Several weeks ago my car developed a hanging idle condition out of the blue. It's exactly like what happens after reflashing the PCM. When coming to a stop the idle speed will come down slowly, and then bump up to about 900-1000rpm and then slowly settle down to idle. On decel, the rpm's will drop to certain level, say at 40-45mph to about 1100-1200rpm, and then just hang there like the car's on cruise control.

The crazy thing is the problem's intermittent. It does it most of the time, but then all of a sudden the idle behavior will be normal. Maybe in the last week of trying fixes it has been less frequent, but it is still occuring. Trying to figure this out, I have:

- torqued intake manifold bolts
- driven for a couple of days with the solenoid purge valve blocked off
- found and repaired a torn PCV boot and replaced the PCV.
- sprayed carb cleaner over all PCV lines and manifold at idle with no effect.
- swapped IAC's with my other car
- physically monitored throttle operation, and scanned throttle position and TPS voltage, all normal
- had a MAF circuit low DTC (no MIL). Swapped on new and reman MAF's

None of the above resulted in any permanent change, although I had a few hopeful false positives. One clue is that my IAC counts are low - often it's at zero at idle and idles perfectly.
Of course I could attempt to compensate for this by tuning but I've run my current tune for a long time with no issues until this occurred out of the blue. It's not the tune. It may have started after I pulled off my catch can and cleaned it, but I can't remember any direct correlation.

I'm up for any fresh ideas. This is annoying and I've done everything I can think of. I know it sounds like a vacuum leak, but where? And why intermittent?

Sorry for the long post - just want to be thorough.

 

Is this for the Formula in your sig with a stock cam and intake? Details on car mods would help.

It's hard to diagnose intermittent problems over the internet, but I would look at two points you mentioned:

- It may have started after I pulled off my catch can and cleaned it, but I can't remember any direct correlation.
Double check all of the hoses and connections and make sure the drain plug is completely sealed. Compare the LTFT's when the problem occurs and when it doesn't to see if there is a difference. Also check all PCV connections and the vacuum hoses on the intake and make sure the MAP sensor is secured and sealed.

- One clue is that my IAC counts are low - often it's at zero at idle and idles perfectly.
This could be the PCM compensating for the condition and is simply out of adjustment range of the IAC. I would compare the IAC steps when the problem occurs and when it doesn't. Remove the intake tract and inspect the TB blade for restrictions (carbon build up, etc) and make sure it moves freely (no binding or hesitation) from closed to completely open. Did you adjust the TB set screw by any chance? I would also check this to make sure it's secured and not loose.

 

Yep it's my A4 Formula - just bolt-ons including LS6 intake, race-style LT's, AIR & EGR deleted. The first thing I found was that the cap on the AIR solenoid vacuum port was split; I thought "Aha, that's it." Replaced it and no help.

I've checked and re-checked all the connections to the catch can and PCV. Found the tear in the PCV boot and the top port of the can was not fully snugged down. Fixed all that, and still, no resolution.

Funny you should mention the drain plug on the can. About a year ago, I discovered that I had left it open for a while. I had driven the car like that for a month or two with no effect on driveability. Since then, I've been real careful about that.

I have several logs now and have been studying and comparing them - no revelations yet. I can take a closer look at LTFT's - thanks.

 

Also, what's the best way to access the MAP sensor? From underneath? I've sprayed carb cleaner back there from up top but can't be sure I made contact.

 

You have a 2000 with the stock PCV setup? Then it's about impossible to know just from inspecting it if there's a leak in the PCV hoses that go around the back of the intake, where they Y together, or where the driver's side hose connects to the valve cover. Best way to check is put a vacuum gauge on the fresh air inlet hose (block off the TB port), and check that you have some vacuum at idle.

 

Ok good suggestion, thanks.

 

I did this and had no vacuum at idle. I did the same on my buddy's T/A and his showed no vacuum either. Even checked with two vacuum gauges to make sure. Still chasing the issue ...

 

Is there any chance the blade could be sticking slightly?

I am having a similar issue and think it is the blade of the POS fast tb sticking just a little sometimes and not closing so the pcm has to use the iac to bring the idle down

 

Have you checked for leaks in the HVAC vacuum plumbing...?

 

A friend of mine had the same issue early this month, the local speed shop accepted the challenge and found it to be a combination of a leaking vacum line, valve cover breather line, and a few minor adjustments to the tune.

 

Throttle blade is fine. I checked the HVAC vacuum line under the MAP sensor by feel - everything feels normal. But as it's crammed under the cowl, it's difficult to tell for sure.

The good news is that the problem seems to be slowly improving - doing it less often. I could attempt to address it through tuning, but that would temporarily exacerbate the problem due to the reflash.

 

Bump to see if there are any fresh ideas out there. Frequency has slowly diminished from doing it about 85% of the time to about 30% now. Still enough to be seriously annoying though. The intermittency makes it the weirdest problem ever.

 

I skimmed through the post, so sorry if you mentioned this already.

What intake mods do you have on the car? And what kpa do you see at idle?

 

Averages about 32kpa at 700-725 average idle. Idle is set to 650, but it idles higher when the condition is occuring. Just like a vacuum leak - that comes and goes randomly.

 

Did you every have any luck with the problem? I'm experiencing the same thing...

 

I'm watching this thread because I've got a similar problem with my car. Mine is constant, xtremely annoying and it makes me just want to blow the car up with a stick of dynamite .

 

I think it was caused by a vacuum leak in my catch can / PCV set up. I never actually determined this, but after I taped and tightened everything up, it gradually went away in about six weeks time. Hasn't recurred since.

 

Mine still does similar things but it is very intermittent so it's hard to try to diagnose

 

I just fixed mine completely the other day. I had three vacuum leaks! One was from the little hose that connects between the PCV valve to under the intake manifold on the passenger side of the engine (apparently this is the most common place to have a vacuum leak). It was still kind of loose with a new hose so I put a hose clamp on each side and it fixed that one. Another was from a very small tear in the bellow that connects the MAF to the throttle body. $50 bucks for that stupid thing... Then the largest leak was from the grommet in the EGR going bad. Replaced all of that and she runs like new now! My mechanic found these by using a smoke test. I think all three leaks happened because I recently installed a BBK intake and removing all of those hoses and grommets after 13 years of them never really being messed with caused it.

 

Fourth-Gen Man, thank you for posting your detailed results. It really helps because those of us suffering from similar issues now know where to look.

Do you or anyone else know anything about this "smoke test?" I've sprayed carb/brake cleaner around hoses and the intake manifold on my 1991 Mustang GT when I had idle speed problems. The harsh cleaners bubbled up the paint on the 5.0 engine. Probably not an issue on my aluminum LS1 .