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Intermittent misfire - Very Annoying!!!!!!


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Hi all,

Had a lot of help and tips from these forums before so hoping for some solution to this... :unsure:

My 93 Euro skier (5.7 Mercruiser) has begun to miss but only intermittently. If I run her up and down the river at any revs there appear to be no probs (a bit of steam and exhaust, and heatercraft 300h wont seem to get up to temp!) but apart from that engine sounds and runs sweet. However, it seems every time i begin to tow someone wakeboarding (though not sure how this can be related) the engine develops a miss. Only appears to happen on one cylinder - big power loss for v. short time, like less than 0.5 sec, then runs, minute or two later misses again, and then continues to misfire more regularly. If I idle home it never misses a beat. After a cool down she runs again fine until towing someone and it starts again. At this stage there seems to be a lot of whitish steam and exhaust out the back.

Due to above issues of heater & exhaust I have pulled and checked the thermostat and fuel filter -both fine, no water etc in filter and stat working as it should. Light grey residue on sparks so thought running lean might be the issue - it wasn't :(

Engine well looked after - winterised and serviced every year, but there are 1100 hrs on the clock.

Any suggestions or similar experiences would be very much appreciated, Thanks! :biggrin:

ps - we live in Ireland so it's cold, like 63F on a good day this time of year.

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Misfire sounds like ignition to me. eg; spark plug wires, cap or rotor. Inspect & replace if there is any question.

No heat could possibly be too low a thermostat? Malibu puts 160s in the engine now days. Heatercraft bumps that up to 180s. Check the water temp in the heater core &/or hoses. Usually you can just grab the hoses by hand & see if their warm. Could also be a blockage in the core or hoses too. Flush as needed.

Good luck. Wish it was 63 around here. Hoping it'll push 50 tomorrow & we can get out for a run.

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Hi all,

Had a lot of help and tips from these forums before so hoping for some solution to this... :unsure:

My 93 Euro skier (5.7 Mercruiser) has begun to miss but only intermittently. If I run her up and down the river at any revs there appear to be no probs (a bit of steam and exhaust, and heatercraft 300h wont seem to get up to temp!) but apart from that engine sounds and runs sweet. However, it seems every time i begin to tow someone wakeboarding (though not sure how this can be related) the engine develops a miss. Only appears to happen on one cylinder - big power loss for v. short time, like less than 0.5 sec, then runs, minute or two later misses again, and then continues to misfire more regularly. If I idle home it never misses a beat. After a cool down she runs again fine until towing someone and it starts again. At this stage there seems to be a lot of whitish steam and exhaust out the back.

Due to above issues of heater & exhaust I have pulled and checked the thermostat and fuel filter -both fine, no water etc in filter and stat working as it should. Light grey residue on sparks so thought running lean might be the issue - it wasn't :(

Engine well looked after - winterised and serviced every year, but there are 1100 hrs on the clock.

Any suggestions or similar experiences would be very much appreciated, Thanks! :biggrin:

ps - we live in Ireland so it's cold, like 63F on a good day this time of year.

Take you exhaust manifold off and look for rust by the ports

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A few things to check and consider: it appears that the engine is running a bit too cold for those ambients, try a warmer thermostat as the cool running temp might be causing a poor atomization of the fuel. It could also be a bad head gasket, the whitish steam might support that theory. A leak down test will confirm any issues there. Debris in the carb will also cause a miss problem, might be time for a rebuild. You might also check the sprk plugs and make sure they are the right heat range, if you are running in cool temps a lot, a warmer heat range plug might be helpful. Different brands can also provide better/worse performance.

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Hi guys thanks for replies.

I'll check for rust at the manifold ports, could account for the steamyness.

Dizzy cap and sparks have all been replaced recently though could be leads. Could also be dirt in cab. Thing is narrowing it down, haven't done any of this yet because engine runs fine most of the time and I've been too busy scratching my head!

I can feel the heat in the hoses and at the radiator in the heater, and the engine is running at a good temp. Might need to change the thermostat to a 180 or there might be a kink in the hoses to the heater. I'll get a good look at it all this weekend.

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So..

Misfire no longer causing a problem, just wasn't getting enough fuel (so it was due to running lean, oops).

Heatercraft heaters need mid range revs on the engine in order to run efficiently - due to water pressure. Also my heater tubes were cut too long. Shortened them, insulated the box and exit tubes and hey presto, even at just 2000 revs (wakeboarding speed) there is a reasonable amount of heat (blast engine to 3000rpm for real hot air).

still a little too much steamy white exhaust though. Water is def not mixing with oil, so head gasket is good for now. I think this is a gasket in the exhaust manifold letting water mix too early in exhaust system, although that is just a guess. Engine is runing sweet though. If I remove the manifold to check the gaskets will I need sealant to re-attach to block (presuming i dont just scrape em off and replace)?

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