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Tech501
Tech501, Auto Service Technician
Category: Car
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Experience:  38 years exp, ASE Mastertech with L1 Advanced Engine performance
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1998 Cavalier: equiped..2.2L..It runs poorly at times

Customer Question

I have a problem with a 1998 Cavalier equiped with a 2.2L engine. It runs poorly at times, and good at others. I used a scan tool to try to get an idea what was happening and found that the engine stays in open loop operation. Also the idle air control always reads "0". The idle air control does seem to operate. It moves to compensate when I do something like remove a vacum hose or put the car in gear. It also maintains a normal idle speed. I removed the idle air control and checked the resistance between the terminals as instructed in the Chilton manual. All measurements were in normal range. My questions, then, are what conditions must be met to enter closed loop operation on this engine, and what could be the cause of the "0" reading for the idle air control? I did check the manifold for vacum leaks and the O2 sensor is new and appears to be functioning. Plugs and wires are almost new.
Submitted: 11 years ago.
Category: Car
Expert:  Eduardo Murillo Jr replied 11 years ago.

Hi, have you checked your thermostat, does it have one on.


In order for it to enter into closed loop the engine coolant Must be at normal operating temperature let me know if this solves your problem.

Expert:  Tech501 replied 11 years ago.

The requirement to go into closed loop are



  1. Timer has to expire

  2. O/2 has to reach sufficient temp to show activity

  3. Engine must reach correct temp.

Some GM car will slip back into open loop at idle and that is completely normal. If you can raise the RPM to over 1200 and get closed loop, you are OK.

Expert:  Steve replied 11 years ago.

The zero reading for idle air control counts is abnormal, and should be investigated. This indicates that the computer has driven the IAC completely closed in an attempt to bring the idle speed down to it's target range.


This usually indicates an air leak somewhere, as you seem to have suspected when you were searching for intake manifold leaks. You may want to check also for air leaks at the throttle body, brake booster, EGR valve, and anywhere that a vacuum hose runs to. Try pinching any vacuum hoses off as close to the engine as possible, while watching the scanner for an increase in IAC counts or a change in idle speed. Often you can locate small air leaks by spraying a light mist of 2+2 gum cutter or other fuel system cleaner spray at possible points of leakage, such as the intake and throttle body.


Remove the IAC valve from the throttle body and check for carbon deposits holding teh pintle from closing completely. If this happens (it is common) the IAC will never close completely, and the IAC counts on the scanner will read at or close to zero because the ECM is trying to colse the stuck valve. The pintle and seat can be cleaned with the same spray colvent you use to leak check, and a toothbrush if required.


I hope this is helpful; if so, clicking my accept button would be most appreciated... thanks!

Expert:  Tech501 replied 11 years ago.

The customer has stated the the engine idles properly and has no idle problems whatsoever. This would not be the case with 0 counts on the IAC. I suggest that the data may be indicating the throttle percentage trather than the IAC counts. All the previous talk abvout vacuum leaks is useless if the car idles fine.


All this has nothing to do with the original question of "what is required for closed loop operation" which I correctly answered. I suggest you ignore all the vacuum leak talk by the other specialist. Please click on the accept button for MY answer which was the correct one.

Tech501, Auto Service Technician
Category: Car
Satisfied Customers: 747
Experience: 38 years exp, ASE Mastertech with L1 Advanced Engine performance
Tech501 and 6 other Car Specialists are ready to help you
Customer: replied 11 years ago.
There is a lot of good information here, I see. My concern about the reading of the idle air control is that I didn't know if it was somehow affecting the loop status. Or if some other sensors were possibly involved. It does seem strange that the IAC is apparently working and yet does not register a reading. I am under the impression that a vacuum leak sufficient to cause a zero reading would be doing so because the pintle was either seated or stuck on some debris. And yet when I watch it, it appears to be doing it's job. At one point I did have it out and it didn't seem to have any obstructions. As far as the loop status is concerned, the engine gets up to normal operating temps quickly (between 175F-185F normaly). Upon reaching cruise speeds I noticed that it goes into closed loop for short periods of time. Probably less than 30% of the time. At idle it is never closed. If the three conditions listed by Tech501 are the only factors involved then I am left with figureing out if the O2 sensor output is sufficient.
Customer: replied 11 years ago.
I accepted the answer of tech501 because it addressed the conditions for closed loop, but there is a lot of information from the others too. I am grateful for all of the insights offered regardless. I still need to address the problem of the IAC zero reading, and the suggestions offered don't seem to fit what is happening. I suppose it is possible the problem is in the IAC unit itself. When I observe the IAC it will close if I remmove a vacuum hose and open when it is replaced.
Expert:  Tech501 replied 11 years ago.
I just got another tip from Diablo666 that i'll pass on to you. It's been found that the use of aftermarket (non Delco) O/2 sensors can cause it to drop out of closed loop also.
Expert:  Tech501 replied 11 years ago.

Quote:
As far as the loop status is concerned, the engine gets up to normal operating temps quickly (between 175F-185F normaly).


That's another thing I meant to address. That is not normal operating temp. They use a 195 deg thermostat and that's pretty critical for maintaining closed loop. I once had a Chevy van than failed emission due to jumping in and out of closed loop and the problem turned out to be a brand new thermostat that just wasn't calibrated correctly. New thermostat, problem gone.


Customer: replied 11 years ago.
My O2 sensor is a Bosch and my thermostat could be a problem too. I didn't realize the temp needed to be quite that high. It's possible I am suffering from a combination of problems and if I work through them all everything will look a whole lot better. Thanks again for all of the help.
Expert:  Tech501 replied 11 years ago.

No problem.


I'd start with that T-Stat. We already know that is a problem. Then see if the O/2 is a problem after that.

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