Once warmed up, the vacuum gauge moves off the peg about halfway to the red when the car is put in gear and idles rough. Runs OK on the road. Does this sound like a vacuum leak? Could it have to do with the vacuum modulator on the transmission?
Sounds like a classic vacuum leak. A small leak is not material at speed because its percentage of the total air volume is not significant BUT at idle that same leak will cause misfiring due to too lean a air / fuel ratio.
Possibilities: anything that's vacuum powered, cracked broken or disconnected vacuum hoses to the intake manifold, intake manifold gasket. Listen for a hissing sound at hot idle.
Very CAREFULLY apply propane to suspect sites and KEEP YOUR FACE DISTANT. A leak will draw the propane into the motor and it will run smoothly, then cut the propane and the rough idle will return. BE CAREFUL - propane can flash!!!
I would check the spark plug wires for possible bad resistor. Measure each spark plug wires for resistance... They should all be in the same range... below 1300 ohms.
Bad idling will cause vacuum problem.
Check the vacuum line TO the modulator for cracks, especially if the transmission shifts hard.
Thanks to T.G. Lambach for the tip - and I will be careful.
"-->> T.G. Lambach
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