Water temp rising and falling
 

Broke in a new 498 blower motor today with a Bravo One with no ciculating pump, a crossover, and new impeller in the raw water pump. Timing, fuel ratio, oil, fluids, all look good. Here is my issue. I run a Merc 140 degree thermostat, same as I did on the last setup which was the same motor except for the heads and a few cubic inches. Now the temp goes from 150 to 200 and back to 150 in about 15 seconds. It then stays at 150 for about 5 minutes and then starts the process all over again. Thermostat? Sender?
--------------------------------------------------------------------------------

Are you running a circulating pump or a cross-over?

Crossover, thanks!

Sorry, I missed it in your first sentence. (It's been a long day.)

Make sure someone didn't put the thermostat in upside-down.
Don't laugh, it happens!

Thermostat could be defective. Try another one.

Yup, I thought of the backwards thing but I was actually showing someone how to put one in at the time so I know it's in correctly!

does the t-stait have three small holes drilled in the outer plate?these holes i am told are there to help keep the engine up to temp and at a constant rate.

Thermostat most likely.
Other less likely possibilities. Collapsing intake water hose or head gasket failure.

Also check for an air leak before the seawater pump, such as a loose freshwater flush fitting if the T-stat shows good.

No, it does not have the holes. It is a stock Merc thermostat came withoiut them and my last one had no holes either.

ghittner,
The holes are suppose to be there to eliminate this particular problem. When I bought a Stainless Marine cross over 15yrs ago.. the holes were there. I am still running that stat today. I havent seen it in a long time, but they are 1/8" I think.. Try drilling three evenly spaced holes in the flange. If it helps, but doenst cure it.. drill them to 3/16"... that should do it.!!
Or call Stainless and see what they recommend..

Hope this helps.

D*ck

the holes are 3/16 and drilled in a three-triangler patteren.i am running a ssmarine cross-over and this is the same t-stait mr gadgets talked about.i amrunning a blower on this motor also.temps are around 140-165 depending on water temp and how hard i run.also are your temp senders on the front of the motor?i ran mine at first on the rear/top of the manafold and the temp-gage always showed too hot....around 150-185.moved them to the front and temps read normal now.hope this helps

AZ Speed and Marine may have a high flow T-Stat. I believe I got a 120 high flow from them.

Front top of manifold.

Just spoke with Stainless Marine, and they say it needs the three holes. I'll post my results, thanks guys...

Just spoke with Stainless Marine, and they say it needs the three holes. I'll post my results, thanks guys...

PROBLEM SOLVED! three holes and another new thermostat. I now run @ 165-170 consistently