Trojan Boat Forum

both my engines are raw cooled, 5.7 Merc's
both risers are in excellent shape, low hours and inspected
new pumps

my situation and I dont know if its normal is that on both my engines my outboard riser is warmer to the touch than the other side.
here are the particulars
raw water enters bottom of each manifold on all 4
I cleaned all 4 nipples (where water enters) and flushed out just in case

engines do not overheat, but,,,, it gets pretty hot on the one that is warmer than the other
I've seen this before on a few boats and not sure if this is normal, assuming the water exists equally to both manifolds after it leaves the thermo housing

It is normal for one manifold to be hotter than the other, within reason of course. This is where an IR temp gun comes in handy on board. Does this happen all the time? Only in idle in gear or out, or only at higher rpm in gear? Check to see if you have poppet valves at your t-stat housing where the hoses hook up to on either side. They would be two white spring loaded plastic balls, one of them may be sticking and restricting flow.

no I dont think I have those poppet valves,, never seen that

You have to remove the hoses to see them. What is the difference in temperature?

I had a similar problem. Turned out to be carbon buildup in the riser blocking free low of water. I had to remove exhaust hoses from the risers and dig and scrape around up inside the output end of the riser to remove the carbon buildup. Worked for me. Good luck.

BTW, I did lots of low speed trolling on old engines which probably caused the problem.