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

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

WHat you need to do first is not bring it back to whom ever replaced the """"new plugs, cables, distributor cap, water pumps""" for a running out of gas symptom. Unless you wanted all that done then disregard my comment.

What you need to do second is a bit more diagnostics.

When this occurrs again, first thing to do is remove the flame arrestor, look at the carb,

1. is the choke open or closed?
2. If choke is closed then that is your problem.
3. if open then activate the throttle and see if gas squirts into the carb? ( motor OFF/not running )
4. if no gas squirting then your carb is out of fuel, (rebiuld the carb) you should see good streams of fuel each time you activate the throttle.
5. if you have fuel squirting then restart engine, It may take a bit as you have now flooded it......
6. throttle up and when it happens again what I would do is say at 3000 rpms ~ 25-30mph and it starts to act like it is running out of fuel then quickly move the throttle (shift handle) back and forth (as if you were pushing on a gas peddel in your car but very quickly) this will do the same thing as step 3 but while the engine is running. If the engine picks up rpms/speed and the engine dieing temporaraly goes away it is a fuel delivery problem either the fuel lines/filters/tank pick up (something happens to be floating and clogging the pick up) or carb float issue and a carb rebiuld is in order or your fuel pump is not providing enough fuel.

Those are the options and things I would most likley do.

You need to check if the float needle valve in the carb is staying open and flooding the engine while you are running. When the engine stalls remove the spark arrester and look down the carb throat. You will see vapor and it will be very wet with fuel.
What carb does this engine have?

Thanks for advice kghost.
Replacement of plugs, cap, water pump etc were all needed as they were all original (20 yrs old).
I will work through your diagnostics when the problem occurs and let you know how it goes.

One more thing!!!

Do you have any water in the fuel tank??? This would also cause this condition!

Plenty of "fuel" but not enough gas in it............

Me i would run it on a external tank of clean fuel. Run it for say 2 hours if good then you know it's gunk and junk in the tank if yes pump the chit off the bottom of the tank.

How old is the gas in the tank? Does it have ethanol in it? I would agree with the Chief, run it off an external tank. I had this same problem last year, turned out to be water in the tank.

I had a very similar problem years ago on a 165hp inline 6. The ignition coil was going bad when it got hot. Ran fine cold and at some point sputtered and backfired and sometimes died completely.

I might be wasting your time, but I had a similar problem with my motorcycle engine just a while ago. I thought it might be the coil breaking down after getting hot but it was actually the CDI unit. Fortunately my bike was a twin so I could test the CDI as both sides were siamesed. Different resistances gave me the heads up. A new CDI later and its now running better than before (probably due to pulling the carbs down and cleaning everything before working out it was the CDI!) Anyway, please take my bit of advice as a second option as mine WAS for a motorcycle engine and not a marine engine.
Good luck with it.
Scott.

I want to rule out the gas before we get into the dist. I don't think it's the coil.

That just my opinion take it for what it's worth free advice ain't worth what you paid for it.

Heres my thoughts, 88 boat shaft in the dist loose ? Before we tear into the dist. lets go with whats below.

Could be the hose breaking down from this new f---ing gas. Been seeing it more and more.

So if this boat doesn't have a fiberglass lined tank " Hopefully no " Then the hose could have a flap on the inside that blocks the gas flow.

What i think is happening is the gunk clogs a hose or the pickup boat stops. Chit falls off the filter and the process repeats itself.

You say the vent is clear " For sure " ?

Again run it on a external outboard type gas tank with fresh fuel for over a hour.

It runs good say then we ruled out the carb. And narrowed it down.

Maybe all new hose pump the tank dry ?

First lets narrow it down.

Just how i would go about it, others here have 5 times better mechanical minds then myself.

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 also checked all the fuel filters, fuel pick-up in the tank, and tank vent - no issues there.

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.

As suggested by kghost I followed his item 6. below: I throttled up and when it happened again I quickly moved the throttle (shift handle) back and forth. The engine did seem to pick-up rpms/speed briefly. The motor didn't die but as it started severly sputtering I brought it back to idle to head back to the dock. When I would accelerate it would sputter more.

I am thinking a fuel delivery problem either the bad fuel, fuel pump issue, or fuel lines/filters/tank pick up.

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 later and after 1 minute at 3500 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.

Tomorrow I will empty the contents of the water separating fuel filter again to see if there are any signs of fuel issues.

Any other suggestions on what I should check?

Check the psi the fuel pump is giving you no less then 4 psi. no more then 7.

Try running it off an external fuel tank. It's very simple and I bet it will fix your problem. If it was a bad fuel pump it would run rough right away, wouldn't it? Just emptying the fuel water seperator is not enough.

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.

I agree. Coils can fail intermittently when the get hot. Change the coil they are cheap enough.

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 though).

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

That is good news. I would've gone with the water in the gas theory since that is the most common problem especially this time if year. Happy boating.

Great news pugetsounder!
It could of been the coil or the CDI. The CDI gives those exact symptoms but that is when its on the way out. Heat will definately stuff up both the CDI and the coil so keep heat away from them.
Good news and good work!
Scott.

Sorry.
That last post was for Searay88. Sorry for the confusion pugetsounder.
Scott.