Cold running engine??

[Keith7481]

My mechanics are stumped, me toooo. I have a 1987 Cobia Merc 3.0 that I've had for about 3 years, my first boat. Since I"ve owned it the temp gauge has not moved while the boat is running. I've tested the gauge and it functions, so this year I put in a new thermostat. I suspected it didn't have one. Well, it did. When getting ready for the summer, I had the techs look at the problem, they cant figure out why the block never heats up. Upstream of the thermostat the hoses/block never gets warm, and the water coming out of the exhaust is about 80, well short of the 140 you would expect when the engine is hot. They don't know why the engine doesn't heat up, but they wonder if water could somehow be leaking/spilling past the thermostat at a rate high enough that the engine doesn't heat up. Considering this problem occurred with the last thermostat, and now with this one, I am questioning this theory. I used the correct gasket, and made sure the housing was clean before installing the new thermo. Any other reasons an engine might not heat up enough that the temp gauge would not get to 100, the lowest temp that the gauge will register??
keith

 

[Don S]

Re: Cold running engine??

The water coming out the exhaust will always be cooler when there is no load on the engine. Most of the water does NOT go thru the engine. Just straight out the exhaust.
Check the actual temp near the sender with an IR temp gun. Then you will know what the actual temp is.
Are "YOUR" mechanics, Mercruiser trained mechanics or the local auto mechanics that don't understand marine but like to work on them????
Just a question, because this happens a LOT

 

[johnbo]

Re: Cold running engine??

I had a problem like this last year. For my engine, it was an old piece of impeller that wedged into the bypass passage within the thermostat housing.

My understanding of what was happening, was that the pressure from the sea water pump would not be relieved by the bypass passage and would force open the thermostat. In my case, the engine would warm up at idle quite normally, but as soon as I opened the throttle, the temperature would drop to the lake water temperature.

The piece of rubber that I pulled out (once I found it) looked like it would lay there in the passage until the flow got high enough, then would crumple up and block the passage.

My two bits,

johnbo

 

[Keith7481]

Re: Cold running engine??

Thanks to both for your replies. I don't know for certain if these mechanics are merc trained, but they work at one of the larger marine service places in this area, so they only work on boats. They did put a gun on the sender area, and they said it never got above 80. As JOHNBO noted, I did find a small piece of impeller lodged in the bypass of the thermostat housing when I replaced the old thermostat. Now, I'm wondering if I have a blockage someplace that forces water past the thermostat and resulting in a cold engine?? I had the impeller changed and they showed me the old impeller, one of the vanes was sheared off, no idea how long it has been that way. The mechanics did not seem very concerned about the cold temp, I think they argued that, as long as I changed my oil regularly, I shouldn't have much sludge build up and other than that, it shouldn't be much of a problem??
Keith

 

[Don S]

Re: Cold running engine??

Does this happen only on muffs, Does the engine get warmer when actually out on the water and under load?
The cooling system on those engines are fairly simple and not too many things can cause your problem. Thermostat (even a new one) could stick open.

 

[Keith7481]

Re: Cold running engine??

Seems to be happening all of the time. The temp gauge never indicates that the motor is warming up. The only time it has heated up is when I had the muffs on and accidently restricted the water supply. Then the gauge showed warmer, which I'm sure it was
The old thermostat was closed when I removed it, as was the new one when I put it in. It seems that there is too much water going through the bypass, which really doesn't seem possible, or something is blocking the bypass and forcing the thermostat open??

 

[Don S]

Re: Cold running engine??

I can't see a rubber impeller building up enough pressure to open a thermostat. If the bypass hole was blocked, your exhaust would be hot.
You can test your pressure theory. Just squeeze the hose that goes from the tstat housing to the cirulation pump.

 

[Keith7481]

Re: Cold running engine??

I agree, not very likely thats the cause, but i'll give the hose a squeeze. That was the only idea the mechanic could come up with. So, why else could it act like the thermostat is not there?? I think these systems are pretty simple, I was thinking of blocking the bypass to see if it would then warm up enough to get the temp up, but that doesn't sound like a great idea...
K

 

[Don S]

Re: Cold running engine??

That bypass is what keeps water in the exhaust. Without that those hoses coming off your exhaust elbow wouldn't last very long.

 

[Keith7481]

Re: Cold running engine??

I thought there was probably a good reason for the bypass, so any other thoughts on why this might occur?? If the mechs are right and it doesn't cause any other problems, I'm not going to tear it apart, but I'd like to have the engine run at correct temp if possible. I mean if it weren't important at all, they would never even put a thermostat in.

 

[QC]

Re: Cold running engine??

Couldn't one use the IR gun to follow the temp changes throughout the entire system and "see" what's happening? i.e. temp on both sides of the t-stat housing, through the hoses, the block etc.? That should show if you are getting a good "seal" in the stat hosuing etc.