Engine misfiring and rough idle

[Ripple17]

Hello,
I got a 1998 Maxum - Mercruiser 3.0 a couple of months ago. I took it out last month and the engine ran great all day.
Last week I took it out for the second time and had no problems for 2 hours, then it made a knocking sound and lost rpm while crusing (from 3200 to 2800). It made it the last 2 miles back to the docks. Oil pressure and temperature guages didn't move from normal. Dropping the rpm below 2000 now causes the engine to quit, but above 2000 it runs well.

I changed the spark plugs, replaced the fuel filter and added Marine Formula Sta-Bil Ethanol Treatment to the gas.

Are there any other suggestions what could cause this sudden problem, tests to try, and solutions?

 

[Don S]

Re: Engine misfiring and rough idle

I changed the spark plugs, replaced the fuel filter and added Marine Formula Sta-Bil Ethanol Treatment to the gas.

Was that before or after the problem showed up? It really sounds like you have water in the carb.
It's almost manditory for marine engines to have water separating fuel filter kits installed. The filter in the pump and carb only keeps rocks and sand out of the carb, water goes right on in.

 

[Ripple17]

Re: Engine misfiring and rough idle

***** Additional Information ******

1998 Maxum - Mercruiser 3.0, owned 2 months. First time out it ran great all day.
Second time and had no problems for 2 hours, then smelled electrical smoke and heard knocking sound - engine lost rpm while crusing (from 3200 to 2800). It made it the last 2 miles back to the docks. Oil pressure and temperature guages didn't move from normal. Dropping the rpm below 2000 now causes the engine to quit, but above 2000 it runs well.

After returning changed the spark plugs, replaced the fuel filter and added Marine Formula Sta-Bil Ethanol Treatment to the gas.

Found the Ignition Wire (purple) on the engine top, bare with melted insulation. It shorted and blew the 15 amp fuze twice in the next couple of days. Took the boat out the third time - maximum rpm was 3200 and rough idle.

Installed a new ignition module - no performance change.

Verified that the spark plug wires and plugs are getting power with a spark wire tester. 2 of the new plugs still look new after couple hours of running and pulling the two plug wires doesn't change performance. The timing white mark is way out of range on the strobe.

Could the shorted ignition wire throw off the timing?

 

[Ripple17]

Re: Engine misfiring and rough idle

1998 Maxum - Mercruiser 3.0, owned 2 months. First time out it ran great all day.
Second time and had no problems for 2 hours, then smelled electrical smoke and heard knocking sound - engine lost rpm while crusing (from 3200 to 2800). It made it the last 2 miles back to the docks. Oil pressure and temperature guages didn't move from normal. Dropping the rpm below 2000 now causes the engine to quit, but above 2000 it runs well.

After returning changed the spark plugs, replaced the fuel filter and added Marine Formula Sta-Bil Ethanol Treatment to the gas.

Found the Ignition Wire (purple) on the engine top, bare with melted insulation. It shorted and blew the 15 amp fuze twice in the next couple of days. Took the boat out the third time - maximum rpm was 3200 and rough idle.

Installed a new ignition module - no performance change.

Verified that the spark plug wires and plugs are getting power with a spark wire tester. 2 of the new plugs still look new after couple hours of running and pulling the two plug wires doesn't change performance. The timing white mark is way out of range on the strobe.

*********** Update 5/23 *************
I found out more about doing timing and realized why my results seamed so far off:

• Unplug the shift interrupter switch leads from the engine wiring harness leads.
• Plug engine wiring harness leads together.
• Install the Timing Jumper Lead.

So I took the boat to a mechanic and was told that I had a blown head gasket.

Can a shorted ignition wire cause a blown head gasket or are the two problems happening at the same time just a coincidence?

Also, since that I just bought this boat is there anything that the Mercruiser 1999 bulliten about blown head gaskets would concern me?
They suggest to have the initial ignition timing retarded to 1? ATDC.

 

[Don S]

Re: Engine misfiring and rough idle

Here is the service bulleting regarding the blown head gasket and timing change. Make sure you go by the serial number of your engine.
View attachment SB 98-14.pdf

Here is some more info on setting the timing.

View attachment 3.0 EST timing.pdf

 

[Ripple17]

Re: Engine misfiring and rough idle

Conclusion: (I hate it when people leave thier problem without a solution.)

The mechanic replaced the head gasket and I picked the boat up today. He said that the boat had a gasket replaced previously but the head wasn't torqued down enough, which caused it to blow. He also said that the plugs and timing were correct.

I put the boat on the water today - It ran an idled better than I have ever seen before. Just in time for boating season!

Thanks for the interest.