What causes the engine to surge at idle and stall? IAC has been replaced. No codes. Interrmitin.

If the operation of your A/C and heat is causing your engine to surge in your 1991 GMC C1500 with a 5.7-liter 350 engine, there could be a few possible causes for this issue:

1. Engine Load: The A/C and heat systems place an additional load on the engine, especially when the compressor engages. This extra load can affect the engine's idle and cause surging. It's possible that the engine is not able to handle the increased load, leading to fluctuations in RPM.
2. Idle Control System: The idle control system, including the idle air control valve (IAC), may be malfunctioning or not compensating properly for the increased load when the A/C or heat is turned on. This can result in unstable idle and engine surging. The IAC valve may need cleaning or replacement.
3. Electrical Issues: Faulty electrical connections, such as loose or corroded wires or connectors, can cause intermittent power supply to the A/C and heat systems. In turn, this can lead to fluctuations in engine performance, including surging.
4. Vacuum Leaks: Check for any vacuum leaks in the A/C and heat system components, as well as the engine's intake manifold and vacuum lines. Vacuum leaks can disrupt the engine's air-to-fuel ratio, leading to unstable idle and surging.
5. A/C Compressor Issues: The A/C compressor itself may be malfunctioning, causing excessive drag on the engine. This can result in surging when the A/C is engaged. A faulty clutch, compressor bearing, or internal component may be the cause.
6. Engine Performance Issues: Other underlying engine performance problems, such as ignition system issues, fuel delivery problems, or worn-out engine components, can contribute to surging when the A/C or heat is in use.

To diagnose and resolve the issue, it is recommended to consult a qualified mechanic or an automotive technician who can perform a thorough inspection of your vehicle. They will be able to assess the specific conditions and components involved, perform necessary tests, and identify the root cause of the surging problem.

It seems like the problem lies with the engine management system, rather than the brakes. Given that you've already replaced the MAF, MAP, and TPS sensors, here are some other things to check:

1. Vacuum leaks: Inspect all vacuum lines for cracks or damage that may be causing a leak. A vacuum leak can cause erratic idling, poor acceleration, and stalling.

2. Idle air control (IAC) valve: The IAC valve controls the engine's idle speed. A malfunctioning IAC valve can cause the engine to stall during idling. Try cleaning the IAC valve with carburetor cleaner and see if the issue is resolved.

Your torque convertor will stop the engine stalling.

not told RPM of surge. (full range of surge)
hot engine surge, or cold or both?
it's old most do this old.
lots of causes.
but why tells the reasons
the IAC is bad or just lost control of engine for 50 other caused.
let say engine as huge vacuum leak
the PCM closes the IAC and can not correct the massive vacuum leak.
or engine goes lean?
or spark misfires so bad IAC can not correcthis.
bad engine, bad spark bad fueling can and will make IAC go dead.
in fact fixing IAC is last, in my book
if all is good just cleaning IAC fixes it. some times.
or replace it.
on old cars the ECU surges the IAC if spec idle speed can not be reached, 800 hot.

Karl, have you confirmed good fuel pressure? It sounds like a weak fuel pump to me. Is your Check Engine light on? If so what are the codes in the computer?

Car Stalls and idles low

http://technoanswers.blogspot.com/2012/10/gmc-sonoma-stalls-and-idles-low.html
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It can be issue with IAC( idle air control valve) causing the problem.

There are many other possibilities causing these idle issues. Engine idles rough.

There are few help links with solutions to troubleshoot such idle issues.

I suggest you refer the link below and follow its troubleshooting details:------

Click the link below:-----

Ford Expedition IAC valve replacing

http://technoanswers.blogspot.com/2012/12/ford-expedition-iac-valve-replacing.html

Bad miss at idle?

http://technoanswers.blogspot.com/2012/01/bad-miss-at-idle.html

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Stumbling and stalling at idle after engine rebuild?

http://schematicsdiagram.blogspot.in/2012/03/stumbling-and-stalling-at-idle-after.html

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1999 Ford F-150 Idles rough and quits when gas pedal is pressed less?

http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html

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Engine stumbles coming off idle when warm?

http://repairhelpcenter.blogspot.com/2012/01/engine-stumbles-coming-off-idle-when.html

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Erratic idle on car?

http://schematicsdiagram.blogspot.com/2011/12/erratic-idle-on-car.html

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Cold start problem and vehicle shudder around 2500 to 3500 rpms

http://repairhelpcenter.blogspot.com/2012/10/cold-start-problem-and-vehicle-shudder.html

