Accelerator Pedal

[Trae]

Ok here

[patrick m.]

i see a snag. The cruise doesnt use the APP to control throttle.
Since the eng does this when using the pedal, and when not, there must be a different explaination.
must find the common denominator.

Since you have no codes, you may need to look closely at the ECM.
I would start with a visual inspection of every input the computer uses to determine "speed requested". such as VSS, APP, stop lamp switch, etc.


for example, if vehicle speed signal is dropping-out during decel, there could be excessive clearance at the tail shaft bushing.

[Trae]

Forgive me if I get the acronyms wrong and please correct me. I can elaborate on the VSS sensor.
VSS (vehicle speed sensor) am I correct in assuming we can eliminate this since the sudden drop to idle happens when in neutral not moving and slowly leting off the accelerator.

[charliepeterson]

The good news is your APP is probably good because when it fails it will set a code along with anything else electronically connected to the IP to keep the truck running.
The bad news is what ever is causing the issue isn't setting a code. One component which will never set a code unfortunately when it fails is the PMD/FSD. This little black box is the brains for the entire operation for the injection pump.
You have high mileage, do you know how many miles are on the Injection Pump? There are a hand full of codes connected to the IP for it failing electronically. Not for it failing mechanically.
I would open up the harness to the IP and check the wiring. The engine has some key ground points at the rear of the right head. Also look at the battery connections very closely.

[patrick m.]

if the condition occurs in park/neutral as well, you can move to other items first, but eliminate nothing untill proper tests and inspection have been performed.
Many times engine speeds above idle in park/neutral can cause a vibration in the tail shaft of the transmission resulting in the VSS picking up a "speed signal".

This has caused problems most often in a vehicle that would need "fast idle" for operation of a PTO for example. As soon as the computer sees vehicle speed input, "fast idle" is aborted.


If your mechanic has determined the APP is the cause, let him replace it. If it turns out the APP doesnt correct the problem, remind him you paid him to correct the problem, not gamble with your money.

If it is the APP, your problem is gone ;)

[Hye]

Welcome.

I agree with Patrick. Replacing the APP won't change anything.

There is another post about this "problem", Cruise Control Wierdness, March 31, 2004. There is a search function located near the top right of this page.

The APP is three sensors in one. If they don't all report the same pedal position, you'll get the STS lite. If the problem persists then you get a code. There's nothing in the book about having a bad APP without the STS lite or/and codes.

Not many long hills around here, but there is one that makes my '94 (289K and counting) do this. Otherwise, it behaves quite well.

[Trae]

I would have to find the service ticket but I believe the pervious owner had the Injection Pump replaced along with the engine at 200k. Im preaty sure the APP was replaced prior to this.
I have done searches on this site that is why I question the mechanics logic since he believes the APP is responsible for this even under cruse control. Guess Ill start with the electrical connections.

[kowsoc]

I agree with Charlie about the FSD. I know someone who's cruise control wouldn't work at all. A new FSD fixed it. The APP should set a code. If you have a spare FSD to try that would help.

[chickenhunterbob]

I too see a snag.

In fact, in my time at TDP, I have heard experienced voice of John Kennedy proclaim that these are very reliable!

Also, in my time here, I hever never once heard of a member erplacing the APP module and solving a problem. Occasionally I have heard some say the erplace and whatever problem they had persisted. And the are very expensive.

I have had codes that indicate APP module, but I am of the suspicion that these codes can also be caused by rather simple to fix wiring problems mentioned by Charlie and Patrick and others, such as battery connections, grounds, and if all else fails, and the APP module is suspect, before shelling out the 500 bucks, I would give a go to the procedure posted bt Chevdooley in the appended link...

http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=005475

[Trae]

Actually the pump mount driver with heat sink will be my first mod. Up until now I have been fixing the little things the last owner neglected such as vacuum leak, exhaust leak, hoses, Etcetera. I plan on having this done before the summer heat hits here in Dallas, TX. I will be very happy if it fixes this annoying throttle problem as well. Ill post the outcome once I have this done in the mean time keep the suggestions coming as they are always appreciated.

[Trae]

Worked on the truck a bit today and found that the lift pump is not running so I replaced the oil pressure switch and now the lift pumps runs. Took it for a spin around town with lots of heavy acceleration and had no hesitation or hiccups. Hopefully that was it in a few days I should know for sure. Now im just wondering why my mechanic didn