mattyd272

I have a new-to-me Mercruiser 502 (carbureted) engine in my boat and it uses an open/raw water cooling system. When I took it out for the first time, I never saw movement on the water temp gauge. Pulled wire off the sender and grounded it, gauge pegged, so I know its good. I then discovered what I assume is why, there is no thermostat in the housing. Water I was boating in was probably in the 50s so that explains it I guess.

I assume I should put a 140 degree marine thermostat in there, right? My question/concern is why previous owners ran without a thermostat. Maybe there is a valid reason and I need to leave it out?

I think I understand how the cooling system is set up in my boat, so maybe this helps get some insight. Belt driven raw water pump sucks in cool water and pumps it to an oil cooler. From there it goes into the front of the block on either side of timing cover. Through the engine, out the t-stat housing, and into the exhaust.

If I install a thermostat, wont the exhaust get too hot since no water would be flowing through it until the t-stat opens?

mattyd272

Here the best photo I currently have, in case it helps.

Blueabyss

160 thermostat. Post a pic of your thermostat housing and water lings and your exhaust manifolds. Most have a type thermostat housing that has multiple outlets and water is bypassed all the time to the exhaust manifolds.

mattyd272

I think I found my answer. Engine has a water crossover kit but there is no bypass. The ports in the crossover tube and tstat housing for the bypass are plugged, so it has to run without a tstat or the exhaust wont have water flow until it opens. If I install a tstat, I need to remove the plugs and install a bypass so exhaust sees water at all times. Now, the question is, do I really need a tstat.

SB

Don’t put a tstat in your system.

mattyd272

Thank you. Any insight as to why I should leave it versus installing a tstat and a bypass?

SB

I’m the wrong person to ask.
‘cause
for most engines i like the merc recirculating pump and tstst housing set up. Usually provides better temp (water and oil) and psi (water) control.

TomZ

If you need to build some heat, you can add a restrictor in place of the thermostat. Moroso makes a few different sizes - you'll need to experiment to get the right balance between block and exhaust temp.

mattyd272

Thank you for the lead on the Moroso restrictors! $17 on amazon for a pack of 3 with diff size holes to find the right one. I will give it a go!

33bajaoutlaw

If that picture is representative of how your boat is configured you can not run a thermostat at all. Don't even think about a resistor plate. You will not get enough cool water to the exhaust. You will overheat your exhaust.

Engineers have designed your motor with a thermostat housing in mind to guess what,,,, house a thermostat. You want the motor to get to temp to achieve best performance. You can read on the internet the negatives of running a motor cold. There are no positives.

There are a number of thermostat housings on the market with internal bypass to insure the exhaust get water as the block heats up.. It will not work with that circulating pump elimination crossover pipe in the picture for what I can tell..