I have a 1996 monte carlo with a 3.4 engine. The check engine light will come on when I am going over 65 mph or going up a steep hill. I hooked my code reader up and it gave me a 304 error code (Cylinder #4 misfire). I replaced the plugs and wires, swapped coil packs, and replaced the fuel injector for cylinder #4. The error is still there and is progressively getting worse.

I hooked my dad's more expensive code reader up and it gave me the 304 error code, but it also gave me the 410 error code (Secondary air injection system). I was wondering if anyone else has ever seen this problem or knows what it could be. Any suggestions would be greatly appreciated.

Other things are: the fuel pressure is 44psi when idling, the fuel filter is fine, and the air filter is new.

 

My first Monte was a new '96 Z34. It had all kind of issues with the engine. The check engine light would always come on and I had a problem with it stalling and also rough idle. The dealership did everything possible to try and fix it, but nothing worked. After 2-years GM finally bought it back under the lemon law. I know that doesn't help you much, but that's has been my experience. I've heard that some of the DOHC 3.4's are just problematic. I've also heard that the harder you run them the better they seem to respond - don't know if I belive that or not. I think I still have my paperwork from back then, I'll see if I can look through some of the service records to see what the dealership tried and maybe it will work for you.

Good luck!

 

Check the cylinder pressure on #4. Possibly a leak or damaged? Also run some gas with Ethenol in it, like Shell or something. I've heard online that dirty valves can cause this but I've never experianced it first hand.

I do know the ethenol keeps the interior of the engine clean. My rockers almost looked like they just came off the shelf at 50,000 miles when I swapped them. Nothing like some others I've pulled off that are coated in gunk.

 

To followup, make sure that Ethenol is up to E10. You don't want to run E85 through it as it will clean the gas tank as well and clog your fuel filter.