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Rough idle went defrost is on

I have a 2003 pt cruiser turbo 5 speed. I get a rough idle issuse when I turn on the defrost. I have replaced the plugs with champion idiom plugs gaped at 0.040, plug wires,coil,cam shaft sensor, engine temp sensor,mass air flow sensor. I had the dealership flash the PCM with the updated program. They think I may have water in the fuel tank. I have gone thru 2 tanks of 87 octan gas and added to bottles of STP fuel injector cleaner. Yet I stiil have the same issuse. Please help It’s our only car. Both my wife and Lost our jobs last year.

I am willing to wager a small amount of money that you engine also idles roughly when you turn on the A/C.
Am I correct?
In case you are not aware of it, your A/C compressor runs when you set your HVAC system to the defrost setting, in order to help evaporate moisture on the inside of the windshield.

Small 4-cylinder engines tend to idle roughly, and they tend to get even rougher when the A/C compressor is activated. Unless this rough idle is something completely new, I believe that you are experiencing a normal situation. If this is a new situation, you might want to have your mechanic check the motor mounts.

Thanks for getting back to me. I have checked the motor mounts,they are good. when I drive down the highway if I stomp on it to pass a car it seems to hesate a bit. small back fire and then picks up speed. The check engine light has come on twice and the code is p0300 misfire in all cylinders. I had reset at autozone.

The rough idle with defrost is due to the AC compressor putting a drag on the motor. It is showing you have a more significant problem. The backfiring on acceleration and check engine light are likely related to the same core problem. Your Cruiser is sick and needs some attention, perhaps more than a DIY can accomplish.

“The check engine light has come on twice…”

Why do so many folks asking for help fail to divulge this very vital bit of information when they post their question?

As Uncle Turbo stated, it is time to take the car to a mechanic since your attempts at repair have not yet worked.
If you don’t spend some money for professional diagnosis and repair, this rough-idling only car of yours will soon be your non-functioning only car.

Misfire can rapidly cause major damage to the engine, and perhaps to the catalytic converter.

When I turn off the defrost the idle picks up and the car runs idles great. The dealership took a sample of my gas today. They showed me my gas next to some that they had on hand. They looked a bit different from each other, mine had a little smell of gas and thiers was a bit stronger. What do suggest I do to get the water out of the system?

  [i] They looked a bit different from each other, mine had a little smell of gas and thiers was a bit stronger. [/i] 

I doubt if it is the fuel, but I guess it is possible.

The trouble might be due noise in the electrical system (misfire codes) due to a faulty ground connection. I suggest you clean the ground connections to the chassis and engine that are under the hood. It may not help in your case but others have fixed problems like you are having by doing that and will only cost you a little of your own time.

Since gasoline rides on top of water, the best way to get it out is to remove the gas tank and drain it. Due to the baffles in the tank, a good flushing solution and a day or two of drying upside-down with the pump/sending unit removed would be sure to clear out any contaminants.

The problem might be with Idle Air Control valve circuit.

The IAC valve is what controls the engine idle speed depending on the load imposed on the engine at idle. So for example, if the steering wheel is turned the power steering pump imposes a load on the engine at idle. The IAC valve then moves to a position to step the idle up to compensate for this load so the engine doesn’t run rough or stall. The same thing occurs when the AC or defrosters are turned on. The AC compressor imposes a load on the engine at idle and the IAC valve moves to a position to compensate for this load so the engine doesn’t run rough or stall. Now if the steering is turned and the AC is on at idle, the IAC valve has to move to another position to compensate for those two loads imposed on the engine at idle. So the IAC valve is a stepper motor that can move to various positions for the various load conditions when the engine is idling.

If the IAC valve fails to go to the proper position where the engine has a rough idle when that load is imposed on the engine, it means the IAC valve isn’t allowing enough air into the engine. This can then cause the engine to run rich and/or cause an engine misfire. This then will cause the Check Engine light to come on.

The first thing to try for an IAC valve problem is remove the IAC valve from the throttle body, and clean it along with the idle air bleed port on the throttle body of deposits with an aerosol throttle body cleaner. Then reinstall the IAC valve to see if that fixes the problem. If that doesn’t work, with the engine up to operating temperature and idling, take the handle of a screwdriver and rap on the IAC valve. If doing this has an effect on the idle speed, there’s a problem with the IAC valve.


Thanks for geting back to me. I cleaned mine out w/throttle body cleaner. It did not seem to help. So I ordered a IAC today. I do not know if it will help or not. It was 45.00 bucks. If it does not work I will return it. I have also replaced the engine temp control sensor and the cam shaft sensor. I also added a bottle of Heet water remover in the tank. I’m going to test the fuel pump PSI and see if that could be the issuse. I will post my findings and i hope I will have good info to pass on to the next person.

So more bad news! It was not the IAC motor, it’s not the fuel injectors. I had the flow checked and they are good. Flow is at 110 per min. I have replaced the the following items. Spark plugs gaped to 0.040, spark plug wires,coil,engine temp control sensor, cam sensor. 3 tanks of gas with water removal additive, new PCV valve, checked fuel presure came back good at 58 psi. Normal is 53 to 63 psi. Sprayed Throttle cleaner on the vaccum lines and saw no change in rpm. I’m at a total loss for words at this point.

Have you checked your gas cap. Believe it or not it can do this if not sealing properly.

Jerry, you just replied to an 8 year old thread .

Jerry is right, it’s the worst when fellow troubleshooters hop onto a thread and it ends with, “Welp, I guess I’m screwed.” Did you wind up finding the issue? Did you just sell the car? An update would be terrific for future DIYers