Open Cooling Gasket Use

[allyolds68]

I've got an open cooling 83 260 Mercruiser that I'm going to replace the thermostat in and I can't find anywhere what gasket configuration is supposed to be. I know the lower gasket is the one with the four rivets (shown below at right) but the upper cover has three different types. Totally closed (I'm 99% sure it isn't this one), the one with the bypass plugged (top photo), and the one with the bypass open (bottom left photo). Based on a few listings I've seen I think the bypass is supposed to be blocked on open (raw water) cooled.

I've been running the one with the bypass open and I've had an intermittant overheat condition. It's not overheating significantly because I checked it with an IR thermometer and it's not going over 165 with a 142 degree thermostat (142 should be correct for this year), it's just going into the red on the gauge but not pegging it. As soon as I back off a plane and take off again it usually doesn't happen again. My initial thought was I was getting fouled with weeds (I live in a pretty weedy area of the lake). My next thought was the thermostat is sticking. This happened quite a bit after I first put the boat in last year but as the season went on it rarely happened. (also leading me to believe it's the thermostat). It's got a four year old SEI drive on it and I've replaced the stock drive pump with a Sierra pump. My gut tells me that the bypass has nothing to do with this issue but if I'm going to throw a new thermostat in I'd like to put the correct gasket in.

Bypass Plugged:

Bypass open:

 

[Bt Doctur]

don't recognize that housing , is it an extention? Is there a number plate just above the starter

 

[Watermann]

Sounds to me like you need to change out the impeller. I just did a T-stat job on my 4.3 L and it has the blocked bypass gasket. The T-stat kit here at iboats has all the gasket. The brass sleeve the t-stat sits inside is a monster PITA to deal with though.

 

[allyolds68]

don't recognize that housing ,

I have the housing you pictured above, I was just using the other photo for the gasket reference.

Sounds to me like you need to change out the impeller. I just did a T-stat job on my 4.3 L and it has the blocked bypass gasket. The T-stat kit here at iboats has all the gasket. The brass sleeve the t-stat sits inside is a monster PITA to deal with though.

I've been told that some of the SEI units perform much better by replacing the entire drive pump with genuine Mercruiser assembly, however the problems seemed to be isolated to early SEI units and mine is fairly new.

 

[Bt Doctur]

OK then, if you have that identical one you use the closed gasket.. with the closed gasket the water flow is thru the check ball T fitting to the risers to keep them cool.
once the therm opens the check balls remain closed and the water goes out the top and thru the cap and to the bottom of the manifolds.

 

[allyolds68]

OK then, if you have that identical one you use the closed gasket.. with the closed gasket the water flow is thru the check ball T fitting to the risers to keep them cool.
once the therm opens the check balls remain closed and the water goes out the top and thru the cap and to the bottom of the manifolds.

Is there any chance that using an open gasket could be related to the intermittant cooling issue?

The check balls are new BTW. I also pulled the manifolds and risers last year and everything is clear

 

[Bondo]

Is there any chance that using an open gasket could be related to the intermittant cooling issue?

Ayuh,..... Welcome Aboard,..... Yes, it can,.....

 

[allyolds68]

Ayuh,..... Welcome Aboard,......

Thank you sir. I've been here for three years or so but I mostly use the site to search for answers. Usually I find them without posting. This issue has been going on for a few years. It's not really bad enough to warrant tearing everything apart but we've owned the boat since 98 and we're selling it this year so I'd prefer to make sure it's running well with no "issues"