Accelerator pedal problem

[stellaontap]

Hi all. I’m doing a LQ4 swap into my 1979 GMC Camper Special. I just did a first start and it idled and ran pretty good. There is a problem I am fighting with and that is when I press the gas pedal the RPM increases a couple of hundred RPM and then drops back to idle. If I keep the gas pedal pressed it just keeps doing that and starts to run rough. Has anybody run into this problem and if so how did you solve it?

Thanks
Eric

[leegreen]

Throttle position sensor working and reading properly?

[stellaontap]

I’m going to check that right now.

[stellaontap]

Checked the gas pedal and when I push it down the blade opens on the throttle body. There are no codes on the scanner. I also changed the accelerator pedal position sensor and still have the same issue.😩

Eric

[leegreen]

Does your scan tool show the TPS moving when you move the pedal?

Any codes stored in computer?

Fuel pressure at idle good?

Do you have a large vacuum leak confusing the computer? (check fuel trims)

Are the computer and harness both from a DBC donor? Not all computers have the chip on the motherboard for the idle air circuit motor drive that controls idle on DBC throttle body. Not clear that would result in the symptoms you have but a bad IAC will mess up both idle and throttle response.
It is possible to de solder said chip from an older computer and add it to a newer computer if you are already invested in unlocking VATS or HP tuners for a newer computer. You'd need to tell the computer about the change via HPTuners, it is not just a soldering iron job.

[stellaontap]

There aren’t any codes and yes the scan tool shows it’s moving. The truck idles fine however my fuel pressure gauge has failed. I’ve ordered a new one which I should receive tomorrow.

I can’t hear any vacuum leaks. I tried a propane torch where there might be vacuum leaks but there was not change in idle speed. I also tried shutting off each injector and also no change in idle until I shut 2 off inadvertently.

The computer and harness are both from a DBW vehicle.

[leegreen]

propane trick won't work with modern EFI, computer compensates idle faster that you can hear idle speed change. Try smoke
or maybe try ether, you might hear it ping (test a shot on an open vacuum line first)

Disconnecting a coil pack or injector at idle often does not tell you much. EFI 8 cylinder is so good at idling you can drop a cylinder and not notice. unless one cylinder has totally failed, then you might notice the change when you unplug another cylinder. or try revving/driving with each cylinder unplug

[stellaontap]

This a picture of the scan where I was checking the gas pedal. The accelerator pedal is push all the way down and the position shows 156%

[leegreen]

not familiar with that scan tool if 156% means out of range for LS TPS. Anything other than 0-100% I'd check TPS with a MM or swap a known good spare.

Gather some spare sensors, coil pack etc by scrounging at the wrecker or whatever and test them when the truck is running good. Keep on hand

[stellaontap]

I will be getting some spare junkyard parts. Just yesterday I had a faulty coil pack, luckily I had a spare. The reading in the accelerator pedal at 156% seems odd to me as well however there are no codes thrown. I’m using hp tuners and turned on all my gas pedal dtc to report on first error.

Thanks for helping leegreen

[TrybalRage]

I have a cable throttle so never had to deal with this myself, but I've read of issues mix and matching pedals that were not with the original ECU. Here's an article about it:
https://www.lsxmag.com/tech-stories/...dies-with-psi/

[stellaontap]

Thanks for the link to the article. This morning I checked all the parts that make up the throttle system and they are compatible with my pcm.

Just got my new fuel pressure gauge and fuel pressure is 58 psi at idle and when I push the accelerator down it stays at 58, maybe jumps up a bit to 60. If I kept gas pedal pushed I hear air rushing through the filter and the engine starts to sputter, cough, then a backfire and stalls. If I release the pedal it will return to idle.

A thought occurred to me while playing around with it today and would like input if it is something that may cause this. I have a returnless fuel system and perhaps the problem is air in my fuel lines. It will be the next thing I look at.

Eric

[leegreen]

Did you check the TPS with a voltmeter? confirm good ground, 5v source voltage and signal output varies smoothly between near 0v and near 5v ?

still no codes on MAF? is it clean?

Is it a stock tune on the ecu? If there is a big missmatch between what the table says and what the sensors are telling ECU it will struggle

if it revs better if you dribble some fuel into intake it would tend to confirm it is not getting enough fuel. Doesn't tell you if it is fuel supply or the computer not knowing what is going on and running it too lean

open the schrader valve when you cycle the pump to see if you get any air

[stellaontap]

Hi leegreeen.

