HT4100 4.1, 4.5, 4.9 Discussion, 4.9 Idle Adjustment on '93 DeVille ? in Cadillac Engine Technical Discussion; My '93 DeVille with the 4.9 is Idling a little too high. I tried to adjust it mechanically but it ...
My '93 DeVille with the 4.9 is Idling a little too high. I tried to adjust it mechanically but it did not want to turn and I was a little afraid to break it. Then a mechanic friend of mine mentioned that it may be adjusted by the computer.
As your mechanic stated, it is computer controlled. If you turn the screw on the idle speed controller, the computer will still set the idle where it wants it - if you thread the screw in, the computer will simply extend it back where it was. If you thread it out, the ISC will retract it.
The only thing you are changing by turning the ISC screw is the upper/lower limits of it's extension. Thread it in too much and the ISC won't be able to extend enough to compensate. Thread it out too much and the ISC will bottom out too early.
While there are many possibilities, the most likely causes of a high idle on this engine are a misadjusted or faulty TPS or a large vacuum leak,
Vacuum leaks won't show up on the codes, but x2 on the vacuum leaks.
I usually use carb cleaner (with the straw) and hit the usual suspects for vacuum leaks; TB base, intake manifold gaskets, vacuum line turns (plastic tube connectors). If the rpm's mellow out, you've found your leak...
Start by checking codes and cleaning the throttle body if it needs it. Usually if the idle is higher then it's supposed to be the computer is trying to compensate for something or someone has messed around with things before. There's a chunk in the service manual that covers how to make sure the ISC TPS and everything else are adjusted and working properly and steps to follow to adjust everything.