4.3l mercruiser engine hydraulic locked

I have a 1998 Bayliner Capri with a carbureted 4.3L. I'm the second owner, I've owned & used it for about 12 years. I bought from my Aunt - she & my uncle were the original owners, so I know it's history from new. It has always been well-maintained & is overall in great condition. However, it has always had one nagging issue - once its warmed up it has a tendency to run-on or diesel when you shut it off. According to my Aunt it's done it since new. A couple shops have looked at it but never been able to get it sorted out. It's always been more of an annoyance than a serious problem, so I've lived with it.

Other than that it has always been very dependable, never a single breakdown. You can use it hard all day & the engine temp never gets over 170.

This week I shut it off in the water & it did it's usual run-on routine; when I tried to restart the engine would not turn over, like it was locked solid. Had to get towed to shore...I assumed a starter issue so pulled that this morning. But the starter is fine, bench tests OK & the gear teeth look new. So I tried to turn the motor over using a breaker bar with a socket on the crank snout & it would barely budge. Pulled the spark plugs & tried to spin it again. I got water out of 4 cylinders & now it spins freely. So it was hydraulic locked.

My questions are:
1. What the heck happened? My theory is that when it did it's usual run-on routine that somehow the motor went reverse rotation & sucked in water? Note it was definitely water, not fuel.
2. What do I do now? My plan is to shoot a little wd-40 in each cylinder & turn it over by hand a few times; then I'll leave it overnight & reassemble in the morning. Try start it & hope for the best. Anything else I can do?
3. Any thoughts on how to cure the run-on?

Admittedly I'm not a boat mechanic, but I'm a muscle car guy. I've built many car engines & have a pretty good grip on how engines work. I also know that hydraulic locking an engine can be really bad & can bend things like pushrods, connecting rods, etc...Now that I have the water out of it it spins nice & free so I'm hoping nothing catastrophic.

Thanks!

too high an idle
too lean a fuel mixture
too poor and octane
too hot an engine temp
too much carbon on the pistons
incorrect timing

Yep. I have heard that Merc rigged 4.3s do this but I've had an OMC rigged one with the Quadrajet and it has NEVER dieseled, not even once. Owned it over 15 years too. I always use premium gas, that might make a difference.....

Pull off the exhaust downspout most likely the flappers are missing. They prevent water from flowing up the exhaust into the engine. What ever side the water was found in. Most likely the flapper are missing on that side.

Bt Doctur said:

too high an idle
too lean a fuel mixture
too poor and octane
too hot an engine temp
too much carbon on the pistons
incorrect timing

Click to expand...

I would go with this list.

Hey, thanks for all the replies.

Put it back together this afternoon. I cranked it with the kill switch off (so I know it wouldn't fire) and it turned over just fine. So I flipped the switch & cranked it. Fired right up, sounds fine. I only ran it for a few seconds tho - I figure I'll change the oil before I really run it, just to be on the safe side. It's due anyway.

Once I change the oil I'll tackle the list posted by Bt Doctur.

FWIW, I got water out of cylinders on both sides. I didn't even know there were exhaust shutters, so I'll be checking those as well.

I've tried every type of gas out there. Where I live the most common octane levels are 87/89/91. Some places have 92 & a select few 94. I've tried them all, with no difference in performance & no change with the run-on issue. Typically I use 89, and every third fill up I use 91/92

Look at the oil and see if it looks like a chocolate milkshake. If so, there's water in the oil.

Jeff

Jarretts70 said:

Hey, thanks for all the replies.

Put it back together this afternoon. I cranked it with the kill switch off (so I know it wouldn't fire) and it turned over just fine. So I flipped the switch & cranked it. Fired right up, sounds fine. I only ran it for a few seconds tho - I figure I'll change the oil before I really run it, just to be on the safe side. It's due anyway.

Once I change the oil I'll tackle the list posted by Bt Doctur.

FWIW, I got water out of cylinders on both sides. I didn't even know there were exhaust shutters, so I'll be checking those as well.

I've tried every type of gas out there. Where I live the most common octane levels are 87/89/91. Some places have 92 & a select few 94. I've tried them all, with no difference in performance & no change with the run-on issue. Typically I use 89, and every third fill up I use 91/92

Click to expand...

Don't wast your money on higher octane fuels. Your engine will make more power on 87 octane.

The exhaust shutters are not going to stop this from happening if your engine runs in reverse when shutting it down.

You can decarb the combustion chambers with some seafoam sprayed in the plug holes and let it sit over night.

This will also help. If you let your engine run at the dock for a few minutes to cool the combustion chambers before shutting it down.

Decarb your engine and you will see an improvement.

Took the exhaust apart today & discovered all that is left of the shutters is the metal pin & a little bit of melted crud...have to order parts!

Now that I know about the shutters I think it's more likely I got water in the cylinders that way, rather than reverse rotation. Either way I'm going to try get the run on resolved once & for all.

As fr as decarbing the cylinders - is the overall consensus Seafoam or Mercruiser power tune? On some of the old cars I've dealt with we just ran a bit of water down the carb with the engine running & to be honest that was my plan, along with a can of Seafoam in the fuel tank.