Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

Searay88 · Apr 23, 2011

Its a carburated engine with electronic ignition.
Boat runs great until after about 1 hr when it starts to sputter and spit. If I bring it back to idle it seems to be OK for a short while, then a few minutes later stalls out. Once the problem starts and I go to idle, if I accelerate again it sputters and stalls.

It seems to happen only after about 1 hr run time. If I leave it to cool down completely it runs fine again for 1 hr, then the problem return.

This issue has happened the last 3 times I have used it. After the first 2 times I took the boat in for service. I now have new plugs, cables, distributor cap, water pumps. I checked the following and they are OK:
- anti-siphon valve - OK
- fuel filters (in tank, in fuel pump, and at carb) - OK
- gas tank vent - OK
- water fuel separator (no water in gas) - OK

Any suggestions? Could it be the coil getting hot and acting up? or carb acting up after 1 hr? Thanks for any suggestions you have.

Don S · Apr 23, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

When it stops running, you need to check it for spark immediatly. If no spark you have an ignition problem
Also remove the flame arrestor and operate the throttle and see if fuel squirts out of the accelerator pump holes. No squirts, no fuel.

Searay88 · Apr 23, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

Thanks Don - I will check for spark and fuel when I go out and it happens again and will let you know what I find out.

6meter · Apr 23, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

When it acts up, put your hand around the coil. If its to hot to keep it there, could be you problem. Would also follow Don S suggestions.

ussoldier_1984 · Apr 23, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

that sounds exactly like the issue I had the end of last season. From everything I gathered using the diagnostic chart for thunderbolt IV it was the ignition module. It was breaking up when it got hot causing spark to go nuts, But listen to the other guys they are good and know what they are talking about.

fsjcowboy · Apr 24, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

If you have a LOP switch, it could be failing. When this switch activates, it kills your ignition. Try bypassing the LOP switch (not your oil press. sending unit),take your boat out for a test ride and see if you get any different results. When looking at the switch, notice if it is dripping any oil.....another sign of a failure.

Happy Easter and happy boating!

Don S · Apr 24, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

What's an LOP switch ?

fsjcowboy · Apr 24, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

Low oil pressure

Searay88 · Apr 29, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

I had the boat out this afternoon for some dianostics. I insulated the fuel line just in case it was getting hot and causing vapour lock - this doesn't seem to be the issue.

I ran the boat at 3000 - 3500 RPM. After 25 minutes is started to sputter again. I pulled the throttle back and it would run smoother. If I accelerated it would sputter again, giving off a popping sound. The engine never failed completely. I idled it back to the dock and kept it idling for 3 or 4 minutes. At the dock I could rev the engine up to 3500 RPM with no sputter. I assume this was because there was no load on the engine. I took it out on the water again 10 minutes ater and after 1 minute at 3000 RPM is started sputtering again. I idled it back to the dock a 2nd time. The engine never failed.

As a side note the coil was not hot and the spark seemed fine.

I am thinking it might be a fuel pump or the fuel itself. Tomorrow I will empty the contents of the water separating fuel filter again to see if any signs of fuel issues.

Any other suggestions on what I should check?

Bondo · Apr 30, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

Any other suggestions on what I should check?

Ayuh,... The gas tank Vent...

6meter · Apr 30, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

Exact same symptoms I had. It was the coil. The second time it happened my coil was HOT, Somehow I don't think the first time the coil was as hot, original coil.

dubs283 · Apr 30, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

i think you are on the right track with it being a fuel issue

have you checked fuel pressure AND vaccuum??

are you using premium non-oxy fuel??

when you say you insulated the fuel line from getting hot, how hot is hot?? is it 90 and humid where you are?? or 60 and dry??

when you say spark seemed fine are you testing it at the coil with a spark tester??

wouldn't hurt to have a FULL tune-up too, you got about 80% of it with plugs, cap, wires, and filters...a new rotor is in order and if the ignition sensor is anything but the newest style, replace that too, it WILL fail

Searay88 · May 1, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

Update: This morning I removed the ignition coil to do a bench test on it.

Mercury Thunderbold IV model 392-7803 manufactures specs call for 9400-11700 ohms between center terminal and either side terminal - I got 10100 ohms so that's good.

Manufactures specs between the side terminals call for 0.6 to 0.8 ohms - I got 1.4 ohms....too high?

Does anyone know if this discrepancy means the coil is on its way out?

Side note: I flushed the fuel tank. The tank was clean. Engine continues to sputter after warmed up, with no power increase when I put the throttle down. I limped it back to the dock again yesterday. Maybe the coil is the issue when its warm.

ussoldier_1984 · May 1, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

I went through all the steps you have done, like I said last season my boat was doing the same thing yours did. and it was the ignition module getting hot and breaking up causing spark to go nuts causing backfireing and running like crap after about 30 mins, anything over half throttle. ask someone to post the flow chart for diagnosing the thunderbolt IV ignition system. It will show you exactly how to test to see what is going on from the coil to ignition module

http://www.marinemechanic.com/site/page251.html

fsjcowboy · May 3, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

Hey Searay88. I'm here to say that I gave you some misleading info regarding a possible low oil pressure cut-off switch malfunction. This switch does not affect or 'kill' engine ignition. Low oil pressure or a faulty low oil pressure cut-off switch will affect dc power to the fuel pump. So sorry for any confusion. That's what a bad memory can do.

Searay88 · May 8, 2011

Re: Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

Update to say good news today - ran the boat for over an hour and no issues. I just might have resolved the problems. Not sure of the exact cause but here are the things I did which may have contributed:

- Replaced the ignition coil and cleaned all the connections to the coil.
- Found a loose engine breather tube which is on the cylinder cover which may have been venting some exhaust directly below the ignition module. This could have resulted in overheating of the ignition module.
- Drained the fuel tank and refilled with new fuel (I don't think this was much of a contributing factor thought).

Thanks to everyone who offered their suggestions. I will let you know if the problem returns.

Mercruiser 4.3L 1988 I/O sputters and spits after 1 hr of run time

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