New season. Still chasing an overheat problem

[nickmo]

2008 Mercruiser 5.0 liter TKS Alpha 1 Gen 2 Raw water cooled.

Engine begins running hot over 3000 rpms. If you back off it will cool down. Will run at normal temp all day as long as rpms are under 3000.

Here's what I've done so far.

1 Changed manifolds and risers. They were 8 years old anyway.
2 Changed thermostat. Old one was ok
3 Changed impeller twice in one season. Old ones were ok. No pieces missing on either impeller.
4 Changed impeller housing with new model SS type with rubber gaskets attached to plate. Did not use any sealer on gaskets.
5 Changed exhaust flappers. old ones were intact.
6 Back flushed power steering cooler when everything was apart. nothing inside.

Here's what I haven't done.

1 Install a clear hose from thermostat housing to starboard manifold to see if there is air mixed with water as per Merc bulletin.
2 Change engine mounted circulating pump. I heard they rarely fail and if they do, usually cause a low rpm overheat.

Is there anything else to check? Has anyone had this issue?

Thanks for your help.

 

[alldodge]

I think your sucking air through a hose fitting or connection. Need to put a clear hose between the power steering cooler and thermostat housing. Just my guess, maybe Bt Doctur or achris could help

 

[JoLin]

One other thing- friend of mine had a similar problem that drove him and his mechanic nuts for 2 seasons. Bottom paint buildup around the edges of the water intake holes on the leg, diverted water away from the holes at speed even though the holes themselves were open. Odd as it sounds, scraping it away fixed the problem. Your issue is most likely something else, but at least take a look. Do what AllDodge suggested. It's a more likely scenario.

My .02

 

[nickmo]

One other thing- friend of mine had a similar problem that drove him and his mechanic nuts for 2 seasons. Bottom paint buildup around the edges of the water intake holes on the leg, diverted water away from the holes at speed even though the holes themselves were open. Odd as it sounds, scraping it away fixed the problem. Your issue is most likely something else, but at least take a look. Do what AllDodge suggested. It's a more likely scenario.

My .02

Good advice. I had some barnacles growing in that same area a few years ago. It did make the motor run hotter.
Unfortunately I checked that also.

Much like your friend, this has been frustrating to say the least.

 

[nickmo]

I think your sucking air through a hose fitting or connection. Need to put a clear hose between the power steering cooler and thermostat housing. Just my guess, maybe Bt Doctur or achris could help

Thank you AllDodge. That will be the next step. I guess I'm looking for a fix instead of more diagnosing at this point. I don't have a trailer for the boat so each test means hauling it and starting over. I did that 4 times last season. I had the cleanest hull in town though.

 

[alldodge]

Thank you AllDodge. That will be the next step. I guess I'm looking for a fix instead of more diagnosing at this point. I don't have a trailer for the boat so each test means hauling it and starting over. I did that 4 times last season. I had the cleanest hull in town though.

In that case remove the hose coming to the thermostat housing and add connect a piece of clear hose. No need to pull the boat

 

[nickmo]

In that case remove the hose coming to the thermostat housing and add connect a piece of clear hose. No need to pull the boat

Do you know if that hose is 3/4"?

 

[alldodge]

No its not 3/4, I would guess at an 1 1/4 but that's a guess. The part number for the hose is 865093, measuring tape should tell you

 

[maicoman]

here you go ! I can guess that your boat has the infamous single point drain system.If it does look up the 7 point system and buy the parts necessary to change over.It is easy to perform ,not too expensive and makes the boat run very cool

 

[nickmo]

No. I don't think so. It's a 5.0 liter TKS. It has drains at each manifold, one on either side of the block and one at lower hose at the circulating pump.

High rpm overheat has me as well as many mechanics stumped.

 

[nickmo]

UPDATE

Before splashing the boat, I decided to look at the impeller once again. As said in an earlier post, I have the newer type impeller housing and plate with rubber gaskets attached. They are not supposed to be permatexed.
According to a Mercruiser bulletin, if there is a high speed overheat there could be air entering at the bottom of the plate. The fix is to glue the lower gasket to the outdrive, but they show it with the old style gaskets and plate. I applied permatex to the lower surface of the rubber gasket and reassembled everything.
I ran the boat at all rpm's up to full throttle with no change in engine temp.
I've been running this boat at 3000 rpm or less for 2 seasons or it would overheat. Feels good when it gets fixed.
Thought maybe this could help someone as all mechanics said not to glue that rubber gasket.