Perkins coolant temperature slowly went high.

Today cruising along at 1650 rpm, after normal running for 4 hours my coolant temperature started creeping up to 225 degrees. Plenty of raw water flow out the exhaust. Exhaust hose temperature 95F. Water exiting was lukewarm. 95 seems about right. Lowering rpm to 1000 barely helped. Coolant pump belt tight.
Opened up and cleaned sw strainer. A little bit of junk in there. Maybe 10 percent at most. Pulled off sw pump cover. Impeller and cam good. All impeller blades intact. Put it back together and again pumping plenty of water.
My best guess is the thermostats. Never seen this type of ts that both opens to pass cooling water and closes off a hx bypass flowpath at the same time. The symptoms are the cooler is hot but the sea water is not picking up much heat. I have 2 new thermostats in hand but have not done this replacement before.
1) ts failure sound right?
2) hard or easy job? I have the Perkins 6.3544 NA with the Multi-Cooler.

New TS showing bypass valve. My guess is that the bypass valve disc fell off the collar on the end of the stem.

I had a similar problem on a cummins. After chasing everything finally determined engine driven freshwater (coolant pump) had failed. Same symptoms you observed. Effectively there is little to none exchange going on in the heat exchanger. So coolant temp keep climbing but saltwater was cool or just barely warm.

READY said:

I had a similar problem on a cummins. After chasing everything finally determined engine driven freshwater (coolant pump) had failed. Same symptoms you observed. Effectively there is little to none exchange going on in the heat exchanger. So coolant temp keep climbing but saltwater was cool or just barely warm.

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Thanks. Did the impeller corrode away or just stop turning?

Thank you HW...will "file" that issue away for future reference

Not hard to troubleshoot. First, make sure coolant level under pressure cap is within an inch of being full. Next, you need a helper, ear muffs and a temp gun. Get engine access opened up. Crawl around engine and figure out the coolant flow path. You need to see the fittings where coolant goes from tstat housing to heat exchanger and from heat exchanger to circ pump suction. Want to shoot metallic surfaces and not hose skin.

Get boat underway and get engine to where it is starting to run hot (like 190-200F). Have helper drive the boat. Get next to engine and read temp at tstat outlet. Can be the actual housing or tube going to HX.

If tstat outlet is like 175F and gauge reads 200F, it is not really overheating and you have a gauge issue.

If it reads near 200F, you do have a problem.

Next go to HX. It has an outlet tube that leads to circ pump suction. Read temp there (not on hose skin as that insulates too much). If temp there is low (like under 120F) you do not have enough circulation flow, due to circ pump or tstat. If temp there is high (like 170-180F), you have good flow in HX, but not enough heat transfer due to low sea water flow or fouled tubes in HX.

Next check temp of SW into HX and SW out of HX. There should be a temp rise of 10-20F. If more than 40F, you don't have enough SW flow.

If you are getting good seawater flow and no cooling, you problem is most likely with the closed coolant loop. In my mind, this points to the belt, pump, stats, or, maybe a failed, collapsed hose.

If that pump failed, you'd /probably/ hear it, see an issue with the belt, or have black belt dust or smoke from it. If that isn't the case, I'd bet on the thermostats. They fail much more readily than the pump.

You could also squeeze the hoses and otherwise check that none have collapsed, and make sure the system has coolant, the overflown isn't overflowing, and the surrounding area in the engine room is clean, just to be sure the header cap hasn't failed and vented a ton of coolant out.

Also, although I don't recommend it unless you are totally confident, comfortable, and prepared, you can look under the header cap with the engine at temperature and see if you see waterflow.

If you have a kit, you might want to pressure test when done and/or keep an eye on the coolant (for no oil floating) and the oil (for no milky coolant mixed in). Not sure how long it was running at 225 F, but that is hot. If, by any chance, it caused a head gasket failure, I'd want to catch that early.

Good luck!

As stated above, I would also be looking at the following:

1) Is there AF in the closed cooling side? pin holes in the heat exchanger can suck AF into the raw water and get discharged slowly. The closed cooling is usually a higher pressure than the raw water side.

2) Is the circulation pump working properly? This circulates the AF through the block and back to the heat exchanger in a circuit. Are you only cooling stagnant or semi-stagnant AF in the heat exchanger, but not circulating the cooled AF through the block?

3) Exhaust manifold? When is the last time this was services/replaced?

Ski's point about checking the gauges...yeah....call that a Big Idea.

Check the quality/condition of your coolant. Often ignored, old coolant/depleted additives will cause problems.

Do you have dual helms and get the same readings on your other helm?

Ski's post makes great sense for logical tracking of the cooling temps and performance.

Thanks all. Yes verified actual high temperature condition and gauges correct. I have some spare parts but no needed gaskets. Parts on order and local mechanic coming Monday.