[Cloud9...68]

My engine is running fabulously, very smooth at all rpm's, and very strong, as evidenced by the fact that I keep lowering my lap times while mopping up the track with what should be much faster cars, often driven by more experienced (which I know doesn't necessarily mean "better") drivers, but there's just one little problem - the engine is really struggling to idle. It completely failed to idle for the first time a few weeks ago, then mysteriously started working again, but when I drove it yesterday, it wouldn't hold an idle for literally more than about a second. I could keep the engine running by keeping my foot on the throttle, but as soon as I release it, it's as if the air supply to the engine was abruptly choked off, and lights out. There's no off-idle stumble whatsoever - the problem is completely confined to the idle itself.



So I removed the idle stabilizer valve today, thoroughly cleaned it (which I had done during the recent rebuild, but I did it again, in case I hadn't done a thorough enough job the last time). I also sprayed contact cleaner on the ISV connectors. When I started it up, it would hold idle, so the cleaning seems to have helped, but it still feels like it's struggling so maintain idle, as though it's gasping for breath.



I also tested the function of the ISV by applying 12V between the center connector and the two ends, and it seems to be opening and closing smartly and solidly when I connect the leads to the connectors.



Before I spend $400 on a new ISV, I want to be absolutely sure it's the source of my problem. Does anyone know of a way to test for the presence of the correct signal going to the ISV? Is there anything else that could explain my car's poor-to-nonexistent idle? Thanks.

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[vliegwielolaf1234]

I had problems . Cleaned again and again .

The i put a new one in , not original porsche i do not remember which car it was from , it is a bosch part and there is a thread on it somewhere .

I orderd it and had no more issue's with the idle .

Clean it can become a problem if it gets in the electrical part of the valve .

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[flash]

if you have a durametric, there is an ICV test in it

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[banditsc]

Do you have a lightweight flywheel in there?

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[flash]

have you checked vacuum?



has the MAF been carefully inspected and cleaned?

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[Lear35A]

It's not always the most expensive part at fault with a Porsche. Although there are a number of things that can cause idle problems, I'm leaning with Flash on this one that a vacuum leak may have crept in there. Check all the hoses that connect to the throttle body and the intake manifold. Hope you find something easy to fix there.

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[Cloud9...68]

Checked all vacuum lines - all look OK. Also, the problem has been intermittent, which doesn't sound like a vacuum problem. Also, the problem is 100% confined to the idle - there isn't even a hint of a stumble as soon as I open the throttle, and it's strong and smooth immediately. Don't have a Durametric. I haven't cleaned the <acronym title='mass air flow'>MAF</acronym> lately, so I suppose I should, but again, wouldn't a problem with the <acronym title='mass air flow'>MAF</acronym> be felt other places besides idle? The inside of the throttle body, and the inside of the intake manifold, are spotlessly clean, I'm assuming because of all the time it spends at <acronym title='wide open throttle'>WOT</acronym> at the track (a continual Italian tune-up, in other words). I do have a light flywheel, but it idled flawlessly after I installed it, and stayed that way for months. This horrible idle is a very recent, post-flywheel install phenomenon.



I actually don't think it's the ISV, because it responds so perfectly when I apply 12V across the connectors. It snaps open and closed so smartly, that I can feel it as well as hear it. It's as though the necessary signal isn't getting to the ISV, but I don't know how to test that. Does the Durametric test such a thing?

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[banditsc]

The reason I ask about the flywheel is the 928 (86 is one of the years) ICV is basically a direct replacement other than the inlet and outlet being like 2mm larger. That extra air seems to help allow the idle to not drop as fast, so with a lightweight flywheel you may see less issues with the idle coming down to quick when you come off throttle. It also has the benefit of being at least half the price.

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[flash]

you will have to conduct an actual test. visual inspection won't do it. hook up a vacuum gauge and look at the needle. it should read at least 17 at idle and be rock solid. if that doesn't turn it up, do a search here for the tool like the one i built to pressurize the system to check for leaks. a small vacuum leak will create issues at idle. a big one would make it race. frankly i suspect the ported vacuum port at the top of the throttle body, or one of the smaller vacuum lines below the intake (like the one going to the AOS)



does your heater work properly?



the durametric operates the ICV through the ECU.

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[Cloud9...68]

But how can I do a vacuum test at idle if the car won't idle? At its worst, it's that bad - literally the instant I let my foot off the gas, it dies, as though someone pulled off the coil wire. But it varies - sometimes it struggles to maintain a very weak idle, and for a couple of weeks after the problem originally surfaced, it idled more or less normally. I don't understand how a vacuum leak could vary that much, but who knows?



Checking the heater operation is a good idea - I'll try that.

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[flash]

see if you can manually make it idle by cracking the throttle plate just a touch.

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[banditsc]

If you can't make it idle, then you can do the pressure test to test for any leaks.

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[Cloud9...68]

Flash, Banditsc,



Sounds good - I'll give it a try. Never hurts to check for vacuum leaks.

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[Rap]

Not the same thing but an idle issue problem never the less. When I come off the track generally after the second run and I'm heading back to park my idle jumps up to 2000rpm's and bounces up and down 3-400 rpm's. Once I park I turn the car off and no problem when I restart the car. Of course it happens again after the next run. So far no luck diagnosing it. Now it's just an annoyance.

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[schwarz968]

Have you checked the Oring located behind the throttle body to intake manifold? Mine were cooked. See the parts catalog, Main Group 1, Sub group 7, illustration 107, index 9 and 10.

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[Lear35A]

I'm not suggesting a vacuum test ( never could figure out how to make that work). Just check all connections and look for obvious cracking or damage to the hoses. 15 minutes max.

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[Cloud9...68]

I really don't think it has anything to do with vacuum, because a vacuum leak wouldn't come and go like this problem has. Also, and I'm sorry I forgot to mention this earlier, because it isn't a dominant symptom, but the idle seems to be somewhat better while the engine is cold, and then it dies as it warms up. If there were excess air getting into the manifold, leaning out the mixture at idle, the exact opposite would happen. The o-ring around the throttle body looks fine as well. l think it's something electrical.

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[flash]

never make assumptions. don't assume a visual test will do anything. remember the DME relay?



do the tests. step by step. eliminate culprits one by one.



make sure the o-ring on your oil fill cap is intact and sealing up.

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[BeBe]

I had an intermittent problem with a different car where it would run fine when cold, but after 5 minutes, start missing. Turned out that one of the plug wires had a crack in it that would expand after the engine warmed up and then the crack would open and let the wire spark to the block. Not suggesting that it is a cracked plug wire, but things expand and get softer when they warm up. You may have something that is physically changing with temperature to open a crack/pinhole or seal less well. Just a thought...

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[Cloud9...68]

I understand about not making assumptions, but I have one over-riding question: A vacuum leak would cause a lean condition due to the addition of extra air, right? But the instant I introduce more air by just barely touching the throttle, the problem goes away. It seems like the exact opposite of a vacuum leak to me.

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