Erratic idle, no codes

Z16MONTE

MEMBER
1991 CHEVROLET CORSICA
95,000 MILES

Some times it will start and idle very high(2500-3000rpms) other times it wont start without partially depressing the accelerator. Showed no codes, but sets one if you unplug the TPS. Unplugging the tps or the IAC made no difference in the high idle. Hard to tell on low idle because its so erratic. Unplugging the MAP sensor caused it to stall immediately when idling low and fairly steady. No codes set when either the MAP or IAC were unplugged. Using voltage readings from my AutoXray, the sensors all seem to be working. This is the only car I don't have a manual for so I'm not sure what the ranges should be. 2.2L TBI.
Ed

6 Replies

KHLOW2008

MECHANIC
41,814 POSTS

Sometimes when a control valve sticks, it does not throw a code and the component that directly affects the idling is the IAC. Try cleaning it to see if it helps.

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Monday, October 1st, 2012 AT 4:36 PM

HMAC300

MECHANIC
48,601 POSTS

Check for a vacuumleak. If youunplug any sensor it will set a code. Clear all y our codes and see if it sets another. Check your fuel pressure it should be 9-11 psi if below that that may be your problem. You might try cleanin the iac hole with choke cleaner it may help. Check your hose goign to map if it has one for being soft as it may be sucking together. You can spray intake with choke cleaenr to find a vacuum leak. Also spray under tbi.

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Monday, October 1st, 2012 AT 4:40 PM

Z16MONTE

MEMBER
3 POSTS

I've checked for vac leaks visual and carb cleaner. Also sprayed the TBI unit. No difference. Car runs great going down the highway as well. Even if I punch it and run it upto 60 or so, no signs of running out of fuel. Need to make an adapter to check the fuel press. Don't have one for this car. Only thing to set a code was the TPS. Neither the IAC or MAp set one. I checked for codes before starting, there were noneEd

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Monday, October 1st, 2012 AT 5:19 PM

Z16MONTE

MEMBER
3 POSTS

I also cleaned the IAC and chamber, then replaced it with a new one. No change.

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Monday, October 1st, 2012 AT 5:21 PM

KHLOW2008

MECHANIC
41,814 POSTS

Here is a list of things you should cehck through.

Check the following items:

Make sure that throttle linkage and/or TPS are not sticking or binding.

Make sure that initial ignition timing is properly set.

Check engine idle speed, both base idle and ECM idle.

Check Idle Air Control (IAC) system. Check for foreign material in IAC bore.

Check for proper operation of EGR system.

Check ISC/ILC operation.

Check P/N switch circuit. Also make sure that P/N switch is properly adjusted.

Check power steering pressure switch circuit.

Check exhaust system for restrictions, such as a damaged or collapsed pipe, muffler or catalytic converter.

If rough idle only occurs when engine is hot, check PCV valve for proper operation, check evaporative emission control system, check for proper spark plug gap and check engine compression.