Hi!! So i have an 05 Malibu. its the LS Sedan with the newer style body. It's got the 3.5 V6 engine in it.
The car was in a nasty accident, it got scrunched up in the front end. something happened to the engine and the gaskets all started leaking. the car ran fine aftrer repairs and all though. So i got the manifold off and replaced all gaskets except the head gasket. i got everything back together and torqued it down. at first i had rocker chatter and i bent a pushrod. but now after getting everything back together my engine is misfiring. i tested the arms and pushrods doing a crank test. nothing broke or fell apart so i am not sure what happened. the engine is very hard to start, the only way to start it is to hold the gas pedal down to the floor. it drives fine but idles very poorly. one could even say it misses at idle. After hooking up a code reader, i got the P0300 Random Misfire code.
has anyone had this problem? ive tried replacing the spark plugs, coil pack, and the wires, but nothing has worked so far. All help is very much appreciated.



when the rod bent





this is what it looked like before the arm / rod removal

 

Why did the push rod bend? It is quite possible that when it bent some other damage was done. Did you check for cracks or a damaged valve?

 

The intake and exhaust push rods are NOT the same length. If the a long rod is installed to the wrong valve It will bend. I don't know if the valve is bent also. Replace the damaged rods, check all rod for correct location and then test compression. If you have an air compressor a cylinder leak down test may be better than a compression test with the intake removed.

 

Yes, the rod bent because I put in exhaust rod in an intake slot. Learned that lesson real quick. So I will run the test. I did find a cracked rocker arm and will be replacing it asap. Will double check the valve. Only one out of 12 valves had a bent push rod. I guess my question is would one bad valve / lost compression in one cylender cause a random misfire or should I dig deeper?

 

if you bent a pushrod then theres a very good chance theres more damage in that cylinder. You said you had the head off? did the piston look like the valve hit it?

 

Even a slightly bent valve will cause low compression and misfire. Start with a compression test , leak down if you can. Also look at the rocker stud, check if it is pulled out of the head some.

 

just did a compression test. as it turns out cylinder 3 has absolutely no compression whatsoever. So I'm guessing next thing to do is check the valves and check / replace the head gaskets and bolts? Rocker stud is in there fine though. had two instances of cross threading on the opposite side that was corrected.

 

Ugh i forgot how much of a pain exhaust could be on a 10 year old car. been working for a day trying to get the manifolds off but no luck yet. Will update when i get the heads off to see what kind of damage I'm looking at.

 

After a lot of work, I got a used head on my engine. Everything went well, besides an occational misfire due to a vacuum leak. It runs and drives again though so it is a good start! Thank you all for helping me to diagnose the issue.