Rough idle after getting on it

[clayperez]

I've got a situation with my car where if I get on the throttle really hard and bring the RPM's up to about 6K and then let the engine idle, it idles roughly and low almost as if it's choking. After a minute or so the "choking" sensation goes away but it doesn't seem to affect the performance of WOT in any case - just idle post mortem.

The car is stock except for a LBE.

Thoughts?

[Porsche-O-Phile]

Running rich under boost?

If so, you could be "loading up" the O2 sensor.

[clayperez]

I assume that you mean by "loading up", an overly rich mixture can contaminat/flood the O2 sensor. Would it make a difference if it were an old/tired O2 sensor?

I was recently fiddling with the mixture screw on the intake to see if it would make a difference, but I can't really tell and I don't want to go so far as to lean it out to the point of detonation.

How about any good, affordable fuel gauges that don't require a lot of installation?

Could old injectors be the cause of an overly rich mixture if the spray pattern is not good enough to allow a full burn?

Thanks.
Carlos

[soxnail]

You don't mention if you are going WOT under load or just in "blipping" the engine in neutral ?
Sox

[clayperez]

It only happens under load after a good pull. Blipping the throttle does not replicate the problem. Even revving to redline and letting off in neutral is not a problem.

[soxnail]

How does the exhaust tip look?
does it have a lot of soot on it and the rear bumper ?
Also if you get the idle stumble, can you clear it by opening the throttle a little bit?
finally is your idle good otherwise?
does the idle stabilization work as its supposed to ?
Lot of questions but I am trying to shoot ...
Sox

[clayperez]

It does clear the stumble by opening the throttle just a little bit.
The idle is good otherwise. Occasionally in hot weather it fluctuates.

The idle stabilizer screw on the top of the throttle body does adjust the idle speed as expected.

However there is another screw on another location that adjusts the AF mixture according to my copy of the shop manual. It is lower and requires a long allen to adjust that I cannot determine whether or not there is an effect. It has obviously been accessed before because the manual states that if it hasn't, there is a plug that must first be drilled out. My care has no plug there.

EDIT: The exhaust tip looks OK - a little black, but I don't have soot all over the place like you would if it was burning a lot of oil or running super rich.

[soxnail]

I was talking about the idle valve...
The AFM screw is not causing you a problem or you would have other symptoms.
How are you testing the idle bypass screw (the one on top of the throttle body) This is used to set the basic or curb idle. It will not have much effect without the ISV by passed. If it is changing the idle rpm (with out) the bypass then your ISV is not working properly.
Sox

[clayperez]

I have only used the idle valve to set the curb idle.

Occasionally the idle fluctuates - typically in hot conditions at a stoplight the idle RPM will increase to about 1000 +/- 100, and then fall back down while in neutral. Sometimes blipping the throttle will fix it.

I have not associated this symptom with the rough idle after loaded WOT, but have instead assumed they were unrelated.

[soxnail]

I think we are talking about different things here.
ISV = Idle stabilization valve this is an electro mechanical device located under the intake manifold. It is not adjustable by user, it is controlled by the DME.
Idle set screw (on top of the throttle body) this is used to set the basic idle. To do so you have to bypass the ISV and then set the basic idle.
Sox

[clayperez]

Ok, I am talking about the idle set screw. I have not had any interaction or review of the ISV. I thought that setting the proper idle speed was done merely by turning the idle set screw at the top of the TB, but I am trying to determine whether you're stating it is actually a multi-step process.

[soxnail]

Yup to set the idle you have to by pass the ISV (there is a good write up on it at Clark's site).
If you can change the idle rpm with out bypassing, then your ISV is not working properly. If so then the air by pass (by the screw on the throttle) may not be enough to stop the car from going rich. (under the condition you describe.) As a working ISV will open to let in more air into the intake (same as you opening the throttle a bit to clear the stumble).
You should read up on my thread on "idle valve" if you have not done so already.

[clayperez]

Thanks Sox for helping me think through this. I will check both your article on the idle valve and also Clark's garage.

Regards,
Carlos

[soxnail]

You are welcome
Sox

[chitown944]

You could also have a vacuum leak somewhere

[nize]

it could also be a faulty TPS.

[clayperez]

Thought about that, but I don't have any trouble with the throttle. If the sensor was bad, it shouldn't be so responsive to when it opens.

SOX, I'm about to open up the ISV, but was wondering if you noticed a difference in your idle when you did so?

[nize]

sometimes when it fails, it's very responsive when it opens, and not so responsive when it's closed, or supposed to be closed. i've had this happen to me once and it took some trial and error to figure it out because i refused to believe it could be the tps.

[soxnail]

Carlos read my post on Idle valve first.
Sox