No throttle pedal responce

[Yukon6.2]

Hi All
Was coming back from a 125 mile tow job with a Uhaul 1 ton cube van last night when my 97 3500 with a 5 speed tow truck started acting up.We have a long hill about 6 miles out of town, i was coming down it loaded and had been noticing that whenever i let off the throttle it would buck/fishbite as the pedal returned to the idle postion.There is a couple of intersections at the bottom of the hill so i like to come around the corner at the bottom a little slower than posted speed limit,i went to gear down and the motor quit as i pushed in the clutch.I had been coasting down in 5th keeping the speed down.I coasted past the intersection and put it in 4th and let out the clutch and the motor fired up,went to give it some throttle and it started to buck as soon as i gave it any throttle.Motor runs fine at idle,give it any throttle and it bucks like crazy.I limped it home trying at times to give some throttle but all it would do is buck.Seemed to run fine at idle pulled up some small hills in 3rd at idle postion,felt like i could give mabey just a minut amount of throttle.coming down a small hill i tried to give full throttle but there was no responce to what my foot was comanding.No SEL and i wasn't able to do much last night,the lift pump was ticking,shut it off and it starts great.
I have been thinking about the bucking on deceleration which has been goin on since i changed the motor,and thought the pedal assembly might be the culprit.
Any ideas i can follow?
Thanks Thomas

[JohnC]

Pedal problems will always set a code. Check the stored codes as it may not turn on the CEL. More likely, though, it's a filter or lift pump or injection pump problem...

[midnightrumbler]

Check out my threads, I just diagnosed a faulty APP. Do not believe what everyone tells you as these units can and do fail. Everyone likes to THINK that the problems are related directly to a faulty IP and I do not blame them as we can only go off by what individuals are describing.

Seems like everyone points the finger at a bad IP though.

[JohnC]

You can monitor the APP signals to the PCM with a scanner. If they are causing a running problem the scanner will show it. If the scanner readings are steady and the performance is not steady then it has to be something else.

The app has 3 independent sensors that all work differently. The PCM constantly compares the 3 signals and sets a code if any one does not agree with the other 2. There are wiring issues, e.g. bad ground or reference connection, that might make all 3 act up in a way that would fool the PCM, but it is virtually impossible for all 3 sensors to fail in a way that would not set a code.

[Robyn]

Get the thing scanned and see what if any codes are present.

If no codes are present then it could possibly be a PMD. This sort of symptom does not fit the classic parameters BUTTTTT.

The fuel filter is always suspect as is the lift pump.

A badly plugged fuel filter will cause what you described as will a lift pump thats failed in conjuction with the Filter.

With the engine at idle open the water drain valve and see if fuel comes out. If the thing dies right off then you have likely found the issue. (Bad lift pump)

The water drain comes off before the filter element so just because you get fuel does NOT mean that the filter is OK

The filter on these passes a lot of fuel during normal operation.

About 35 Gallons an hour goes through it.


Check the filter before you spend $$$$ on anything else.

Missy

[midnightrumbler]

As stated, get the codes.

Here is a post I just put into another forum, It may help you with your issue?


Here were the symptoms I was getting with the truck.

1995 Chev Silverado 6.5 TD Ext. Cab Long Box Dually 4X4

The truck would start and idle great. I would get up to 60-80km/h and the truck would start to buck or hesitate as if I had air or lack of fuel in the lines. Going up grades would be fine but running on straights or down grades I would get the bucking hesitation. I set a DTC-25 which lead me into the APP but I also reveived a DTC-35 shortly after which made me and others believe it could be the IP. I wanted to start with the easy things before working into an IP.

I tested fuel pressure at the WIF drain at idle and while driving and the numbers were well within spec which informed me the Lift Pump and OPS were functioning properly. The fuel pressure did not drop off while the truck would buck or hesitate.

Numbers at WIF Drain Valve within Spec

I am seeing 6-7 psi at idle and 2-3 psi at hard acceleration. When the jerking sensation occurs, it did not effect the fuel pressure.


The PMD was new when I bought the truck but was installed on the IP. I wanted to relocate the PMD to the front bumper of the vehicle to rule this out as a possible factor as the truck would not buck or hesitate right away so I thought maybe the PMD was heating up and causing the issue.

