Here's the story: 2005 Monte LS with 3.4 liter engine, about 120000 miles. Did the LIM/ headgasket job and had it all put together Sunday night. I did allow the car to prime itself before attempting a start. Tried to fire the car up and noticed a backfire through the intake, Seems consistent at low RPM, was able to *** it to a high idle and it seemed to alleviate the problem but not entirely certain, perhaps wishful thinking. Did not notice any metal on metal sounds of valves hitting the piston.




From what I can find on other threads I should look for fuel delivery, firing order and vacuum leaks. I'm at a bit of a lose on how or where to start.


I don't think the issue will be a bad fuel injector, as I did not remove them or the fuel rail from the LIM. Fairly certain I placed all the pushrods in the correct location there would be a metal ping sound if that was true, at least I think.
getting strung pretty thin here and would appreciate any help you folks can offer.

 

My gut says to inspect your firing order first. You may simply have a plug wire crossed.

 

Re checked the firing order and everything is in the correct place.
Performed a fuel pressure check, key on/engine off and got repeated readings of about 45psi, which is below the 52-59 psi range.
After each test the pressure began to bleed down. Not sure if this is normal or another problem

 

hmm, I don't have a thought at the moment. It's weird it's back firing through the intake. By chance, have you checked that all vac lines are connected properly (should not be many of them)?
If you fuel pressure is too low, I would not suspect that cause a backfire via the intake. Something to check (as it's happened with the 3800 engine) is if the fuel pressure regulator is failing. Pull the vacuum line from it and check for signs of wet gas in it. If so, time for a new one.

 

Might be a crazy question but did you have the backfire issue prior to the gaskets being replaced? Or did this begin after reassembly?

 

issue began after reassembly. This weekend I had some time and replaced the fuel pressure regulator as Maniac advised, it was leaking fuel into the vacuum line.
Have not retested fuel pressure because the battery died after a few cranks.
I think it may be time to drag it to a local shop cuz I'm stumped. Any last hail mary advise would be greatly appreciated, really don't want to let this one beat me!