Overheating 350 Mag MPI

I have a 2006 290 SLX with twin 350 mag MPI motors with Bravo 1 drives. I recently developed an overheating issue that I cannot overcome. It began by running well while on plane but would overheat when I returned to idle (manatee zones). I was able to overcome this by backing down slowly on this engine while returning the other to idle. It became worse on the next use and overheats every time the engine is run.

I replaced the impeller in the sea water pump, even though it was perfect on inspection. It was open so why not. I replaced the water circulating pump as well. I back flushed the water distribution hoses to make sure nothing had gotten sucked up in them (impeller pieces, etc) and nothing came out. I felt high temp on the power steering cooler hoses, but when I opened them and flushed/backflushed and the cooler tube water flowed without restriction.

I have replaced the thermostat (twice, just in case). I even removed my thermostat entirely to see if it helped...it didn't. When I disconnect my hoses to the manifold risers (1.7 inch) water flows with what seems like adequate supply. I do notice it is not as much as the normal working engine but it definitely seems to be adequate.

I am able to keep my hands on the risers endlessly though this although my oil filter is very hot. The temp of my exhaust water does not increase (once up to operating temp) even though my temp gauge rises to alarm. I have had my manifolds off within the last 6 months so no rust build up in them. No water in my oil. Fresh oil change.

The only thing that helps is if I open the thermostat housing drain and let water flow into the bilge the temp will come down and run within normal range. I found this by accident but it is not a reasonable thing to flow seawater into your bilge when in the water. Bloop...bloop

I am out of ideas. Any help you can give would be appreciated.

i have very similar experience... several times excessively flushing, disassembling..

finally found a piece of broken impeller form PO not getting it all... had to pull out with long nose pliers and was in the fuel cooler exchanger just after the raw pump...

literally look though each piece..

Check water hose inlet.

How many hours on the motor?
Original manifolds? If not, how old are they?


Check your water pressure readings and compare from one motor to the other.

Can you remove the sealing plugs on the underside of the exhaust manifolds? You can check for rust and debris inside the manifold by removing these.

I would also suggest removing the engine block drain hoses and flushing the block. The quick connect fittings have a small produce and they clog easily.

Thank you for such quick responses. Happy Dayz, I must have missed this in the line. I will go back and check it and report back.

Thank you for taking your time and for the suggestions.

Thank you for your quick responses. Espos4, about 350 hours on the motor. Original manifolds and I know what the general consensus is on this subject. I had them out and tested them. They were very clean. I could always hand touch them and no hot spots when running after they were reinstalled. I could remove the ports/caps to test for rust but since I just had them out and pressure washed the inside and saw clean metal I am sure this is not the problem. This was also a rapid increase in pressure, not a slow increase over time as manifolds typically are.

I do not have a pressure gauge on my system but I could rig one. I will get this set up and reply back with readings.

However, the block drains are where I had my sites on my next visit. I am going to add the suggestion about the fuel cooling drains. I will report back either happy or with more information.

Thank you for your suggestions and time.

To Craig...Thank you for this picture. Is this inside or outside? Motor removed or outdrives? I have heard the stories about this problem. The only problem is that this problem seems like it would come on slowly and not in one use 1 week apart. How did yours happen.

You guys are awesome!

That photo is inside on the transom. Drive needs to come off. In salt water it can happen pretty fast. If its never been changed there is probably some restriction there.

To Craig: Yesterday I removed the fuel cooler supply lines and flushed the last remaining obstruction in the lines. No difference was made. I began to remove the S-hose for inspection and diagnosis, but could only get the top bolt off. I will be back tomorrow to give it another try. Still overheating.

Sanibel Life said:

To Craig: Yesterday I removed the fuel cooler supply lines and flushed the last remaining obstruction in the lines. No difference was made. I began to remove the S-hose for inspection and diagnosis, but could only get the top bolt off. I will be back tomorrow to give it another try. Still overheating.

Click to expand...

What was the "last remaining obstruction"?