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Temperate control in 5.7L MKS carbed engine

sandkicker

sandkicker

Outstanding Contributor
The trouble apparently started two years ago with a slip and fall that ended up with my foot neatly breaking off the end of the much hyped Blue Plastic drain valves on my circa 2006 MERC factory (5.7MIE) FWC engine. Anti freeze started running out! Fortunately no one had a camera or it would have been on You Tube. Finger over end to stop the leak, stretch to reach cell phone call to wife with other hand to get a suitable can, scramble with a large coffee can to catch the antifreeze and transfer it to another larger container until the level dropped to almost nothing. I replaced the blue valves with brass plugs and refilled the system with antifreeze.

It took me awhile to notice it but yesterday I paid attention to when the engine went from enrichment 1200 RPM idle to 650 RPM. I had checked the manual and this transition point should be in the 130 to 140 degree range. Now it does not change (the "hiss" starts to moderate and engine RPMs drop) until Temp rises to just over 160 degrees and then drops slightly, i.e., when the T'stat opens. IF you watch closely, you can see the temp gauge go up and down a few degrees as the T'stat opens and closes at idle. (Was recharging a bilge pump depleted house battery)

It seems to me as if I might have a air pocket in the engine. Some other marine engines I've owned had a air bleed valve near the T'stat housing. So far as I can see, this one does not have one. The heat exhanger is full of antifreeze on the engine side. I don't think that the sensor is funky as it would too much of a coincidence for it to fail by changing from 130 to 165 degrees.

I first noticed problems last year with a wandering idle when traveling at idle for long distances , i.e., idle would go from 650 in gear to 1100 and then shift back after 30 secs to a min. I'd posted this last year on this forum and never found the problem. It had been a ratty boating season due to personal/family schedule so the boats not been used much at all ( 3 times all summer! July to Nov!)
 
If it has a TKS carb the enrichment parts may have decided to malfunction. If there was air in the engine water jacket the coolant temp. sensor would show a drastic drop in temp. when it was sourrounded by air and not coolant...it needs to be submerged in coolant for the thermister to work. Make sure there is a good ground for the sensor--if the sensor is in the T'stat hsg. the ground is created by the lower hsg. gasket--it has two brass buttons to complete the hsg. to engine ground. Does it have a coolant overflow tank? Is the coolant level in the tank fluctuating when the engine has run a while? If not test the coolant pressure cap.
 
Bob, that is a good tip on the brass buttons.
They must complete the negative circuit in order for the sensor device for work correctly.

Also, as for trapped air within the cooling system, look closely at the thermostat.
It should have a very small air bleed hole in the body.
Some will have a brass or SS pin that is crimped so that it remains there.
4236%20thermostat%20lowres.jpg

This bleed hole allows for any trapped air to excape after initially filling the system with E/G/h2o.
It should also bleed any air that may have accumulated afterwards.

.
 
Use a digital volt/ohm meter on the lowest ohm scale to check for continuity between the side of the sensor and the engine block. Any ohms above "0" means the gasket buttons are funky or corrosion has built up aroung the sensor. If you pull the sensor do not use any sealer or tape on its threads when reinstalled. I always scuffed the contact areas of the T'stat hsg. and intake manifold where the gasket buttons make contact. A thin film of marine grease on both sides of only the buttons prohibits corrosion.
 
You can also remove any paint beneath the bolt heads, and install star washers for continuity.
Do both, and you'll have some redundancy.

I agree with no teflon tape, but thread sealants should not prevent continuity.
 
Boat due to be hauled this Sunday afternoon. Will have some debug time Saturday morning so will do recommended checks... Rick, good tip on the bleed holes in the T'sat. I'd not considered that even though I know they should be there. I was less than satisfied by the job done my the mechanic who installed the engine. The FWC is not MERC factory, it's a kit, so who knows if it has the correct t'stat, i.e., one with holes.
 
sandkicker said:
... Rick, good tip on the bleed holes in the T'sat. I'd not considered that even though I know they should be there. I was less than satisfied by the job done my the mechanic who installed the engine. The FWC is not MERC factory, it's a kit, so who knows if it has the correct t'stat, i.e., one with holes.
Click to expand...
If the stat does not have a bleed hole in it....., one can be drilled. Just make sure that it's small.
The stat won't even know that it's there!
 
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