To answer your question first, it is not a stock tune and the engine is modified. I have 60lb injectors from Deatschwerks that came with data. All the data is in the tune. I also have a gm hot cam and have adjusted accordingly as well. The idle is a bit rich but overall sounds good. I can’t proceed until I can increase engine beyond idle

I have been doing some testing with the multimeter. My grounds are good (I think) at 0.3 ohms back to the engine block or the battery. At the throttle body 5v reference circuits show 5.0 volts.

At the TAC module I unplugged the APP sensor and have 5.02 volts on the 5v reference circuits. When I plug the APP into the TAC module I get a different reading. One of the circuits reads 4.16 volts and the other is 4.86 volts.

When I check the APP sensor 1 signal the voltage starts at .66 volts and sweeps to 3.76 volts as I press the gas pedal. The APP sensor 2 signal starts at .619v and sweeps to 3.34v as I press the gas pedal. The APP sensor is brand new.

Do these voltages indicate normal operation or point to a potential problem? I am not sure if the voltage reading lower than 5 volts with the APP sensor plugged in is acceptable or not.

Eric

[leegreen]

The 2 circuits are there for redundancy so things like a wiring short can't send a vehicle to WOT. If the ECU thinks there is a mismatch on the readings you get limp mode. I don't know what the tolerances are for a mismatch to be an issue

from a quick google search, primary should be from .3v to 4.5v and secondary should be .3 to 2.5 . I'd take that as needing further research but it may indicate your app is not good

Is it a trustworthy brand/source pedal sensor? the voltage drop with pedal plugged in and mismatch might be most easily diagnosed with a known good spare.

[stellaontap]

The APP sensor is a Dorman, which is the one that the voltage measurements were taken on. I have to others that I have had on the gas pedal and they behave exactly the same. I’ll put one of them on and see what the measurements are tomorrow.

[bigmoe]

Does your pedal come from a car or truck. They are one wire different.

[stellaontap]

The gas pedal is from another truck.

[bigmoe]

Originally Posted by stellaontap

The gas pedal is from another truck.

As long as you're sure your pedal and TAC are compatible.

[stellaontap]

Hi guys.

Here is yet another update. I have hooked up a fuel pressure gauge at the end of the fuel rail. The pressure there is 58 psi, at idle it’s 58 psi and when I push the gas pedal it also stays at 58 psi but the rpm increase only momentarily and then dies.

I have checked continuity on UART data wires and they are fine.

These are the components that I am operating the engine with
1) P59 computer with OS 12592618
2)Throttle body is GM part 12507080
3)TAC module GM part 12588923 (TMD1-160E1)
4) Accelerator Pedal Position sensor Dorman 699-101

I have searched this to death and from what I’ve found these parts are all compatible.

Any advice would be appreciated.

Eric

[leegreen]

you have HP Tuners. Did you save the stock tune? flash it back to find out if the issue is in the software or not?

Did you compare voltages on one of your other pedals? I know you said they behaved the same, grasping at straws now

[stellaontap]

Leegreen- yes I saved the stock file. I did flash it back but the truck wouldn’t even run and when it did sputter it backfired and quit. The file that I have created with the injector data, idle air flow and idle rpm starts right up.

Eric

[binford]

Unplug the MAF sensor and start the truck.
If the gas pedal now works its a tunning issue.
This could be caused by using a differant air intake setup.

PS: the ECM will figure out quickly that you unpluged the MAF and will limit the throttle, about 1 minute.

your MAF sensor shows 0 in the pic above?

[stellaontap]

Hi Binford. In the pic the engine is not running, I was trying to show that the throttle body when the gas pedal was floored was 100% and the accelerator pedal was reading 156%.

I have the maf disconnected so it should go to the VE table fuelling, but same thing happens every time no matter what I seem to do.

I bought a smoke machine and hooked it up to the intake to see if I could see smoke anywhere but there was none, which leads me to believe I don’t have a vacuum leak.

I’m running out of ideas here. Should I get a new throttle body, tac, and gas pedal sensor to replace the whole works? Also thinking to get a new computer.

Eric

[leegreen]

from post #1 > If I keep the gas pedal pressed it just keeps doing that and starts to run rough

if you press the pedal a bit to where it starts to run rough then squirt some fuel into the intake does it die or rev up?
3 scenarios and what I think they indicate:
1) nothing changes. The computer is doing an OK job managing fuel and the problem is elsewhere
2)it revs up. then something is getting in the way of adding fuel...can you swap back to stock injectors as a test?
3)it dies. then it is already rich and the extra fuel is making it worse....can you swap back to stock injectors as a test?

other than injectors what else is not stock?
does HPTuners show RPM properly as it starts to rev?
can you tell what fuel trims are doing just as it starts to run rough?
what do the plugs look like after it dies?
just fishing now

were you able to further research what the right throttle voltages were? or what seeing 156% in torque means?