Erratic Idle

[bigfoot]

Hi all. New to the site and new to the Corvette. I will try to keep this short and to the point but want to add all necessary info. The car is a 1994 lt1 with 80k original miles. I bought in April of this year. From what I understand is that the car was entirely stock except for a new fuel pump last year. The car starts right away every time and seems to idle fine (about 700 rpm). About 2 months ago when the weather started to get warmer I would run the AC and all was good. But at some point I noticed that when I came to a stop the idle would get erratic and bounce from 700-1400 or so rpm. That happened one time and was fine after that. I installed a cold air intake and still all was good. About a month ago (give or take) it happened again but I also felt the car lunge like it wanted to go while I was holding the brake (car is an auto btw). Did some research and found some info about the IAC so figuring it was 20 years old I replaced it. Car ran fine for a few days and then 3 days ago I am coming home with the AC on and coming to a stop on the freeway the idle starts bouncing and the car actually died on me (that was a first). But it started right up. Still stuck in traffic I alternated between drive and neutral to keep it running. It seemed a bit better in neutral. Off idle and accelerating was fine. Drove another 7 miles at high speed with no problem. Got about a mile from home and a new symptom. While accelerating from a stop to about 40mph the car was bucking a bit. Got it home to the garage and while sitting there the idle was still bouncing. Threw the code reader on and with no check engine light on I got a 43. From what I found that is a knock sensor. I don't think that would be the cause of my problem but a symptom of the real problem. But hopefully someone on here will know better tan me and have an idea.

I took the car out today for a short run and it was fine with and without the AC. However it is a cooler day herein Denver and the temp gauge never got over 200 degrees. I have not gotten a FSM for it yet but I will be. I have worked on cars for quite some time and am not one to just go throwing parts in until it is fixed. So if anyone has some insight as to what the problem may be or where to start I appreciate it. I have looked at past posts and haven't really seen a problem quite like mine. From what I have found on the web it seems there a some who just replace plugs, wires, computers and everything else until it works or they give up.

Sorry for the long post but thanks if you took the time to read it.

Oh I forgot to mention the previous owner was good about maintenance and stated they did do a tune up last year. Plugs, wires, cap, rotor.

 

[ecss]

If the idle problem occurs with the AC off, disregard this info. :ugh

If only with the AC on I'd say you are low on refrigerant due to a small leak which is causimg the compressor to short cycle. When AC is called for, the PCM increases the engine idle speed. If the AC short cycles, the compressor cuts off in a matter of seconds then starts
again then cuts off again etc.... This can cause an erratic idle.

A simple test is to turn the AC off when the idle problem happens. If the engine idle stabilizes you need to attach a set of manifold gauges and verify the hi and lo side pressures.

The C68 auto climate control system monitors the number of times the clutch cycles. If the clutch cycles to many times within a given time frame the compressor clutch is disabled and a code 09 low freon will set in the AC programmer. A led will flash on the HVAC control head.

For C60 manual AC there is no low freon code.

Here's a AC performance chart. You would need a thermometer and AC manifold gauges to verify the system pressures.

 

[WhalePirot]

I am still a fan of using a vacuum gauge as a basic troubleshooting tool (old school). Much of the engine is electrically controlled, vacuum operated so all those old rubber hoses..... well, you know. The ECM not knoweth all, as you surmise. Unmetered air will cause a variable idle as the ECM tries to optimize the mixture to achieve stochiometric fuel burn. You already know that a hot or cold engine can vary in vacuum leaks.

 

[Schrade]

Welcome onto the boards there.

Loping idle (Closed Loop, WARM MOTOR) NORMALLY would indicate a vacuum leak

But not a lope up to 1,400 rpm's.

Mods, like cold air intake, with some problem is NEVER the thing to do. Replacing parts, like IAC, with some problem, is NEVER the thing to do.

If there's ANY problem, ALL operating parameters must be STOCK, with baseline operation, to do diagnostics.

Start with FSM diagnostic procedure for 43. SKIP NO STEP. If you do, you can draw NO conclusion. (which module DTC 43? Module 1? Module 4?, Module 7? ASR? SIR?)

