So about a week ago my car (05) started to ride rough at around 40mph. Idle its totally fine. Got a diagnostic and said I was having a miss fire. Checked the spark plugs the top left has a little bit of oil on it the others were clean including the wells. Replaced all spark plugs and even changed out the valve cover and gasket. Went on a test drive today and still running rough when I get up to speed. Idle still fine. Gonna replace ignition coils next but if that does not fix the problem anyone have other ideas?

 

A misfire could be the result of a couple things.. When you say that you 'got a diagnostic', does that mean that you took it somewhere and they read the codes? Is your check engine light on?

 

Yes, got a diagnostic done they said it was the valve cover. So did that myself as the shops around me wanted 1200-1500 for it. The check engine light only comes on when it starts to ride rough, and when the roughness starts it lasts for a few minutes and then rides smooth once the car starts to smooth out the check engine light turns off. My first car (3g eclipse) did this because of oil on spark plugs so when I changed the valve cover on that car it fixed the issue but so far it hasn't fixed it for my 350.

 

OK, then you have a stored code. Even thought the CEL is intermittent, that code will still be stored in memory.. Go to Advanced Auto/Auto Zone/NAPA/National Auto and have them hook your car up to an OBDII scanner tool. They'll read it for you for free. Report back.

 

I had a 3G Eclipse also. Went through all the typical issues with it, but for the most part was really happy to own it and was kind of sad to see it go when I traded up for the Z.

It's pretty easy to check for bad coil packs and injectors using a relative compression test. You'll need a scope and some current clamps (not that cheap), but it's cheaper than changing out your coil packs and injectors just to see if they might work.

Like Matt said, "A misfire could be the result of a couple things..." A relative compression test on a modern scope will save you a lot of money in the long run.