Fast idle after disengaging clutch from reverse

[Mike Petonic]

My new-to-me '04 996 ttx50 has been doing this to me ever since I've received it. This may be "normal behavior", and that's why I'm throwing it out here...

When I start the car up in the morning, I have to back it out of my space in an underground parking garage. I'll back it up maybe 75 feet (necessitated because of the parking garage configuration), and when I engage the clutch to switch it to first, the engine's idle picks up from 1K to about 2K. It's a bit embarrassing because the garage is indoors and even an unmodified exhaust of a Porsche is pretty loud.

Is this normal behavior? This tends not to happen when the car is warmed up. I understand that when the engine is cold, RPMS are generally increased to warm up the cat converter faster.

Thanks,

-Mike

[phillipj]

No that is not normal. You should never have a 1000 rpm change during warm up. You should check for codes..

[Mike Petonic]

OK. Good to know. I will pick up a reader.

It only happens at start up. Not when it's been running.

[Mike Petonic]

I bought an OBD2 scanner for my iPhone and checked it. No engine codes. Would I need to take it to a P-car mechanic to get extended codes?

My car consistently does this in the morning. I live in a moderate environment (San Francisco) and it's in an underground condo parking garage, so the temperature is a VERY steady 65 degrees, 24x7.

It's an enclosed space, so when I pull out, it really sounds as if:

• I don't know how to drive the car
  
• I'm trying to showboat the engine
  

Or both.

Any more advice?

Thanks!

[Loren]

Yes, you will need Durametric, a PST2, or PIWIS tester to see pending codes and also have a look a what the fuel trim is doing at that time.

[Mike Petonic]

I'm reviving this old thread that I started. I've found and have been using a "compensating control" for the phenomenon that I described.

For the first 4 minutes or so after I do a cold start, when reversing, I get a pretty fast idle when I lift off the gas -- about 1800-2000 rpm or so. In order to stop the high idle, I step down on the brake rather hard. As soon as I do that, the fast idle stops. A more gentle press on the brake doesn't stop the high idle. Eventually, the car will stop idling fast, but the fast way (<1s) to do it is to step on the brake.

I'm guessing that there's some sort of sensor on the brake that feeds into the ECU that enables this. Can anyone explain how this might be possible? From this post, I realize that my car's behavior is not normal, but I did get a PPI about 7 months ago when I bought the car, and there were no codes that my indy told me about (regarding this). The car has always had this behavior.

Thanks!

[Loren]

Vacuum leak in the brake booster circuit?

[Mike Petonic]

Thanks, Loren. My brakes seem to work well at this point, and I'm wondering if this is something that I should get looked at, and whether it's a potential safety issue. If I do have a vacuum leak in the brake booster line, what's the worst case failure if it degrades? Could I have inoperable brakes, eventually?

As things currently stand, I don't mind hitting the brakes hard to stop the fast-idle. But if it's going to degenerate into something potentially dangerous, I will definitely jump on it. Thanks!

[Loren]

That was my question - if touching the brakes stops the high idle then it could be a vacuum leak. The power brakes use engine vacuum.

If just letting off the gas stops the issue then it is something else.

[Mike Petonic]

OK, we're getting closer to the root cause. It's definitely stepping on the brake that causes the fast idle to stop. A gentle step on the brake won't stop it. If I just let off the throttle, that's when the fast idle actually kicks in.

[keeganlow]

Mike , I also have a 2004 TT that is new to me and it does the exact high rev after I back it out of my garage here in Reno. Did you ever confirm it was a vacuum leak causing the issue? I just assumed it was some sort of programmed action related to warming up the engine, as you did.

[Macster]

My 03 Turbo does this. I have some half baked theories as why but since it has done this since I bought the car in 09 and after having put 90K miles on the car it has not be a real pressing concern to get any senior tech input. But I would like to know...car goes in for new tires soon and if I can rememberI I'll ask about this behaviour.

[keeganlow]

Macster, I am also going to see if my tech can look for vacuum leaks or has any ideas, which I will share here. I have owned several 911s, although this is my first TT, and the brakes are decent but seem not quite as good as my other cars, but not sure if this is related in any way. Seems to be a pretty common complaint, should have a common fix. Thanks for sharing.

[Rich L]

Reviving an old thread in hopes there has been some resolution.

I have had the same issue since I bought the car 4 years ago. Happens only on cold starts. Give it about 30 seconds then start to back out of the garage. I don't press the accelerator pedal, but as soon as the clutch pedal is pressed in, the revs jump to 2k rpm and slowly settle back to idle. Again, I never touch the accelerator pedal.

The symptoms aren't present after the vehicle idles for a few minutes or at any time when warm. No CEL or codes.

2003 X50 with plugs, coils, MAF all new within the last 4k miles.

[wross996TT]

When did you last clean the TB?

[Rich L]

When did you last clean the TB?

I have not done it and cannot see where it was done in the maintenance records. Will add it to my list of tasks for the week. Thanks.