post your truck yr engine .. test your gas pressure post on here.
also post any codes .
the IAC is not gonna control cold starts .. when the engine gets to normal temps the IAC will control a small amount of the eng rpm.....so you stated engine warm idle is good ....

 

Hi,
it’s a 2000 gmc with the 5.3 ltr. no codes, checked by myself and a parts store.

 

Hi,
it’s a 2000 gmc with the 5.3 ltr. no codes, checked by myself and a parts store.

Just curious, if there’s enough fuel pressure to run the engine at 1000 - 1500 rpm, wouldn’t there be enough pressure to run it at 450 - 500 rpm?

 

Not necessarily. At slow speeds, every miss is exaggerated and takes more oomph to overcome. At 1500 RPM, you can hit or miss and the flywheel keep the motor running until the next sufficient bang.

This has been my experience, anyway. My truck (2000 Silverado w/ 5.3L) would run at throttle, but I would have to feather the pedal to keep it happy when at idle. New ACDelco pump and it's been good to go.

 

Thank you very much!
BTW, Are you referring to a new AC Delco fuel pump? Thanks again!

 

Thank you very much!
BTW, Are you referring to a new AC Delco fuel pump? Thanks again!

Yes, a new fuel pump. My previous NAPA brand trio had been... Bad. The last one whined the day I put it in, but I was broke, cheap, and desperate. 😁 Survived about a year and I had to replace it.

 

Thanks again. As soon as I have a few hours I’ll check the fuel pressure with an eye on a new AC Delco fuel pump and a new filter.
I never would have thought of going that route because of the way the truck preforms at high power and high speed.
Thanks again! Nothing beats experience!

 

You might have a PCM problem, not one that will require a new PCM, just a reboot
When the engine is cold, the fuel mix is determine from software tables in the PCM, as the engine warms and the O2 sensors turn on, the PCM uses the O2 data to control mix.
I'd try a PCM reboot (battery disconnect) to see if that helps.

 

Thanks! You are a valuable resource.

 

don't throw parts on your truck.. you have no codes well you must check the fuel pressure ..key on eng off pressure must be 60-62psi .. pressure should hold up a few min..pressure port pass side..
if the pressure is good and then it drops could be the fuel regulator on the upper intake manifold on driver side.pull vac hose off the reg... if it smells of gas or has liquid gas many had these failures..
if your pump has low fuel pressure then replace the fuel pump and the fuel pump relay...change the fuel filter.

 

Thank you for good, specific advice. My work schedule prevents me from getting right to it but I will get to it ASAP.
Thanks again.

 

@J, I had a similar situation (low RPM fail) w/my burb (‘94 5.7 K1500). We finally traced it to an intermittent short in one of the plug wires. Replaced the wire, all is good. 😉

 

on the GM OBD1 1994 vehicles the ECM does not do a good job on diagnostic testing. OBD II 1996 and newer the PCM will throw codes quickly and then diagnostic data to check on the items having a defect.
the newer trucks do indicate fuel pressures but on a 2000 GM truck no fuel pressure data ..he has to do the fuel testing ..do not throw parts on it ...

 

Thanks for the tip. I imagine that wasn’t an easy find. I appreciate your taking the time to provide guidance. It has been a while since those wires were replaced.

 

Thanks again. Hopefully I’ll be able to get to my diagnostic testing in a few more days to a week. As you can imagine, I miss my truck... but no for much longer!

 

Jwalker,

Were you ever able to test your IAC?
I have a similar starting issue. Once the engine is warmed up it operates normally.
I have pulled the IAC and it well coated with carbon.
I tested continuity on A/B and C/D pins. Both are 46.3 Ohm +/- 0.1.
I can not find a source for what the resistance is supposed to be.
Given the PITA taking the IAC out I would like to have a better clue as to if it was just dirty and stuck or failed.

tb2
-------------------------------
2003 Chevrolet Express 3500 Cargo Van; LQ4 6.0L V8

 

I tested continuity on A/B and C/D pins. Both are 46.3 Ohm +/- 0.1.
Given the PITA taking the IAC out I would like to have a better clue as to if it was just dirty and stuck or failed.

Addendum:
Looking at NEW IAC parts on-line and what videos I can find on replacement, the IAC piston that controls the air bypass is set in the shortened position.

I understand this to indicate that without being energized the air bypass is open and only as the vehicle warms up does the piston restrict and/or close the air bypass at the throttle butterfly valve. So the fact that the piston is fully extended, on the IAC I have removed, lends me to believe it has seized, closing the throttle butterfly valve idle air bypass.

