so i buy a slightly used (140 hrs) boat, very clean...has 2 4.3efi merc engines, drive it around the lake near where i live, everything checks out....change out all the filters, gas, oil, outdrives, etc....take it to Lake Powell, hot there, day temps around 100 F, everything goes well first day....second day, i notice one engine tachometer fluctuating from 3800 to as much as 6000 rpm...not the engine itself, just the tach, no problem i figure, just a connection somewhere...short time later i start getting some serious pops (backfires) out of the throttle body, pull power back, backfires go away, power back up, runs for about 5 minutes, then same thing....runs good at idle and up to 2400 to 3400 rpm, then problem repeats itself depending on how long its been running...runs good in the morning when its cool, but as day gets warmer, problem repeats....anybody with 4.3 or similar problem on another engine have experience that might give me a place to start ?....i'm thinking electrical, but not plugs, wires, or rotary cap because for first hour engine runs great, gets full rpm, but after cruising for hour or so, problem begins as described...any ideas ?
4.3 efi mercruser engine
- Thread starter slyrydr
- Start date
premium fuel from name brand station...thanks for input though
has distributor, backfiring reminds me of a lean condition, but it's definitely related to temperature of running it for 45+ minutes 
has distributor, the coil comment has merit i think, will be looking at that angle....the fuel pressure also...thx
The backfire is being caused by the engine being out of time. Either the distributor timing is way off, you have a worn distributor or possibly it has jumped time on the timing chain. In any case, backfires on a boat are a good way to blow the whole thing up.
yup....that's what i'm trying to prevent
J
Jaynelson
Well-known member
I would agree on the timing. I have heard of guys trying to advance timing on those motors....maybe someone has done that and gotten carried away? Maybe it's on the edge and presents itself more when hot? Total guess.
On another note....I have that same motor in my boat and the tachometer does the same thing, but never had a running issue. Some times if it's acting up I can tap it and it fixes itself. So the 2 issues may be completely unrelated.
On another note....I have that same motor in my boat and the tachometer does the same thing, but never had a running issue. Some times if it's acting up I can tap it and it fixes itself. So the 2 issues may be completely unrelated.
Or "She done blowed up" in NASCAR speak.
Backfiring is usually caused by bad ignition parts. I'd scan the computer first, expecting to see codes between 300-306 if your boat is using the odb2 system. I'm not a boat guy so have no idea what system they run. Plugs, wires, cap and rotor would be where I'd start if there were no codes or only the 300-306 codes are present.
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thanks guys for the inputs, i'll be trying to run them all down 
finally figured it out...replaced all ignition parts, plugs, distributor, coil, modules etc, expensive also cleaned tank. fuel lines, filter, checked pump etc,,,every thing i could think of....no luck..because the problem was associated with some heat, and rpm and running across a thread on a boat forum, i disconnected the lead from the coil to the tachometer (source of rpm protection for engine)....FIXED....engine runs good, never would have thought the engine rpm limit protection was in the tach....had to open my file for things i hadn't experienced before.....
also cleaned tank. fuel lines, filter, checked pump etc,,,every thing i could think of....no luck..because the problem was associated with some heat, and rpm and running across a thread on a boat forum, i disconnected the lead from the coil to the tachometer (source of rpm protection for engine)....FIXED....engine runs good, never would have thought the engine rpm limit protection was in the tach....had to open my file for things i hadn't experienced before.....
I would imagine that the tach was simply shorting out the coil not causing the engine to hit a "rev limiter".
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