Motoring without overheating - The Heat Exchanger Upgrade

July 7, 2009

Universal diesel heat exchanger and overheating

The heat exchanger had been acid washed, the hoses cleaned out, thermostat is A-ok - why is our trusty Universal M25 Diesel engine still overheating if we push it up to spec?

I thought it might be that 3-blade prop - could we be overpropped? The boat's original 2 blade prop had been replaced with a 3-blade way back in 1987. Then on haul-out in 2008 it was replaced with the same due to a chunk out of the prop on the boat. Nah, can't be the prop, we'd been plagued with overheating since about 1992.

Heat exchanger upgrade from 2 to 3 inch

Before I went down the path of having the prop pulled and analyzed, the first step was - search like mad for other issues of engine overheating on a 1984 Catalina 36 with the Universal M25 diesel and I found the answer from a Catalina owner post in a forum archive - BULLSEYE - there's a service bulletin (#195) out on it due to overheating problems. Sounded like us alright.

I didn't upgrade to the 3" heat exchanger right away - our Universal M25 with the 2" exchanger was just fine IF we kept watch on the RPM's and temperature, but anything over 180 degrees was going to be trouble.

Finally bit the bullet in July and had the 2" replaced with the 3" heat exchanger. OMG. Reality Check is like a new boat - Sweet! The overheating issue that plagued us since 1992 was solved.

photo of Universal M25 Diesel engine
Universal M-25 Diesel engine with 2" heat exchanger
Universal M25 Diesel engine 3 inch heat exchanger solved overheating problem
Universal M-25 Diesel engine with 3" heat exchanger

The 3" heat exchanger made a HUGE difference. undersized 1/2" hose from the salt water pump to heat exchanger was replaced with a 5/8" hose. That would have made a huge difference supplying cooling sea water to the 2" heat exchanger

7/8 inch hose from 3 inch heat exchanger to fresh water pump
7/8" hose from 3" heat exchanger to fresh water pump

What about removing the heat exchanger?

I didn't do the original replacement of our 2 inch heat exchanger, but I got my chance to remove the 3 inch later. It turns out it is pretty simple - how to do this is on the removing and reinstalling the heat exchanger page.

What about that 3-blade prop?

Mechanic says yes, we are overpropped - on a sea trial after the heat exchanger upgrade we're motoring at 7 knots full throttle with no sails up, and RPM's 2800 max - not 3000 as per spec. That 3-blade (15 R 10) should probably be one size down. But since we are making hull speed at only a couple of hundred lower RPM's he says that's not a bad thing. Hmmm, not sure about that but - o-Kay.


Universal M25 Diesel

Heat Exchanger

Engine Compartment Insulation / Soundproofing

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