High idle at speed, not when stationary

[Thklinge]

I recently swapped my engine (it's a 996 Carrera), and got a strange problem. It idles high, at 1500 rpm whenever I clutch (with accompanying poor mileage). It will continue to stay at 1500 rpm, making me look like an idiot around town, until the car has been standing still for 10-20 seconds. It will then idle down to approx 900 rpm like it should. I swapped over the throttle-body from the old engine and it worked perfectly before.

I checked the throttle valve and cleaned it out, but it did close properly already. Tried to clean the ICV as best I could, but without liquids. I did notice a clicking-sound from it that I'm not sure I've heard before? It may be related?

Any ideas?

[RFM]

As always, reed the actual values and error codes first, it can be anything as air leaks, etc.

[Thklinge]

I'd love to, but my OBDII-reader says no errors and the CEL isn't lit.

[Loren]

Have you removed cleaned and made sure the Idle Control Valve rotates properly?

I would start there.

[Thklinge]

I did, and there was some buildup of gunk in the ICV, but nothing too major. Did a wipe without liquids. If I push the valve with a screwdriver it will easily swing open, and swing back when I remove the tool. It will sometimes close completely, other times leave a 1-3 mm gap. I'm not sure if that's normal behavior and that when it's connected it will use electrical force to close? Or is the proper operating mode that the spring shall close the valve completely when the actuator isn't electrified?

Edited August 16, 2012 by Thklinge

[Loren]

If it were the throttle valve sticking or mechanical accelerator cable sticking then I would think it would happen all the time.

If you do not have the tools to test the ICV then perhaps a replacement is in order.

[Thklinge]

What tools are required to test it? The ICV is 350 dollars and I'd hate to replace it if something else is wrong. From what I've been able to google people have pointed at vacuum-leaks, MAF (which doesn't come into play at idle?!), AOS, amongst many.

[Loren]

Drive Links on Durametric, a PST2, or PIWIS tester will trigger the valve and you can see if it is actually moving.

If you take the valve off the car (out of circuit) and apply 12 volts (DC) to it you can likely accomplish the same.

Still not a 100% test but if the solenoid does not move the valve then you need a new ICV.

[Thklinge]

Ok. But since the idle is correct 10-15 seconds after the car has become stationary, what else can I look at? I'm guessing the valve works correctly but the DME doesn't give the correct valve-setting until it reads 0 mph? Strange..

[Loren]

Doubtful the DME would cause that without many other symptoms.

I suggest you try the tests I suggested.

[Thklinge]

I removed it from the throttlebody and turned on the ignition. The valve makes clicking sounds that I'm not sure was there before or not. It is able to open, as it idles higher when cold (aprox 1050 rpms) and is able to keep a steady high 1500rpm idle when I'm driving. Just not sure that it closes like it should.

When the ICV is in the closed position, is it fully and completely closed? Or will it have a few mm's of crack? Can I spray carb-cleaner in it without making things worse? Some will probably end up in the intake, is that a problem for the engine or can I spray happily away?

[Thklinge]

To conclude.. The problem fixed itself over time. Guess the ECU adapting to the new engine or something. Idle normal now. :)