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?

 

Forgot to mention in first post -
PCV and hose were recently changed. CEL is not on.

 

Is it possible I could have a slight vacuum or fuel pressure problem that is not bad enough to throw an error but, bad enough to cause rough idle?

 

I've discovered that the loud clicking noise I'm hearing is the AC compressor clutch kicking on and off. From what I've gathered that's probably caused by a low freon charge. Apparently I have two different issues going on.

 

Did you ever figure out the cause of the rough idle. I'm having a similar issue with my ranger.

 

An 81 month old thread by a guy who didn't give mileage nor mention how long he's owned the vehicle or if it'd ever been trouble free.
He didn't describe his vehicle's condition or level of upkeep. I bet a PM would go unanswered.....just means he's gone.

1) He has a good scanner. I couldn't discern fuel trim levels without a guide in my hands.
2) Learning what his AC hardware sounds like reminds me to do the same with mine. Will I be listening for a relay or a clutch engaging or both?
The "lurch" could be said AC engagement bringing vacuum or sensed rpm beyond a threshold which makes the engine stumble. Bad motor mount or two amplify this?
Don't want to be lurching an auto whilst shifting
3) Was a quality IAC valve used? Knowledgable people on this board say OE or Hitachi ONLY.
4) "rough" advance? Is the crankshaft positioning sensor in the advance circuit? I had one go out in a car and it made my engine run rough as can be...often with sputtering out.
My bad CKP/CPS threw a code however.
5) I've seen videos where a MAF was easily checked with an ohm meter. Can ours be unplugged and checked at the plug?
A TPS can be checked at the plug too. It'd be 12 v passed through or resistance. They need a 12v supply and a ground too, no?
There's also a check for the coolant temperature sensor. I think a bad temp sensor would lead to rich operation, not rough operation but you'd eventually foul plugs. Have you inspected your plugs?

Maybe check your fuel pressure regulator. There shouldn't be gas smell on the vacuum hose side.
Unplug your idling engine's IAC valve. Your engine should drop in idle or even die. I believe you do this one warm. An increase in idle means you have a leak. The PCV system is an intake leak source.
Is yours a DD? Maybe start by giving your vehicle a good dose of Techron.
Is your air filter over 30K old? They do last so long people forget about them.

Mileage of your rig please. What kind of tooling have you?