Pontiac Repair: idles rough, intake manifold gasket, pontiac bonneville


Question
1992 Pontiac Bonneville SIR automatic; The bypass fitting in the intake manifold for cooling system broke apart, I lost all water and the engine heat went into the red.  A mechanic replaced the part, but now, the car seems to run fine except a rough idle, almost like a surging.  The mechanic hooked it up to his scopes, etc, doesn't get an indication to what could be wrong. Any clue to the problem?

Answer
Don,

   We always assume the mechanic may have skewed something during the repair, so you may have a vacuum leak. A rubber hose connected to the engine which is disconnected or something like that. If that is not the case I might disconnect the battery for a few minutes and see if this helps. If not you may have something damaged due to the overheating. Maybe a intake manifold leak (vacuum leak) this can be the case. If you have a vacuum leak using a scanner which displays data would show the idle air control motor reading as real close to "0" counts. The reading would normally be over "0". "0" means the engine computer is trying to lower the engine RPM but since "0" is the last count it may be that extra air is entering the engine which is not designed to and the system can't control it. I don't know the reason but if it's not a skewed vacuum line somewhere it's likely a side-effect of the overheating and could be a intake manifold gasket as I stated or something else unknown. Hope this helps.
                       John