Help -- A/C guru needed

[bike01]

My '91 has an intermittent problem. Sometimes when I turn on the A/C my rpm drops to around 400 rpm and it will stay there, this also causes it to stall sometimes. Sometimes when I turn on the A/C it will idle at 650 and be fine, but it seems to screw up for a day or so then be okay for a day or so. My question is if there is a relay somewhere that I can check or replace. If so where is it physically located and what does it look like. I've bypassed all of the pressure switches on the A/C line so I don't think they are a problem. The ECM just isn't getting the signal to increase the RPM when the request for A/C is on. Car runs and idles fine with the AC off.

[Tom]

I would pull the Idle Air Control Valve and clean it and the passage, these can affect the idle control when they get some deposits built up. It is easy to remove the valve and a little carb cleaner and a rag is all you need. Heed the shop manual warnings and do not try to manually move the IAC Valve pintle.

[bike01]

actually, now i think it's another problem. I can start the car and the idle shows about 850, things are okay, then I can turn it off, restart it again and the idle is at 450, shut it off, Idle at 750 okay, shut it off idle at 450, etc. etc. Bad injector, bad coil, knock sensor I don;t know.

All I know when I start it and it initially idles at 750 or so, it will run great untill I shut it off again. IAC has been replaced (twice) throttle body cleaned, new TPS, new plug wires & plugs, new Air temp sensor, new speed sensor.

Took it to the local corvette shop, after 1200 bucks, they still could not find the problem, got ripped off there.

[Patton]

PCV valves,... If you havent done that,... definetely a source of rough idle problems.
Maybe a coil pack? Especially if you have original coil packs and spark plugs, just change em all, even if they arent the problem.
I heard if the Camshaft position sensor is bad then it gives you bad idle problems. As a matter of fact I pulled that DTC from my car once,... but I have never had bad idle problems. I think it was trigged from something ealse that was wrong.

I had that exact problem on my 89 IROC... Changed the O2 sensor and BAM,... problem was solved. Does the Z even have a conventional O2 sensor? Havent heard anybody talk about it. Geesh I need a shop manual. If it does have one, I HIGHLY recommend replacing it or whatever accomplishes its like task. Also,... do you have the factory chip or a "performance" one.

[Jeffvette]

Francisco, here you go

http://cgi.ebay.com/ebaymotors/1994-Chevrolet-Corvette-Service-Manual-Two-Volume-Set_W0QQcmdZViewItemQQcategoryZ34215QQitemZ8010382 203QQrdZ1QQsspagenameZWDVW


And yes the ZR-1 uses two O2 Sensors.

[Patton]

Jeff, you are a kind a beautiful man.

KICKASS :thumbsup: :dancing :handshak: :cheers: Especially because they are listed as "Out Of Stock" everywhere.

Bike01, I would get hot on them O2 Sensors,... Thats just my experience though.

[Tom]

My car had similar symptoms where the idle would move around and the engine would sometimes die. My problem was caused by the MAP sensor. The sensor would read at one side of the proper values but not go out of limits and set an error code. Replacing the sensor fixed the problem. You should probably check the vacuum line to the sensor carefully as well and remove the oil that gets in it and consider replacing it. The original rubber fittings on the line get soaked with oil, get soft, lose their grip, and do not seal very well and are then prone to slipping off.

[Patton]

Come to think of it, I replaced the MAP at the same time as well,... Hell could have been the MAP.

[Jeffvette]

Francisco, you should have used the buy it now.

[bike01]

Thanks guys, I put a new TPS in the car 3 days ago, and still had the idle probelm, but I didn't really drive it beacue the inspection was out of date. Today I cleaned it up and would still experience the idle problem. Start idle at 450, restart idle at 750, restart idle at 450, etc. Well I drove it today, about 5 miles to the inpection station first, the idle was fine, then after getting the new inpection sticker I drove to lunch, drove the the Chevy dealer and got my forsy look at a new Z06, and drove it some more around town. What a nice day, no clouds, temps in the lower 80's, low humidity.


Anyway, to make a long story short, once i put some miles on it, the idle really held in there with the A/C on or off. the car idled perfectly. Could there be some 'learning' time required time for the ECM when you replace the TPS.

Like the thread on the manuals, I have a set of '94 manuals that cover the LT-5, do the specific LT-5 manuals have that much more data, I was under the impression in '94 they just combined the LT-5 stuff with the regular Corvette stuff.

[bike01]

Oh yea,

I forget, yes I replaced the MAP sensor and cleaned the hose, haven't changed out the PCV valves, I'll do that next if it screws up again. Question, There is a map sensor looking thing under the ECM, is this another MAP sensor or it it something else.

[tomtom72]

The look-a-like MAP under the ECM is the MAP sensor that the ECM uses as a reference signal. I don't know what the reference is referring to but it comes off of the vacuum hose assembly that is under the plenum. The line from the ECM MAP sensor is traced back to the secondary vacuum hose assembly and is "T-eed" off of one of the secondary actuator vacuum supply lines...if I remember correctly.

In any event it does originate in the secondary vacuun hose assembly and I would guess if the actuators fail it might set a code for secondary actuation malfunction. I'm just guessing, here. The only thing I know for sure is it is another MAP sensor under the ECM & it gets it's vacuum signal from the secondary vacuum hose assembly under the plenum.

:cheers:
Tom

[tomtom72]

I'm not C4/ZR-1 literate enough to really assist in the issue you're having but when I re-did my vacuum stuff for the secondaries I learned from Jeffvette that there is a vacuum control valve under the plenum that supplies vacuum to the HVAC system. The thing I'm driving at is if that valve had a leak could the resultant vacuum leak affect the idle quality of a motor???

Sorry for not having more than this dumb theory/question to contribute.

Tom