Morgan OI 28 Great Lakes

[Ibrew4u]

I bought this 1976 Morgan with a 30hp A4, after getting it running good it runs hot 200-208 from time to time and even tries to go higher but by throttling down it will drop back to 190 where it stays most of the time, I removed the thermostat and inspected the interior of the housing which looks rather clean but that made no difference other than takes longer to raise temp, inspected water pump and impeller looks good and has a decent amount of water coming out with the exhaust, anyone have any suggestions what I should try next, seems no rhyme or reason, will run normal for hours at full throttle then just start jumping temp, thanks for any impact or similar experiences, Tim

[joe_db]

I would as a first step give it a really good vinegar/acid flush and then try it with this valve installed:

https://moyermarine.com/product/cooling-system-bypass-kit-late-model-csot_01_61/

Remember - shutting the valve = COLDER

[Sam]

Visually, it took me a long time to discern lower water and exhaust flow due to a worn pump [shoe] or an older impeller. I have a stand pipe type exhaust. I would change the impeller, should be done periodically, with a new one. It won't hurt and might help you problem. Other area is the water inlet from the pump - if you have the old fashion 3/8 in. cooper tube it does get clogged time to time as does the block inlet.

I have an early model A4, t-stat is 140 F. Other later model A4's in the harbor have 160 F t-state [with different plumbing]. You could have a higher and tired t-state.

[Al Schober]

Ahh, yes - those intermittent blockages. Time to flush the crud. Disconnect the hose between the water pump and the block inlet, then hook it up so you can run a garden hose right into the block inlet. My experience is that overheat conditions are due to blockage/low flow, not scaling. I'd skip any vinegar or acid flush at this point.
First, I'd check for a blockage at the outlet of the manifold. Remove the outlet fitting, then give it a flush with the hose.
Second is for crud in the block backing up against the thermostat. Remove thermostat housing and thermostat then give it another flush.
I see the high temps as being intermittent as a good sign, ie: loose stuff trying to get out. If the temps were always high, then I'd be suspicious of seriously blocked cooling passages requiring removal of the head and mechanical removal of scaling.

[zellerj]

My Great Lake boat always ran hot - 210 F for 15 years. I changed impeller, changed pump shoe, checked for blockage at the exit elbow, checked for blockage at the side plate T, gave it a HCl acid flush, and all my efforts still did not result in a cool running engine.
Then my alternator quit, so I removed it for an exchange. As long as the alternator was off, I decided to take off the side plate and the cylinders walls were covered with decaying organic matter blocking the flow of water. Cleaned that out with a nut pick and skinny screwdriver, and now it runs at 140 F. I think acid flushes work OK if the blockage is carbonates, but not decayed organic matter. Picking that out is the only option.

Just be careful that you don't break off any bolts while removing the side plate. I broke off two of them.

[Ibrew4u]

Wow, you guys all over this, what an awesome forum, so I pulled the 90 elbow water out off exhaust manifold and that wasn't plugged, water really looking good out the back when it's running at the dock, of course while under power off shore I can't tell due to the back water over the exhaust, as a master brewer I have all the acids and caustics at hand, yes acid is good for removing rust, calcium and other mineral deposits, and caustics work great at removing organic materials, certainly don't want to remove alternator and side plates at this time, have any of you any imput on a caustic flush, probably followed by an acid rinse to maintain positive electro magnetic field with the metal interior of the engine or is that going to far not being stainless steel, it's just weird how it runs for so long at proper temp with or without the thermostat then out of no appearent reason jumps, the impeller and pump housing appear to be in real good shape, I did notice on dry dock when I finally got it running that a lot of rust scale and particules did come out the back end but the po said that happens every year at launch time? baffled but I have a new water pump with impeller on the way and if a good flush doesn't work I'll go ahead and install that and let y'all know how this progresses, do any of you think it could be the water riser which all that rusty crap blew out of could be the culprit, it's solid clean metal from the outside? Thanks again, Tim

[JOHN COOKSON]

One of the first things I would do is remove the thermostat. This will take the (possibly defective) thermostat out of the picture. Than we can look to other causes for overheating if necessary. I'll bet you will be a lot happier without a thermostat.

True Grit

[ndutton]

If removing the thermostat be sure you restrict the bypass hose between the sideplate and thermostat housing (late model cooling system).

The first thing I would do is get the MMI Manual.

https://moyermarine.com/product/service-and-overhaul-manual/

[zellerj]

caustic is fine with cast iron

Caustic will not harm cast iron. In fact, most chemical plants store caustic in iron tanks. Hydrochloric acid is much harder on steel than caustic. I was not aware that caustic would decompose caked on organic matter. Good luck with that, and if it works, please report back.

[joe_db]

Before you do anything else:
Remove the thermostat.
Clamp off the bypass hose.
Run the engine hard.
Now you know your baseline cooling capacity. I can't budge off of 120 unless I run full blast for over an hour when the water is warm when I do this.
Almost forget - Get an IR heat sensor gun. Your *gauge* might be way off!