spark plug question

[craig newton]

1996 mercruiser 4.3lx 6cyl 4bbl
Was told running a different spark plug (not as hot) may help with a deisleing problem I am having
The manual calls for a AC-45MRT plug. The replacements have a longer shaft and I'm concerned about them making contact with the piston. Is that possible?

 

[Bugzilla46310]

IMO, a replacement should be the exact same length. Take all the plugs out and use a scale through the plug hole to measure the distance to the piston at TDC. Even if the plug wouldn’t hit, I would find a replacement that is the same length. Would a longer plug affect the flame profile? I don’t know. Might, might not.

 

[nola mike]

1996 mercruiser 4.3lx 6cyl 4bbl
Was told running a different spark plug (not as hot) may help with a deisleing problem I am having
The manual calls for a AC-45MRT plug. The replacements have a longer shaft and I'm concerned about them making contact with the piston. Is that possible?

The longer plug is for the vortec engines...Your '96 PROBABLY is non-vortec, but that's right around the time Merc switched the heads...

Dieseling isn't uncommon after running for a while and not letting the engine cool down for a full minute or so before shutting down.

 

[Pmt133]

I would use the correct length plug for your application. IF the dieseling persists even with letting it cool after running, you can shut down in gear and that should take care of it. My 3.0l used to diesel a lot.... I suspect it was heavy boat with a small, over worked, engine.

 

[craig newton]

It is in fact a Vortec engine and it's in a 21' Blue Water CC that I regularly run to Catalina Island so I can't really rule out the overworked idea but at least now with the new plugs in I'm not afraid to try to start it. Thank you people.

 

[Scott Danforth]

Dieseling or run on is from either carbon hot spots and running rich, or running hot and running rich, or timing is off and running rich

 

[Scott Danforth]

It is in fact a Vortec engine

Vortec head intakes have 8 bolts that are vertical, non vortec head intakes are 12 bolts at an angle

 

[bajaman123]

lol...the plugs are not the cause of your 'dieseling'...you are trying to fix a symptom of a bigger problem! GO FIX THE PROBLEM instead of trying to band-aid it!