951Porschiste

I've noticed lately that after a long highway trip, the idle stays quite high (1500 to 2000 RPM). It feels as if the throttle plate cannot close back down on its own. Could the throttle return spring get overextended after a prolonged period of open highway throttle? Or should I instead look to the TPS or idle control valve for an explanation?

Matthew West

Here are a couple of things that you can check:
Throttle plate is one explanation. Check for slack in cable after the car is warmed up, poke the cable with one finger and get a small amount of deflection prior to the cam rotating.

Make certain that the throttle rolls all the way back down to the set screw when closed gently. If not snap it shut and see if you get a different reaction.

Make cetarin the throttle bypas screw is not backed out, a little blue locktite is nice.

Look for vacuum leaks.

951Porschiste

Thanks! I'll look for slack in the cable.
Dumb question: Which is the throttle bypass screw?

Sam Lin

I've currently got the same issue on a new 951, the throttle plate closes fine. I suspect it's a combination of the idle stabilizer valve dirty and sticking, a coolant temperature sensor not reading properly, and an improperly set idle. That's what I'm addressing to try and solve the problem, I'll let you know if it works.

Sam

Sam Lin

Throttle bypass screw is the idle screw on top of the TB.

Sam

951Porschiste

Sam,
I did the vacuum/venturi delete back in July. The idle was a little high afterwards (about 1K) but never to the extent it gets to now. During the venturi delete, I also changed all the coolant temp sensors (engine to DME, engine to cluster gauge, turbo coolant sensor).
When the negine is cold the idle is fine. It creeps up once the engine warms up or has been run at high way speeds for about 10 to 15 minutes. Last week after a 2 hour highway drive it would not come down below 2K.
The throttle bypass screw does not appear to have backed out.
How can one clean the idle stabilizer valve?

hosrom_951

The ISV should't be a problem as long as the car idles fine when cold and when the a/c is turned on (those are the times the ISV is switched on/activated), but someone else may want to jump in on this one?

How is the Throttle Position Sensor? try adjusting it : http://www.clarks-garage.com/shop-manual/fuel-06.htm, It's worth a shot since your sure about the vacuum lines along with the idle adjustment.

Just to clarify things, you should adjust your idle by bypassing the ISV and when the engine is WARM, after 10-15 mins of driving and a/c is off........try that after the TPS adjustment.

Chipster

Check ALL of your vacuum lines - including the blow-off valve. You might want to get a kit and replace all the lines if you have't already. They get old and brittle and new lines are cheap insurance - plus you replace all the connectors at the same time.

Chip

Tomas L

First check all vacuum lines for a leak, then check that the TPS closes the idle switch.
After that adjust the idle according to the correct procedure, if you search the net I'm sure you'll find it described somewhere. IIRC you should disable the ISV by bridging two pins in the diagnostic connector and then adjust idle with warm engine.
If you still have idle problems then the ISV is probably not ok.

Tomas