Please help me diagnose what the problem is

Hello,

I have a 1995 K2500 Extra Heavy Duty with the L65 engine option, 6.5l turbo diesel.

Recently while driving the truck, I have noticed that the accelerator doesn't always respond like it should. ie. you are driving along, truck is running fine. You take your foot off of the accelerator, slow down, then go to accelerate and there is no response. If you really push your foot into it, it will then leap back to life.

At the same time, I was driving along, only to get the "Service throttle soon" light up. In a couple of cases, the truck has gone into a limp mode of staying stuck at 1000 RPM regardless of how far I try to push the accelerator.

If I turn the truck off, it always restarts and then sometimes the "Service throttle soon" light will not appear and the truck will run fine for a while, only to reapeat the accelerator and "Service throttle soon" nonsense.

Recently, I disconnected the batteries, let the truck sit for a while and re-connected the batteries. The truck ran fine for quite some time and ran very smooth.

Only to find this morning, soon after I started it up, this nonsense came back of the "Service throttle soon" light and going into this 1000 rpm nonsense.

So, I took my Jeep to work instead.

I haven't tried to find a Diagnostic Trouble Code as of yet, but plan to.

Also, 7000 km ago, I replaced the PCM on the injector pump and it has run fine until this other nonsense appeared.

Worse yet, I just had some work done on the front end, ie. brakes and other stuff.

Any suggestions?

Thank you to anyone that responsds.

2Lane

First thing to do is read the stored codes.

My guess is that "Service Throttle Soon" is more than just nonsense...

Quote:

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Originally Posted by JohnC

My guess is that "Service Throttle Soon" is more than just nonsense...

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X2! It seems to me that your truck is doing its best to tell you what the problem is... :D

Point well taken, it is probably not nonsense. However, there has been times when the vehicle has given false readings. One tends to feel that it is crying wolf sometimes.

What is meant by "Service throttle soon?" What is involved - assuming that there is no false readings.

Thanks.

2Lane,
I had this same problem too, after a pump/pmd was replaced by the dealer.

Definately go down to Autozone or your local parts store that has a scanner. Get the code so that you know.

But, also check your grounds!!! My grounds that go to a stud on the intake manifold back by # 8 cylinder had loosened up-big time. Once I tightened my problems went away (9months)- I also has some other symptons as it would die and the Service Throttle light came on...my volt meter, oil pressure gauge were behaving erratically.

Bad grounds will do crazy things...check them before you start chasing parts.

Brent

If you do replace the throttle assembly, (your footfeed is basically a spring loaded, electric potentiometer) call the parts yard. New ones are $300. Used can be found for $50.

And since you are in Canada, a new APP module can be had for slightly more than 300 dollars.

I was quoted 670.

But as suggested already, read your codes, if you have a GM service manual the procedure is in there, and quite simple with OBD 1.

With key in off position, connect the top right two pins of the diagnostic plug (under the dash up around your left knee when you are seated in the driver seat) with a paper clip or other suitable wire, and turn key to "run" (don't crank engine) and count the flashes of the SES light.

Be ready with a pen and paper and record the flashes.

Each series will flash 3 times, with a little longer pause between first and second digits, example, code 33 would be "flash flash flash" pause "flash flash flash" (and over again twice before on the next code).

And since APP module are so expensive, they are worth trying to salvage the existing one, if at all possible.

I'll try and see if I can find a previous post on a possible repair, and edit to include in this post.

From reading the information on these boards the last few years, the APP module seldom fail, but anything can. Electrical gremlins can pop up at any time, and can often be repaired by simply unplugging and re-seating electrical contacts, and regular maintenance of battery connections and associated grounds and such.

Edit: tried search feature, appears as though I can only search back a few months, and two searches using identical keywords net two different results.

At any rate, the procedure (initially posted by Chevdooley) you can try if it is established the APP module is the culprit is to carefully drill a small hole in the plastic cap on the end, spray in contact cleaner, swoosh around and drain and dry out as best you can, plug the hole with silicone or such, and try again.

It has worked for some.

