Cold Start Grinding Noise & High Idle

[JKCutler3]

The last couple of days when I start my 1.8T the motor idles high at 1,200 rpms and I hear a rattle/grind on the passenger side of the engine bay. It grinds for a couple seconds, stops, grinds, stops, grinds etc. My fear is that it's coming from the turbo.

I shut the motor off for fear of breaking something and start the motor up again. The idle drops to 800 rpm (normal) and the noise goes away. I drove 40 miles highway and city with no problems. And when the car is hot, starting is no problem. This only happens on cold starts.

Any ideas?

[Let it snow]

SAI pump?

[old guy]

The cold start fast idle is normal. It's part of the SAI cycle. You mentioned that the noise only occurs during the fast idle. That would be the only time that the SAI pump is engaged so the noise very well may be from the SAI pump.

[JKCutler3]

The shop that just did my timing belt/water pump and thermostat said that my SAI was removed by a previous owner and said I should consider putting one back in. They said it was mostly for emissions.

[jonan]

i replaced all the vacuum lines and check valves with all the stuff in the a4darkness thread, pleased to say i passed emissions last week...

that thread should serve as a starting point for anyone who isn't deleting any of those components...

[old guy]

I guess that rules out the SAI pump. And yes it only for emissions. The SAI system gets the cat up to temperature faster. It has nothing to do with performance.

On the next cold start get your head under the hood and see if you can get a better idea as to where the noise is coming from.

[MacFady]

Ignoring the idle, since what you describe is normal, you have an intermittent grinding/rattle sound from the front passenger side only on cold starts. Are you sure it is coming from the engine bay? See if it changes at all based on your climate control settings. Something else I would see is if the car is warmed up, and your idling, leave the car in neutral but give it a little gas to move the RPMs back up to the approx. 1200RPM the car would be at during a cold start and see if you can replicate the sound. If so, I'd suspect something is just vibrating against something else when at 1200rpm.

[JKCutler3]

So, I lied. Or I was lied to. Anyway upon further research I think Let It Snow and Old Guy are correct. I remember hearing the "jet engine" noise previously when I would start it up and after reading that somewhere I realized that I haven't heard that sound in a little while. Here is some video and based upon the location I definitely think its the SAI pump. What the video doesn't show is that once I turn the car off and on again, the idle drops, no noise and car runs smooth.

[JKCutler3]

All assumptions confirmed. Check engine light just came on, tested at auto parts store. Bad SAI system.

[seanf86]

All assumptions confirmed. Check engine light just came on, tested at auto parts store. Bad SAI system.

Not sure how plentiful pick and pulls are where you are but if you want a used pump get one from a mk4 2.slow, they are mounted under the engine cover and are hidden from the elements unlike ours and are up high enough in a warm enough area they don't take on water and rust out which sounds like what has happened to yours, I would check your combi valve too as it can let exhaust back into your pump and cause it to corrode and make the noise you are describing. Best part is the mk4 pump is the exact same part and because you tell them its off a vw there is a chance it will be substantially cheaper.

[old guy]

I would check your combi valve too as it can let exhaust back into your pump and cause it to corrode and make the noise you are describing.

This is a very important point. When the combi valve leaks it causes exhaust condensation to collect in the SAI lines and ultimately the SAI pump rusts and locks up. If you don't correct this condition the next pump will do the same thing. It might last 6 months to a year depending upon how bad the combi valve is leaking but it will eventually fail again.