Overheating MC

[jayski]

Friend has a '19 6L Mastercraft and religiously overheats between sets. First set runs perfectly fine, drive it back to the dock, it sits for 10 min shut off, restart and by the time it idles to the end of the lake, maybe 30 sec temps are climbing into high 200's...almost like it gets air locked or something, increasing revs (without driving) brings it back down and then is perfectly fine till the next time...always happens between first and second set, sporadic from there on

 

Any ideas to fix???

[MISkier]

Did it ever shred an impeller previously? If so, remove hoses and start looking for debris from that which may be impeding water flow.

[markn]

Sure you have checked the usual suspects; impeller, plugged strainer or strainer tight and not sucking air. Also may want to check temp sensor to be certain temp reading is accurate. The fact that increased revs brings down temp, it sounds like lack of flow...impeller or sucking air. Good luck.

[MISkier]

One other thing: has the boat been run extensively in muddy water, etc.? Over time, a lot of mud in the engine will harden like concrete and block water flow, which may be overcome by higher revs finally pushing enough through the narrowed channels. If is gets bad enough, you get hot cylinders and melting pistons.

[ColeGiacopuzzi]

@jayski That sounds like the check valve in the back of the motor between the two hoses. Usually if you rev the motor it will clear the air pocket that gets stuck in there and you can literally watch the gauge fall in temp instantly. We had this problem in our '20 6L and I bought the higher end check valve straight from illmor and its been good. "knock on wood" The one that comes from the factory is a cheap plastic one where the illmor one is made of brass. the actual name is pressure relief valve.

[Mastercrafter]

Sounds like impeller, water pump gasket/o-ring, or air leak in raw water intake. Check all your connections starting at thru-hull intake, trans cooler, water pump.

 

The PRV previously mentioned fails in an open state, allowing water to flow through the exhaust without being pushed thru engine. Get the brass one.

 

How old is your impeller? I’ve had impellers that appear good, fail to work.

 

I was watching a 2018 5.7 with computer for a few passes this weekend, and I was at 165 down the lake, and 175 at idle between passes. 170 degree thermostat.

[jayski]

Thanks all, but I think @ColeGiacopuzzi has the answer...same issue, seemed to cure it...will get him to order one and see

[bigtex2011]

or just get a malibu

[jayski]

@bigtex2011 I'll pass that on lol

[boscoman]

ColeGiacopuzzi, where is this valve located...back of engine, correct? toward the stern? I don't see it on my ilmor 6.0 mc x14v

[ColeGiacopuzzi]

@boscoman I don't think that's the same engine, so yours might not have it. The newer 6.0L is called the MPI I think. If yours does have it; it would be right on top towards the back of the motor. Two hoses come from each side and between them is the valve.

[Mastercrafter]

On the 5.7, here’s the valve location and update kit PN