What is possibly causing this misfire??

[Pac_Man_13]

96' Saturn SC1

Check engine light: P0304 : Cylinder 4 misfire

The car is also idling very high, the rpm's jump up and down and the temperature slowly rises to almost 3/4 when the car is stopped but running, the engine shakes when idling (Just changed the upper motor mount), it's also burning a lot of oil (more than normal). The car struggles up hills causing the check engine light to start blinking when the car downshifts.


Check engine light has been on for months, I can not find the root of this problem. Changed all the plugs and wires, changed both ignition modules, took the throttle body off cleaned it and the inside of the intake manifold, they were both covered in a layer of black soot. Also switched the throttle body gasket although the old one did not look very worn when I spayed brake cleaner between the throttle body and intake manifold with the car idling the idle dropped to around 900 RPMs (Usually idles at least 1300)

The only other option I haven't done yet is to change the intake manifold gasket. Does anyone think this is the problem? I've read it is a fairly time consuming job so I don't want to start if that is not going to fix the misfire. I have to take this car on a 1500 mile trip in a couple weeks, any suggestions are appreciated.

[SL19302]

I would start off with a compression test focusing in on cylinder 4.

[Signmaster]

It sounds to me like a second test would confirm or deny the vacuum leak at the gasket. It might not be the entire problem, but it certainly isn't going to help you find the other problems if they exist.

I really think you have have answered your own question in regards to the gasket.



Compression test wouldn't be a bad idea still, since your misfire seems to be fairly bad and you are burning lots of oil. Short of that swapping injectors to see if the misfire follow the cylinder or injector might be worth a shot.

[54packman]

normally the intake gasket will cause a high idle as well though, would do a compression test and see what happens there,, maybe switch the plug wires between #1 and #4 coil towers and see if the problem moves to #1 cylinder..

[fdryer]

Pac_Man_13, being a new member with an old car with a mystery problem, did anyone ask about your engine coolant temperature sensor? Search posts about it.

[wingsauce7]

Spray some starting fluid around the IM gasket with the car idling. If you have a leaky gasket, your rpms will increase drastically as the staring fluid gets sucked into any breaches.