Mercruiser 260 Overheating over 2000 RPM

[CaliFormula]

Hello everyone, I’ve got a problem. My 1985 Mercruiser 5.7 is overheating under load at about 2000 RPM. I have done new risers and manifolds, new impeller already swapped my 160 thermostat for a 142 thermostat. I checked and the flappers and they were fine. I have freshwater cooling and verified that raw water is flowing through. It runs 165-175 at idle and lower rpms. As soon as I rev up and get on plane between 2000-3000 rpms it creeps up 200+.
I also recently rebuilt the heads I thought it might be a valve issue, maybe too tight but I’ve since adjusted them and I believe I have ruled that out. Any ideas on what to check next would be really appreciated.

1985 Formula LS3
Mercruiser 260 Serial# 6765655
Alpha 1 Drive

 

[alldodge]

You have a restriction or your sucking air. The 165-175 (if accurate) is bad news.

Assuming temps have been verified using a IR temp gun, then it as stated above.

Being a 260, is it a Alpha or Bravo drive?

 

[CaliFormula]

thanks for your response. I think the temps are correct and I’m running in saltwater. I can verify with an IR temp gauge. Where is the best place to take that measurement? Are you saying those temps are too low?
Alpha 1 drive.

 

[alldodge]

If temps are correct then they are high and your overheating

Best place to take measurement is where the sensor is at the thermostat housing

When was the last time the impeller was replaced?

 

[achris]

thanks for your response. I think the temps are correct and I’m running in saltwater. I can verify with an IR temp gauge. Where is the best place to take that measurement? Are you saying those temps are too low?
Alpha 1 drive.

No, too high...

Do you have an engine mounted raw water pump or is it still in the drive?

Chris......

 

[CaliFormula]

Ok got it on the temps, impeller was changed less than a year ago.

 

[tank1949]

Impeller changed by whom? Has it ever worked 100% correctly? If the above has all been accomplished, make sure NO corrosion on temp sending unit connectors. Back pressure with garden hose at risers. Since boat is anti-freeze cooled, you should place garden hose at places where salt water comes to motor and see if water flows freely backward. You may have sucked a piece of plastic or "ate" impeller. Maybe muffs came off by accident? I always use inferred on several parts of motor looking for hot spots. If heat evenly spread, I'd look for water congestion points. If inferred indicated 170 at temp sending unit AND stat housing, exercise connections at helm gauge. Yes, they get green and corrode too. With muffs and hose/water on, pull every hose connection on motor/manifolds and check for easy flow. Caution not to leave connections off too long, only a few seconds. All else fails, pull circulating pump and examine its impeller blade.

 

[Scott Danforth]

with fresh water cooling, when was the last time you pulled the HX end plates and cleaned out the Raw water side?

my guess is this is an aftermarket HX installation. is it a half system or a full system?

 

[achris]

We still don't know if it's got an engine mounted raw water pump or uses the pump in the drive... ¯\_(ツ)_/¯

 

[Scott Danforth]

We still don't know if it's got an engine mounted raw water pump or uses the pump in the drive... ¯\_(ツ)_/¯

we also dont know if its a Mercruiser supplied unit or aftermarket. original to motor or a retrofit.

 

[Rick Stephens]

On my own boat I would pull the raw water line off at the thermostat and fire up on muffs to see what kind of flow I am getting from the impeller. I just let it dump into the bilge with the plug out. There are, somewhere, actual specs for gallons per minute, but a good flow is enough.

 

[achris]

.... There are, somewhere, actual specs for gallons per minute, ....

Any of the (genuine) engine manuals, under 'cooling system'...

Chris..........

 

[alldodge]

Any of the (genuine) engine manuals, under 'cooling system'...

Chris..........

For clarity, boat sitting in water, not garden hose

 

[Rick Stephens]

For clarity, boat sitting in water, not garden hose

I've never had opportunity to test pump flow in the water. Running my motor for a minute with the inlet hose hangin in the bilge while on muffs has always been a dead giveaway on impeller and inlet plumbing conditions for me. Can't disagree with the float your boat specs, just never close enough to water to perform it that way.

 

[CaliFormula]

Update! So I finally had enough time to get this sorted. Thank you to everyone who chimed in time help. I paid someone to change my impeller one more time thinking maybe I screwed that up somehow. It was still overheating. I went through all the hose connections and everything seemed alright. Then I went to where I should have gone in the first place which was listen to the suggestions here and disconnect the water inlet hose and run the boat on a garden hose attached to the heat exchanger. Then clear hose to the water supply coming through the transom to fill a bucket. ( my boat is in the water at a dock). Filled the bucket way too slow! Took well over a min. Then I revved and watched less and less water flowing. I was definitely sucking in air/exhaust at the impeller.
I rented another trailer and split the stern drive and inspected again. I replaced the water pocket and copper tube and impeller housing and impeller. I think the old housing had a small crack in it. Put it back together and it’s all good!

 

[Rick Stephens]

Rough trip. Glad you got it.