Car won't hold an exact rev
 

Hi,

Car is MY02 STI.

Noticed the other day whilst doing a noise test, I was told to hold the revs at 4500rpm. I did this and the revs go up and down, it won't hold the exact rev.

Yesterday I tried it again at 1500rpm and it does the same. Goes up and down.

Not really a big deal, but still curious as to what the problem could be.

Car idles just fine.

Noticed today, with the car not running but with the ignition on, that the revs on the AVCR also go up and down.

Could it be the throttle position sensor ?

I have noticed with the avcr that the boost levels rise ever so slightly with igntion on.

bump

How far is it going up and down? If you're holding it within +/- 50rpm you're doing well.

Don't know the exact answer to that question Paul, but I will go and have a look and report back :)

It's not constant, and the OE tacho needle is damped to remove such quick and minor oscillations. If you're holding it at 4000rpm you may be in the closed loop lambda range, and the AFR will be oscillating a little. On a classic, the AFR swings so far that you can hear the tone of the engine change as it does it, and the RPM will fluctuate without any load on. When cruising, those oscillations are damped by the weight of the car, the minor fluctations in power aren't enough to significantly change the speed of teh car. Although on late classics many people report surging on light throttle, much of this is the closed loop AFR, and it's more pronounced when the lambda sensor is tired and makes larger, slower swings.

paul