Mercruiser Overheat Problem
 

I bought my first boat. It has an over heat problem. I am out of ideas

1990 Baja 250 ES
7.4 Mercrusier
Bravo 1
23 pitch prop
low water pick up
New hose between the out drive an the raw water pump
New impeller
New hose between the raw water pump and the oil cooler
New thermostat (140)


At idle it is about 155
At 1000 it is 160
At 2200 it is 170
At 3000 or more it slowly increases. I have only let it get to 190 before backing off.
When you stop the temp slowly returns to 155.

Any one know what is wrong?

Thanks

Gene Bopp

Check your exhaust risers for corrosion. Generally this takes disassembly, but one way to check on a running engine is by feeling the hoses going to the exhaust manifolds (input side). If they are fairly hard when the engine is running, you have restriction in the risers. This is caused by normal scale and rust buildup.

Two ways to service this. Replace risers, or soak existing risers in muratic acid for 2 hrs, rod out, soak again in muratic acid, then flush with water. Really not a hard job. Just be careful upon reassembly to use the correct riser gaskets and clean sealing surfaces well. Failure to do so will result in death of your engine.

BT :cool:

One thing else to look at,
check the oil cooler for possible
blockage from"failed" sea water pump
vanes.
Take the sea water pump outlet hose
off, and take the sea water inlet hose off
of the thermostat housing, put a garden hose
in the hose removed from the thermostat housing,
turn on water, this will backflush the cooler.

hope this helps,
jt

If it was docked, get a screw driver and take off the water inlet grate. There might be a ton of Zebra Muscles in it like how ours was.

How many hours of use in fresh water will typically clog the risers? I have 480 hrs on mine and it runs cool (160) and after a few minutes at WOT it runs 175 and the risers feel warm not too hot to touch, but very warm. Is this normal. The motor is out and I've rebuilt it over the winter.

cheitman ... There is a rule of thumb for this, and going from memory I think it was.... service exh manifolds and risers every 2 years if run in salt water and every 4 or five years in brackish. 6 to ten years for fresh water.
500 hours and a sterndrive engine should get a valve train rebuild no matter what, and collateral repairs would be exhaust manifold service.

BT :cool:

Do you have an engine flush out system?
If so and it is the type that you remove a cap and screw a garden hose onto,check and make sure the cap is on tight and has a washer in it.
If it is leaking it could suck air a cause a overheat problem and /or cook to seawater impeller.
Jeff

Also remove the hose fitting on the transom plate and make sure the plastic bushing that holds the hose in the transom plate is not collapsed due to corrosion around the hose.
This with restrict the water flow.
Jeff

Overheating
 

If you elect to boil out the manifolds and get them good and clean you also wll need to inspect the mating surfaces between the risers and the manifold. Typically it is a safe bet you will need to resurface these areas because the acid does eat into these areas. Keep in mind that the older the manifolds the more corrosion takes place. If you leave them in the acid too long they may end up being junk. You may find after tear down they are junk anyway.
The water inlet hose connection mentioned previously is a great one to follow through on. I had the inlet hose reduced by corrosion down to 1/8 of an inch. Would run at idle and then gradually raise the operating temp.
Good luck and post your findings.
Brian

I have been out trying the cheep solutions. I have a perko flush kit (clear Plastic tube). At idle there is nothing but water in the tube. When I run the engine up 1500 or so there are lots of airbubbles in the water. I would assume this is bad. I have run the engine up, running on sea water, and used the garden hose to pour water all over each connection. The bubbles don't seem to change. My entire outdrive is underwater, so fresh air can't be coming from outside the boat.

Are the bubbles normal? I would think they might be the cause of my overheat problems. My hatch opens forward so I can't check for bubbles while under way.

I am open to any and all suggestions. If you have a long solution and it would be easier to tell me on the phone let me know.

Thanks

Gene