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Low idle when cold out side?

http://repairhelpcenter.blogspot.com/2011/12/low-idle-when-cold-out-side.html

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GMC Sonoma Stalls and idles low

http://technoanswers.blogspot.com/2012/10/gmc-sonoma-stalls-and-idles-low.html

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GMC Idle Air Control (IAC) Valve Replacing?

http://schematicsdiagram.blogspot.com/2011/12/gmc-idle-air-control-iac-valve.html

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1996 Camry intermittent engine failure while idle?

http://repairhelpcenter.blogspot.com/2011/10/1996-camry-intermittent-engine-failure.html

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Engine quits at idle?

http://repairhelpcenter.blogspot.com/2011/10/engine-quits-at-idle.html

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Car overheats and Air blows hot at idle?

http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

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High Idle Problem

http://repairhelpcenter.blogspot.com/2012/10/high-idle-problem.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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Idle flair is noticed on car? Rough idle? car will not idle? Erratic idle speed?

http://technoanswers.blogspot.in/2012/02/idle-flair-is-noticed-on-car-rough-idle.html

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High rpm in neutral? Inconsistant high RPM in neutral?

http://technoanswers.blogspot.in/2012/02/high-rpm-in-neutral-inconsistant-high.html

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Vehicle stumbling at idle and shows code 54?

http://schematicsdiagram.blogspot.in/2012/03/vehicle-stumbling-at-idle-and-shows.html

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Rough idle, staling and Erratic shifting?

http://technoanswers.blogspot.in/2012/03/rough-idlestaling-and-erratic-shifting.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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These details will help.

Thanks.

Have you looked at your idle air control valve What is it? This is an electrically driven valve that allows additional control over the throttle at idle Where is it located? The IAC is located on or adjacent to the throttle body and consists of a small device held on by two bolts with an electrical connector.
How does it work? When the throttle is closed the throttle plate closes off any air from the intake manifold. In order to stop the engine from stalling due to lack of air the IAC opens a small valve allowing air to pass from the air ducting to the intake manifold. Often the IAC components consist of an air inlet port in the inside wall of the throttle body, immediately in front of the throttle plate, with a similar exit port behind the throttle plate. Connecting the two ports is a motor driven valve. When the valve is opened air effectively by-passes the closed throttle plate to maintain the engine at idle. The IAC also augments the throttle when additional power demands are put on the engine such as steering.


Symptoms of faulty IAC

• Poor starting: If the IAC fails to open or the inlet/exit ports are clogged shut with carbon the engine may start and then promptly stall. The engine can be kept running by pressing the accelerator. However any additional demand on the engine such as power steering or putting the car into drive (automatic) will cause the engine to stumble badly and often stall.
• Engine cut out: Even with the engine warmed when the vehicle is stationary such as at a red light the putting of the transmission into of 'drive' to pull away before the accelerator is pressed can cause the engine to stall. (Note this differs from the TPS fault which causes engine stall due to the accelerator being pressed too quickly)

How to check? Physically remove the IAC but keep the electrical connection to it. Temporarily block off the now open ports on the throttle, turn the engine on and examine the IAC valve action in response to additional loads (power steering inputs etc). The valve should open and close according to demand.

How to fix? If the motor of the IAC has failed then replace the IAC. If the valve is gummed closed by baked oil and carbon then clean it thoroughly with carburetor choke cleaner spray and a cloth. Similarly if the entry and exit ports on the throttle body to the IAC look blocked again clean them out thoroughly.

Replace the Intake Air Temperature Sensor.
Depending on your size Engine (2.4L, 3.0L or 3.3L) It should be located where the Air Filter is. When you take the cover off, it wil be mounted into the cover. It has the only wires going to the cover.
This sends information to the Computer which adjust the air/fuel mixture - causing surges.
The 2nd recommednation: to remove, clean and check is the Idle Air Control (IAC) Valve and replace if necessary.

Let me know if this helped....

those sensors could cause the issue, clean the MAF 1st, also clean the idle system using the following procedure, the IAC can cause cruise surge. I use this to cure these problems all the time.

Here is the most common cause of surges and stalls and low idle, it is the idle speed control air-bypass valve (IAC) and throttle valve, they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

Sounds like a bad IAC (idle air control valve/sensor) that controls your idle when going from throttle to idle. If bad your idle will act crazy and can cause stalls easily. Pretty sure that's it.