omc 5.7 running hot

[jajin2004]

I have a 1988 trophy flybridge with a omc 5.7 liter (4 valve per liter) and a cobra outdrive. Just acquired the boat. It runs HOT over 220 degrees when wide open. When you let off and go to 1/2 throttle, temp comes down until 170-180 still HOT. It runs 140-150 at idle, 170 degrees at 1/2 throttle (20 mph) then rockets up quick at full throttle. I have an infared thermometer and the hoses from the outdrive is 90 when operating temp is 200+. Hoses are cool, manifolds are scorching and the risers are also above 200 so the relay or sensor and guage are working correctly. I have changed the impeller, I have changed the risers, I have changed the thermostat. I took off the oil cooler and checked for blockage- still nothing. Should I replace the water circulating pump (I have a spare but I do not see any leaks and the temp gun shows 100 or so at full throttle) or do you think it is a head gasket? I also have a spare set of manifolds- could that be it? (I also took the hose from oil cooler to thermostat off and held it 15 inches over transom bracket and it pumped 2-4 inches). I heard that if you take the thermostat housing off and fill up the block with water to the line of the thermostat and start the engine- if there are bubbles, it means a blown head gasket- does that work? No water in the engine oil- what gives?

 

[bruceb58]

Re: omc 5.7 running hot

The fact that your manifolds are getting scorching hot means you have a water delivery problem...not a headgasket or circulating pump problem. Probably time to take the risers off and take a look in there.They may be blocked up.

 

[danond]

Re: omc 5.7 running hot

With that kind of heat, exhaust flappers are surely fried, broken and possibly jammed down in your exhaust passages somewhere. Time to remove the drive and check.

Also you can pull one of the exhaust manifold hoses, start the engine with a water supply and see if you get water from the hose. If you don't, something between the drive and the hose is plugged or not operating.

Replace the thermostat. It's cheap.

Replace the impeller in the outdrive. Cheap and probably needs to be done anyway.

Then if no joy, pull the exhaust manifolds and see if they're plugged up. If they are, gotta replace them. If not, replace the engine recirc pump.

 

[seattle2855]

Re: omc 5.7 running hot

** nevermind, you already did this ***

I just went through similar overheating fun.

I would find the raw water hose fed from the outdrive inside the engine compartment. Disconnect that hose and run the engine for a few seconds. You should see a nice stream of water coming out of there. If not, then you have a problem with your impeller, housing, or hoses in the outdrive.

 

[seattle2855]

Re: omc 5.7 running hot

Is this a closed cooling system, half closed, raw water cooled?

If you have a heat exchanger, I would open on the ends up and make sure its clear in there.

If you have raw water going into your manifolds, I would disconnect one of raw water lines to it and run the engine of a few seconds to see the water delivery there.

How did the manifolds look when you changed the risers?

 

[Bt Doctur]

Re: omc 5.7 running hot

5.7 is a 350 cubic inch motor with 2 valves per cylinder
double check the impeller installation,and the connecting tube, may be sucking air .

 

[Wet Wonder]

Re: omc 5.7 running hot

Also check the intake hose feeding the uptake pump. It's a high vacuum line and can get kinked or collapsed, reducing the water flow enough to raise temps at all rpms.

 

[nazz2005]

Re: omc 5.7 running hot

sorry to cut in guys just a curious question after reading a few forums not found an answer.. is it normal for one riser to get hotter than the other? inboard merc 5.7. One is bearable to touch at operating temp while the other is not meaning prettty dang hot..? does not overheat..complete cooling system just redone

 

[bruceb58]

Re: omc 5.7 running hot

Ok...so I see now that your risers were replaced. It is still some sort of water flow problem and probably not your head gasket. Pull the hoses going to your manifolds and start the engine...do it quickly so you don't overheat the exhaust...and see what kind of water flow you get.

You checked the outlets at the thermostat housing for blockage right?

Like you said before, the exhaust manifolds could also have blockage. Might be a good idea to replace those as well.

 

[Thad]

Re: omc 5.7 running hot

I had this problem with a Merc Alpha I drive. I found that the water intake hose from the drive to the transom bracket was kinked just enough that when I ran at high rpm's, the motor could not get enough water. Slow down, and it cooled off.
Can I assume that the Cobra drive also has a hose between the drive and transom plate?
If so, and if you can see it, look there.
Just a thought.

 

[jajin2004]

Re: omc 5.7 running hot

UPDATE AS OF 6-17-09

Last night I took off the oil cooler and blew it out- didn't looked plugged at all and re-installed. Took it out and same thing- hot at WOT- I have only run it that hot for 10-15 seconds and then limp back to the dock. A couple things I have found: In examining the old Cobra water pump/impeller all the vanes are there but there is some melted veins on the metal screen/basket. I did replace the unit last week- same thing. Also, when the water tube comes from the transom plate and attaches to the oil cooler, it comes up vertical and then bends almost 90 degrees and immediately attaches to the oil cooler- Could this be the restriction? Lastly, I did replace both bellows before I floated the boat. The connection to the water tube is solid- new o'rings etc. I don't think it is sucking air. When I first started it and ran it the first few times, there was a lot of "carbon" black s*&^% coming out of the exhaust. When I changed the risers, the risers had a lot of black carbon on the exit side where it connects to the water passage tubes. Do you think the exhaust manifolds are similarly plugged- I have a good set of spare manifolds. Should I replace the manifolds and water circulating pump first, then do a compression test for the head gasket. It really runs well- starts right up, idles smooth and runs smooth all through full WOT, just hot. What about the exhaust flappers? How do I access them? Thanks for the help

 

[jajin2004]

Re: omc 5.7 running hot

UPDATE AS OF 6-17-09

Last night I took off the oil cooler and blew it out- didn't looked plugged at all and re-installed. Took it out and same thing- hot at WOT- I have only run it that hot for 10-15 seconds and then limp back to the dock. A couple things I have found: In examining the old Cobra water pump/impeller all the vanes are there but there is some melted veins on the metal screen/basket. I did replace the unit last week- same thing. Also, when the water tube comes from the transom plate and attaches to the oil cooler, it comes up vertical and then bends almost 90 degrees and immediately attaches to the oil cooler- Could this be the restriction? Lastly, I did replace both bellows before I floated the boat. The connection to the water tube is solid- new o'rings etc. I don't think it is sucking air. When I first started it and ran it the first few times, there was a lot of "carbon" black s*&^% coming out of the exhaust. When I changed the risers, the risers had a lot of black carbon on the exit side where it connects to the water passage tubes. Do you think the exhaust manifolds are similarly plugged- I have a good set of spare manifolds. Should I replace the manifolds and water circulating pump first, then do a compression test for the head gasket. It really runs well- starts right up, idles smooth and runs smooth all through full WOT, just hot. What about the exhaust flappers? How do I access them? Thanks for the help

BTW it is a fresh water cooled- not a closed system-