Sunfire GT 2.4 Cylinder 4 Misfire…

[MarooZ24]

I am getting the misfire got P0304 Spark plugs are in great condition, replaced the whole IDI with everything on it from my currently disabled cavalier. Still runs horrible, you can feel the engine shake in neutral and in drive and on the brake you can hear it running rough, giving gas little or alot sounds perfect. Compression on all cylinders is around 175ish nothing lower than 150. Cannot find a vacuum leak, tried spraying everywhere on intake.

Topic

[R.G.Young,Jr]

have you checked the egr valve? it may be stuck in the open position.

[MarooZ24]

I would have but from 99 to 02 the 2.4 gm engines never had a EGR.

[R.G.Young,Jr]

do you have access to a dvom, and a fuel pressure tester? testing the fuel injector for voltage switching on and off and resistance and also fuel pressure drop test per injector would be a good place to start. keep testing and posting and good luck

[college man]

Try doing a power balance test. do it both ways fuel and spark.

[MarooZ24]

I did check to see if the injector wires were firing right, which they were. but will a clogged Fuel injector still fire right and click evenly?

[R.G.Young,Jr]

It can, in a lot of cases the clicking will sound dull compared to a good injector. however this isn’t always the case. If you can disconnect the injector while it is at idle, you should notice a sharp drop in rpm. the ideal is to disconnect an injector from a known good cylinder and note the rpm change. then reconnect and then do the same with the affected cylinder and see if you have close to and equal reaction. that is a quick test. that being said, the truest test is to hook up a fuel pressure gauge and recycle the ignition switch a couple of times to get the fuel pressure up (do not start engine, turning the switch on and off will achieve what is needed for this test) and then activate that one injector by itself externally and note the fuel pressure drop. Then recycle the ignition switch again to restore the fuel pressure and activate an injector from a known good cylinder in the same manner, and note that pressure drop. the pressure drop should be close to the same in that injector, if the suspect injector is working properly.
if they are different, you may need to replace the injector, but before I would do that, I would try cleaning the injectors. If the suspect injector is dirty and not totally blocked, then the spray pattern will be streaming instead of a conical spray pattern. If this is the case, a good cleaning should restore harmony back to the engine.
Good Luck and keep posting.

[R.G.Young,Jr]

It can, in a lot of cases the clicking will sound dull compared to a good injector. however this isn’t always the case. If you can disconnect the injector while it is at idle, you should notice a sharp drop in rpm. the ideal is to disconnect an injector from a known good cylinder and note the rpm change. then reconnect and then do the same with the affected cylinder and see if you have close to and equal reaction. that is a quick test. that being said, the truest test is to hook up a fuel pressure gauge and recycle the ignition switch a couple of times to get the fuel pressure up (do not start engine, turning the switch on and off will achieve what is needed for this test) and then activate that one injector by itself externally and note the fuel pressure drop. Then recycle the ignition switch again to restore the fuel pressure and activate an injector from a known good cylinder in the same manner, and note that pressure drop. the pressure drop should be close to the same in that injector, if the suspect injector is working properly.
if they are different, you may need to replace the injector, but before I would do that, I would try cleaning the injectors. If the suspect injector is dirty and not totally blocked, then the spray pattern will be streaming instead of a conical spray pattern. If this is the case, a good cleaning should restore harmony back to the engine.
Good Luck and keep posting.

[Justin Farrington]

It could be as simple as bad intake/exhaust gasket. A simple way ($5-7) test to check if it’s the intake gasket is to spray some carb spray into the intake. If the idle is smoother, it’s the intake gasket most likely. A free test for the exhaust is to feel around the header (obviously don’t get too close as it will be hot!). If you feel some exhaust coming from the sides, on top of, or beneath, you have an exhaust gasket leak.

[EricTheCarGuy]

Yes, don’t rule out a mechanical issue. So often I see that a mechanical issue is overlooked in a situation like this. A simple compression test can help you nail down if that’s an issue. I would think if you had a gasket issue you would have found it when you looked for vacuum leaks but it probably wouldn’t hurt to check again. If you know the ignition is good I would look to the mechanical next. Keep us posted.

Compression testing

Leak down testing

Finding TDC

[MarooZ24]

Well I did fix this awhile ago it ended up being a bad injector. Replaced them all and haven’t had a misfire since. Engine does still shake but not nearly as bad and makes the weird rough running sound very lightly. I believe the motor mount is bad.

[college man]

Thanks for the update.Keep us posted if things change.

[Author]

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