Mercruiser 5.0 High Speed Overheat

[cookiesareyummy]

Mercruiser 5.0L raw water cooled with an alpha I drive. No pump on engine itself. I'm not sure of the year or model # but there isn't much more to the cooling system than what I already mentioned.

The engine runs fine at low speed but begins to climb at about 1800+. When I pull the speed down the temp will return to normal but not fast. The engine is getting hot, it is not just a bad sensor.

I already did the pump impeller twice as well as the pump base, the thermostat was done, the risers were removed and look brand new, the manifolds also look brand new. I checked the y-pipe for any blockage at the bottom but it is clear, the exhaust bellows doesn't connect to the drive so I don't have to worry about blockage in the drive area though I did look and it was clear. Power steering cooler is completely clean.

I was thinking about connecting a clear hose from the spot where the water enters the boat (don't know what that little plastic 90* thing is called) to the power steering cooler to see if there are any bubbles at high speed.

Any other ideas what this might be? Or any other ideas to diagnose?

 

[BIGALF]

Re: Mercruiser 5.0 High Speed Overheat

I had a problem with my Mercruiser 5.7 EFI and found out the problem was the water line from the outdrive to the engine was a little to long and it had a kink that restricted the amount of water being pumped to the engine. it worked fine at low RPM but it overheated at higher RPM's.

 

[dubs283]

Re: Mercruiser 5.0 High Speed Overheat

water line from the outdrive to the engine was a little to long and it had a kink that restricted the amount of water being pumped to the engine

definitely check that

also, when you replaced your impeller, did you just replace the impeller, the whole kit, impeller and gaskets??

do you have alpha one or gen II??

 

[wpowell323]

Re: Mercruiser 5.0 High Speed Overheat

try taping off the top 2 holes on the outdrive. Sounds crazy but i just fought this problem on a 2005 Monterrey. At WOT it was sucking air. We welded the top 2 holes shut and the problem was gone. Our problem was a little different because it wouldn't get hot until above 4000 rpm's, but a little duct tape around the top 2 holes is cheap enough to try

 

[cr2k]

Re: Mercruiser 5.0 High Speed Overheat

Look into the heat exchanger from the transom side and see if there are any remnants from an earlier impeller failure. This will restrict flow.

Usually clear tube is placed between heat exchanger and thermo house.

 

[cookiesareyummy]

Re: Mercruiser 5.0 High Speed Overheat

I did the whole impeller kit with gaskets and wear plate. The upper housing was fine, and it is a gen I. I will check that hose coming from the drive, I had it off and didn't see anything wrong with it but will look again when it is on for kinks. As far as the clear hose, I have a standard cooling system and therefore don't have a heat exchanger. I had the power steering cooler off and it was very clean, it wasn't the style with a screen but a large opening that really nothing could get stuck in.

 

[Brentathon]

Re: Mercruiser 5.0 High Speed Overheat

Where's all your boat info???
Year, what's been replaced, checked, condition/age of all components. etc.
I did a clear hose from PS cooler to T-stat housing, as well as clear hoses to manifolds and elbows. I was able to see flow aerate at high rpm's in hose going to T-stat housing. (turned-out to be a new, yet defective drive impeller; rubber was very stiff).
T-stat housings go bad, engine circ pumps fail,....

 

[soundview]

Re: Mercruiser 5.0 High Speed Overheat

try taping off the top 2 holes on the outdrive. Sounds crazy but i just fought this problem on a 2005 Monterrey. At WOT it was sucking air. We welded the top 2 holes shut and the problem was gone. Our problem was a little different because it wouldn't get hot until above 4000 rpm's, but a little duct tape around the top 2 holes is cheap enough to try

wpowell,

Which top 2 holes on the outdrive are you referring to? I have a similar problem I've been chasing and would like to try this with duct tape.

Thanks,

SV

 

[Don S]

Re: Mercruiser 5.0 High Speed Overheat

wpowell,

Which top 2 holes on the outdrive are you referring to? I have a similar problem I've been chasing and would like to try this with duct tape.

Doesn't matter, that's on a Gen II drive, and this thread is about the original Alphas.
Either way, if you have a question, either PM the poster that said that, or start a new thread of your own. Don't hijack someone else's thread.

Cookies:
Hook up a clear hose between the thermostat housing and the hose coming from the transom shield. I would be that around 1800 rpm you are going to start seeing air bubbles.

I already did the pump impeller twice as well as the pump base

Did you replace the base, or just reseal it? What did the gasket surface under the base look like?

 

[Bt Doctur]

Re: Mercruiser 5.0 High Speed Overheat

water pump base gasket sucking hot exaust or base gasket installed backwards.

 

[cookiesareyummy]

Re: Mercruiser 5.0 High Speed Overheat

I did the whole pump base not just the gaskets and seals. From what I remember it wasn't possible to put the base gasket on backwards as there is one hole in it meant for a peg and if it were backwards it wouldn't go on that peg. I cleaned off the gasket surface under the pump base and thought it looked alright. I still haven't hooked up that hose but am still planning on it. I'll keep updated when I do.

 

[cookiesareyummy]

Re: Mercruiser 5.0 High Speed Overheat

I decided to take it apart again since I really believe, and it seems like others do too, that it is something to do with the drive or pump. I took pictures though I don't know if there will be anything noticeable. I didn't see an o-ring between the upper wear plate and the upper plastic housing. I always thought these had one but maybe I am thinking of gen II.

 

[cookiesareyummy]

Re: Mercruiser 5.0 High Speed Overheat

Here are some more pics

 

[NHGuy]

Re: Mercruiser 5.0 High Speed Overheat

Turn on your camera flash. I'm getting eyestrain.

 

[cookiesareyummy]

Re: Mercruiser 5.0 High Speed Overheat

Turn on your camera flash. I'm getting eyestrain.

Haha sorry about that. This is actually a bore scope camera with the light on the end turned all the way up, no flash though.

 

[cookiesareyummy]

Re: Mercruiser 5.0 High Speed Overheat

I just thought I would post something else I noticed even though I'm pretty sure it doesn't really matter. The old pump base that I took out had a number 46-42579 stamped on it. When looking into this number I found that it is a mercury number and that the number for the mercruiser pump base is 46-44292A3. I put in the mercruiser one, thats the 46-44292A3 even though that wasn't the one I took out. Other than the part number they look identical, would this matter and should I put back a pump base with the mercury part number instead?

 

[Bt Doctur]

Re: Mercruiser 5.0 High Speed Overheat

http://www.boatfix.com/merc/Servmanl/6/6covr3.pdf
3B-51 thru 3B-53

 

[cookiesareyummy]

Re: Mercruiser 5.0 High Speed Overheat

http://www.boatfix.com/merc/Servmanl/6/6covr3.pdf
3B-51 thru 3B-53

I actually have that book and have been looking at it but haven't found anything I might have missed in it. In your picture it kinda looks like the gasket under the base is on backwards which is what someone else was talking about above. Maybe it is just the picture but that is what it looks like to me.

 

[cookiesareyummy]

Re: Mercruiser 5.0 High Speed Overheat

All fixed, I'm not sure what did it but it is running at normal temp now. I did 3 things different.
1) I replaced the upper pump housing. I'm pretty sure this is what actually did it, the upper sleeve might of had more wear than I though would be ok.
2) I used that other pump base. If you read my previous posts you will see the base that I took out had an odd number on it. This one actually had a little tighter fit going in.
3) The alignment peg under the pump base (attached to the drive itself) had a little debris between it and the "wall" of the drive. I didn't notice it the first time but this time I scraped it out real good.

So not sure which one did it but one of them fixed it, thanks everyone for the help