Rough idle.

90-06stangs

New Member
Aug 25, 2016
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Harrisburg Il
I drove my Stang to get gas the other day. When I started it back up I heard what sounded like a miss with rough idle. I limped it home and popped the hood. I heard a ping from driver's side. Got the valve cover off and noticed the number 8 exhaust rocker arm just laying there. Replaced the rocker arm stud. Pushrod looked good. Still have a miss. Maybe a stuck valve?
 
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jrichker

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Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Notice the brass fittings on the picture below – they screw into the spark plug home and then the test gauge screws into them



Your local auto parts store may have a compression tester to rent or loan if you have a credit card. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.


With the engine warmed up, remove all spark plugs and prop the throttle wide open with a plastic screwdriver handle between the throttle butterfly and the throttle housing. Crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 PSI. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

I generally use a big screwdriver handle stuck in the TB between the butterfly and the TB to prop the throttle open. The plastic is soft enough that it won't damage anything and won't get sucked down the intake either.

A heavy duty battery charger (not the trickle type) is a good thing to have if you haven't driven the car lately or if you have any doubts about the battery's health. Leave it connected while you are cranking the engine and it will help keep the starter cranking at a consistent speed from the first cylinder tested to the last cylinder.
 
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