New so need some help. Just bought a 1998 z28/ls1 from my neighbor. It's throwing code p0300 for multiple misfire. My neighbor told be had changed a lot of parts and couldn't figure it out. He took to a shop and they told him it had a bad exhaust valve from one of the heads. It drives good but idles like crap and sometimes dies. Takes a while to start sometimes. It also makes a popping sound from the exhaust like if It was unburnt fuel. What do you guys think? I don't wanna tear the heads off just yet.

 

i am not sure hope you got a good price ,could be doing damage to the engine,i would bring it to someone

 

Yeah I got it pretty cheap. Hopefully not. He told me that two shops told him it was the exhaust valve from cylinder #7.

 

It may have a bunt valve but that would result in a single cylinder misfire, Multiple cylinder misfire is usually the result of either an ignition problem or a lean condition. If the ignition checks good, then look at items that control fuel delivery, such as fuel pressure with engine under load, O2 sensors, MAF. These sensors may not be setting codes, but can be giving the PCM wrong information.

 

It's important to find out what cylinders are misfiring. Check with spark tester. It could be that you have a bad coil or bad spark plug wires.

Hear is beating a dead horse!

 

A burnt valve can result in an improper air/fuel mixture in that cylinder. That is then picked up by the O2 sensor, which is reading an entire bank of (random) cylinders.
What sensor/process picks up a/f readings to just one cylinder? What am I missing?

 

i would think a full compression check could confirm a problem with valve sealing.

 

My neighbor told me the car has brand new spark plugs spark plug wires new iac new tps new fuel pump and there's some other parts. He has new o2 sensors he never installed. I don't think it's a bad valve. Wouldn't it be making a noise or smoking out the exhaust? Has small back fires/ popping sound and idles like ****. When I put it in drive and hold the break it almost wants to die but once I'm driving it feels like nothing is wrong until I come to a stop. I can't check the car right now because I'm out of state.

 

A bad valve would make the engine run/idle like **** and the exhaust would have a popping/sputter noise.
No, a badly seating valve wouldn't be causing smoke to come out the exhaust

 

Yeah that's what it does but no smoke. If it turns out being the heads what "cheap" heads would be a good upgrade? Engine is completely stock.