Cold motor idle - Open Loop - means nothing.

 

[BD104X]

I had an Z28 that had a bad mass air flow sensor that would behave like that. If you tap the MAF sensor with a screwdriver handle while the engine was running, the idle would change. Replace MAF and all was good.

 

[Schrade]

I had an Z28 that had a bad mass air flow sensor that would behave like that. If you tap the MAF sensor with a screwdriver handle while the engine was running, the idle would change. Replace MAF and all was good.

Good call on the MAF BD...

But since he has a DTC (43), the tap test won't work for him.

 

[BD104X]

To be honest I don't remember if I had a MIL light or not, but the symptoms sound awfully familiar. It's easy enough to try. I DO remember that after the guy who diagnosed it did that test, it really ran like crap after that until I changed the MAF!

 

[bigfoot]

Everyone, thank you for the input. Ok, here is an update of the problem. I check the AC and it is ok. The control unit in the dash for the AC is a little wonky and does not always want to turn on and off. I was able to turn the AC off and found that was masking a bigger (in my opinion) issue. With it off I took it out for a drive and it was a hotter day and all seemed good while driving. When I stopped I noticed the temp gauge getting way to high and the idle became rough. I drove back home with the AC on and the temp seemed better until stopping. Without the AC and stopped the idle was bad and gauge was almost pegged. Popped the hood and noticed without the AC the fans were not running. Turn on the AC and they both come on. I have not had a chance to check anything further. I'm thinking possibly the fan switch, or a relay (though I have never seen a relay fail, but it is possible).

 

[ecss]

Everyone, thank you for the input. Ok, here is an update of the problem. I check the AC and it is ok. The control unit in the dash for the AC is a little wonky and does not always want to turn on and off. I was able to turn the AC off and found that was masking a bigger (in my opinion) issue. With it off I took it out for a drive and it was a hotter day and all seemed good while driving. When I stopped I noticed the temp gauge getting way to high and the idle became rough. I drove back home with the AC on and the temp seemed better until stopping. Without the AC and stopped the idle was bad and gauge was almost pegged. Popped the hood and noticed without the AC the fans were not running. Turn on the AC and they both come on. I have not had a chance to check anything further. I'm thinking possibly the fan switch, or a relay (though I have never seen a relay fail, but it is possible).

The PCM controls both fans.
Primary fan (driver side) turns on when engine temp. is around 228F
Secondary fan (passenger side) turns on when engine temp. is around 235F

Not positive about AC operation but Primary fan on when AC is On
Secondary fan On when AC high pressure exceeds 235 PSI (this reading is from the AC high pressure transducer).

The PCM monitors the engine coolant temperature sensor. It's screwed into the driver side front of the water pump. It has two wires going to it. Yellow wire is 5 volt reference. Black wire is ground.

Press the gauges button on the driver information center until Coolant is displayed in the speedometer LCD. This is the temperature the PCM is looking at. Monitor that reading and see if primary or secondary fan comes on with AC off.

 

[bigfoot]

Thanks for the info. I will try and look at it in the next day or so and report back.

 

[Schrade]

Kinda' confused here about A/C on or off, fans 1' and 2', and taking a drive, etc., ... (but I'm always confused)

You had a code 43; which module? There's a diagnostic flow chart for codes which will show the problem in a few minutes.

Seems like your fishing in pond 'B', when the fisherman are catching fish in pond '3' ???

 

[bigfoot]

I've probably made it confusing so I'll give a quick recap.

1. Had an erratic idle and did not see an CEL or SES light. Put the code reader on and saw no current codes but history had a 43 knock sensor message.
2. Erratic idle only happened when stopped and not all the time.
3. I have found that the erratic idle only happens when the car is running hotter than it should.
4. With the AC off the cooling fans do not come on and the analog temp gauge is almost pegged and the erratic idle is present.
5. With the AC on both cooling fans will run.
6. Don't want to be fishing in the wrong pond when I would rather be out enjoying the car as the weather starts to cool off, and appreciate any help and advice.

Jeff