High Low Idle?

BadSax

BadSax

enjoys 3 martini lunches
Ok, so ever since my intake cleaning project, my MK seems to have a slightly higher low idle. I would say the car used to idle around 650ish RPMs, and now it seems to find it's happy place at 1000 RPMs...

I for one don't really care other then with the strait pipes the car was pretty quite while idling, and now it's slightly less quiet, but then again, If I was a big fan of quiet I would have kept the stock exhaust... :D

Anyway, just more of a curiosity thing then anything else.
Any ideas what's going on?

Thanks!

:D -J
 
driller said:
Yep, I'd say you have an unmetered air leak.
Click to expand...

I like the term "unmetered"... :D
I was thinking some sort of air leak, and since there were many many things I forgot to plug back in during the reinstall, it is I suppose not outside the realm of possibilities that there is still something unplugged.

Any thoughts where to look?

There are no codes, no CE light...

Thanks,
:D -J
 
BadSax said:
I like the term "unmetered"... :D
I was thinking some sort of air leak, and since there were many many things I forgot to plug back in during the reinstall, it is I suppose not outside the realm of possibilities that there is still something unplugged.

Any thoughts where to look?

There are no codes, no CE light...

Thanks,
:D -J
Click to expand...

Hope it's not a manifold gasket leak...though more likely a vacuum line left unplugged. I don't think a gasket leak could ever be extreme enough to cause 1k rpm idle.

By the way, 1k idle in a Mark could be VERY dangerous. Besides the fact that the tranny and TC are being way over-worked, your brakes are heating up quite a bit more - maybe to the point of warping the rotors - and maybe even causing brake failure due to fade. I had something similar happen in my '85 5.0 T-Bird. The throttle linkage came apart, and locked back up to where my foot off the throttle was actually 1/4 throttle. The brakes faded to almost nothing in minutes...VERY scary.

I personally wouldn't drive it until that gets addressed.
 
It's not even noticeable when you drive it, I probably wouldn't have noticed at all except that the pitch on the exhaust is slightly higher at idle then it used to be.

The car doesn't pull against the brakes, or shift hard into drive from park or anything like that... It may be putting a little added stress on the TC but it's not going to cause anything to fail...

:D -J
 
I thought the IAC might be the culprit...

I'll try to poke around at it tonight.

:D -J
 
Ok, just started the car to head to "sorta work" and thought I'd mess with the IAC for a sec...

As soon as I unplugged it, the idle dropped back to where it used to be so I guess I'll take it off the car later and make sure it's free...

:D -J
 
Definitely something IAC related. Before I left I could hear the car occasionally surge, and not like the little difference you might hear when the compressor turns on and off...

hmmm...

:D -J
 
The surging may point to the IACV, but in my case it was two vacuum leaks. One on the large line at the throttle body going to the fuel vapor canister and the other at the small line going to the FPR off the intake manifold. The IACV was fine.
 
driller said:
The surging may point to the IACV, but in my case it was two vacuum leaks. One on the large line at the throttle body going to the fuel vapor canister and the other at the small line going to the FPR off the intake manifold. The IACV was fine.
Click to expand...

Well I don't know what the vapor canister is or the FPR... :D
but I pulled the IAC off and it seems to move just fine, but it's starting to get a little late for starting my car up repeatedly and I try to pick and choose when I piss off my neighbors at night... :D

I'll fool around with it more tomorrow.

:D -J
 
So, I've been driving around with IAC unplugged and the car drives totally normal, no high idle. Other then the CE light, you'd never know it wasn't doing it's thing...

So is the IAC just suddenly bad, or is there reason that the car is telling the IAC to let in more air?

Thanks!

:D -J
 
BadSax said:
So, I've been driving around with IAC unplugged and the car drives totally normal, no high idle. Other then the CE light, you'd never know it wasn't doing it's thing...

So is the IAC just suddenly bad, or is there reason that the car is telling the IAC to let in more air?

Thanks!

:D -J
Click to expand...

The IACV is basically a controlled air leak. The PCM modulates the signal to control the valve opening determining how much air bypasses the throttle plates thus controlling the idle rpms.

With the IACV plugged in you have a high idle speed.

With the IACV unplugged, you have a more normal idle speed.

That tells me the IACV is opened when it is plugged in and closed when it is unplugged.

Now let's throw in another air leak.

Now with the valve closed (unplugged) you still have enough air supporting idle. With the valve open (plugged in) you have more air causing a higher than normal idle.

It is possible either the IACV is responding to a incorrect signal or it is not capable of closing all the way when it is plugged in. Now I know the IACV will regulate from a certain level to 100% open but I'm not certain it is ever fully closed.

There are times in normal driving the IACV opens fully but I don't know of any strategy where it is 100% closed. So if you accept the theory the IACV will never close 100%, the symptoms would be exactly as you describe IF you had an additional un-metered air leak.

From your description of the engine surging, in theory that is the IACV trying to regulate the idle but it is confused by the added air leak.

So I think we have three possible scenarios.

1. A bad IAC valve.

2. Vacuum leak.

3. A bad IAC valve and a vacuum leak.

I don't think buying an IACV would break the bank, and as easy as they are to access on a Gen2, I recommend replacing the IACV as a troubleshooting method. If it turns out the IACV is good, having a spare is not such a bad thing.

One further idea... is the throttle closing all the way??? :rolleyes:
 
driller said:
The IACV is basically a controlled air leak. The PCM modulates the signal to control the valve opening determining how much air bypasses the throttle plates thus controlling the idle rpms.

With the IACV plugged in you have a high idle speed.

With the IACV unplugged, you have a more normal idle speed.

That tells me the IACV is opened when it is plugged in and closed when it is unplugged.

Now let's throw in another air leak.

Now with the valve closed (unplugged) you still have enough air supporting idle. With the valve open (plugged in) you have more air causing a higher than normal idle.

It is possible either the IACV is responding to a incorrect signal or it is not capable of closing all the way when it is plugged in. Now I know the IACV will regulate from a certain level to 100% open but I'm not certain it is ever fully closed.

There are times in normal driving the IACV opens fully but I don't know of any strategy where it is 100% closed. So if you accept the theory the IACV will never close 100%, the symptoms would be exactly as you describe IF you had an additional un-metered air leak.

From your description of the engine surging, in theory that is the IACV trying to regulate the idle but it is confused by the added air leak.

So I think we have three possible scenarios.

1. A bad IAC valve.

2. Vacuum leak.

3. A bad IAC valve and a vacuum leak.

I don't think buying an IACV would break the bank, and as easy as they are to access on a Gen2, I recommend replacing the IACV as a troubleshooting method. If it turns out the IACV is good, having a spare is not such a bad thing.

One further idea... is the throttle closing all the way??? :rolleyes:
Click to expand...
well said
 
+1 well said and I agree... likely a vacuum leak hopefully not the dreaded intake gaskets, which is common in the second gen.
 
I just want to thank everyone who posted!

Particularly JP...!

When it looked like that car was going to be sold I stopped fooling around with stuff... interestingly when I delivered the car Friday the idle seemed totally normal with the IAC plugged in. So I don't know what was going on with it, but now the car is gone, so... :D :D :D

THANKS!!!

:D -J
 
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