The OPS was also new when I bought the truck but also wanted to relocate this for reliability. I installed an 8" grease line to the bac of the motor where the OPS original was and mounted the OPS to the fuel management unit and installed a relay with this to take the load off the OPS.

The truck has 336,000 km's on the dial so I wanted to replace all fuel lines as they appeard to be original and I put in a new fuel filter and air filter. All grounds, and I mean ALL grounds were thouroughly cleaned. Removed the Fuel Filter Management Unit and thouroughly cleaned this out as there was a slight blockage in the feed line and alot of debris on the bottom of the unit. Cleaned the CDR with a little fuel and also installed a High Idle Switch. I also installed a fuel additive directly into the Fuel Filter Management Unit as well as 2-Stroke oil to help lubricate the pump.

I was also having an issue with lack of power and black smoke so wanted to rule this out. Ended up i had a faulty Wastegate Selonoid so I bypassed vacuum and installed a home made TurboMaster and a Boost Gauge. Do not run without a Boost Gauge.

Want to install a TurboMaster?

http://www.dieselplace.com/forum/sho...51#post2322951 (http://www.dieselplace.com/forum/showthread.php?p=2322951#post2322951)


Much to my surprise none of this helped the issue at all


Installed a High Idle Switch.....

The thing I found odd is I could not hold rpm's at all with the fuel pedal and yet with the High Idle Switch the truck held 1250 rpm's no problem. This lead me to believe I may have a faulty APP as if it were the IP, the high idle mod should also have issues holding rpm's, or atleast we would think.

I would recomened if you are having similar issues, install a high idle mod and see if rpm's can be held

Installation of OBD-I High Idle Mod

http://www.dieselplace.com/forum/sho...d.php?t=340699 (http://www.dieselplace.com/forum/showthread.php?t=340699)


Want to pull your codes on an OBD-I ?.

Paper clip on the A-B connectors of the Diagnostic port - Two upper right connectors are the A -B. Put a paper clip in A and lead the other end into B

Put the key in the ignition but do not crank the motor. Check engine light will flash

1-2 1-2 1-2 means 12 which is no codes any other codes will flash as to what they are indicating 2-2 2-2 2-2 means 22 etc.

All codes will flash 3 times. If you have never done this before, have a pen and paper handy, you may have lots of stored codes but generally they reset after 50 key on engine off procedures.

You can clear the codes by either disconnecting the batteries for a short period (10 minutes) or leave the paper clip in A-B ports of the diagnostic and simoutaneously press the brake and fuel pedal 4-6 times. This will clear all codes.

Do not remove the paper clip with the key in the on position. Take the key out of the ignition and remove the paper clip. this prevents any shorting to the PCM.



APP Voltage Signal & Reference Numbers

1995 Silverado 6.5 TD 4X4 ext.cab Dually

My original APP Signal/Reference

KEY ON ENGINE OFF

Terminals G and A have 5.03 Volts and do not fluctuate at all when pedal is at WOT

Terminals D and B have 5.03 Volts and do not fluctuate at all when pedal is at WOT

Terminals E and J have 4.90 Volts and can drop down in volts to 0.64 when pedal is at WOT

Between terminals F and A have 0.66 volts and increase to 3.44 volts

Between terminals C and B have 4.40 volts and decrease to 1.72 volts

Between terminals K and J have 3.38 volts and decrease ro 0.62 volts


So we know that the APP has 3 sensors, my readings throughout APP sensor 1 and 2 appear to be correct, it is APP sensor 3 where the problem seems to be but I am not sure if it is the pedal or the wiring

Incase some are wondering what signal wirings are what

G, F and A are all in Sensor circuit 1

D, B and C are all in Sensor circuit 2

E, J and K are all in Sensor circuit 3


EXACT APP TESTING VOLTAGE

APP Sensor Circuit 1 produces a varying voltage of between 0.35volts-0.95volts dc at idle and 4.0 volts dc at WOT

APP Sensor Circuit 2 produces a varying voltage of between 4.5 volts dc at idle and 1.0volts dc at WOT

APP Sensor Circuit 3 produces a varying voltage of between 4.1 volts dc at idle and 2.5 volts dc WOT


If I compare the information based on the above readings to what I have recorded, I think we have diagnosed a faulty APP as the readings in APP Sensor Circuit 3 are way out. APP Sensor Circuit 1 and 2 are within specs....