The throttle assembly has 3 tps units that all work together.
The computer needs to see the same reading from all of them within a certain allowance.
If one dissagrees then the code will set, if 2 are hinky then the computer sets the limp mode.
Beings this is a fly by wire system they have to have a safety to prevent a runaway condition.
Get the code and most likely you will just need to replace the TPS unit.
Good luck
Robyn

The Chevydooly TPS/APP [AcceleratorPedalPosition/ThrottlePositionSensor] service procedure referenced by C-H-Bob will at least help in troubleshooting. Grounds and batt cabling is good initial service (after codereading).

The PCM[PowertrainControlModule] is in/behind/over the glovebox. It has multiple functions (CPU+) including directing engine operation after incorporating sensor input, like APP and ECT and boost.
The PMD/FSD[PumpMountedDriver/FuelSolenoidDriver] comes OEM screwed onto side of FuelInjectionPump. It energizes the solenoid-valve on the FIP that squirts pulses of diesel fuel down the injector lines.

The codes are your friend... happy motoring!

Quote:

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Originally Posted by tom mac 95

The Chevydooly TPS/APP [AcceleratorPedalPosition/ThrottlePositionSensor] service procedure referenced by C-H-Bob will at least help in troubleshooting. Grounds and batt cabling is good initial service (after codereading).

The PCM[PowertrainControlModule] is in/behind/over the glovebox. It has multiple functions (CPU+) including directing engine operation after incorporating sensor input, like APP and ECT and boost.
The PMD/FSD[PumpMountedDriver/FuelSolenoidDriver] comes OEM screwed onto side of FuelInjectionPump. It energizes the solenoid-valve on the FIP that squirts pulses of diesel fuel down the injector lines.

The codes are your friend... happy motoring!

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Ooooh. Bad choice of words. Steaksauce always used that closing. Still makes me gag.

Tim

Code 25 came up last night. Neighbor accross the street is a diesel mechanic and worked for General Motors. I asked him for some help and it was really easy from that point forward.

APP was the culprit and replaced it. $400 bucks Canadian - cash 'n carry at my local GM Parts to go counter. I had thought of a wrecker, but some of the local wreckers want almost as much for a used part than a new part. Some may say it's a waste of money, but it's my truck and I'd rather have a new part on there. Infact, I have been upgrading alot of stuff on it as I go.

What is interesting, is how much more throttle and power this truck has gained access to. The previous APP was never as sensitive as the new unit. The slugishness that the truck used to have (lack of over all throttle response) is gone. I put my foot into the accelerator today, like I used to and got a bit of a chirp from the GoodYear Workhorse tires and jack rabbited out into the intersection when the light turned green! Whoa!

I also notice that when the truck starts up cold, it idles slightly higher and the rpm's remain consistent at start up. Before, it never did this and went through a coughing and weezing while the rpm dropped and then bounced up and then fell back down, continuing this until the truck finally warmed up.

Thanks to everyone who replied and gave me some feedback. Very much appreciated. Happy motoring...

2Lane

A manual that I use says:

"If the PCM has determined a fault in one APP sensor the PCM will store a DTC but, it will not turn on the "Service Throttle Soon" lamp. The vehicle and throttle pedal will operate normally.

If the PCM has determined two APP sensors are faulty, a DTC will set and the PCM will turn on the "Service Throttle Soon" lamp. Some engine performance will be lost.

If three APP sensors malfunction, DTC will be stored, the "Service Throttle Soon" lamp will come "ON," and the PCM will only allow the vehicle to idle.

In case of an intermittent fault (a fault not recognized by the normal APP DTC setting criteria), it will then only allow the vehicle to operate under limited performance and store a DTC 84."

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I checked the wiring diagrams and it seems all the three potentiometers have their own dedicated wiring to PCM (+5V, GND, output). Considering this, having a sudden total APP failure does not sound logical. Instead, you'd loose one first, then the second and finally all three leading to limp mode, which well may be the case here.

I would check the APP operation with a multimeter first measuring at the corresponding PCM connector. Then with a scan tool to find out what the PCM readings are.

-ari

>>Bad choice of words.

Apologies. Cogito ergo moot?