Renix idle creep suggestion

Went through this last weekend on a friend's '89 Limited with the classic 'idle creep' symptom and figured it may be of help to other folks.

Searching through other threads suggested loose manifold bolts or a cracked main vacuum line as possible cuplrits. Not to say that they aren't, but there's something else you may want to check: the condition of the valve cover gasket and bolts.

He had a major oil leak from the rear of the valve cover. Upon removing it, we found two things: that about 60% of the valve cover bolts were hand-tight, particularly along the passenger side as you work towards the rear; and also that the remains of the valve cover gasket were down to about 1mm thickness along most of its length. Either one or a combination of both will cause low vacuum and a host of other problems. It's surprising how well the brakes work right now as an example.

Having replaced the gasket, the idle sits at around 1100RPM - definitely high, but at least pointing towards a blocked EGR valve as the next-most-likely problem child. As it no longer climbs to over the 2000 mark in a vain attempt to build proper pressure, this is a major improvement. Just something to consider if you've torqued and re-torqued the intake bolts to no avail as we did.

You have that backwards- an EGR valve (used up to 1990) functioning properly is closed at idle, and has no effect on idle. If the EGR is malfunctioning, it will stick open at idle, and the engine will bog right down from too much exhaust gas getting recirculated back into the engine at low rpm. Short version: shot EGR gives a low idle or stalls, runs OK at higher rpms.
My 1990 has had a high idle each time I needed a new TPS. It is on the third TPS since new. There are other possible causes for high idle (intake leak or open vacuum line, IAC stuck, etc.)

4xBob said:

You have that backwards- an EGR valve (used up to 1990) functioning properly is closed at idle, and has no effect on idle.

Click to expand...

Interesting - I've read elsewhere that a dirty EGR valve can cause high idle; searching suggests this as a possible cause, specifically in this thread.

I'll admit that this particular XJ is my 'Renix Learner' as I'm mostly familiar with the runout (2000-2001) models, but I've seen similar symptoms on EGR-equipped cars that I've owned. Not to be difficult about it, but could you expand on this? Not discounting the TPS suggestion by any means, but just curious as to where it slots in with regard to compound problems.

I always suggest taking the throttle body off and cleaning it, which includes taking the IAC and TPS off. Clean the IAC and the IAC passage and see if that has any affect.

K