General Thermostat Question

[jayhanig]

I have a bad tendency to convert working equipment into expensive paperweights. Before I pull the thermostat cover off, I'd like to have at least a replacement gasket available. Identifying what I've got without opening it up has been a challenge. The service manual says to use a 140 degree thermostat for an open water system and a 160 on a close cooled engine. Mine is open cooled, only two years old, and has always been flushed with Salt Terminator after every use.

What I'm curious about is that once warmed up, my engine runs at 175 all the time; no matter whether at idle or on plane. If the proper thermostat is a 140 degree one, does it seem reasonable that the engine runs 35 degrees warmer? It has always done this. It seems to me that although the thermostat might open at 140, there's nothing to say that the ultimate temperature reached would be the same. So the question becomes: how much higher than rated opening temp should the engine operate? What is to be expected?

The engine is a 1998 GM built Mercruiser 3.0 liter four cylinder. And while you're considering this, does anybody know off the top of their head what part number gasket and potentially thermostat I need for this? I'm inclined to think my thermostat's OK but I know I'll need a new gasket if I open it up no matter what. I tracked this down once in the past but now I can't find the information.

 

[Fishermark]

I don't have the part numbers handy, but have you owned this engine since new? On the temperature - perhaps the previous owner put a higher temp thermostat in. Or perhaps your gauge is reading incorrectly. Those are just a couple of possibilities.

 

[jayhanig]

I bought the engine as a long block still wrapped in plastic a couple of years ago and had a local guy finish it out for me with a combination of new and gently used parts from another engine with a known history. It is entirely possible it has the higher temp thermostat in it... I won't know until I open it.... but I don't want to open it until I have a replacement gasket in hand.

 

[Alumarine]

If it uses the 3 hole thermostat cover I think the part no. is 27-475901.
A serial no. would confirm that.
Nice looking boat.

 

[achris]

Yep, need the engine serial number. To know what the actual temperature is, you need to check it with an IR gun. What the gauge shows is just a rough indication and is not a guarantee of complete accuracy...

 

[jayhanig]

Yep, need the engine serial number. To know what the actual temperature is, you need to check it with an IR gun. What the gauge shows is just a rough indication and is not a guarantee of complete accuracy...

The serial number is 1M07188RX. I happen to have an IR gun. Does it matter where on the engine I point it?

 

[mr 88]

You should be pointing it in the area of the thermostat,preferably by the temperature sending unit. Pointing at the cylinder head by the plugs will give you a really high number and pointing at the exhaust manifold may give you a lower number.

 

[alldodge]

The serial number is 1M07188RX. I happen to have an IR gun. Does it matter where on the engine I point it?

Unable to find the S/N listed, also never seen one with an RX. S/N should be a number, then letter, then 6 more numbers (1X000000). Tried removing the RX and replacing with a single number and no luck. Here is the list of S/N at Mercruiserparts

0L097000 THRU 0W301999

0W302000 THRU 1A0019999

1A002000 THRU 1A020543

1A020544 & Up

 

[jayhanig]

Since it might freeze tonight, I went ahead and dumped the water out of the block and manifold today. while I was at it, I dug into the thermostat housing. 1) The cover is held on with two bolts and lock washers. 2) The gasket is a solid one, ie, no hole in the center. 3) There was a white plastic spacer that the cover pressed against to hold the thermostat in snugly. 4) The thermostat itself is a stainless steel one stamped "160", which I believe properly is only used on closed water systems... which mine is not.

I have to wonder why that thermostat is in there instead of a 140 degree one. Was it because that's what the guy who installed it had on hand? Or did he have a technical reason for doing so? In any case, there's two reasons to leave well enough alone: 1) It's running fine. 2) The needle on the temperature gauge points to the 12 o'clock position when the engine is warmed up, allowing me to see at a glance that all is well without really having to read the gauge.

Tomorrow I will take the housing cover with me and attempt to find a replacement gasket at a local boat shop.

 

[Bt Doctur]

guessing its the one on the right

 

[jayhanig]

It is just like the one on the right with the exception of gasket "c" actually looks like a gasket version of the housing cover "b". So if you were looking at a perfect exploded diagram, it would appear as if there were two cover "b"s; one made out of steel and the other made of gasket material. What confuses me is that everywhere I've seen similar diagrams, the version I have doesn't appear to be associated with my engine but with larger ones.

 

[Fishermark]

It is just like the one on the right with the exception of gasket "c" actually looks like a gasket version of the housing cover "b". So if you were looking at a perfect exploded diagram, it would appear as if there were two cover "b"s; one made out of steel and the other made of gasket material. What confuses me is that everywhere I've seen similar diagrams, the version I have doesn't appear to be associated with my engine but with larger ones.

On the gasket - it doesn't matter if it is solid like you describe, or with the middle hole as pictured. It simply seals the cover and no water is going through it. Who knows? Maybe someone made their own gasket from gasket material.