Rough / Fluctuating Idle
I'm having an intermittent idle problem. I have a scan tool that I've used to try to find the problem. My STFT and LTFT are within +/- 5%. MAF graph on scanner seems to be good. It increases when I accelerate and decreases when decelerating and sits at .5 lb/min when idling. The problem is intermittent and I have not found any rhyme or reason as to when it happens. Because it's so random I have not had many opportunities to check out the scanner while it's happening. I'm getting a somewhat rough idle a lot of the time, but occasionally it gets very rough. Other times the idle will speed up and slow down over and over again. If I'm driving while these changes in RPM occur I feel the vehicle sort of lurch a little bit. If I'm out of the vehicle letting it idle while this happens I can hear a pretty loud clicking sound, like a switch or relay, followed by the change in RPM. I've recently changed the IAC valve. As I mentioned before the MAF graph and fuel trims on my scanner seems good. TPS seems to be working too. The only thing I really notice out of the ordinary while the problem occurs is that the Spark Advance gets really rough where normally it is flat lined during a normal idle. I've read that you can test the TPS and MAF by unplugging them while the engine is running. If it continues to run or even improves while either is unplugged the sensor is bad. Not sure if this test is foolproof, but I gave them both a try. The engine dies when I unplug either sensor. Is it possible the rough/random idle is causing the Spark Advance variations or could it be the CPS causing the rough idle? Is there a way to determine cause/effect?