Heat Exchanger 2.0

[Striper Sniper]

So the saga continues.... 4140 Mitsubishi,
so here's all I've done so far.
Replaced the heat exchanger with a new unit, checked all houses to confirm they are in the proper locations, checked thermostat in hot water... it's 180° and didn't start to open until 190° fully open at 200°, replaced the heat exchanger cap from a 15 lb cap to a 20 lb cap, replaced the sea cock because it was not fully opening. Removed the thermostat to see if running it with out it would keep it cool.. Today was the 5th time I trucked it to the lake with high hope only to have it run up to 200° when running at 3/4 throttle, back off and it drops down to 140/150 range. I guess it's time to pull the exhaust manifold. You thought guys

[Ship]

Pat, where/what are you checking the temp with? 200* is not that bad IF you're not going above that and depending on where you are taking the measurement. That it returns to 140/150* says it's doing what it should.

[Striper Sniper]

The temp gage sensor as I recall is in or around the thermostat. I'll check in the morning.

[tuner]

Did you replace the raw water pump? Maybe the flow is not enough at higher loads? Is the pump belt driven or crank mounted?

[Striper Sniper]

Did you replace the raw water pump? Maybe the flow is not enough at higher loads? Is the pump belt driven or crank mounted?

Belt driven, supposedly it was new when I bought it, I'll have to check it out. I may of been mistaken to assume it was in working order.

[Quik Fix]

I'm with Steve on this. Sounds like an impeller to me. Check the hoses in the RW side for blockage.

[Ship]

I thought you said you had checked those early on but if you didn't, that should have been your first check.

[Ship]

From intake thru Hull to manifold. I know you checked the HE. Did you check the trans HE too?

[Fish Witch]

One of the things that could be causing a high temp issue at cruise speed like you're experiencing I've seen posted on another forum about my Yanmar (I can't remember where, Diesel Dudes on the other site or Boatdiesel), and is kind of sneaky is that some after market SW impellers are a bit wider than the OEM impellers and cause more friction running, which can actually cause the rubber impeller to slip on the hub at higher speeds. You really can't tell it by just looking at the impeller so kind of hard to figure out. You said you have a new pump so less likely but worth a change of impellers if all else fails to correct your problem.