1997 Mercruiser 5.7 EFI - TBI engine, just shy of 1300 hours on it. This is not a Vortec engine. It recently developed a hesitation/bog/backfire upon acceleration. It occurs after the boat has been running for 45 minutes to an hour. At low speed, I can feel the engine running mildly rough (no one else notices it but me) and when I accelerate to plane off, it will stumble/backfire. Once the boat planes off, it is fine and does not backfire.
I thought the fuel pickup tube may have had a crack as once I filled it up, it ran perfectly until the fuel level got around 3/4 of a tank. I pulled the fuel pickup and it was fine - there was no cracking or holes of any kind.
Thinking it may be ignition related, I swapped the coil from a Chevy 350 which has a 350 TBI and HEI ignition and there was no change. I then put the original coil back in and inspected the coil wire from the coin to the distributor. The coil end had some black crud in it which I did not recognize. It could have been dried dielectric grease. I cleaned it out, flushed it with brake cleaner and wiped it out. A light coat of dielectric grease on the coil terminal and I installed the coil wire.
I put the boat in the water and it ran fine for about an hour, then stumbled once and did not do it again for the rest of the time on the water. I checked the temp. of the coil with an IR thermometer and it was between 110 and 135 degrees which was comparable to the intake manifold temperature. I replaced the spark plugs (AC MR43T) last season and they have only 100 hours on them.
I replaced the fuel water separator last fall when prepping the boat for storage and I haven't pulled it to see if there is any crud or water in it but I'd think if that were the case, the engine would act up constantly, not wait until it was running for 45 minutes to an hour.
Searching the archives, there were many posts with stumbling/backfiring but those were constant on acceleration, the backfiring did not go away once the boat planed off.
What should I check next? Distributor cap & rotor?
I thought the fuel pickup tube may have had a crack as once I filled it up, it ran perfectly until the fuel level got around 3/4 of a tank. I pulled the fuel pickup and it was fine - there was no cracking or holes of any kind.
Thinking it may be ignition related, I swapped the coil from a Chevy 350 which has a 350 TBI and HEI ignition and there was no change. I then put the original coil back in and inspected the coil wire from the coin to the distributor. The coil end had some black crud in it which I did not recognize. It could have been dried dielectric grease. I cleaned it out, flushed it with brake cleaner and wiped it out. A light coat of dielectric grease on the coil terminal and I installed the coil wire.
I put the boat in the water and it ran fine for about an hour, then stumbled once and did not do it again for the rest of the time on the water. I checked the temp. of the coil with an IR thermometer and it was between 110 and 135 degrees which was comparable to the intake manifold temperature. I replaced the spark plugs (AC MR43T) last season and they have only 100 hours on them.
I replaced the fuel water separator last fall when prepping the boat for storage and I haven't pulled it to see if there is any crud or water in it but I'd think if that were the case, the engine would act up constantly, not wait until it was running for 45 minutes to an hour.
Searching the archives, there were many posts with stumbling/backfiring but those were constant on acceleration, the backfiring did not go away once the boat planed off.
What should I check next? Distributor cap & rotor?
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