Mercruiser Problem

[andrewa]

Took the boat out today first time in 4 weeks so was getting withdrawal symptons but an engine fault spoilt my day. Before I call the mechanic in could any forum member advise if they have had a similar problem.

My engine is a Mercruiser 7.4L it started fine and ran great below 1500 rpm, however when above this I detect a misfire and it won't rev past 2100 rpm and it sounds gruff and strained. Got back in ok keeping below 1500 but on the way back detected the misfire and engine sounded gruffer than normal. If no load on the engine i.e out of gear it revs effortlessly to 5000 no problem and no hint of the misfire. Could it be the fuel pump or blocked filter? Or something more serious. Any input would be appreciated.



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[Freebee]

I would check fuel supply including change filters first. Engine will run to full revs on no load because it does not need as much fuel.

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[duncan]

could be fuel of electrics - load/no load would be effected by them both to a degree.
I would be tempted to run from an auxillary fuel tank direct to engine (ie bypass main fuel filter if you can) in the first instance to rule out a few things and change the plugs too.

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[h4nym]

More likely to be fuel filters than fuel pump... not to mention easier to resolve!

H

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[ArthurWood]

Andrew - Take a look at your plugs. If some are black, you prob have ign prob. If any plugs not nice sand colour, replace. I had a similar prob a couple years back with one of my 7.4L Mercs. Turned out to be corroded contacts in ignition module attached to side of distributor. If yours same as mine, there is a small rectangular box attached to side of distrib. It has a rubber cover where the leads go through and a clip on the side which holds it on. Release clip and pull connector off. Spray electronic cleaner over contacts. If that doesn't work, check inside distrib cap for dirt and wear on contacts and rotor. Replace cap and rotor if necy. Good luck

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[adrianm]

I've had similiar problems in the past. Both times it turned out to be the HT leads breaking down. New leads cost me less than a tenner.

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[DavidJ]

Sounds like a breakdown in the HT to me. The problem is that it could be breaking down anywhere from the ignition coil, the main HT lead, to the distributor to the plug HT leads and finally the plugs.
Once the HT has tracked it leaves a carbon trail which the further HT can easily follow down. If the tracking is on the outside of the component then at night you should be able to see some sparks.
If you haven't replaced your leads and plugs in the last 3 years then thats a good place to start anyway.
best of luck
David

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