Rough running engine.

[Ceramic Guy]

Car: 1993 Geo Prism, 172,000miles, Automatic, All Original

Car ran well for 168,000miles then the engine seemingly developed a strong vibration. As strange as this may sound, the car runs more smoothly after running it then turning off the ignition and letting it sit for a couple of hours or so. When it is started up again that second time it runs much better although not quite at 100%. However, if allowed to sit overnight or through a workday the vibration is very strong.

It seems that the cold weather has something to do with the problem but it may be that the problem is getting worse over time and that cold weather is simply coincidental.

Engine mounts and wiring have been checked an excluded(?) as the cause.

Any suggestions would be greatly appreciated.

[freakinbox]

kind of sounds like it's misfiring. My mom's van does that. Did it recently again when some ice built up in the engine compartment and dripped water all over everything, yay for shitty ford wiring.

[digital_racer]

Don't know if this will help or if it is the right thing to do:

You could do a quick check with the car idling. Remove the high tension wires on the spark plug one by one. If you removed a plug and the idling doesn't change, that cylinder needs fixing.

[freakinbox]

might just need new sparkplugs.

[Flashmn]

Might be a cracked distributor cap, or moisture condensing inside it.