Thread: Engine Miss Puzzler...

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Hi All,

Mercruiser marine engines of this vintage operate with copper wire core spark plug wires. If you check for resistance you should find zero. I checked the wires and they were both good. Wiggling and shaking the wires did not produce any change in continuity.

There were no burn thru marks on the insulation of either of the wires. I pulled the boots from the plugs and distributor and was getting about a 1/2" spark jump at both points.

Distributors on most nearly all marine I/O engine dont have vacuum advance and rely on the distributor centrifical advance.

This engine relies on fresh water cooling to keep the heat under control. Water usually enters into the oil and turns it milky should the head gasket leak.

As far as the number of hours this should be a fairly low time engine, > 100hrs I suspect.

I think a leaking cylinder head gasket would show up as a loss of power on the dyno but, stranger thing have been known to happen...

If the gasket is just seeping it won't show up in the oil. Difficult on a boat, maybe, but water will show up in the exhaust gases if you put a cold piece of glass near the outlet. Much easier with individual pipes, of course. Or antifreeze, it stinks. Hard to find a lakefull of the stuff, though!
Just the fact that they are side by side keeps pointing back to the head gasket.

Originally Posted by R Pope

If the gasket is just seeping it won't show up in the oil. Difficult on a boat, maybe, but water will show up in the exhaust gases if you put a cold piece of glass near the outlet. Much easier with individual pipes, of course. Or antifreeze, it stinks. Hard to find a lakefull of the stuff, though!
Just the fact that they are side by side keeps pointing back to the head gasket.

Just have to put a set of " Ears " on the motor, and connect them to a garden hose. they will provide the nessasary water supply to the engine w/o having to be in the lake.

I just retired from this industry, and I am leaning toward what R Pope is saying. When you have two adjoining cylinders missing at only idle, it could be telling you that there is a little coolant getting into each cylinder. The reason you only notice it at idle is because at higher rpms the engine can burn off the water easier and the plugs won't be fouled so easily. I had a 3 liter Mercruiser that only fouled # 4 cylinder at idle, at more rpms it ran great. Turned out to be a cracked exhaust manifold.

I would suggest a leakdown test and also checking the exhaust manifolds and risers to see if they are bad. Sometimes a leakdown won't even show it as the water only gets sucked in under compression, like when the engine is running.

Also look for tell tale rusting on those two plugs. Could be something else, but I am betting a new head gasket or manifolds will be the fix. Here in Florida manifolds only last about 3 years typically due to the salt water, so we change these and head gaskets quite a bit. Let us know what you find out.

Don