Rebuilt 2 brand new GM 350 Mag MPI long blocks, both standard rotation. To fend of corrosion this time around I installed 2 Monitor Pro 5320 full closed cooling kits running 50/50 mix antifreeze. Right out of the gate I had overheating issues. Upon cold starts, engines heat up slowly, hit about 180 and then come down to normal operating temp around 170. My issue is after running the engines and returning to idle, 650 RPM, both engines would heat up to the point of the overheat alarm and I would have to shut down. If I increase RPM to 1000 engine temps come back down. I have a 15 minute idle back to marina, and the overheat alarm would happen almost immediately.

Thinking it was a circulation issue, I've gone to a smaller pulley on the circulating pump and have also ran an additional line from the circulating pump to the top of the intake manifold just before the temp senders to add cool coolant to hot coolant coming out of block. I still get the increase temperatures after returning to idle about 190, but no alarm at least. Just the warning light. This kit is the one designed for these engines. I'm starting to think that the heat exchanger is not size correctly and I'm getting to much heat soak. Temps do eventually return to normal but not for at least 10-15 mins. Not sure what I should do at this point. Monitor hasn't been much help.

Also, the thermostat gaskets that come with the kit did not have the brass ferrules on it, so I was have a grounding issue with the senders. I've since installed temporary grounds until I can pull everything apart and install the new gaskets.

I'm new to closed cooling so anyone with some experience, your advice is appreciated.

What are you running for raw water side?
How is it plumbed, are you running the closed side through any exhaust components?

raw water comes in through sea water pump, up through power steering cooler, down through cool fuel and then up into heat exchanger before going out through SS risers. Its a full kit, so coolant runs through circulating pump into the block, out intake manifold, then down through exhaust manifolds before coming back through heat exchanger. The temperature senders are at the point the coolant is coming out of intake manifold. So its before the coolant goes through exhaust manifolds.

What do you have for a raw water pump? standard merc? you may not be getting enough volume out of it to cool the addition of the heat exchanger...also the pressure on the raw water side might be something to look at...possible obstruction in one of the other heat exchangers or the water is rushing through too fast and not pulling heat away from the heat exchanger efficiently....just a couple ideas

If you have the brass raw water housing...I have seen those personally wear grooves in the housing side and the cap side to where the impellor would suck air and cavitate hence loosing water pressure and volume...have also seen that on the plastic housings too.

Personally that's where I would start looking because if it was a closed cooling issue I would think you would see it even at idle and by adding your hose that you added you are just masking a raw water issue and what makes me say that is that you said if you rev it up to 1000 or so it cools right down...that tells me the heat exchanger and closed cooling is working properly and its a raw water flow issue...just my thoughts

I get what your saying, but its able to cool at a higher RPM when the raw water is rushing through even faster then at idle and its able to cool effectively while the engine is creating even more heat. Thus my thinking that the heat exchanger is not of the right capacity. I'm getting enough flow, its just not enough volume to extract the heat from the coolant after running the engine at a high RPM thus why I'm getting heat soak after returning to idle. If the heat exchanger is too small, the hot coolant coming out of the manifolds doesn't have enough time in the exchanger to cool it down thus its going back into the engine hotter than it should.

I think you have a water movement problem somewhere. I would start by checking water temps at every hose with your hand. Also touch the exchanger,oil cooler and risers. I only have a half system in my boat so I’m not sure how much heat the exhaust adds to the coolant water, but if it can keep up at wot the cooler isn’t the problem. I think you need to pump more raw water at idle. What oil coolers are you using?

When I installed an SJE full closed cooling system on my Merc 7.4 MPI engines (twins), the temperature was all over the place, going from a low temperature to an overheat situation and then back down. I finally attributed the problem to the thermostat. It had very little bypass before the temperature got up to the 160-170˚ range. Then the thermostat would open and all of the super heated water from the exhaust manifolds would start to flow now that the thermostat was opening. This rush of very hot water caused the temperature to soar. Then it would come back down and then rise again.

I solved the problem by taking the thermostats that were supplied with the kit out. In their place, I put the original thermostats and made a slight modification. I drilled 3 bypass holes in the flange of the t-stats so that some water would still flow to the exhaust manifolds BEFORE the engine reached its operating temperature (160˚). This completely fixed the problem and I never looked back.

your the 2nd person to advise drilling t-stat. one suggested removing them completely, but with MEFI3, will constantly be in cold start mode. I guess start with 1 hole and go from there. Shouldn't need anymore that 1 or 2 holes