Rough idle - 560SEC

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Bill Bunn - 01 Jul 2007 19:48 GMT

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?

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-->> T.G. Lambach <<-- - 01 Jul 2007 21:57 GMT

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!!!

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-->> T.G. Lambach <<-- - 02 Jul 2007 04:09 GMT

Check the vacuum line TO the modulator for cracks, especially if the
transmission shifts hard.

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Bill Bunn - 02 Jul 2007 21:01 GMT

Thanks to T.G. Lambach for the tip - and I will be careful.

> 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
[quoted text clipped - 9 lines]
> cut the propane and the rough idle will return. BE CAREFUL - propane can
> flash!!!

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Tiger - 01 Jul 2007 23:40 GMT

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.

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