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Overheating - 45 hp Isuzu Diesel - at a loss

5603 Views 11 Replies 9 Participants Last post by  Nutella
Ok...I have a '68 Colvin. Ranger is her name and she has an overheating problem. We bought her and was trying to deliver her ourselves over the Christmas holidays and she overheated on us coming out of the channel headed for open water. We came back to the dock and troubleshot the cooling system and found nothing more than the coolant being a little low. I wasn't aware that there was a fill cap on the heat exchanger. I topped it off and ran the engine for a couple of hours with no worries. I lashed the boat to the dock and put it under a little bit (not very much..maybe 1500-1800 rpms) of a problem. I decided that we did not now have time to take her home ourselves, and hired a captain who is on the boat now trying to bring her to me.

The first day, when faced with a strong current, he bumped her up to about 2000 rpms and she overheated within about 2 minutes. Drop her back to idle and she cools right off. He brought her back to the pier and checked all he could, didn't find anything wrong. We called a mechanic, who came out and looked everything over and they tested the thermostat, checked out the fresh water pump find nothing obviously wrong. I am getting at least some heat transfer to the exhaust water. The raw water pump is pushing plenty of water. The fresh water pump appears to be ok. The heat exchanger was made by the previous owner, who really seemed to be johnny on the spot when it came to such things.

At the end of the last 24 hours, she has used about a quart of coolant in the freshwater side. Any thoughts as to what is happening here? Is the little bit of loss of coolant the culprit?
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A common problem on our Westerbeke model is restricted coolant flow due to debris in the internal cooling system. Specifically degrading pieces of the heat exchanger's zinc collect over time in a joint, gradually reducing flow. Backwashing the system solves the problem.

I once debugged a cooling problem to the situation that the mechanic who winterized the system somehow sucked up the little cardboard cap from an antifreeze jug into the cooling system. An hour or so underway in the next season, the little cap found a bend and blocked the system.
I have used this product to solve an overheating issue I had. It worked well.
A common problem on our Westerbeke model is restricted coolant flow due to debris in the internal cooling system. Specifically degrading pieces of the heat exchanger's zinc collect over time in a joint, gradually reducing flow. Backwashing the system solves the problem.
I have heard of the zinc problem before. A friend of mine, who delivered yachts in a previous life, was relating that to me as well. How do you back wash a system like that? Also, this is the second heat exchanger that we have put on with the same results. I am at a loss.:confused:
Is it really hot?

Might want to put the sender in boiling water and use a meat thermometer and watch the gage as the water cools.
Has anyone been fooling with the prop recently?
An overproped boat will overheat the engine.
Long shot but if it is your problem nothing inside the boat is going to fix it.

Another long shot is low compression. Really bad news of course but easy to check. Another one of those things you check last.

We had a raw water pump that looked find and pumped fine but the overheating problem went away after it was replaced.
How do you know for sure you are getting enough water.

Also some systems have a raw water thermostat that could be stuck.
I figure that the raw water pump is working fine because I am getting what I think is a good solid, steady stream of water out the overboard discharge. I don't want to think about low makes my wallet ache...HA Actually it is easy to check and I will have to take a close look at that. The delivery captain seems to think that it may be blowing some of the coolant out into the raw water, since it did lose about a quart over night.

Using a heat gun, he checked the temp at the block and it was about 210 ish. This was after he noticed the water boiling out through the cap and brought it back down to idle then it cooled back down to normal operating temp. The high temp goes off over 180.
The quart missing from the freshwater is worrisome and possible to be minor or a pretty bad thing. But if both fresh and raw water systems appear OK it could be a clogged exhaust elbow, how many hours on the engine & has the elbow been checked/cleaned in the recent past. Picture how fast an engine would heat if the exhaust was restricted by half, not too bad at idle but run it up and put a load on and things get hot in a hurry.
I had the same problem, the alarms went off telling me it was over heating. After checking and changing everything that could be changed it still would read hot at higher RPMs. The sending unit was bad. After someone here suggested I use an infrared thermometer this confirmed that indeed it wasn't truly over heating just a bad sending unit.
The exhaust elbow is something that I definitely need to check. and I want to thank you all for all your help.
Scanning through the entries, I can't find where anyone did this:

Start the engine cold. Leave the filler cap off the cooling system.

Rev it up just a little and watch for bubbles coming up through the coolant. There should be none. If there are bubbles coming up through the coolant that is an indication of a cracked cylinder head, blown gasket, or maybe even a crack in the cylinder walls.

What happens is, the engine is pushing hot gas into the cooling system. Under no-load conditions the engine can handle it fairly well, but when things start happening rapidly and lots of hot gas gets mixed with coolant, well, you know.
I use to own "Ranger" in the keys from 1990 to 1994, a Colvin design and built, 43' coreten stell hull with a gaff rigged main and a marconi misen.
I also experienced over heating problem and find out that the brass fitting going to the muffler was gloged by black residue and restricted the output of cooling raw water. from the helm it seems that it is pumping, but the flow is not fully released and after 15min the alarm ring.
let me know if that help.
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