What would cause the engine to surge at idle?

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.

Clean Throttle Body (both sides, not just what you can see) and MAF (mass air flow sensor) [spray and shake to dry].

https://www.google.com/search?q=2001+Ford+Focus.+running+rough+and+power+surges
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Hello. It seems your Cadillac is getting too much air into its engine; causing it to 'surge'. This could be due to a malfunctioning/defective vacuum brake booster. If your Cadillac's catalytic converter is 'clogging'; this will cause the engine to 'surge' too. Good luck and keep me informed; if you want to.

(which engine. ?????)_
as always check all fuses. and scan the ECU for DTCs. first.
there is no need to fix OBD2 cars like 1980 cars (hard rows to hoe)
it can be 2 problems this......
lets fix the easy one first. idle. (and hope for moving issues to fix for free) it idle car is parked, so is 100% safe tests. (not moving)

so it surges at idle.
questions.?
1: hot or cold or both.?
2: low extreme RPM and high at idle.
400 to 800 RPM? surge.
1000 to 2000. what? numbers tell the tale.

first off, idle is the last thing to work right on any engine.

there are many ways to find what is wrong.
fix idle surge way.
i make sure that the engine holds closed LOOP status.
hot idle, if not , i fix that first.
keep in mind if not in closes loop and lean
all lean engine surge and guess what the ISC will add to the surge
because idle is way too slow, so the ECU open the ISC wider
and wider and its still too slow, so the ECU resets the ISC
this cause a worst hunt (surge)
the ECU adjust spark time and the ISC to get 800 rpm. (spec. look up yours)
so say , i see closed loop failed, OBD2 scan page 1
i look at the O2 sensor an its stuck lean.
if stuck lean and it really is (o2 bad?) then its lean and it will surge
.
examples.
1: i find 02 sensor bad, (only effects idle and light steady cruise)
2: i find fuel pressure at 15PSI , hopeless low .
3: i find all injectors clogged. and lean.
4: with MAF EFI, ( i find maf reading too low, (dirty) causing lean.
5: if MAP car, i find the MAP stuck hard high vacuum causing lean.
not knowing what engine you have or the idle hot surge range,
i can not go farther.

the driving bog tells me,
1: scan the ecu ,get DTCs; do that first.
2: clean MAF if car has one, ask how..
3: check fuel pressure.


my guess is engine is lean. and starving for fuel all the time.
cept WOT.

MAF cars starve for fuel if there are any vacuum leaks.
MAP cars do opposite, they race fast with 100% full power.
MAF is air density EFI
MAP is speed density EFI.
apples and oranges diffr. so what engine.

cold engine idle surge. (problem)
RPM range of surge not stated
300-700 or 800 to 1500? (range is critical to diagnosis)

bad fuel
running summer fuel in winter (running old fuel?)
needs a tune up bad, see the list in the operator guide.
idle speed controller,sticking or EGR valve same. (clean them)
location not stated, so cant guess about local air temps.?
no other symptoms, lack of engine power? etc?
was it ok last week, or better, when was the last time it was OK?
1week, 1year?
surge , causes, on a tuned engine might be:
1: too lean. so lean the ISC can not control idle. rpm low, stalling
2: too rich, flooding, stalling, spark plugs foul (look at them)
3: weak compression in 1 or more cylinders.
4: bad spark.
5: air leaks, in the plenum causing , ISC to lose control.
why high RPM?

especially?
well that means it fails hot too.
and there is a simple tests. for hot surge.
is engine fully hot? 180F and holds solid there?
say rpm is about 800
and you turn on head lights, heater fan and (not AC) and defrost.
the RPM must hold or the ISC is failing.
ISC -= idle speed controls. (all EFI has this function)

the compressor that pumps up the air suspension does make a horrid noise, and if you can turn it off, like I could in my crown vic, just long enough to see if that is indeed the compressor making the noise you hear, then try that. My switch was located in the trunk.

Sounds like the IAT (Idle Air Control Valve) or the MAF Mass Air Flow Valve

spark plugs

Dirt may have got onto the O2 sensor, this could cause a surge without the light coming on. Some debris may be on a sensor preventing the computer from making correct adjustments.....

Here is the most common cause of surges and stalls and low idle, it is the idle speed control air-bypass valve and throttle valve, they get full of gunk over the miles and cause idle issues (stalls) 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.