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Rough idle at start - appreciate your diagnosis input


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Hello, new owner of an '02 Targa. PPI checked out and seems to run perfectly, except at start up. Cranks fine, engine fires but settles into a stumbling idle a under 600rpm. Light stab of gas pedal usually stalls the engine, then on restart it zooms up to 1500, settles back down at 700 and operates fine. No stalling yet during regular driving.

I did some archive searches but either I'm not too good at it or didn't pull any good explanatory posts. One mentioned the idle control valve. What do you all think?

Thanks,

Piccia :(

'02 Targa, Seal Gray/Black

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  • 1 year later...

My '99 996 C2 exhibited the same symptoms...plus some idle hunting after a cold start...cleaned the throttle body and all is fine.

So my question is: Why does a dirty throttle body have such a profound effect? Is it that the accumulated gunk doesn't let the butterfly come to complete closure, and the throttle position sensor is confusing a computer somewhere in the car?

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There are tiny air bypass ports near the butterfly (or in MY99 C2 an idle control valve) when these get gummed up with carbon deposits they can affect the idle. I usually run a bottle of Chevron Techroline through my car about every 3,000 miles - that seems to keep the carbon build at bay.

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Loren,

Please excuse my ignorance, but would you kindly explain how adding Chevron Techroline to the gasoline can help reduce the carbon buildup in the throttle body?

Doesn't only air passes thru' the throttle body? Does gas from the gas tank go thru' the throttle body as well?

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Loren

Thanks for all the input, but to clarify. The original question was bad idle after cold start. Once the engine is warmed up, it idles fine. Is this the symptom you see when in need of cleaning ??

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Loren

Thanks for all the input, but to clarify. The original question was bad idle after cold start. Once the engine is warmed up, it idles fine. Is this the symptom you see when in need of cleaning ??

Yes, it can be. On egas cars the idle ports are tiny holes near the throttle butterfly together with minute movements of the throttle butterfly establish a stable idle. It certainly wouldn't hurt to clean the throttle butterfly.
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Fuel vapors also pass back into the throttle body.

I guess it is those vapors - because I can tell it makes a difference in how often I have to clean the throttle body.

Thank you, Loren. :cheers:

Ife: I had the same symptoms, I cleaned it and now I'm a believer! :thumbup:

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Fuel vapors also pass back into the throttle body.

I guess it is those vapors - because I can tell it makes a difference in how often I have to clean the throttle body.

Thank you, Loren. :cheers:

Ife: I had the same symptoms, I cleaned it and now I'm a believer! :thumbup:

Do you need to take the throttle body out to do a good job. I tried to clean it in the car, but it is pretty hard to get at all the surfaces. didnt seem to change anything. If you do need to take out the throttle body, where do you get the wrench for that wierd fastener (anybody know what that bolt head is called ??).

Thx

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You probably have a torx bolt in there, like in my car. Try a 5/6 hex socket, should fine. Be careful with the lower left bolt. You need a 10 mm socket to remove the nut that is attached below and the small black bracket can fall off when you. It will make sense when you get into it.

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