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Mercruiser 37 liter

paradise_beach

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"Why doesn't my heat excha

"Why doesn't my heat exchanger get hot, while the temperature gauge bounces to extreme temps in a few seconds? I have a 165hp 3.7litre 224cid. I have an external waterpump and an external alternator all driven by fanbelt on the crankshaft. I've checked the flow of water by putting a hose and running the water and everything flows ok. Even thru the heat exchabger!!! But when i start the engine it starts nice and beautiful to hear it idle!!! But after a minute or two my temp gauge exponantially climbs to higher temps, therefore i must shut off the engine when it reaches 220-240 degrees!!!! I put my hand on the heatexhanger and surprisingly it's cold or room temperature as they say!!! How is this possible???? And it was running nice on the sea...I took the boat out of the water so that i can sandblast the head,had the cilinder head shavended off nicely again and put a new head gasket with interval torques to 150. and now i want to keep on enjoying the beaches and the engine overheats in barely five minutes!!! Any one with suggestions?"
 
My 1st Guess would be that the

My 1st Guess would be that the T-Stat is Sticking........
 
"Yup... thermostat. Pull it ou

"Yup... thermostat. Pull it out and put it in a pot of boiling water. If it doesn't open up all the way, that's your problem.

Since the Hx is cool, that would indicate that you're getting plenty of raw water flow, just no fresh water being diverted from the engine to Hx to get cooled by all the nice cool raw water flowing through the Hx tubes."
 
"And if it's NOT the t-sta

"And if it's NOT the t-stat, then you have air trapped in the system that won't allow the ciculating pump to move coolant about. If so, you need to "burp" it by releasing air from the highest point.

Jeff"
 
"Thanks for the suggestions...

"Thanks for the suggestions...i knew it had something do to with the flow from t-stat to heatexchanger but i couldn't figure out why with a hose it flowed, meanwhile with the engine running it didn't. I must say..i did a test with a hose pressuring water into the waterpump inlet and it did flow thru everything untill it came out the hose going to the waterpump inlet. And the hose to the heatexchanger did get warmer but still too slow compared with the heat in the exhaustmanifold area..But i will check the t-stat to see if works!!!! Tnx once more and i'll keep youz up2date ..."
 
"Sergio: Is the coolant hose

"Sergio: Is the coolant hose from the engine warm and the hose from the exchanger going back to the engine cold? If the exchanger is working and not plugged internally, the hose from the engine should be very warm to hot to touch and the return hose should be less warm but not cold.

Is the heat exchanger in your boat very old? Has it ever been cleaned with a rod to remove scale buildup internally? I would be looking at the exchanger if the T'Stat is working.

Also consider the engine circulation pump. It may not be circulating the coolant fast enough.
Guy"
 
"I'm going to check the t-

"I'm going to check the t-stat today..the heatx has been cleaned, flushed last time i took it off. I also put in new spark plugs, complete engine gasket set, valves had been sandblasted and polished, head also!!! The engine was running perfect...but i couldn't run it on full power for more then ten minutes otherwise the temp rose so much i must run it for 20 minutes on 1/4 speed so that the temp came back to normal numbers. That's when i decided to take it out and do an complete engine cleanup and change of gasket and had the head sandblasted ect ect. And now the temp rises before i put into the sea!!!"
 
I opened up the outlet to the

I opened up the outlet to the heatX and to my surprise there was no T-stat present...Thus the water was flowing to the exhaust manifold without passing thru the heatX. Fixed that!!! Went to the beach and now when i start the engine it idles at 170degrees and when running slow or at half speed it stays constant at about 200 degrees or a bit more. I didn't risk opening up the throttle as yet to full power! Is it normal for it to run at 200 degrees at half power??? I have two questions...which oil should i put in the engine 40W or 30W?? and is it possible that the impeller in the sterndrive is damaged or old in a way that it doesn't suck sufficient cold sea water into the heatX????? Where I live the outside air temperaure is 34 degrees celsius everyday all year round. Must i put a thicker oil or thinner??
 
"Sergio: I'll bet you don

"Sergio: I'll bet you don't own a winter coat! Living in 93 deg. F. year round must be nice, especially being able to boat all year.

Glad you are progressing on your repair. No it isn't normal to run at 200 degrees F. at half throttle. As for the sterndrive water impeller, if it has been over a year since you replaced it, it can't hurt to replace it now since you boat year round. Changing the OD impeller should remedy that issue. Good insurance.

The next time you are out in your boat and running at half throttle, have someone with you so you can look at the engine hoses. Look and see if one of them is collapsing, especially the engine circulation pump hose. It should not collapse which restricts coolant flow. Have you checked or changed the pressure cap on the coolant reservior? If it does not open the pressure can cause the hose problem.

Concerning which weight engine oil to use, in the hot climate that you live in I would stick to the 40 weight oil over the 30 weight. Mercruiser recommends 25W40 weight oil but your engine oil is already warm at 93 deg. F. You could use an expensive full synthetic oil which would help the engine run cooler but you might have difficulty finding it in a straight weight. You will find a 15W40 and even a 20W50 which you could probably use since it is hot where you live.

Guys: What oil and weight do you recommend for Sergio's engine? Guy"
 
"tnx....i'll check if ther

"tnx....i'll check if there's a collapse in the hoses and also the pressure cap. I'll do the oil change today...cause i put in 30w sothat it can go every where and flush out unneccesary stuff in the engine. So I thought!!!

I do own several winter coats, cause i studied in holland for five years...winter is nothing for me!!

I'll keep you guys up2date on how it runs.....

Tnx once more"
 
"Sergio: Check the serial num

"Sergio: Check the serial number on the O/D on the Mercruiser website. My 1987 4.3L 175 HP O/D has 3 Alpha 1 decals. I would assume you also have the same O/D with the 3.7L 165 HP or an MR O/D which predates the Alpha drives. The Bravo drive was designed to handle the higher HP and torque of the big block engines with beefed up gearing, bearings, etc. Guy"
 
"Tnx for the comments,....i&#3

"Tnx for the comments,....i've flushed the oil again and put new recommended oil in it. Now i'm busy dismanteling the lower housing on the sterndrive to change seals and impeller from the outdrive. Cause my gearoil comes out milky every pass two days of running the engine...so i decided to do the whole change up!!!"
 
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