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Old 09-10-2018, 08:09 PM   #1
spacetrace33
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Default Lopey idle with ac running

Havent posted on here for a while so here it goes the scenario.

Start car, turn ac on, blows cold (constantly), idle is perfect. Start driving-which seems normal. Put in neutral for a stop and the rpms jump from 200? To 900 rpms repeatedly. Turn ac off, no issues. Ac clutch spins freely by hand.

Where to start?

Back history ....

2000 sl1 with a 99 dohc manual swap. No cel. Kept all original ac components hooked up. Cleaned tb and egr valve apx 2500 miles ago. New etcs and thermostat.

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Old 09-10-2018, 08:33 PM   #2
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1997 SL1
Post Re: Lopey idle with ac running

Possible weak tensioner and belt is slipping under ac load causing alternator voltage fluctions.

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Old 09-10-2018, 09:44 PM   #3
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Default Re: Lopey idle with ac running

What does "...from 200? To 900 rpms repeatedly." mean?

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Old 09-10-2018, 10:17 PM   #4
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Default Re: Lopey idle with ac running

The idle fluctuates between near stall (200) and 900 rpms.

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Old 09-10-2018, 10:17 PM   #5
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Default Re: Lopey idle with ac running

Fdryer

200? To 900 rpms repeatedly means:

In neutral, the idle bounces back and forth between 200ish and 900, several times.

Sometimes the idle will find itself....othertimes its jumping around like its at a trampoline factory. 😀

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Old 09-10-2018, 10:28 PM   #6
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Default Re: Lopey idle with ac running

Quote:
Originally Posted by toggenburg View Post
The idle fluctuates between near stall (200) and 900 rpms.
Exactly


I should add that once i start going again, say light turns green and its still acting up at idle...once the wheels get moving again(in gear), it "seems" fine.

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Last edited by spacetrace33; 09-10-2018 at 10:38 PM..

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Old 09-11-2018, 10:35 AM   #7
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Default Re: Lopey idle with ac running

Ok. I saw 200? as possibly 2000. Sometimes grammar gets in the way.

Shifting idle rpm may be the idle air control valve. If its sticking or faulty, it may not work smoothly and create a varying idle. Testing is easy but relies on having an initial steady idle after the engine is fully warmed up (no ac).

With the intake air tubing removed, engine idling, block the port in front of the throttle plate with a finger. Idle should drop down to around 500-600 rpm. this is the factory throttle stop setting without supplemental air being added from the bypass air port. The iacv is controlled by the pcm to retract or extend to fine tune idle speed with loads (electrical or mechanical). When the bypass air port is blocked, the pcm detects the lower rpm and retracts the iacv to allow more air thru the bypass air port passageway but your finger is blocking it. Remove your finger and idle should jump up immediately, higher than normal idle rpm. As soon as the pcm detects higher rpm, it extends the iacv to reduce air flow until a normal idle returns. Blocking and unblocking the air port in the throttle body forces the pcm to command the iacv to retract and extend the valve quickly. If engine rpm doesn't jump up and return to normal idle after blocking/unblocking the port or changes slowly, the iacv may be faulty.

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Old 09-11-2018, 12:26 PM   #8
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Default Re: Lopey idle with ac running

fdryer I agree with everything you added in that reply, but also have an idea. I recently (as you know) had an IAC issue. But I also had a 2000 in the garage with exactly same issue as spacetrace described. I did the usual cleaning of the TB and swapped in a known good IAC. No result. As I inspected further, I noticed that the a/c clutch at idle kicked on and off frequently. When it was engaged it nearly stalled the engine. If I held throttle open near 2,000 RPM the clutch would stay engaged. Only at idle did it cycle on and off. I got the A/C gauges on it and verified levels of R134 were good. I recovered the gas and replaced the compressor. After I recharged the system the clutch no longer cycled at idle (unless commanded to). Just my 2 cents. Good luck with the repair.

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Old 09-11-2018, 12:54 PM   #9
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Default Re: Lopey idle with ac running

uluz2a6, you have an excellent point. My suggestion to test the iacv is to test it for possible interference/fault to either reveal it as the problem or to move on with other troubleshooting. Your suggestion of possible ac compressor loading down the engine idle is the mechanical load the EFI system detects as a drop in programmed idle rpm and corrected by the pcm with spark timing, injector pulse width, iacv adjustment - all on the fly.

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Old 09-11-2018, 03:58 PM   #10
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Default Re: Lopey idle with ac running

Nice. Thanks for great leads.

I know almost nothing when it comes to A.C. systems.

With what uluz says, it sounds like the ac clutch should stay engaged during idle....no on and off rapid fire. Correct?

Thanks again. Y'all given me things to check and off to a great start.

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Old 09-11-2018, 04:04 PM   #11
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Default Re: Lopey idle with ac running

What causes the clutch to kick in and out?

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Old 09-11-2018, 04:26 PM   #12
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Default Re: Lopey idle with ac running

Usually, I'd say it depends on 3 factors: actual need of the cold air draw, or short cycling or IF the R134a is really low, not out, just low, or a flaky refrigerant switch over by the drivers side rear engine compartment.
Equipment (ac compressor clutch) I'd presume has a low failure rate.

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Old 09-11-2018, 04:36 PM   #13
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Default Re: Lopey idle with ac running

Saturns normally have ac compressors running constantly. Variable displacement compressors change output according to heat loads. Heat loads are detected by the thermal expansion valve to regulate cold temps while the compressor detects loads internally to vary displacement. All happening magically while we enjoy ac during summer heat without soiling our clothes and smelling rank in meetings with the board or significant date.

The main reason a Saturn compressor cycles on and off is............the leak no one wants to address but runs to auto stores for refill kits. If your compressor cycles on and off while running, your best option is to buy and use an inexpensive uv blacklight to find dye marking the leak site. All Saturns have dye installed at factory assembly. When damage or wear and tear occurs, every ac system that came sealed and pressurized eventually leaks. Refrigerant, oil and dye are released with oil and dye marking the leak site. When refrigerant leaks out to a certain point, lower pressures are detected to begin a cycle of powering off and on where pressures are too low to run/compressor cycles off then as pressure builds up the compressor cycles on, repeating until enough refrigerant leaks out to stop compressor operation altogether. The pressure sensor in ac systems detects pressures and sends signals to the pcm to disable or enable compressor operation. A leak or damage that allows refrigerant release is the main reason for compressor cycling. You can observe this at the compressor clutch when it cycles on and off - the center compressor plate will not turn when ac isn't running, turns when ac is on and cycle off and on with a distinctive metallic click every time power is applied and removed to the clutch.

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