Hi Harvey,
If you are maintaining 180 degrees, I doubt you have a problem. The normal operating range is 165-195. Here in FL, the bay water can reach 85 degrees or higher in the summer. My problem was the temp would very slowly increase under a load. I let it go once to see if it would finally top out. At 205, I chickened out and pull the throttle back and it slowly came back down. Anyway, here is the article.
Don, #6293
M-18 ENGINE HEAT EXCHANGER UPGRADE
Anyone with an M-18 that operates in warm sea water knows that the original 2 inch by 12 inch heat exchanger is only marginal to keep the engine at normal temperature. The combination of a high output alternator and 85 degree seawater equals reduced engine RPM or an overheat will occur.
I ordered a new 3 inch by 13 inch heat exchanger from i-netmarine.com for $295 plus $20 shipping. Part number is UNI299835CN. It is made from copro nickel and suitable for salt water.
The old heat exchanger had a 1/2 inch line from the raw water pump to the exchanger. The new one is 5/8 inch, so a new fitting for the raw water pump and 2 feet of 5/8 hose were purchased.
On my 1986 model C-27, there is an access door in the starboard quarter birth. I find it easier to remove the entire frame (four screws) to give maximum working room.
I drained some of the engine coolant out of the manifold reservoir with a small plastic hand pump. I climbed into the port cockpit locker and removed the forward
access panel. I wedged a one gallon bucket under the exchanger and removed the lower coolant line and let it drain into the bucket. Now back to the quarter birth to remove the other coolant line and raw water lines. I removed the two hose clamps that attach the exchanger to the bracket and remove the HE. I removed the two bolts that attach the bracket to the bell housing. The bracket radius needed to be changed to mate with the 3 inch HE. I had a piece of 3 inch pipe, but I could have purchased a coupling or nipple from a hardware store of the same size. I put the bracket on the pipe and whacked it with a big hammer until the radius matched the new HE. The bracket had two sets of holes. Originally it was held on with the forward holes. I attached it with the aft holes to get clearance of the exhaust pipe.
The lower coolant hose barb on the old HE was angled off at about 30 degrees. The new HE barb is straight with the body. The coolant hose just did fit. Had it not reached, I would have replaced it with 7/8 inch exhaust hose from West Marine.
I added a piece of thin rubber, like inner tube material, between the HE and the bracket and finished the installation. I opened the pet cock on the thermostat housing this allows air to escape when servicing with coolant. I closed the pet cock and installed the pressure cap. I started the engine and watched for water out the exhaust. At 140 degrees, I shut it down and checked for leaks. It is not unusual for this engine to air lock after the coolant has been drained. If this happens, the temperature at idle will go from 160 to 180 or more very quickly. Mine did just
that. I advanced the throttle to 3,000 RPM for about 30 seconds and the temperature quickly fell back to 160. I have heard this called âburping the systemâ and it really works. Now the air pocket moved to the manifold reservoir so I shut the engine down and after it cooled for a few minutes, gave it some more coolant. I only âburpedâ it once, but sometimes it takes 2 or even 3 times to get all the air out.
It was a good full dayâs work to accomplish the change, but the results are wonderful. The temperature sets rock solid at 170-175 degrees at all power settings.
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