By pjkbrit on Monday, September 10, 2001 - 04:09 am: Edit

Symptoms;
Lumpy idle, occasional stalling followed by won't idle at all unless ECU is allowed to reset, (ignition switched off for 20 seconds).
All other items check out or have been renewed, I even did a compression test yesterday...160psi on all cylinders...94 Disco, 125K. Car drives well and does 17 mpg...passed emissions test on Friday, although the tester told me at idle the emissions went too high so he ran it at 2500rpm to acheive the needed result.
So before I sping for Mr. Crabtree's sensors...what do you all think....and that bloody idle is driving me crazy!!!!!!
Pete

By Jon Williams (Jonw) on Monday, September 10, 2001 - 05:21 am: Edit

A faulty Idle Air Control Valve (IAC) can cause the symptoms you're experiencing at idle. Oxygen sensors have little to do with idling, and old catalytic converters would likely be the cause of your high base-idle emissions. I remember somebody tried to rig a GM IAC valve onto their Disco, but I don't remember if it worked well or not. Basically it was cheaper than the Genuine part, of course. But if you're getting 17 mpg, I seriously doubt your oxygen sensors are bad, as you would be getting dismal gas mileage (around 5-10 mpg, depending). I'd check the IAC valve first.

By pjkbrit on Monday, September 10, 2001 - 05:58 am: Edit

IAC valve is new....but made no difference...1 converter was replaced last year...other one checks out OK...also, car always revs to 1500rpm or higher on initial startup then settles back to a lumpy 850rpm.....driving me nuts...save me from going to the dealer...somebody!!!! PLEASE!

By Jon Williams (Jonw) on Monday, September 10, 2001 - 06:36 am: Edit

Oh..... well, sounds like it's starting and returning to idle normally. Can't figure-out what would be causing the loping idle, though. I've heard of instances where a bad connection at the diagnostic readout (under your front passenger seat) caused the engine to hesitate under acceleration and idle poorly ...something about the connector pins would spread over time and create a loose connection to the ECU. Also a bad ECU ground can cause erratic idling and performance. Is the truck running fine over 1500 rpm or so? Does it miss (sputter) under acceleration? My last guess (and you'll love it, I'm sure) is that your ECU is ready to expire. But then your truck would be running crappy all the time, and not just idling bad, which leads me to believe there's a sensor somewhere that's not giving the computer the correct voltage at idle. Unless some other folks have ideas, you may just end-up having to take it to the dealer...

By Robert Mann (Oldscout) on Monday, September 10, 2001 - 06:38 am: Edit

Sound like a major vacuum leak. The fact that the RPMS race up makes me think, Too much air is getting by the throttle plate via a leak , then the IAC can adjust for leak to 850. The lumpy Idle is because the leak is not constant and the IAC is having a hard time adjusting. Spray Carb cleaner on the manifold/intake and listen for a change in idle quality.

Check the brake booster for a leak, by disconnecting/plugging the vacuum line.

If you disconnect the o2 sensors the ECU will go into open loop mode and if runs better, the o2 sensors need to be replaced.

By pjkbrit on Monday, September 10, 2001 - 07:01 am: Edit

Thanks for your ideas chaps....will try out and report back.
Pete