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Q: Issue with firing - 1991 Volvo 240

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Hello! My car doesn't seem to be able to fire properly, i've just changed the rotor and distributor cap, and the sparkplugs, it seemed to run a lot better after i did so, the light which shows that it burns unclean also disapeared, but the problems returned (within a day) and it doesn't run on idle mostly, but sometimes it runs just fine (although still the light which indicates it burns unclean still is there) and sometimes it idles badly, but it doesn't die, sometimes it just dies instantly after driven then coming to a stop, other times it just vibrates and seem to be fighting to keep runninh, but while driving i feel no problems, what could the problem be and/or how should i approach troubleshooting?
My car has a manual transmission.

A: Misfires can occur from fuel supply, spark,...

Misfires can occur from fuel supply, spark, and camshaft timing problems. You have replaced most of the ignition components that supply spark to your cylinders, but you could still need spark plug wires. They may look good and not be. Also, disturbing them by replacing the cap and spark plugs can make them work for a short time then fail as they did before.

What need to happen to figure out why your car is running bad is some diagnosis. This will require checking fuel pressure and doing a cylinder balance test. The cylinder balance test can be done in a myriad of ways. In your case, I would use a test light to short out one cylinder at a time. The car should run significantly worse when a cylinder is short out. If the idle of the motor changes very little when a particular cylinder is shorted, this cylinder would be suspect to having problems.

As stated in the first paragraph, it could be a bad injector (fuel supply), poor connection of the injector connector, bad spark plug wire or even a compression issue. The main thing that needs to happen here is some definitive diagnosis before any more parts are replaced. If you should need some help with this, a certified mechanic from YourMechanic can come to your location to perform a misfiring diagnostic.

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