Overheating nightmare!

[stephentyler20]

Engine/drive details: Mercruiser 5.0 Litre V8 (GM 305) mated to Alpha 1 drive.

So this problem technically started last summer. I would start to run hot at low speeds (such as docking or chugging out of the harbor - very troublesome), but as soon as I increased engine speed it would cool right down.

I changed the thermostat, replaced a hose I thought might be bad, but nothing fixed it. Finally, I prematurely changed the water pump (impellar was only a couple months old and looked fine on inspection), and for some reason, that solved the problem. So I thought.

This winter, I redid all the seals on the upper unit, and threw in some new gears. Pretty big project, but went well. The boat runs fine... Except the overheating problem is BACK! Same exact symptoms, only this time, I changed the water pump and it didn't go away.

The only thing I found is that the seal where the copper tube goes into the upper unit was broken. I'm assuming this is bad, and maaaaybe lets some water leak out so it's not cooling as well under low rotation speeds, but I'm dubious. Obviously I'm sourcing out the part and will replace the seal post-haste to give it another go.

Any other thoughts? Is there a chance my engine idle is just too low, or is that unlikely? Today on the water it was starting to heat up, so I put it in neutral and just revved it a bit, and it cooled right back down instantly... Anything else to try? The only plus here is I'm becoming a whiz at changing the water pump!

 

[ziggy]

Re: Overheating nightmare!

is that the seal where the copper tube goes into the upper unit was broken.

Any other thoughts?

that sounds like a waterpocket cover. if so. use extreme caution when removing the 4 ss bolts. they have a tendency to snap right off.
i do think that a water pocket cover leak can cause an overheat condition. best i understand it is that hot exhaust water and gases are escaping the drive going right by the water pipe and water pocket cover. if there's a leak at that point it reintroduces hot water and hot gases back into the system. that's the way i get it anyways. others might be able to clarify the reason.

like i said. be real careful removing the bolts (stainless steel bolts mounted into cast aluminum). you'll have a whole different problem as they start to snap off. my 1 drive snapped off 3 of 4. was a real project to get the broke ss studs out and ended with helicoils..

 

[MikDee]

Re: Overheating nightmare!

I don't know what a waterpocket cover is? so I'm curious.

Anyway, when a Mercruiser is overheating I always mention to check the main raw waterfeed hose up behind the drive, where it goes into the boat from the back (thru the transom plate) and make sure it's not kinked here on the bend! I've had this happen on more then one older Mercruiser. The replacement hose is a special type, & length, Mercruiser part.

 

[badger42]

Re: Overheating nightmare!

It would seem to me any small leak or crack on the raw water intake line (copper connector) could cause some problems when only a little water is being drawn up (low rpms). Kinda like a tiny crack in a soda straw; if you suck really hard (new water pump or high rpms) you'll get water up to the engine to cool it, but not if it's only pulling a small amount- you'll only get air.

 

[ziggy]

Re: Overheating nightmare!

I don't know what a waterpocket cover is? so I'm curious

it mounts to the bottom of the upper with 4 stainless steel 1/4-20 bolts, is made of rubber. it's the where the other end of the copper water pipe thats stuck into the water pump housing. it's the 'in' side for water flow through the upper. the 'out' flow side would be where the oring goes between the upper and the bellhousing then onto the waterhose.. lateeda

s/m #6
pp 3A-10
item 43

edit. for sure on checking the water hose for a kink. i've had that problem too...

 

[MikDee]

Re: Overheating nightmare!

it mounts to the bottom of the upper with 4 stainless steel 1/4-20 bolts, is made of rubber. it's the where the other end of the copper water pipe thats stuck into the water pump housing. it's the 'in' side for water flow through the upper. the 'out' flow side would be where the oring goes between the upper and the bellhousing then onto the waterhose.. lateeda

s/m #6
pp 3A-10
item 43

edit. for sure on checking the water hose for a kink. i've had that problem too...

Thanks Ziggy! Now I know what you mean, it holds the water tube in place, if you hafta take those bolts out, it sounds scary

 

[stephentyler20]

Re: Overheating nightmare!

Thanks for the responses guys...

In regards to the water pocket - I replaced that this winter when I was redoing the seals on the upper unit. I don't need to remove that to replace the busted seal - the seal just pops out the bottom. Interesting thought about the hot water/exhaust leaking back INTO the cooling system. That would definitely hurt. Also a good thought that any small leaks might affect low pressure draw.

Where I'm at right now is, I've obtained the new seal ($2, thanks local small-town marine shop!!!), and I intend to reinstall the lower unit tomorrow after double checking my water pump.
I will also check that intake hose to make sure its not kinked. I'll report back, but please if there's any other ideas throw them my way... Easy to check this stuff when the boats in my garage. Not so when its back at the dock!

 

[CharlieB]

Re: Overheating nightmare!

When checking your water pump, be sure that the lower plate and gaskets are flat and seal, so's the impeller cannot suck air at low RPM.

You could temporarily splice a short section of clear hose in the supply water line near the engine water pump and observe the water supply for bubbles indicating the raw water pump sucking air.

 

[Bondo]

Re: Overheating nightmare!

I will also check that intake hose to make sure its not kinked. I'll report back, but please if there's any other ideas throw them my way... Easy to check this stuff when the boats in my garage. Not so when its back at the dock!

