[gydiesel]

Here's a problem that I have with my 94 GM 6.5 TB diesel and I hope someone can help.

When I push on the accerator petal from rest position aprox. 1/4 - 3/8 of an inch and hold it there. I noticed a slight jump in rpm. This increase in rmp is more noticable when I'm going down a hill at 80k/h. or when the engine is under low loads. This increase in rpm is also noticed when the cruise control is on. What happens here, when I'm cruising let say at 80k/h and I start to go down a hill. The truck will speed up and slow in a jerking fashion as the cruise control is trying to control the speed. This jerking motion can be simulated with the accerator petal and always at the same petal position.
I can give an example here from 0-10, 0 being the accerator petal at rest and 10 is accerator petal at full throttle. The throttle will seem to go from 0 to 2 and skip 1. This is where the jerking motion is. As I go pass 2 everything is fine.

I have no 'accelerator petal' fault light and have no faults on the 'check engine light' The bulbs in these socket check out OK during start up.
I changed the accerator petal and the pmd anyway and still have the same proble. I'm starting to suspect the fuel solenoid valve might be the problem as it is allowing too much fuel in when the throttle position is at 1.

The truck starts up fine and idles great. once I'm past that throttle position the truck runs perfectly.

Any suggestions

Thanks

 

[gmctd]

Welcome to the forum gydiesel...........

Go thru the Diagnostics Checklist in Reference and FAQ's section, post the results - provides more information for diagnosis

 

[gydiesel]

Here are the result of the Diagnostic

1.Lift pump test - describe results- Works Fine
2.Service Engine Light - on/off/intermittent-Off
3.Model year-94
3a. Odometer reading-200,000 Km
3b. Miles on major engine components if been replaced
4.Do you have any engine codes? Yes or No.-No
5.If you do have engine codes - what are they? - N/A
6.Air Filter condition (visual check)-Good
7.Fuel filter condition (freshly changed or condition unknown) - Replaced recently
8.Condition of Battery terminal connections (cleaned and tightened)-Good
8A.Known condition and age of the batteries. - 4 years
8B.Are batteries of differing age or are they a matched set? - different ages
9.Upon cold start, does the radiator hose get hard quickly? Yes/no - No
10.Upon cold start, do you have excessive white smoke? Yes/no - No
11.Do you have excessive cranking time before the engine starts? - No
12.Have you used the block heater, and does this effect engine starting? - None installed
13.During hard acceleration, do you have excessive black smoke? Yes/no - No
14.Do you have any unusual exhaust smoke issues? - No
15.Turbo check out - pass/fail - Pass
16.Do you have an EGR on the engine? (An F or an S engine code) - ?
17.Indicate if you know if it’s a 1500, 2500, 2500HD, 3500, 3500HD. - 3500HD
18.Indicate fuel that you are using: Bio-diesel, #2 Diesel, #1Diesel, SVO/WVO, other - Regular diesel fuel from fueling stations
19.Are you using any fuel additives? If so, please list. - No
20.Please indicate geographic region you are in: (example: Texas or Canada) - Canada, West Coast
21.Do you have any service history available that might pertain to the problem you are having?- IP replaced 2 years ago, under warranty
22.Please indicate any modifications to the vehicle that might help us diagnose better. - PMD and Acceleration Petal recently replaced, but found out they were not the problem.
23.Upon unscrewing the fuel cap, do you have a large vacuum formed in the tank? yes/no - No
24.Location of PMD/FSD? On the pump/remoted over intake/remoted out of engine bay (please specify specifically) - Remote location, bolted to the Intake manifold using the Cooling Fin method
25.Are ALL glow plugs in proper working order as per this thread? - Yes

Here is a site that I found, when I was doing some research on my problem. Its a Student's Work Book on the 6.5 TB diesel. It won't solve your problems but it will help you out

http://fsunw3.ferris.edu/~millerm/SWB_a8_m16_Final.pdf

Thanks

 

[gmctd]

That is a good site for some 6.5 EFI education - thanks for the link.

