Repetitive Cylinder Detonation/Pre-Ignition

emahaffey

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I have a Gen IV (I think) 7.4 L Mercruiser that keeps detonating cylinders. 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?
 

Don S

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Re: Repetitive Cylinder Detonation/Pre-Ignition

Timing or lean fuel.
EFI or carbed? Serial number of your engine?
 

emahaffey

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Re: Repetitive Cylinder Detonation/Pre-Ignition

Hi Don. I have the serial off of the original motor (0F282059), but not the most recent one. It's a carburated 7.4L 454. I'm thinking it's lean fuel as well. We set the timing while under way after the install of each motor. Ran fine at first, but each time when I went to wide open throttle the motor would stumble. After idling briefly I could get back on plain again. I think the stumble was actually when the detonation occured.
 

Don S

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Re: Repetitive Cylinder Detonation/Pre-Ignition

Are you replacing the complete bobtail engine (Ignition, fuel system, exhaust and all) or just the long block and keeping the original accessories?
 

emahaffey

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Re: Repetitive Cylinder Detonation/Pre-Ignition

I've replaced just the longblock twice now, and reused most of the components (carburator, fuel pump, ignition, etc.). Although the second time around the carburator was brand new (first one was rebuilt).
 

Don S

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Re: Repetitive Cylinder Detonation/Pre-Ignition

More questions, is this a Bravo outdrive, or a straight inboard?
What was your Maximum wide open throttle rpm?
 

emahaffey

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Re: Repetitive Cylinder Detonation/Pre-Ignition

It's Bravo III outdrive. Normally I could run about 4300 RPMs wide open from what I remember.
 

Don S

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Re: Repetitive Cylinder Detonation/Pre-Ignition

Your rpm range is 4200 to 4600 so you are just barley in range, Merc likes to see near the top of the range, and since you are having problems............
That could be a slight problem along with the fuel we have now. Might want to check your jet sizes in the carb as well. May not hurt to go a little larger from specs. Don't forget, those engines are probably bored, and if not OEM rebuilds will probably be .060 over.
Have you been inspecting the plugs to see if you are getting a lean burning condition?

webber.jpg Webber2.jpg

With all these new engines, and still playing with a USED fuel pump hmmmmmmm.
 

emahaffey

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Re: Repetitive Cylinder Detonation/Pre-Ignition

I know, I know. I'd like to put a brand new fuel pump on it, but it's the mechanical one that also provides the raw water pump housing. I think it costs around $500. Definitely cheaper than a new motor and all this frustration. It is, however the second pump that I put on it. When I first started having fuel issues (broke down on July 4th 2 years ago), I had the carburator rebuilt, and swapped out the fuel pump with one from a motor with not too many hours on it.

I looked at the plugs and you can tell which one is burning lean. This last go around it was the number 5, I think the first time it was number 1.

I really think there is some blockage some where between the pickup tube and the fuel pump. The motor still runs at this point so I plan to run a pressure test on the fuel line after the pump to find out if it's creating enough pressure. Although a blockage could mask a non-issue with the pump. Think I'll check the pickup tube and supply line first.
 

emahaffey

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Re: Repetitive Cylinder Detonation/Pre-Ignition

True, thanks Don. I think when we pull the existing motor, I'll order a new pump just to be safe.
 
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