Ayuh,.... While you have the Lower unit off,...
Stuff a garden hose into the hose coming From the outdrive, at the T-Stat Housing,+ Backflush it....
You just might flush some old bits of impeller from the P/Steering cooler...

 

[sarantis]

Re: Overheating nightmare!

and please report back your results.I've an overheating nightmare too and i' ve started suspecting the gauge or an electrical problem!Tomorow i'll put the temp sending unit in a boiler together with a thermometer and i 'll trace down its resistance at all range from cold water up to boiling water with a digital ohmmeter.After that i'll connect the sending unit with two long cables (of course their resistance will be counted in) that i 'll lead to the dashboard and connect there with the ohmmeter (they are very cheap) in order to have as accurate picture of the temp as possible.Together with the sending unit you may sink in the boiler your thermostat to check the point it opens.Good luck!

 

[PitchFork]

Re: Overheating nightmare!

My 1994 5.0LX was doing the same thing two years ago when I first bought it. The problem turned out to be the check balls where missing completely from the Design II thermostat housing. Not sure what year your boat is. 140? Thermostat right?

Diagrams at:
http://www.perfprotech.com/store/catalog/5.0LX--(4-BBL.)--GM-305-V-8--1988-1995,3172,model,816596-900.aspx

 

[stephentyler20]

Re: Overheating nightmare!

Alright guys well little update...

I replaced the busted seal I found, reinstalled the drive and ran it and noted the engine remains coolER, but still has trouble. It seems now that it has no problem running cool from a cold start, and no problem running cool at high RPM.

But when you run it at high rpm for a little while (Say on plane), and then slow down to a slow cruise, THAT's when it starts to overheat. So essentially, it's when the engine's hot and you slow down the fresh water source.

I forgot to check the raw water intake hose to see if it's kinked, so I'll do that next. I replaced the T-stat last year, but I'll check that too just in case. I also replaced the check-balls last year and improved their seal, but I'll double check them anyway because it's easy. I'm inclined to say the sending unit and gauge are fine because I'm noticing behavior-based changes, not random fluctuations. Next time I take the drive off I will definitely backwards flush the cooling intake hose to see if anything comes out, but I've never had a blown impeller.

One more thought... what about just removing the thermostat altogether? Wouldn't that keep the engine cooler all the time?

 

[MikDee]

Re: Overheating nightmare!

No, the thermostat is only a 145* unit, so it can't run much cooler! You did everything but check the most important thing, the raw water supply hose Outside the boat, turn the drive to the right, & raise it, then look underneath and to the left side of the transom for the hose, check the bend where it goes in to see if it kinks when you put the drive back down. You could take it off there, and back flush it from the inside.

Inside the boat, at the bottom of the transom plate is the continuation of the hose that's clamped onto a fitting (probably hard to get at), but if you follow the hose to the front of the engine, you'll see where it goes into the engine water pump,,, this is where you can take it off, & backflush it, Or, run the engine on the hose, to check for water flow.

 

[stephentyler20]

Re: Overheating nightmare!

No, the thermostat is only a 145* unit, so it can't run much cooler! You did everything but check the most important thing, the raw water supply hose Outside the boat, turn the drive to the right, & raise it, then look underneath and to the left side of the transom for the hose, check the bend where it goes in to see if it kinks when you put the drive back down. You could take it off there, and back flush it from the inside.

Inside the boat, at the bottom of the transom plate is the continuation of the hose that's clamped onto a fitting (probably hard to get at), but if you follow the hose to the front of the engine, you'll see where it goes into the engine water pump,,, this is where you can take it off, & backflush it, Or, run the engine on the hose, to check for water flow.

OK. We did replace the intake hose over the winter when we redid the bellows and gimbal bearing, so I'll take a look and see if we put a kink in it by accident.

When you say backflush from the intake hose near the water pump - the drive has to be off to do this, right? Otherwise i'll just flush garbage into the water pump and potentially eat up my impeller?

Also I did run it on the hose, and water flow appeared to be fine. Plenty of water coming out of the exhaust outlets, and the engine stayed cool, although that's easy to do when there's a higher water pressure at the source and no load.

 

[MikDee]

Re: Overheating nightmare!

OK. We did replace the intake hose over the winter when we redid the bellows and gimbal bearing, so I'll take a look and see if we put a kink in it by accident

So it could be a suspect, if it's not the right hose, too soft, or the wrong length (as in the service manual)

When you say backflush from the intake hose near the water pump - the drive has to be off to do this, right? Otherwise i'll just flush garbage into the water pump and potentially eat up my impeller?

Yes, or what I would do, is take off the raw water hose on the outside of the transom, then flush it, and while you have it off, put the muffs on it, and crank it, or run it just long enough to give it a quick flush from there, this way flushing it from both ends, then button it all up, and you should be good to go.

Also I did run it on the hose, and water flow appeared to be fine. Plenty of water coming out of the exhaust outlets, and the engine stayed cool, although that's easy to do when there's a higher water pressure at the source and no load.

Once under load, it all changes,,, I normally had no reading on my temp gauge on the hose, but under power it came into the propper range.

 

[stephentyler20]

Re: Overheating nightmare!

Just a quick update on this for posterity... After replacing the busted seal, and *carefully* reinstalling the lower unit, and doing NOTHING else, the boat seems to finally be running cool all the time. At worst, I'll notice small fluctuations in heat after slowing down from a long cruise, but that quickly levels back down to normal and never OVERheats.

Safe to say - problem solved!