Your symptom could be caused by failing FS, or bad fuel - try a brew of 16oz cetane booster and 16oz of two-stroke motor oil.

OR - if the short black ground wire that was on the top of the Inj
Pump is not still there, pull it back thru the harness and re-ground it to the IP.

 

[qwomack]

I also had the same symptoms with an over agressive optic bump. How long have you had the truck? Is it possible that the previous owner tweaked some things?

 

[Cowracer]

Sure sounds like a dying fuel solenoid. I hate to be the bearer of bad news, but I just went through that mess. I had the exact same symptoms, and it just got worse and worse till the truck was undriveable.

The fuel solenoid is not considered a user replaceable part on the pump (though some claim to have done it). The fix requires replacing the Injection pump.


Tim

 

[gmctd]

Use a mirror and get the DS4-xxx-xxxx number off the blue Stanadyne model number tag on the rear face of the IP - it faces the intake manifold.

Post that number here - could be the entire problem on your 3500HD

 

[zaskar_ofn]

it sounds to me like one of the pots of the acelator pedal is on its way out.
Do you own a digital multimeter??
I'll post the pins you need to test with the ohmmeter later on tonight or tomorrow.

 

[zaskar_ofn]

I don't think the fuel solenoid is going. the way it works is this:
the fuel solenoid is just an electromagnet that is driven by the FSD or PDM. the FSD (to me is a more appropiate name) is driven by the CPU of the truck by a square wave signal. the FSD gives another square wave back to the CPU to let it know it fired the fuel solenoid. so, if the fuel solenoid goes, it would be a shorted out situation and it would not go, period. there is a spring behind the plunger inside the fuel pump that would keep the plunger open at all times. and that would not allow the truck to run. now if the FSD is going it would be another set of symphoms you'll be experiencing...

 

[gmctd]

On both APP modules?

If you'll read from the beginning, all his posted data can be used for analysis.

There are at least 6 DTC's for the APP module - he has none..............

 

[zaskar_ofn]

sure. if you noticed there is no guaranty on new electrical items. it could very well be that the "new" one is not that good.
There is a doctrine of testing before buying parts to pinpoint problems.
I've had the error code being set for problems with the app but had no drivability problems whatsoever; all it took is to "reseat" the connector (with some electrical paste.) I don't have my book near me but if somebody provides the wires that are supposed to connect the CPU to the app that would at least put him away from having the app or its wiring faulty.... cheers

 

[gydiesel]

I hope I not going to cause an arguement here.

In response to 'gwomack' I brought the truck from a dealer in 2000. The IP was replaced in 2005 (under warranty) so I assume the Optic Sensor was never adjusted. Though the possiblity of the sensor adjustment vibrating loose is a thought on my part. Will check it out in a couple of weeks and will post the results.

For 'zaskar ofn', the accelerator petal and PMD were both bought new. When I first experienced the problem about a year ago it wasn't as bad as it is now, yet it is still a liveable problem, I know the accelerator petal position where it happen so I either slow down or speed up pass that zone, it just doesn't work well on Cruise control unless I'm pulling a load.
I agree that there is that possiblity of buying a faulty new product, but the symthoms are still exactly the same. No codes or faults and same petal position. In the mean time I'll be checking the wiring for bad or loose connections

If it is the Fuel Solenoid Valve (FSV) then I agree I'm stuck buying a new IP. What I'm also planning to do, is see if I can measure the voltage signal to the Fuel Solenoid Valve. If the signal is smooth when I pass the accelertor petal through the 'Fault zone' then I can rule out which side of the fence the problem may be comng from. FSV or the control to it.

Thanks

 

[gmctd]

Good response - but why not post your DS4 nomenclature, so we can get this show on the road?

 

[gydiesel]

Here's the Model # of the IP, D24-8E1-5521. Just a note, this IP was not the stock pump on my 94, 3500HD dually. It was replace a couple of years ago with this model.

I haven't check the Optic Sensor yet, I'm going on a trip so I don't want to mess with it now.

