low idle bounce
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Joined: Feb 2010
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From: Pennsylvania
low idle bounce
so i've been noticing recently when i turn my car on (1985.5 944 w/ 91 S2 eng) the idle bounces and the tach falls below normal and bounces as well as the engine. it sounds like its dying or gasping for air and when i rev it, it revs and then gets even lower and almost shuts off. also, sometimes i notice the idle is a little higher than normal, this doesn't happen when i first start her up though. it doesn't return to the normal 900 rpm when i put in the clutch, it slowly lowers down to about 1050 or so. so sometimes low and sometimes high idle. any help? could it be vacuum hoses or something? thanks
Burning Brakes
Joined: Dec 2003
Posts: 1,036
Likes: 12
From: Chicago, Il
Clean the ISV
Most all of our Porsches have an idle stabilization valve (ISV), including both my 968 and my 993. I experienced your symptoms with my 993, and with some spare time one weekend decided to take a look at the ISV. What a mess! When you stop to think about it, the ISV sees all the vapors that get passed off through the engines vapor vent system. So, there is oil, dirt, air, and other pollutants that flow through the ISV. Over a period of time, the valve begins to coke off and you will see the symptoms as elevated or wandering idle.
The ISV's on all these cars are remarkably similar since they are all basically the same Bosch design.
Most of the valves do not break.....they just coke up and the valve ceases to move.
The fix is to clean out the ISV. There are lots of threads on how to do that, usually using a can or two of either carb cleaner or brake kleen. You need to remove the ISV from the car and then repeatedly, using compressed air and solvent remove all the coke from the guts of the valve. You will know that you have a clean valve when you are able to shake the ISV in your hand and hear a distinct rattle from the valve that sounds exactly like the metal ***** found in aerosol paint cans that you shake up to aggitate the paint before spraying.
If you don't hear the rattle, you are not clean! Just stay at it for a while (took me about 30 minutes to restore the ISV out of my 993),
Five dollars worth of solvent and about an hour of your time is much more cost effective than shelling out $200 to $300 for a new ISV!
The ISV's on all these cars are remarkably similar since they are all basically the same Bosch design.
Most of the valves do not break.....they just coke up and the valve ceases to move.
The fix is to clean out the ISV. There are lots of threads on how to do that, usually using a can or two of either carb cleaner or brake kleen. You need to remove the ISV from the car and then repeatedly, using compressed air and solvent remove all the coke from the guts of the valve. You will know that you have a clean valve when you are able to shake the ISV in your hand and hear a distinct rattle from the valve that sounds exactly like the metal ***** found in aerosol paint cans that you shake up to aggitate the paint before spraying.
If you don't hear the rattle, you are not clean! Just stay at it for a while (took me about 30 minutes to restore the ISV out of my 993),
Five dollars worth of solvent and about an hour of your time is much more cost effective than shelling out $200 to $300 for a new ISV!
Proprietoristicly Refined
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Joined: Oct 2001
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From: ~Carefree Highway~
A couple of ICV/ISV solutions.
1. Spray Gumout Throttle Body and Fuel Injector cleaner directly into the ICV-car off--start car.
Do not use "Carb Cleaner" (If ICV is still on the car. Pulled, use whatever...)
2. Pull the ICV and clean:
http://www.texasblake.com/porsche/tech/icv.html
http://www.theexitwound.com/944/inde...t_from-&ucat=2
OR
https://rennlist.com/forums/924-931-...-on-87-na.html
GL
John
1. Spray Gumout Throttle Body and Fuel Injector cleaner directly into the ICV-car off--start car.
Do not use "Carb Cleaner" (If ICV is still on the car. Pulled, use whatever...)
2. Pull the ICV and clean:
http://www.texasblake.com/porsche/tech/icv.html
http://www.theexitwound.com/944/inde...t_from-&ucat=2
OR
https://rennlist.com/forums/924-931-...-on-87-na.html
GL
John
Trending Topics
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Joined: Nov 2003
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From: CT
I'd start with the ISV too (clean it or maybe even replace it since it's a pain to get to. Wait - is it a pain to get to on the S2 engine? It's a pain on the S -have to remove the intake manifold to get at it ...).
As said, TPS needs to tell the DME that it is time to idle, so check that (at least that's not a nightmare like it is on the S).
Also be sure the throttle plate is fully closing. If it rubs the bore it could leak air on idle or prevent TPS from engaging, or both. At random. This was a fun one for me.
Two other things that made a surprising difference for me coming off throttle to stable idle were O2 sensor and AFM. My view on idle, especially for the dynamic idle stabilization on the S/S2/968 is that the DME really doesn't like to be lied to.
Good luck.
As said, TPS needs to tell the DME that it is time to idle, so check that (at least that's not a nightmare like it is on the S).
Also be sure the throttle plate is fully closing. If it rubs the bore it could leak air on idle or prevent TPS from engaging, or both. At random. This was a fun one for me.
Two other things that made a surprising difference for me coming off throttle to stable idle were O2 sensor and AFM. My view on idle, especially for the dynamic idle stabilization on the S/S2/968 is that the DME really doesn't like to be lied to.
Good luck.
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Campeck Rulez
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Joined: Sep 2003
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From: Woodstock, GA
mine used to do that. It started one day after I turned it off and it didn't want to restart. When it did start it had this problem and I don't remember how we fixed it, if we did. Sorry...no help.
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Joined: Nov 2003
Posts: 1,182
Likes: 2
From: CT
Yeah, Campeck reminds me of another thing to look at - measure your fuel pressure.
When FPR fails, fuel pressure leak down is much faster, which makes hot starts take longer.
But also when FPR fails, fuel pressure goes way high and idle control might then get wonky (each pulse of fuel injector delivers more fuel than expected by DME).
When FPR fails, fuel pressure leak down is much faster, which makes hot starts take longer.
But also when FPR fails, fuel pressure goes way high and idle control might then get wonky (each pulse of fuel injector delivers more fuel than expected by DME).
