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.

[gstar]

In answer to the last post, the impellor is brand new, never been run dry or attached to a hose and I
checked it before I changed the thermostat and it was fine.

I just installed the 143 degree thermostat today and it has helped reduce the operating temps to between
140-160 depending on what I'm doing with the boat. At this point I'd say problem solved, but for the
fluctuation of 20 degrees, which I never noticed before. Is it normal to have some movement up and
down depending on speed, load etc.?

... And new problem today after I took two massive waves over the bow, the boat ran fine and the bilge pump emptied the water out quickly, but when I went to start it after sitting on the lift draining for a few minutes, it wouldn't start. Specifically, the blower was running but the starter didn't even make a clicking sound. Pure silence when you turn the key (except blower running).

I'll start a new post for this issue, but obviously something got wet that didn't like it. Any ideas?

[gstar]

Thought this issue was resolved and maybe this is normal, but I had the boat heat up to 220ish degrees while idling for
about 15 minutes. As soon as I started driving it went right down. Is this normal? I'm paranoid now that it overheated once.

[Blackntan90]

A couple of years ago I had my Mariah(351PCM) overheat from an impeller gone south- I had the same issues as you are describing-temp fluctuate 10-20 degrees. I took all of my hoses off looking for the blockage, finally I used compressed air to blow an impeller vane from deep inside a hose where I did not see it! Best of luck to you- let us know how it turns out!

[earl b]

I have a 1990 Mariah and just repaired a heating problem. I replaced the impeller and the thermostat and remove parts of the impeller from on top of the thermostat .Went to the lake an the temp was back to normal. After about a hour the temp when over 200 deg. When back to the house and took all the hoses loose and ran water threw each hose and transmission cooler no blockage. Took the raw water pump back out and the impeller was good.But the inlet to the raw water pump hosing was full of old impeller.You might check there. I still don't know how it got there since it is not a closed loop system. Hope this helps.

[devovino]

When I bought my Saltare, the PO told me that "if I wanted to get better MPG, to install a thermostat" which I assumed meant there is no thermostat on my 454. She runs like a champ, never had a propblem wih her heating up, in fact I assumed the only gauge not working on the Saltare was the temp gauge, then yesterday I had 7 people and 1000 lbs ballast on the boat was running around doing some surfing, temp never registered. But once I stopped to just float and listen to the radio, I noticed the temp gauge was registering ar 150, so I now see that it works, I think the engine is always really cooled from constant flow of water. In talking to a few mechanics, they think the only real problem with this is it might never really reach a good operating temperature, and maybe run rich,does anybody else see a real problem with a really coo engine, I figure better cool than too hot! Thanks Devon

[docdrs]

Check all your intake connections to make you have no places you could be sucking air. Check your impeller housing gaskets and cover plate screws so no possible air leaks

[DAFF]

I am wondering if the reason the first impeller went out was from lack of water. I have a fealing you should inspect the lines from the hull to the impeller. Check for obstructions, cracks, broken or missing hose clamps ect. Remove every hose and look through it.

Don't over look the strainer perhaps some seaweed or debris stuck in there.

[rludtke]

When I bought my Saltare, the PO told me that "if I wanted to get better MPG, to install a thermostat" which I assumed meant there is no thermostat on my 454. She runs like a champ, never had a propblem wih her heating up, in fact I assumed the only gauge not working on the Saltare was the temp gauge, then yesterday I had 7 people and 1000 lbs ballast on the boat was running around doing some surfing, temp never registered. But once I stopped to just float and listen to the radio, I noticed the temp gauge was registering ar 150, so I now see that it works, I think the engine is always really cooled from constant flow of water. In talking to a few mechanics, they think the only real problem with this is it might never really reach a good operating temperature, and maybe run rich,does anybody else see a real problem with a really coo engine, I figure better cool than too hot! Thanks Devon

Hi Devon,

Running under the design temp is almost as bad as excessive temperatures. The parts of the engine are sized to work at a certain operating temperature range. Below or above this range, the parts will be either to large or to small for the space that are placed to work in. The engine uses a lot of steel and aluminum parts in near proximity to each other (such as bearing surfaces of the crankshaft) neither of which expand or contract at the same rate when heated or cooled. Excessive clearances between moving parts, and/or inadequate clearances between moving parts will result in less than optimal performance, excessive wear or damage to your engine.

I recomend you install the correct thermostat for your engine (and replace it every several years), and confirm that you gauge is functioning correctly. A simple test for this is to remove the senser, and while connected to the wiring (with power and ignition switch on) place the sensor into boiling water, and observe the temperature gauge response. If the gauge indicates 212 degrees F (or thereabouts) you should feel confident that the gauge/sensor is functioning and is reasonably acuright.