7.4L Mercruiser Repetitive Cylinder Detonation/Pre-Ignition

[emahaffey]

Anyone,

I have a Gen IV (I think) 7.4 L Mercruiser that keeps detonating cylinders (#1, then #5). I've gone through 2 motors so far, and hesitant to replace the motor again until I figure out what's causing the pre-ignition. I'm thinking it's a fuel issue, but can't seem to isolate the problem. On the first motor I had the carburator rebuilt, and replaced the mechanical fuel pump with a used one, and it still happened. The second motor has a brand new carburator and the same fuel pump as the first motor. I had the cylinders scoped, and sure enough another cylinder is cracked. I plan to pull the entire fuel line to check the syphon tube, etc. Anything else I should be looking at?

 

[Happy Hour IV]

I believe that a lean running condition would burn exhaust valves long before any detonation took out the piston. If you do not have evidence of burned valves the other common cause of detonation is timing. Look for base timing issue or possible the advance curve in the module is defective.

I guess you could have some spark plug wires reversed so two cylinders are firing early but that would affect 4 cylinders so it would barely run.

Maybe two plug wires are cross firing but again with two cylinders firing that early it would not run well.

I suspect the advance curve is an issue. You could use an adjustable timing light to see what the total advance is. I would guess it should be in the area of 34 degrees. A lot more and you have found your problem.

 

[CliffA]

i would also run a higher octane fuel if you are running regular grade now....my 5.7 will run OK on regular grade fuel but at higher RPM's it starts to spark knock (detonate) and the knock sensor will start retarding the timing...i now run high octane (93) grade fuel and no longer have that issue....and my ignition timing is set dead-on to factory specs....

cliff