 

the IAC 2003/04 the resistors of the corrosion .. up did it GM 120 percent damaged it ..

 

??
Did you leave out some words?

 

Local Chevy Parts do not sell an IAC.
They indicate the part was never sold separately but was part of the the throttle body.
And the throttle body is no longer sold.

Sources for replacement IAC valves?

 

IAC IDLE https://www.amazon.com/Standard-Mot...-fkmr0&vehicle=2003-47-449------------5&vehicleName=2003+Chevrolet+Express+3500

 

 

Ooooo... there may be one with an integrated IAC...

2 docent options on RockAuto. Never seen this electronic one before. My 2000 Silverado uses the plain one.

 

2000 Silverado - cable throttle

2004 Suburban - drive-by-wire

 

OpalLacod,

I have been over the ground you describe.
I do have a cable throttle body.
The piece missing in your feedback is that I am driving a '03 Chevy Express 3500,
For some reason there is a hole across all sources for an IAC replacement.
The closest I have gotten is to trying to match engine and throttle body from a 2500.

With all that, is there a way to test an IAC, and would I be correct to qualify it as failed if the piston is fully extended?

 

The piece missing in your feedback is that I am driving a '03 Chevy Express 3500,

I went back to the OP vehicle description. Oops...

 

Testing
I did not know this, but it makes sense... Apparently these valves are stepper motors. So, it should be "stuck" in place. If it's not "stuck", then the threads are stripped or something inside is broken. You can test the windings for resistance to make sure they are not shorted out inside. You'll need a proper pin-out for the IAC you have (or a similar one). All the GM ones appear to be very similar. If you want to play with function, find out which prongs are paired together and then apply 5V across them and run the IAC in and out.

TMI
I had already run down this rabbit hole before the testing question, so here's the rest of my train of thought.

2003 3500 Express Throttle Body has the same casting markings as...

2001 Silverado.

The only difference I can see is the part number on the butterfly. Everything else looks identical...

Idle Air Control Valves are all 4 pin (based on wiring for 2003 3500 Express and the IAC Valve shown in the 2001 3500 Silverado category).

I think you can get an IAC from another model 3500 and be OK.

 

quote:
Testing
I did not know this, but it makes sense... Apparently these valves are stepper motors. So, it should be "stuck" in place. If it's not "stuck", then the threads are stripped or something inside is broken. You can test the windings for resistance to make sure they are not shorted out inside. You'll need a proper pin-out for the IAC you have (or a similar one). All the GM ones appear to be very similar. If you want to play with function, find out which prongs are paired together and then apply 5V across them and run the IAC in and out.
end quote

I have done a continuity test across the two circuits and they are both 4.3 (+/-) Ohm.
All the new IAC valves I see and one replacement video show the new IAC piston to be retracted. In the video the failed one is extended. Since my issue is the van is not idling correctly at start-up, I imagine the piston is closing the bypass air passage. BUT once warmed up the engine idles normally. So by that, I assume one of the IAC HIgh Low circuits is working (?).

 

If you can run the IACV in and out with a separate battery source, then I would think it's an issue with the plug or the controller wire.

 

If you can run the IAC in and out with a separate battery source, then I would think it's an issue with the plug or the controller wire.

You mentioned in a previous post to use a 5v source.
How did you come up with 5v as an operating voltage?
I read one post where someone tried this with 12v and cooked the IAC.

 

I've seen this resistance value a couple times, but I think it applies to older model units. Will keep looking...

 

I've seen this resistance value a couple times, but I think it applies to older model units. Will keep looking...

Sounds as if my 4.3 Ohms is a partial burnout.

If I tasked you, what IAC would you find to put in the pull cable throttle body of a 2500 w/ a LQ4 (6.0L LS)?

 

The butterfly does appear to have a different size hole in it...

 

The unit you show on your truck has the same butterly stamp as the CARDONE 641075. This is the same unit as shows for buying a new throttle body and putting it on my truck (2000 Silverado 5.3L). Your intake manifold also has the same vacuum line setup as my truck...

What is the part number for your ECM? If it's a similar model to any of the other trucks out there, I'd be willing to bet the pinout is the same...

 

We are getting close ...
RockAuto does not have a throttle body for your 2000 Silverado 5.3L. = : /
I will have to dig for the ECM and while we are at it, previous references have been to a PCM.
I assumed that a PCM was what I understood to be an ECM. Are they different?