Rough Idling!

Hello all!

I have a '91 Volvo 740 that's been having an intermittent rough idle -- the car's steering wheel shakes up and down about a quarter inch and I can feel it throughout the car. I also notice that while I'm waiting at a red light the tachometer shoots up and down about 1/4".

I've replaced the distributor, wires, ignition coil, ignition amplifier, fuel filter (maintenance items), and 99% sure there aren't any vacuum leaks either (replace most of the lines with silicone hoses). I've also cleaned the throttle body and IAC valve.

Any help would be greatly appreciated!

Actually, now that I think of it -- there's one hose that's still not replaced, and it's the PCV breather hose...

Perhaps check the idle stabilizer under the intake mainfold and the hoses that attach to it and that the base idle adjustment is correct - there is a set screw for the butterfly valve on the throttle body itself. They can be incorrectly set and cause a low idle situation. The IAC can also be out of spec with sludge or buildup causing it to stick.

Turbo'd 24erty4er said:

Perhaps check the idle stabilizer under the intake mainfold and the hoses that attach to it and that the base idle adjustment is correct - there is a set screw for the butterfly valve on the throttle body itself. They can be incorrectly set and cause a low idle situation. The IAC can also be out of spec with sludge or buildup causing it to stick.

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I've cleaned out the IAC valve and throttle body, I'm currently replacing the exhaust gasket and checking the PCV & egr system. One thing I noticed too is that the rough idle only happens when I'm not in park (i.e. in drive, brake on, at a red light.)

I had a similar problem like that in my 92 740.I tried almost everything and replaced many sensors that i did not need. It turned out to be a spark plug wire boot.

I think you mentioned you replaced the spark plug wires already.The lesson i learned is never overlook anything when trying to find idling issues!

HighMans said:

...I noticed too is that the rough idle only happens when I'm not in park (i.e. in drive, brake on, at a red light.)

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Engine is not loaded....

So, pull off injector electrical harness, one at a time...engine running, emergency brake on with blocks on wheels, and in gear.

Do the same with spark wires...might wear rubber gloves.