ANOTHER METHOD of testing the APP - Very similar to above

http://www.dieselplace.com/forum/att...0&d=1272309290 (http://www.dieselplace.com/forum/attachment.php?attachmentid=102560&d=1272309290)

[Yukon6.2]

Hi All
Thanks for responces.I do have a new Dtech PMD which is mounted remote,and the injecton pump was a new one,which has about 5-10 K on it.The grounds were all cleaned when i did the engine swap.I noticed the hesitation on deceleration right after the donor motor went in,which also was about the time i remote monted the PMD.
You have given me a few things to check,i'll see if any codes are present.
I do have the high idle mod ,i did try it when i was crawling home but got the same bucking while i was driving.
I changed the pedal assbaly but haven't taken for a drive,probly on the back burner till next week now.
Thanks Thomas

[Yukon6.2]

Hi
Well i'v switched a few things and am still having intermittent problems.
Changed fuel filter
Changed throttle pedal
Changed The dtech fsd with a stanadyne.
Pulled some codes and got
First time P0219
P1216
P1217 Seemed to be ok after the code reading,went on a tow job and ran fine Went to use it today and it wouldn't start as it ussally does,finally got it going and the SEL light was on,had to go help a guy lift a 6.2 out of a motorhome so i took off anyhow,it didn't have any power,wasn't making any black smoke worked about an hour changin the motor in and out of the motor home and it idled fine during the lifting and moving around,on my way back to the shop it stalled and wouldn't restart,my wife happened along with her sunfire so she pulled me back to the shop,i tried to restar a few times but it caught once but wouldn't run.
So i pulled the codes again and got P0251 and P0370 on top of the original three.
Anyone have a direction to steer me?
I'm thinking of looking for a mechanical pump if this continues,i'm out in the middle of nowhere with this truck at all hours and am losing confidence in the computer run system.I 'm not much of a fan of computers anyhow was hoping to use up the ds4 pumps befor i shelled out some $$$ for a mechanical pump and lines.Will the lines from a 6.2 mechanical pump work on this?
Anyhow sorry for the long post,and thanks for any help
Thomas

[JohnC]

Your codes are all over the map, but they all point to the pump wiring.

Where is the ground wire from the FSD harness connected? Have you checked the ground point at the rear of the passenger's side intake manifold? what about the engine harness connectors?

Check the stalling thread in the 6.2/6.5 Tech forum.

[Yukon6.2]

Hi
Had a few minutes to check a few things.The grounds are still good,ground is attached to the pump.The harness is still hooked up,didn't pull it apart.I jumpered the lift pump,pluged in the block heater charged batterys and finally got it to start.I really don't know how to use my scanner,but i managed to find a couple of tests that test the CEL and shut of solinoid and both of them work. I cleared the codes and three comeback right away,i left my notes in the truck but i'm pretty sure they wereP1217,p0370,p0251,Still runs like crap when i did get it running.
On a lighter note i did win a new 6.5 military pump on Ebay db2 style,and do have a throttle pedal in a wreck out back,i'ts a gasser pedal but i think it will work.I think the lines i have of a 6.2 will work,so i just need to see if i can make or transfer a timing pointer to this engin.So if all else fails this truck will be getting a mechanical pump.I would rather fix the problem then do the conversion at this time.It's the weekend now so i wouln't be touching it till Monday,weekends are stricktly for working on our house.
Thanks All

[JohnC]

Codes from 2 separate systems within the injection pump. Either you've got a load of really crappy fuel that made its way through the filter, or you've got a wiring problem with the pump. Unplug the engine harness connectors and give them a long, hard look for corroded or loose connectors...

Wouldn't hurt to check the fuel for contaminants and give it a heavy dose of fuel conditioner, too...

