Here's some background on the situation... Usually I start the car- idle flares up to 1500 rpm and then ramps down over 10-15 secs to 800rpm. Well a couple of weeks ago, on the first cold day(20*) out here the car started "funny" - Started car- no flare, right to idle @ 800. Then when I started driving, every time I would come to a stop the idle would drop to ~400 for 2-3secs and then bounce right up to 800 nice and steady. The car did that for two days, and then back to normal for the last week. I was thinking it might be temp. related, but today it did it again (it's 55* outside). I grabbed the laptop and it shows IAC counts 95-120 @ idle. Usually I see ~20-35. What's going on here?

Dave

 

your IAC is getting stuck closed. replace it. cheap and an easy do-it-yourselfer. cold weather usually freezes a worn out IAC motor. the high pcm iac counts at idle means the pcm is struggling to get your IAC open. they should be at 20-40 at idle. park and drive.

 

What is normal behavior for iac, my old one that i broke cleaning used to flare up to 1500 when started and then drop down slowly but seemed to always be a hundred or two hundred rpms high. I put a new one in and it seems to flare to 1500 and immediately drop to 800 and slowly work its position to the correct spot.

 

that sounds like normal behavior for your new one phenomic. to really find out if your iac is working right, you need to scan the pcm and monitor the IAC counts. 20-40 at idle = normal behavior. Assuming your "new" IAC is fresh from the parts store, then it should be fine. if your idle is too high, or too jumpy, you may have a vacuum leak somewhere.

 

JDBlaze85,

Isn't the IAC closed when the count is "0"? I was under the impression that low IAC readings indicate a vacuum leak, and high counts indictate a rich condition. What I'm trying to find out is why this changed "all of a sudden" Help! it's been 55*+ for the last few days and the car is still malfunctioning, so I have a feeling it might not be temp related.

Any other input?

Dave

 

dave, iac counts are not the best indicators for a vacuum leak. you may be mistaken with the 02 sensor. a low 02 reading indicates a lean condition, possibly vacuum leak. a high 02 reading indicates a rich condition. your 02 should be constantly fluctuating high and low at operating temp and closed loop. 200-800mv. now your IAC counts- the counts you see on your scanner is displaying what the pcm is trying to do. the counts DO NOT represent what the IAC motor is doing. the counts are showing what the pcm WANTS the IAC motor to do. so for example, if you see your pcm showing 0 counts, then the pcm wants to completely close the IAC which usually only happens when you have a stuck open IAC motor, or a good vacuum leak and the engine idles higher than what the pcm wants to idle at. if you see your pcm go as high as 100+ counts at idle, then the pcm wants to open the IAC because the idle rpm is too low. usually a misfiring, badly tuned engine will cause the pcm to open the IAC this high, ORRRRRR a faulty IAC motor or restriction in the IAC intake passage or a stuck closed IAC. if your engine runs normal other than the idle problems, then I really believe the IAC motor is at fault or the throttle body is clogged up where the IAC screws into and sucks air. pull the IAC and check for black sludge and see if your throttle body needs a cleaning out. sometimes you can clean the IAC motor needle and WD-40 the flip out of it, but it is highly reccommended to just replace the motor if it is old.

btw, you dont have a throttle body bypass mod right?

 

Did you check the readout for the knock sensor?
Mine did the same thing for well over a year. I always assumed it was the IAC and just lived with it. However I received a SES light and found out the knock sensor was pulling up to 12* of timing whenever I would let off the gas. I replaced it and things have been fine ever since.

 

Update:

Yesterday I tried the IAC "reset" procedure at lunchtime- no change. I tried it again leaving for work and the car has been back to it's usual self! (and it's cold again today!)I would still like to know why it did it...

JDBlaze- IAC was cleaned ~4mos ago... however, I DO have the TB bypass mod. I really don't think that the TB blades icing up is an issue... How much quicker than the top of the engine does the coolant warm up???

Although I suppose it could explain the situation if some ice A) sealed up the blades better than usual, so the IAC was compensating or B) actually froze up the IAC motor...

SS RRR- Knock is 0*.... Well sometime .1* between high rpm shifts...

Right now I suppose the plan is to keep an eye on things. If I have trouble again, I will prob. replace IAC.

Dave