Re: GM 4.3 misses bad after warm re-start up any ideas
I had a 1996 Chevy Blazer with the 4.3L in it for years. At about 200K I had the exact problem you are seeing. I went through and replaced the O2 sensors as it was throwing a code that insinuated this was the problem. After changing them out, it still ran the same.
The symptoms were - Cold start - Touched the key and it fired right up immediately. It ran very strong but as soon as it warmed up it would have a hesitation at low RPM's, it'd miss constantly when pulling out from a stopped position. Finally it became nearly undrivable. I did some more research and everything I could find pointed to a vacuum leak, but I couldn't find one. All signs then pointed to an Intake Manifold leak.
So, I tore the entire intake off, cleaned up the block and the intake, laid a new gasket and put all the pieces back together. Truck took a couple cranks and fired right up. It ran strong for about another 8K then showed the same symptoms. I then ended up getting rid of the truck. My best guess was that the intake was warped. I should have checked it to make sure it was flat, but it didn't cross my mind.
My guess is that as the engine heats up, the intake and block expand, the intake being aluminum, it expands more than the cast block. with the seal already bad, it just increased the problem void enough to cause a leak big enough to make the engine run poor.
Also note - a warm restart, i'd have to crank on it for 5-10 seconds to get it to start, it would crank and sputter until it finally took hold and kept running. Man I still miss that vehicle, great mountain runner.