1998 996 Replaced AOS and now it won't idle.

MaxHeadroom · 07:26 AM

I just replaced my AOS and have gone for a drive to blow out the oil from the exhaust that made its way there when the AOS failed. However the car just won't idle. It's not that it's a poor idle, it's that it simply won't idle at all.

I've tried blocking a few of the vacuum hoses to eliminate them, and have cleaned and checked the function of the idle air valve that's attached to the throttle body. None of this has made any difference.

Obviously I'm concerned that there is a massive air leak somewhere behind the throttle butterfly, but I'm not keen to go there until I've checked out everything else. I'm also going to try changing the plugs as they were massively oil fouled from when the AOS let go.

If anyone has any ideas of what to look at next, I'm keen for your input.

I guess what I'm asking is this: Is there an electrical cable/wire/connector somewhere that would cause the engine to die instantly at idle speed?

JFP in PA · 11-27-2016, 07:25 AM

I would start by doing a vacuum check on the new AOS. We have seen new units that were duds right out of the box. You need to check the vacuum level at the oil fill cap, it should not be more than 5 inches of water; any more than that and the AOS is bad. If you used an aftermarket AOS, I, would almost bet that is your issue; wwe have had zero luck with aftermarket AOS units.

MaxHeadroom · 11-27-2016, 08:43 AM

The problem with that is that the car won't idle at all, so I can't leave it idling and then pop round to the back and look at the cap. I could wedge the throttle open at a speed where it will self sustain, about 1500 rpm and try the cap test there.

I guess I could also just remove all the pipe connections to the inlet manifold/plenum and blank them off and see if it idles then.

MaxHeadroom · 12-01-2016, 04:54 AM

I just discovered that the hard hose from the AOS to the intake plenum had become disconnected at the AOS. It's a b1tch to get to but hopefully I can reconnect it in a way that will mean it stays reconnected. Time to pull the LH inlet manifold again..

JFP in PA · 12-01-2016, 09:41 AM

You might want to consider just replacing the hose with a new one. It uses a combination a pinch clamp and an O-ring, either of which can become problematic with age. Be sure to apply a small amount of lubricant to where it attaches to the AOS, and it should seat with a click.

MaxHeadroom · 12-02-2016, 02:50 AM

Thanks - you're absolutely right, it took a lot of effort to push it on, but when 'home' it clicked, and I don't remember there being a click when I pushed it on the first time around.

I've now got the inlet manifold back on but frustratingly, one of the very small bore plastic vacuum pipes that comes out of the back of the rubber hose that connects the plenum to the manifold has come out of the thing it's connected to somewhere in the rats nest near the starter motor.

JFP in PA · 12-02-2016, 05:43 AM

Quote:

Originally Posted by MaxHeadroom

Thanks - you're absolutely right, it took a lot of effort to push it on, but when 'home' it clicked, and I don't remember there being a click when I pushed it on the first time around.

I've now got the inlet manifold back on but frustratingly, one of the very small bore plastic vacuum pipes that comes out of the back of the rubber hose that connects the plenum to the manifold has come out of the thing it's connected to somewhere in the rats nest near the starter motor.

You are now beginning to understand why we require "carte blanche" when we do one of these; any hose or connection that looks even questionable goes in favor of new pieces. Not the cheapest approach, but it prevents "come backs".