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

ck the inlet of the coolers for debree,ck thermostat ports and make sure there clear,put a manuel water pressure gauge on it so you can tell if its pressure loss,or possibly just a leaking head gask!!

Open the drain cocks on your engine block and make sure water flows easily out them. If it doesn't, it could be an indication that there is a lot of sad that has accumulated in the block.
Just another free and easy thing to check.

I think I found the problem. I was going to replace the manifolds like everyone suggested. There was a pair on EBay and a phone number for questions. I called the guy, he said that the manifolds weren't the problem. He was sure that the raw water pipe would have sediment right behind the fitting that comes in from the out drive. There wasn't sediment there, instead the end of the tube was crushed and mangled. I dont know how the thing ever got water.

The reason that I say, I think I fixed the temp problem is because while I was out testing it the drive coupler let go........

Anybody want to buy a boat.......


Gene Bopp

Believe it or not baja bopp, the coupler problem this boat apparently has may have indirectly cause overheat problem (ie crushed hose). When installing an engine the hose is usually dangling down in the bilge while the engine is lowered. If not careful, the end of the hose can get crushed under the motor mount. I have done that before.

So, if this coupler thing has happened before, but the engine was not aligned properly, you get another bad coupler out of the deal and an engine to pull. Unfortunately Gene, this is exactly how many of us became knowledgable about boats...

Good luck,

BT :cool:

Baja Bopp,
It is all part of boating.
Keep in mind that you have several viewers that are on this web site for the shear love of powerboating.
Also keep in mind they are willing to offer advice free of charge.
ANYBODY CAN GO OUT AND BUY AN NEW BOAT BUT NOT JUST ANYONE CAN MAKE AN OLD BOAT LIKE NEW!
Good luck
Brian

My boat is a 1990. That plastic fitting that is bolted to the big metal tube that appears to run though the transom is actually just a big tube with a small plastic tube inside of it. What Profile (above post) said is what can happen. Salt water causes corrosion between the plastic hose and the metal casing and closes off the water inlet. In my case, the end of the hose was all bent and mangled. I don't know if I had the corrosion problem, but it definitely was blocked. I couldn't get a small Philips screw driver in the hole. I ground the plastic away to make the hose normal sized again and my heat problem seems to be fixed. At least for now.

Gene