learned that the non-serviceable cruise buttons can actually be serviced if you carefully disassemble them. So glad to get this thing lined out. Nothing is more frustrating than weird, intermittent problems.