I was able to narrow down the noise to the fuel rail. I put 1/2 a can a Sea Foam in the tank and the otehr half in the crankcase. Now I am getting a P0304 "pending" code. The car does not havemuch power. I took it out on the hiway this afternoon and when I floored it from a stop, it had a hard time getting going.

 

Still no progress. I put new plug wires on the car last night, and still have a P0304. Also, I noticed that when it is revved up to 4000 RPM, is loses power and the RPM's bounce between 3000 and 4000. I am at a complete loss over this. I have never had a car be so stubborn. I'll probably have to take it to the dealership and have them look at it next week. This SUCKS!

 

I just Googled P0304. Here's what I got:
OBD-II Trouble Code: P0304 Cylinder 4 Four Misfire Detected
It claims mis-fire in cylinder 4.
It's been a while since I looked at this thread, so I took a brief look at we discussed. It looks like we ruled out coils and wires possibly. But we have not ruled out injectors.

You mentioned changing the UIM gasket. When you did this, did you replace the injector O-rings? If not, they may be the source of the problem (sure they may have seated, but are worn out and note making a good seal).

You also noted finding a noise in the fuel rail. I wonder if perhaps injector #4 is messed up. I would try trading injector #4 with a different one on the rail and see if the code changes.

If you took it to a shop, I believe a GM Tech II scan tool can turn off one injector at a time. The goal is to find out which one causes the least amount of change in engine performance (but again, that code is really pointing you to cylinder #4).

Also, double check the spark plug. I know a guy that went round and round with a misfire after installing new plugs. After 1-2 months it was just a bad plug from the get-go.

 

I switched the number 4 injector with the number 6 injector, and it made no difference. I still get the P0304 code. I did not change the injector o-rings. I'm at my wits end with this thing. I am reasonably savvy when it comes to fixing this stuff, but this has got me completely at a loss.

 

This is really not adding up. I would think it's plug, wiring, injector or coil.... Since you compression checked each cylinder earlier, tells me we shouldn't have a gasket leak some where.

Just to be certain, I believe cylinder #4 is on the back of the engine (side closest to the firewall) in the center position. That's the one you have been checking for #4, right?

 

I should have said earlier. I have the 3.4. The number 4 plug (according to the coil and the numbers on the wires) is the center one in the front of the car. Man, I am really at a loss on this one. It's got fuel, spark and compression. I have no idea why it is misfiring. I did unhook the battery awhile back, could that have something to do with it? I know there's a re-learn proceedure for the computer. But, with the way the car is acting, I would think it is something else. I'm wondering about the cat and the PCM.

 

Well, I took the car into the shop this morning. They called and said that they found the problem. Apparently someone had done some work on the cylinder head ot one time and stripped the bolt holes out for the clips that hold the rocker arm in place for cylinder #4. They repaired to with helicoils, but they didn't install them correctly. The helicoils pulled out of the head and the valve wasn't opening and closing properly because of it. They said it needs 2 new rocker arms and new bolts for those rockers. They wanted $384 to fix it. My Dad is a machinist and he says that he can fix it. So, I'm going to try and fix it this weekend.

 

Had some trouble finding rocker arm bolts, but I finally found them. Now I can replace the piece of all thread rod that someone installed in my cylinder head instead of the correct bolt. I'll post pictures tonight of the mess I found under the valve cover. How could someone actually feel comfortable repairing a car this way??

 

wheres the 02 sensor(bank 1) located? and how can i tell if its bad?how much for replacement? sorry for stealing your thread

 

Bank 1 Sensor 1 is on the rear exhaust manifold. I believe it's recommended to replace approximately 70,000-100,000 miles. The reason to change it is as it ages, it's readings become less accurate. If it's really bad, it trips a check engine code. Otherwise, I don't know how to tell if it's bad.

@mairj40 - Sorry I missed some of your previous posts. Sorry to hear the news, but at least you hopefully have some answers AND a way to fix it. FYI - for what it's worth, sounds like the problem is a bolt hole is stripped on the head... Might be worth getting a new head from a salvage yard. Just a thought.