I have a 2003 Cavalier.
Check engine light comes on. To get the light to turn off, all I have to do is turn on the heat or AC when I drive and light goes off and stays off. (Basically if the compressor is engaged the check engine light goes off)

Looking for experiance on what the problem is.

I went crazy buying new parts and still same issue. Car idles rough at stop light. I tend to put it in neutral and it gets alittle better. Car idles around 500rpm. Driving the car it drives great. Plenty of power.

Compression is good on all cylinders.
Fuel pressure is good.
New:
Timing Chain Kit
Balance Chain Kit
Water Pump
Thermostat
Map Sensor
Purge Valve
IAC Valve
Fuel Injectors
Plugs
Boots
Coil
Ignition Module
ECU

I'm out of ideas....... Please help.

Hi Carl,

You probably have some corroded contact somewhere, possibly in some ground connection.
Go through them and clean them, around the engine and the ECU.
Also, instead of throwing parts at the car (you did not say why all those things needed to be changed), plug in an OBD code reader, it will give some indication where to look for the trouble.

Good luck, Peter

I feel as though it's my idle causing the roughness. I idle at 500rpm. If I put the car in neutral the rpm goes up to 600 and is much smoother. Also by turning on the ac it kicks in the compressor that raises the idle to 600/650. I have replaced all sensors that have anything to do with fuel and air monitoring.... but it seems it really comes down to the idle speed being the only issue. So even if I manually give the car alittle gas to raise the idle much better. Just annoying always seeing the check engine light.

Hi Carl,

I guess I was too ignorant to see the error code in the title

Looks like you have already replaced everything that could cause that misfire too!
Here is a video to check out some relevant things to misfire on that engine, but do check for intake leaks!
Your text to link here...

I think that on this car (but I could be wrong), when you turn on the AC, it sends a signal to the ECU to increase the idle speed. So that could be the reason to the increased idle when turning on the AC.

Peter

Yeah, I've gone through cans looking for intake leaks... nuthing.

I went ahead and ordered air intake sensor, and crankshaft position sensor. Since rite next to the crankshaft position sensor is the oil pressure sensor, and knock sensor I just ordered those 2 also since I had to take the starter off to get to crankshaft position sensor.

I'm completely out of Ideas. Every article I've read (many)... says:
Air, Fuel, Spark, Exhaust

I was going to replace o2 sensors..... but, turning on the air to kick on the compressor would not affect the exhaust in any way.
Same with catalytic convertor, people say that could cause it... but once again... turning on the air wouldn't affect the exhaust to change in any way.

I shouldn't have said I just replaced everything. I've replaced everything that could cause this issue.

I considered the throttle position sensor.... but on my scan tool its rite on the money were it should be.

I'm about out of ideas. It really seems to be the idle is just a little to low, and causing this issue.

Hi Carl,

I looked up the service manual of my '05 Cav, same engine/drive train.
It looks like that the low idle could be the result of the engine misfire.
The engine misfire is being continuously monitored by the ECU when the engine is warm, from RPM 460 to RPM 6400.
The CEL light is being turned on or off after two cycles of misses or non-misfires.
The algorithm uses the crankshaft position sensor, it checks the minute variations in engine speed.
If there were problems with the IAC circuits the diagnostics would report any electrical problem.

For engine misfires the manual suggests to check for vacuum leaks, cleaning the connections/grounds around the ECU.
As for the specific cylinder misfire it suggests to swap coils and plugs and to see if the problem moved over to another cylinder.
I would also check the gap of the spark plugs.

Peter

Thanks for the response Peter.
If you read all the other comments I posted on here (alot)

I am in the process of installing a new Crankshaft position sensor rite now.... Well see what happens.
I already did everything else. Checked for vacuum leaks. All new sensors, bla bla bla. I Hope this finally gets it. This car has been a champ and hate to end up ditching it. I will clean my grounds for the heck of it. Cant hurt.

I'm also gonna rule out vacuum leaks, because if I turn on the compressor car runs fine. If it were a vacuum leak the issue would still be there.

Hi Carl,

I would be really surprised if that crank position sensor resolved your issues, but I guess anything is possible!

The crank position is so fundamental to engine control that if it were flaky, it sure would have resulted in a whole slew of other error codes as well!

You error code is misfire in cylinder 2.
As you mentioned that could be from ignition, or from fuel (improper mixture, vacuum leak).
I would try to figure out why is that code set!
Follow the suggestion in the manual about possible ignition failure.
You mentioned that you checked for intake leaks, but is there a chance that there is a leak at the cylinder 2 intake on the underside, where it would be hard to reach. Did you check that part of the intake?

It is also interesting that your idle is so slow.
The IAC valve is a stepper motor actuated valve. The ECU controls all four of the connections. It does some checking, though the manual (the error code description) is a bit vague to nail down exactly if there were something wrong with the electricals.
I would also expect the idle control to maintain proper idle speed, even if one cylinder is misfiring, it would be rough, but at the correct idle RPM.
What happens if you remove the IAC valve connector, does the RPM change? Is that IAC system operational?

And finally the fact that when you turn on some electrical load, the fault goes away.
That is an indication of some connection where the resistance has increased, the load current causing a voltage increase.
How that makes the error go away and the idle correct that is a good question. There is simply not enough info about the internal workings of the ECU to come up with a good explanation, but cleaning contacts could not hurt.

Good luck, Peter