Ford 5.0 overheated, now seized.

1993 Four Winns, carb'd 5.0 Ford, OMC Cobra drive.

The boat had been running a little hot according to the gauge (~180-200, maybe a little more at idle/low speeds), but everything had seemed fine, and whenever it creeped up to near 200 while idling it usually came down to the 180 range once on plane. Impeller looked great. Suspected thermostat. Planned on figuring it out eventually but didn't seem to be pressing, and thought it may just be an inaccurate gauge.

Today (90 degree weather), I had it idling after I launched it and was parking the car, and it crept up to around 200. I got underway, thinking it would come down once I got on plane, and it crept up to the top of the gauge (240). I was worried, but again, was not entirely convinced that the gauge was accurate; I proceeded to throttle up and plane the boat for about a minute, but the temp did not come down, and I noticed a burning smell. I quickly took it down to neutral and shut the boat off. Lifted the engine cover and white smoke came out (not tons; a decent amount, but not billowing by any means).

Drifted in the channel for a bit with the engine cover up and blower on, allowing it to cool. We started to drift into a dock, so I tried to fire it up for a quick maneuver, and the engine wouldn't turn over. Just a quick chug then nothing. Thought maybe it was vapor locked or something; so I tied up the the dock and waited for about 20 minutes and tried again; still wouldn't turn over. Just a quick chug, then nothing.
Waited about 45 more minutes, tried it, same thing, except I got a second chug out of it. Waited a bit more and tried it, and got a few more chugs out of it, but the condition never improved past this point even after several hours. Ended up getting towed back to the launch ramp and bringing the boat home.

So that's where I'm at. The engine seems at least partially seized, though the starter can turn it a little. Oil looks fine on the dipstick.
I plan on pulling the spark plugs, seeing if there's any water in the cylinders(hydrock), spraying PB in them and seeing if I can crank the engine by hand.
Any ideas/opinions on what else I can do or check? Does this sound like hydrolock or seized due to overheating?
How likely do you think it is that my engine is toast?
Is it possible that the heat just caused expansion and therefore friction, without doing any serious damage?

Any opinions are appreciated,
Thanks.


P.S. After doing some googling, I realize that I'm an idiot for trying to run the boat with the gauge showing the temp that high, and probably stupid for trying to crank it after it overheated, but I'm a bit of a newbie and didn't know that overheating could cause such serious issues.

P.P.S. While waiting for the boat to cool off, I pulled the thermostat housing, and it looked pretty old/corroded, so I'm thinking that was probably the cause of the overheating.

Re: Ford 5.0 overheated, now seized.

You've already realized your error in failing to heed your gauges. I'm sorry you have trouble.

I suggest you carefully check your exhaust manifolds and risers for debris, sand and corrosion. Even if the t-stat is OK a clogged discharge can impede cooling and overheat the engine.

Re: Ford 5.0 overheated, now seized.

Ayuh,.... Pull the sparkplugs, 'n turn the motor over on the starter to expell any water,...
Hope it ain't Saltwater....

If it don't turn over with the plugs out,... Pull the drive....

'n,... the 1st place to look, in Any overheat situation, is the Impeller,... Change it....

Re: Ford 5.0 overheated, now seized.

DBreskin said:

You've already realized your error in failing to heed your gauges. I'm sorry you have trouble.

I suggest you carefully check your exhaust manifolds and risers for debris, sand and corrosion. Even if the t-stat is OK a clogged discharge can impede cooling and overheat the engine.

Click to expand...

Thanks, I know I did it to myself though Can't tell you how upset I am. It's my first boat, I've been working on it every day for the past 2 months, thought I was pampering it, and now I think I may have ruined it with a stupid mistake. I always work on my own vehicles but I don't really know that much about engines... had no idea that overheating could cause this much damage or I would have taken the gauge much more seriously.

Bond-o said:

Ayuh,.... Pull the sparkplugs, 'n turn the motor over on the starter to expell any water,...
Hope it ain't Saltwater....

If it don't turn over with the plugs out,... Pull the drive....

'n,... the 1st place to look, in Any overheat situation, is the Impeller,... Change it....

Click to expand...

This is exactly what I plan on doing, thanks. It is freshwater, thankfully.
When you say pull the drive, is that because the engine will turn over more easily with the drive removed, or because you suspect the drive may be seized, or both?

Re: Ford 5.0 overheated, now seized.

I overheated my boat just a touch too much last year.
Impeller blades let loose.
When hot the riser gaskets failed.
I got it all back together, had it run for a minute.
Then it ran rough.
Stalled and would not start.
Riser leaks let the center cylinders fill with coolant water-hydrauliced.
Not that you are going to feel "lucky" but maybe... you could be in same situation.
I retreived my impeller blades, replaced impeller, replaced riser gaskets and exhaust couplers since those got close to melting.
Got all the water out of cylinders and was good to go again.
Really wished I had not been so mean to my starter prior to figuring it all out.

Good Luck!!

Re: Ford 5.0 overheated, now seized.

Thanks - I just pulled the spark plugs and water came out of cylinders 3 and 5. Motor cranks fine now, I cranked it 8-10 times to expell the water.
What should my next step be? Put new plugs in and try to run it? Should I change the oil or anything first?
If I do try to run it, I will remove the thermostat, as by the looks of it I think that may have been why it overheated.

Re: Ford 5.0 overheated, now seized.

Your going to take the drive off anyway so I would do that and start it dry. let it run about 15 seconds to dry it out. Then do your tests.

Re: Ford 5.0 overheated, now seized.

I'd want to take off the risers and see if you can tell if the water got in that way, you would see rust trails down the center section of the exhaust manifold and if so, replace the riser gaskets on both sides. Also look carefully at the sealing surface for the exhaust manifolds and risers to make sure they are not eroded by rust. Make sure the cooling passages are not blocked by rust either. While you are at it, look at the rubber exhaust hoses and see if they are burned. I'd also look down the exhaust Y pipe to see of the exhaust flappers melted and fell down and could be blocking the exhaust. Once you go through the whole cooling system, start it up and change the oil. Most OMCs came with a 160* stat. That means it should run from 160-175 on the gauge. 185+ is definitely too hot.
If you still have heating problem, you need to look for water supply restrictions. Put the boat in the lake, then temporarily disconnect the hose that goes from the transom mount to the thermo housing. Have someone hold that hose up while you briefly start the engine, the water should shoot up at 2-4". If not you have to look at the cooling intakes on the drive, the raw water intake hose and power steering cooler.

When I change my impeller I do this test: remove the housing, then hook up the muffs. Next turn on the water and sqeeze the muffs tight to the drive. Water should come out of the right side opening in this pix. Next I disconnect that raw water intake hose on the thermostat, put a hose in it and turn it on, water should shoot out of the left side opening. You need water in, water out and a good impeller to start with.

Re: Ford 5.0 overheated, now seized.

How do I get to the Y-pipe to inspect the flappers? I don't even see it anywhere.

Re: Ford 5.0 overheated, now seized.

you remove the risers, then remove the rubber exhaust tubes and then look inside, the Y pipe is what the rubber exhaust tubes are attached to. If the flappers melted they can block the exhaust. In this pic, the Y pipe is just below the black rubber tube...

Re: Ford 5.0 overheated, now seized.

Thanks a lot!
Just ordered riser gaskets, a new thermostat and a water pump kit.

Sprayed oil fogger into the spark plug holes and checked the compression; all cylinders were 140-170psi.

Hopefully once I install these new parts and get everything back together, it will run normally...