[Yukon6.2]

,Hi
Checked out the 15 pin conector and didn't see anything bad.
Begane with no codes today,it wouldn't start at first,pluged in the block heater and finally got it to start and run,runs like it's pluged up,2500 rpm max,like the timing is off??Have to hold the throttle pedal down to get it to start.Got the codes P0251,P0370and P1216 when i read them again.I also got a timing # which was Des timing @ 747 rpm was 8.7.Is that in the range it should be?
I changed the fsd twice and it didn't make any differance,added 5 gals fuel and that changed nothing.
Sometimes when it wouln't start it will smoke white smoke,which may be a timing issue??
How about the crank sensor?
A couple of times as i was flooring the throttle and letting it off it would really rattle and revup like the timing was changed,that would only happen on deceleration.
Anyhow,hopefully someone sees something that makes them say AAHAA try this.
Thanks for the help
Thomas

[Yukon6.2]

Hi
Todays findings.
Got it started without being pluged in.Didn't fix it'self overnight.So i checked the plugs on ecm,removed the filter on the optic sender wiring harness,I ran it wiyh the optic sender unpluged and it acted the same,still not right.
Cleared the codes,did a KOKO,scanner had -0.1 as the tdoc.
Got it running,runs crappy at low rpm's better at 2300rpm and up.
Was watching the Des timing and Act timing on the scaner,they were at 13-14 degree sand up when it was running smooth at higher rpms,and dropped to around 8 when bucking at low rpms.Then it went into limp mode no rpms over 2500 and the Act timing went to 0.These codes came up P0251,P0370,P1216,P1217,and P1218.So anyone have the magic answer fingers crossed??
I'm thinking that the optic sender is the problem i did notice that when in limp mode the cam refrence signals missed were over 8.
Anyhow look forward to anyones thouhgts.
Thanks Thomas

[Yukon6.2]

It Runs
So i got some diagnostic printouts from alldata today from a guy i know for the P0251 and p0370 and always came to the change IP,he had a bad pump i talked him out of so i could see what it looked looked like inside.After reading how to do it ,I pulled the optic encoder out of it and was fairly straight forward.So i looked at the truck and said what the heck i'v got nothing to lose.So i pulled the one out of the truck and replaced it with the used one.got everything ready to spin the fuel filter off in case it ran away.Hit the key waited for the glow light and it fired right up,better than it started for a while.Went for a short drive and seems to run great,it had a hicup/fishbite on deceleration that seems to have disapered.It will take a few trips to be confident that it is cured but looks good so far.Talked with the guy that installed the pump on the motor,and it was a new pump and max had 10k miles on it.Makes you wonder why it would die so quick.Talk about some fine holes it reads light through.
Anyhow thanks for info that helped me through this problem,i guess i can put the MFI conversion off for a while.Have way more important things to fix/build than reinvent a fuel system right now.But the parts list is a little smaller have the pump and throttle cable on the way.
Thomas

[Yukon6.2]

It Died
So it's a few years later,same truck...actually the same hill started these problems. Was doing the usual coast down the hill and noticed the engine sound was different.Pushed in the clutch and tach went to 0.I assumed out of fuel i was pushing the end of the tank.So i stopped at a good spot and dumped in my reserve.Keyed the pump a couple of times and it started no throttle response over 1k so i figured trapped air.A couple of minuets later still the same so i decided to limp the 5 miles to my shop.First thing i do is swap the fsd and fire it up and all was great:D,go finish my tow.
Yesterday my wife wants me to move our planters by the pumps.These are cones from a cone crusher about 5' across and a few inches thick steel.I was going to use the twin winches to lift and level them,but when i went to start the truck nothing,not even a wisp of smoke.
So i'v tried a known good fsd,checked the fuel pump,checked the connections,pins in extension harness and pulled these codes today
P216,P219,P1216 and P1217.
I'm still in the same boat where i would like to put off the pump change.We have just moved into the house i built and I'm in the process of reclaiming my garage that became a 40x80 storage building that you could just drive through.I haven't found my injector lines for the swap so I'm hoping for a fix.
In rereading this thread i will try plugging it in tomorrow if i can get it to start then maybe it's the optical sensor again.
Thanks for any help.
Thomas

[Warren96]

Have you checked for air in the fuel? When you walk up to the truck do you notice that it smells like fuel? Are ALL the fuel lines DRY? The only time mine was replaced was when it was under warranty and GM's fix was to change the injection pump not the PMD. Good luck and keep us in the loop!

[Yukon6.2]

My relationship with the DS4 is over,at lest in this truck.
It is now running under my command,and i hope the computer is screaming under the dash...need input whats going on....
I love the way it starts now,crisp and no smoke.There was always a fair bit of cranking followed by white smoke at startup with the DS4.
It was a bunch of hours to get it done,but now i have a truck that i can go drive out at -40 to the middle of nowhere with out worrying about some stupid computer or electronic failure leaving me in a bad spot.
Thomas