Cooling System Issues...Help

[gstar]

I have a 1992 Mariah in good condition with low hours on it.

The impeller went out on it the first time I drove it after I had it looked over by a marina. This in turn burned out my muffler and a couple of rubber hoses between the manifolds and the muffler.

Since I had this fixed, the boat was running great, but this weekend I noticed the temperature gauge moving up and down quite a bit. I never noticed this before.

Specifically, the temperature was holding steady at 160 and then when my wife gave it throttle to pull me out of the water, she said it immediately climbed to just over 200 and then came back down to 160 for my whole run. When I let go and she circled back to pick me up it went up to 180. After we drove it around at 1500 RPM for a minute it went back to 160 and then as she was going back and forth having trouble lining up with the lift it started heading over 200 again. We cycled around at 1500 RPM again and it went right back down to 160.

Long story short, it never used to do this. It would go up maybe after shutting down, but regardless of speed it always held 160 during operation of all kinds from full throttle to idling. I checked the impeller and this is still fine. There is no play in the water pump shaft and the pulley spins smoothly.

Any ideas? Thermostat? Partial blockage? Bad Gauge? Bad Sensor?

The boat starts and runs perfectly but for this temperature fluctuation issue.

Any insight would be greatly appreciated. I'm scared to drive it at this point.

[wotan2525]

As long as it doesn't go too far over 210F, you're fine.

Could be bits of the old impeller stuck in the thermostat. Might want to replace the thermo with a new one as long as you are taking it apart. The temp in my boat goes up and down between 160 and 180.... I just assume it has to do with the thermo opening and closing.

[gstar]

Do I order a 160 degree thermostat? Would this explain the erratic temperature readings?

[docdrs]

yes order a 160 one

[jet]

Check ALL of the above.

[scottchapin]

On a water test run the boat would heat up too much (190) while idling but would cool off as soon as we started going so I thought the impeller was bad. When I took it apart the impeller still looked good but I put the new one in anyway (NAPA). After putting in a new impeller the same problem was happening. Turned out to be the EZ flush water system. I’ve got the type where the garden hose connects to a fitting mounted to the transom and then T’s into the fresh water intake hose. The plug that you put in when the hose is not connected was missing and the system was letting the impeller draw air through the line. When I got a plug and stopped the impeller from drawing air in through the fitting, and thus through the T in the fresh water intake hose, the problem went right away. I think the T is supposed to have a spring loaded valve to prevent this but apparently it needs replacing so it’s on the list for someday.

[gstar]

I'll order the new thermostat and change it out this weekend. Do auto part stores like NAPA carry marine thermostats or does this need to be ordered through SKIDIM?

What is the best way to manually test the temperature and bypass the sender and gauge?

[wotan2525]

The gauges and senders don't often go bad. You can use a 160degree auto thermostat. Any NAPA should have it in stock.

[gstar]

Quick update on this. I installed the 160 degree thermostat (what a pain to change one that had never been changed in nearly 20 years) and this has seemed to help, but the temperature is still running higher than it used to and ranging between 175 and 190 (used to stay steady at 160). I ordered the 143 degree to see if this would explain the higher temperatures. The boat is running great, but I would prefer to see the temp fluctuate less and be down where it used to be. Is there any concern about prolonged operation up to 190 degrees? Is it normal for the temp to fluctuate 10-15 degrees as you transition between different speeds, usage etc?

[rludtke]

How much use does your new impellor have on it? Has it ever been run dry? Has it ever been run at rpms above idle while connected to the hose? These things can accelerate the wear on the impellor. Are your exhaust manifolds hot enough to burn skin (or nearly so) when the engine is running? If you can say yes to any of these questions, then I think you should replace your impellor. Chances are, your temps will come down, and remain more consistant with a new impellor installed in the raw water pump.

The impellor should be changed often, arguably once a season (cheap insurance). Thermostats should be changed out as well, but perhaps the invterval can be a little longer. Neither of these parts are expected to last the life of the engine, and require replacement as part of normal maintenance.