2003 350 magnum popping through intake at 3400 rpm

[homelite]

I installed a new marine GM long block (not a reman) from a a reputable distributor. Did it right. New exhaust manifolds, elbows, plugs, wires, cap rotor, etc. I must say, I'm an ASE certified auto mechanic and know what I am doing, but I am getting stumped, as is another tech friend of mine that is helping me. It's getting frustrating!

Runs good at idle and low speeds. Runs good over 3600 rpm. At roughly 3400 rpm it has a popping through the air intake (no sparks, just popping) like a backfire pop. I replaced again just to rule out, cap, rotor, plug wires. I did not replace coil (yet). I have a tech mate and have no codes and as I monitor all sensors I can see they look good. TPS voltage on tech mate is fine in that rpm range. It's almost as if that is the "lean" spot of the fuel delivery system? I do have a bottle of Chevron with Techtron fuel injection cleaner in the tank to clean the injectors, but at standard recommended strengh. I do have fresh gas in the tank as i drained it prior to firing up the engine.

I'm asking a marine tech, any issure here that would help me? Coil, crank sensor (it was new and came with the engine), knock sensor? In the book, I don't see a way to check the coil?

it runs great power wise and at top end so Ihave a hard time believing its a fuel issue.....(unless I am just too lean at that 3400 rpm range)?

I don't imagine I am the only one with this type of problem. Any suggestions? Thanks in advance.

 

[Don S]

Re: 2003 350 magnum popping through intake at 3400 rpm

What happened to your original engine? Have you checked fuel system pressure.
Fuel system cleaner keeps injectors clean, it doesn't clean up dirty plugged injectors. Might consider sending the injectors out for cleaning and rebuild.

 

[Flysfloatsor]

Re: 2003 350 magnum popping through intake at 3400 rpm

Sounds like and intake valve staying open. or an exhaust valve staying closed.
Lifter pump up? Valve adjustment issue? Cam lobe damage (hope not)

 

[homelite]

Re: 2003 350 magnum popping through intake at 3400 rpm

Thanks Don. Original engine was bad when I got it. Locked up due to overheating and head gasket failure. Then sat 2 years, hence the reason for the new engine job. I did not check fuel pressure (yet). I didn't because it runs great at higher rpm. No sign of starving for fuel. I had each injector out and it appeared ok. Where's a reputable place to send out if I want to have checked? I can't beleive for the life of me it's a mechanical engine issue....

 

[Fun Times]

Re: 2003 350 magnum popping through intake at 3400 rpm

Did you R & R all the old engine accessories to the new block? If so, what is the old engine serial number?

Also when you put the new spark plugs in, did you adjust the gap? i know you did new plugs but you may want to check or carefully install new one's that are pre gapped to (0.060 in.) as a test. Merc's newer part number 884019001 (AC 41-993).

With these type of spark plugs on marine engines has been known to give a backfire sound when their bad.

Also verify that the rotor is pointing at the correct timing mark, 8, on the HVS housing between the 2 lines shown below,
http://www.marinemechanic.com/merc/distributors/mercurymarine/sterndrive/gmefi-nostart.PDF