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.

If engines do not run hot/overheat at rpm/load than I don't think the heat exchangers are too small.
Stupid question I have to ask, are the thermostats in the right way?

Install 160 stat, are your exhaust manifolds included with closed cooling. Most systems don't due to too much heat. I built a 400 small block which had closed cooling for the exhaust manifolds with the engine and it had similar reactions that you are with overheating issue so you can eliminate that and run the exit from heat exchanger to bottom of exhaust manifolds.

Kits come with 160 stats. Its a full kit so exhaust manifolds are included. After coming out of block, coolant runs down through manifolds before returning to the heat exchanger.

Final dump from heat exchanger usually goes to bottom of exhaust manifold and goes up and out riser. If your exit water is going too riser then your manifolds are closed with motor.
gasket between manifold and riser is either open between the two or closed, closed means just that, closed with engine. Manifolds create way to much heat to add with engine temp. I hope this makes sense. If not call me@ 386-785-6986. Kurt

I did something similar years ago, had to take the exhaust manifolds out of the loop and temps were great after that.
As others said, to much heat with the manifolds.
Best of luck!

Is your exhaust dumping threw y pipe, or straight out transom.
way more heat built up with y pipe verses straight out.
​​switchable to straight out maybe an option too eliminate heat.