Is the 140 deg the correct thermostat for a 1988 Merc 7.4L RWC? Had overheating prob above 3200rpm and pulled t-stat and ran cooler w/ none installed so think may have been part of my overheating problem. Runs about 110 at 1500rpm, 160 at 3200rpm and 180 at 3600-4000, is this about right?

Also how hot should risers get? Temp gauge only reads about 165 but they are definetly too hot for me to touch for longer than 5-6 seconds. If temp seems right but risers too hot what should I look for? Based on touch both are running the same temp. The cooling lines from the t-stat housing to risers (top ones) are cool when running and the lower one to the manifolds are warm. Thanks

Never ran without a T-stat but that seems hot for none installed. I would suspect a flow rate problem first. How is the impeller?

RMM i would run a 160 degree... some say 180 degree...i think your engine will run a little better. 140 degree is to cold IMHO. my risers at 160 degree operating temp are also hot to touch for very long...been that way since new and it is exhaust so it will be hot.

oh yea it could be plugged lines...flush em out while your in there

Thanks guys, I did go ahead and flush all the lines. The impeller and circ pump seem to be fine, I pulled the lines and ran for a second to see how the flow was and looked good. May try pulling the risers and cleaning the passages out too. Someone said try soaking in muratic acid?

Two questions:

What does the little rod with the spring and little balls in the t-stat housing do?

What are the +- of running a hotter 160 vs a cooler 140 t-stat? Checked the manual and it calls for a 143deg t-stat.

Going to get a infrared thermometer to measure the temp of the risers. Can anyone tell me what temp would be in the proper range? Assuming would be hotter than the water temp guage as is exhaust.

RMM, how old was the T stat, sounds like it was stuck if the temps qouted were with it installed? Also, when was the last time the sea water pump impeller AND housing were changed? They can look good and still not be performing up to standards. If they are more than a few years old I would change them just to be on the safe side, especially if you beach the boat ocassionaly. I used to try to get by, but found it to be less trouble to simply change them.