1996 Disco with 4.0L and Automatic. When idling with truck in park or neutral, tach reads about 300 rpm. If I shift into reverse or drive, the tach reads 200 rpm and the oil light flickers on and off. Shift back into park and the light goes out and rpms go back to 300 rpm.

Friend suggested it was the idle air valve (stepper motor?) in the throttle body. I removed it and cleaned it and the entire throttle body, but it made no difference. When I was cleaning it, I tried to move the plunger by hand but it doesn't seem to move. Are these so strong that it can only be moved by the electrical signal? Seems like it's just a simple solenoid arrangment.

I've checked all the obvious things like vacuum hoses and electrical connections. I'm thinking the low idle speed is causing low oil pressure, making the oil light flicker. 205,000 miles and other than this, the engine runs fine. Any ideas on how to test or diagnose that idle air valve?

 

the plunger can be set by turning a set screw underneath the little cap on the throttle. just turn it till its set to the right idle speed around 600 to 800

 

Just tried that. If I screwed it all the way down, it would slow the idle speed a little less. I turned it back up several turns and it wouldn't bring the idle speed up past the original 300 rpm. Don't know how many turns you can back it up, but seems that several would had made a difference. You did mean the larger cap on top of the throttle body and not the smaller cap in the front just above the idle air valve?

 

Update.......I checked the Idle Air Valve by removing it and keeping it plugged to the harness. Then I turned the ignition on and saw it move the plunger. Good indication that it was working. Next I screwed the plunger in a few turns and reinstalled. The idle speed increased to over 1000 rpm's. Took it back out and backed off a couple of turns and got it down to about 800 rpm's. Short story is that by using a combination of Rovin4life's suggestion above and adjusting the plunger length, it's now idling about right. I'm going to take it for a test drive tomorrow and see how it does after some stop and go driving and idling at stoplights. Thanks.