I put a volt meter across the two wires leading to the Fuel Solenoid Valve. The readings appear to be linear, meaning that the voltage did not seem to jump excessively through the problem zone.

Just a thought here... If I install a lower resistor in the PMD socket, I'm wondering if it will at least damped the surging effect. I know it will reduce
fuel consumption therefore lower HP, but it might be worth the trade off.
The existing resistor is a #6

Thanks

 

[chrisk1500]

You have the wrong IP on the truck.....unless your chip was changed....

The IP you NEED (if the PROM chip is stock) ends in the numbers 5068....

The wrong IP will cause the symptoms you are describing...

 

[gydiesel]

In response to 'chrisk1500'

The IP was replaced under the GM warrantty at a GM dealer 2 years ago. Upon recieving the truck after the IP replacement, there was a drastic change in HP. I could move forward, but the truck lacked major HP backing up. I twas like no fuel was getting to the injector. I return the truck back to the dealer and told them of the problem. They later explained to me a chip had to be replaced on the truck.

After the chip was replaced the truck ran normal. So there may of had been a good chance the original IP was the 5068 series IP.

I phoned the GM dealer who replaced the IP on my truck a week ago and asked them if they had any records on the type of IP that was originally on the truck and thier answer was 'No'

But as far as I can remmember, after the IP was changed it ran fine. at least until a year ago.

Thanks

 

[gydiesel]

Hi folks back from my trip. now back to my truck,

Started thinking along the lines of how the Optic Sensor works on the IP and when the member WOMACK mentioned the problem may be an overly aggressive optic sensor bump, I though ' OH well I'll check that'

After removing the cover to the Optic Sensor, I moved the sensor about 1/16" towards the driverside and then put it all back together. I started the truck up and drove for a while and it seemed like the problem was still there. I returned home after the short ride and waited to go to work. Well! after I started the truck up again and drove to work, I noticed the problem has disappeared at least to the point of not noticing the problem.

So without jumping to conclusions here, I still need to test run the truck a bit more and I'll post the results.

 

[knkreb]

In retrieving codes, you did get a code 12 correct? Your symptoms mimic mine before IP replacement.

 

[gmctd]

Each adjustment needs about 30mins of drive time, so PCM can 're-learn' the driveability quotient - try it for a couple days, see what happens.

In response to an earlier post - a faulty new component will give the same DTC(s) as a faulty used component - a faulty connection will give the same DTC(s) as a faulty component, used or new - removing and replacing components often burnishes the connector contacts, restoring circuit continuity, whereby the component gets the bad rap..

No Service Throttle Soon indicator would preclude a third APP module swapout - logically, it's time to move on.

The Fuel Solenoid will give those exact symptoms, if you'll search and read thru the posted descriptions of same, becoming scary in the increasing failure - read Cowracer's thread for more on that.

Each item eliminated as not causative is not the problem - the 5521 IP eliminates age of the oem IP as being causative.

May also be load(s) of bad fuel.

 

[gydiesel]

Hi again,

I drove the truck today and it looks like the problem is solved. The truck controls well in cruise control mode with no hesitations. It feels a little wierd because I knew when the engine was going to surge and now it doesn't.

The Optical Sensor is a bit of a mystery to me, but I quess it Retards/Advances the fuel injection timing, similar to turning a distributer cap on a gas engine.

In response to KNKREB, I did not get a DTC code at all and after reading the threads here I replaced the PMD and a GM mech replaced the Accelerator Petal (APP) and that was a total bill of around $1000. If I could not find the problem I was planning on buying a rebuilt IP and that would of cost me around $1500 installed.

The money spent was well worth the knowledge in understanding how my truck fuel injection system works now and I have a spare PMD and APP also which I will take on those long holidays with the 5th-wheel. So I will say thanks to this site and to those who added thier thoughtful insite into helping me with this problem.

Bless you all.

PS. If I get another problem I can't solve I'm comming back. but I'll keep tabs on other threads here anyway.