I just bought a 94 Suburban 2500 6.5 TD (F engine) with 160,000 miles on it. It is driving perfectly as far as I can tell but every once in a while the Service Engine Soon comes on temporarily while I'm driving. I pulled the codes after I got home (no SES light was on then) and it gave codes 35 Injection Pulse Width Error (Response Time Short) , 84 Accelerator Pedal Position (APP) Circuit Fault, and 99 Accel Pedal Position (APP) 2.5 Volt Reference Fault.

Any Ideas?

Although I do not know when the Injection Pump was last replaced it does have the new green tag model installed with the PMD mounted directly on it. Is it still necessary to attach a heat sink to the PMD on the new model IP's? Could the code 35 indicate an impending problem with the PMD?

As for codes 84 and 99, I have no clue what it might be. Any suggestions would be greatly appreciated.

DTC 35 could be airbubbles in the fuel lines. The other two codes could be a connection problem, The APP is $400 and finding a bad ground or melted wire is likely.

Although I do not know when the Injection Pump was last replaced it does have the new green tag model.
Green Tag, I'll bet its a 5288 model, with the old 'new' optic sensor in it, and probably the old 'new' PMD as well.

DTC 35 could be air or PMD problem, but its most likely a pump problem, fuel solenoid/poppet valve spring are worn out of tolerances.

The other two codes indicate a problem with 2 of the 3 sensors in the Accelerator Pedal Position Sensor. So it probably needs to be replaced too.

Thanks for the feedback.

Today I noticed that the cruise control is not working. In fact the part of the "multi function" lever that has the cruise control switch on it feels very loose. I wonder if this could be related to the codes 84 and 99 or if it is strictly due to a bad APP. Maybe I can find a used APP at a junk yard instead of springing $400 for a new one.

As for code 35, if it turns out not to be air bubbles (I assume that there is not much that can be done about air bubbles in the lines short of replacing them and the tank) and the injection pump is going bad I guess I'll have to get a LOT more comfortable with disassembling the fan, intake manifold etc. so that I can replace the pump myself. Paying a shop $2000 to do it is NOT a possibility for me unfortunately.

You can determine if it has air bubbles by installing a piece of clear tubing right before the IP fuel inlet.

Changing the IP is not that bad. It will take you a day the first time.

1st the Good news. I found a short in the wire from the Multi function (cruise control) lever and spliced it out. That fixed the Cruise control and so far I am no longer getting DTC 84 or 99):h !

Now the bad news. This morning while I was enjoying my newly restored cruise control the Service Engine Soon light came on. As soon as it came on Cruise control stopped working again. It stayed on for longer than I've seen so far. The longest it had stayed on for in the past was 2 minutes. I bet cruise control will work again once the SES light turns itself off. It still seems to drive perfectly with the exception of cruise control not working while the SES light is on.

As soon as I got home I checked the DTC codes. Code 35 is back but now it is giving DTC 36 too :eek: ! Is this the first signs of the dreaded Injection Pump failure or will remote mounting the PMD do the trick. Either way I'm ordering a remote mounting heat sink for the PMD this morning.

Thanks again for the continued help.

I used to see the same thing on my first IP. I keep driving it and it would get impossible to re-start hot, then eventually the fuel metering screw broke and the truck surged when you slightly touched the fuel pedal. Start looking for a replacement IP.

We'll do thanks.