OK. I've read many posts about misfiring and realize that many possible answers have been given, but none of them have helped me. Here is my problem:

I have a 97 Yukon, 5.7 V8, two wheel drive with 143k on it. Up until this year, I haven't had to do anything to the car, and until last week it was still running on the original plugs, wires, distributor cap and rotor. For a while now the car has been getting harder to start. When I first bought it, I barely had to touch the ingnition and it would start. It has progressively gotten to the point where I now have to crank it for several seconds to get it to fire. About a year ago I had the manifold gasket replaced, which I'd hoped would clear up the starting problem, but it didn't. Last spring I had the transmission rebuilt and a year ago this past summer I had to have the fuel pump replaced. Each of things, based on the posts to this forum, were bound to happen and did.

Now back to three weeks ago. I noticed that the car was missing as I drove at highway speeds. It wasn't happening all the time so it was hard to identify the specific conditions that surrounded the occurrences. As I continued to monitor this situation I found that when was cold it would start and run perfectly, but as the temperature reached around 100-120 degF it would begin to miss and sputter. This went down hill rapidly. I took the car to my mechanic and asked them to identify the problem. The first thing they said was that the car needed a tune up. OK. They also said that the hard starting was probably caused by a bad fuel pressure regulator. OK. They also identified a leak in the radiator. Fine. I picked the car up and proceeded to repair what I could.

I now have new plugs, wires, distributor, and rotor (as previouly stated) and I still have a misfire condition that just keep getting worse and a leaky radiator.

My thoughts are that something is getting warm and malfuntioning. With so many sensors on todays cars and the interrelationship of all the systems (not like the good ole days) its seem from reading the manual that it could literally be anything.

I need help desparately. I love the car and it has served me well, but I can't keep dumping money (parts) into it in hopes of stumbling across the problem.

Can someone please help me?

Thanks.

I don't think this will help, but I am doing the same thing right now. P0300 Random engine misfire, had it for about 2 months, on my 1996 Yukon. It's getting steadily worse, I did a full tuneup, new ignition coil & module.

Funny enough, I also had to replace exhaust manifold gasket 6 months ago, 2 of the O2 sensors, and a newly rebuilt transmission.

My mechanic is working on it now, he thinks it's the actual distributor itself. I think it's the fuel pump, but no one believes me.

I got it fixed. At some point the Crankshaft Position Sensor wiring harness was taken from its normal mounting position, which allowed it to lay across the passenger side exhaust manifold. When the car got hot, the wires shorted out. Now that I've returned the harness to its normal location, the car runs great again. I found this by checking the DTCs and finding a problem with the CKP (P0339). If I hadn't checked the codes I would have never found it.

ALWAYS CHECK THE CODES AND DO THE SIMPLE STUFF FIRST!!!