P0304

[JonJon68]

1997 Grand Prix 3.8 N/A 181,000
I have been getting a misfire only on cylinder 4 which has become more frequent. It "only" happens after the car has warmed up and I turn it off for a few minutes, for instance going in to pay for gas. When I start, it immediately starts to miss but as soon as I hit the accelerator it evens out and runs great, every time. I replaced plugs and wires two years ago with stock. Upper and lower intake manifold gaskets were replaced around 110,000. All three coil packs have been replaced with original GM packs. I replaced my EGR valve last year due to the original was sticking. My throttle body was cleaned about four years ago. I've seafoamed through brake booster and the rest in gas tank.
Any thoughts on this? I'm stumped that it corrects itself right away by giving it gas. The SES light always come on but goes off on it's own after a few cycles.

[Tech II]

Coolant could be getting into the cylinder.....upper plenums are known to leak where the EGR stovepipe passes through it.....

So, are you losing coolant?

If not, pull the vac line off the f/p regulator and check for fuel in the vac line......

[JonJon68]

Yes it seems I am losing coolant but very slowly. I didn't want to think upper plenum again but it looks more and more that is my problem. Thanks

[JonJon68]

Update: Replaced upper intake manifold and gasket last week and so far no misfire on cylinder 4. Running even better now..Thanks Tech II

[Levithan9]

Thanks for the updated fix. I've done a few intake manifolds on these models. The fix is usually just an updated manifold which addresses this problem.

I would have checked injector resistance, and lab scoped the injector and coil. But hey, looks like you found it.