Overheating Engine

[scootie1]

Just bought a ''84 Catlina 30. engine is Universal M25. Overheats after about 10 minutes of use. I''ve replaced the water pump, raw-water impeller and thermostat. No obvious leaks. Someone suggested that the heat exchanger coolant should be checked. Not being a diesel mechanic I have no idea where it is located. Can someone please advise? Thanks.

[RichH]

First check for a stuffing box that set too tight!
Then start with a bucket and a stopwatch. Run the engine at your ''crusing'' rpm and measure/verify the volumetric water output at the exhaust outlet ... about 3 gallons per minute. If less, then you probably have a restriction in the circuit (piece of broken impeller stuck in the circuit, pinched hose, worn lobe/cam in the water pump, etc.). If approx 3 gpm, then consider to ''desalt'' the heat exchanger - use a non-acidic/non-destructive ''descaler'' such a Marsolve, RydLyme, etc. ... just pump it through, let sit, etc. Do the descaling on BOTH sides of the cooling circuits - fresh water and raw water sides. You can use an acid to ''pickle'' but risk dissolving the cast iron of the block, etc.
Hope this helps.

[Jeff_H]

Maybe this is only me, but I can''t see how overheating is related to "a stuffing box that set too tight". The engine might not turn the RPMs but I can''t imagine that it would overheat.

I do think you touched on a number of posibilities. A clogged exhaust elbow is another possibility as well. I would start at the intake valve and make sure it is open and free of debris. Then move up to the intake water strainer. Those two are pretty easy to check. Obviously the impellor is pretty easy to check as well. The fresh water filler cap is on the top of the heat exchanger. Most have a separate reservoir that is bulkhead mounted and that is connected to the heat exchanger with a rubber hose. You can recognize the overflow bottle by its anti-freeze color. Actually add new water or anti-freeze at this reservoir.

Jeff

[RichH]

Must be you!

Any ''heavier load'' for the engine to attain the same rpm will require more fuelinput (power). The heat exchanger (at the same rpm and @ constant water pump flow) is a constant heat sink - only ejecting a fixed certain amount (BTUs) ... (yes, the log mean temperature differences LMTD between input and output will be different, but lets not go there at this time). The more fuel to attain the given rpm, the higher the heat load; but, since the exchanger/raw water pump is operating at a fixed condition the temperature will rise in the engine ..... its a simple question of thermodynamic balance.

Whats happening is that the engine is using more horsepower/fuel to attain the same rpm; more fuel consumed = more heat, the exchanger is (essentially) constant in heat removal ability.

Next time you drive your car up a long hill at a reference mph/rpm, take note of the engine temp. Now run the hill again at the same mph/rpm but with your foot on the brakes or pulling a large trailer .... and note the difference in engine temp.(and how much further down you have to depress the accelerator to attain the same mph/rpm).

[Jeff_H]

Actually, one oddity of most internal cumbustion engines is that more fuel means less heat. A lean engine mixture burns hotter but produces less work than a rich engine. The excess fuel delivered under high load in a diesel actually cools the cylinders which is why you see black smoke under load.

Jeff