7.4L Merc OVerheating

I have a 1995 Searay Sundancer with a 7.4L (Non-FI) Bravo III with an open cooling system. I recently gave the boat a tune-up and it's now running great on all cylinders. But the temp gauge now runs up to 210 degrees (and sets off my alarm),whereas before it always ran 170. I have confirmed that the heat range of the spark plugs is OK (I used Autolite 144's instead of the AC MR43T's).

I have the Mercruiser repair manual that tells me it could be the water pump, thermostat. hoses, water inlet on the stern drive, and/or engine timing.

I did not change the engine timing during the tuneup - just the distributor cap, plug wires, and spark plugs.

Which one of these other causes is most likely the problem? Can you give me a ranking or order of what you would check first, second, third, etc?

Any help would be much appreciated

Re: 7.4L Merc OVerheating

Have you ever ran the engine without water going to it? When was the last time it ran at 170? ? This year? Last Year?????

Re: 7.4L Merc OVerheating

No, I have never run the engine without water. The boat ran 170 just before I did the tuneup a few weeks ago. I have now changed the thermostat (didn't help). I guess the belt driven sea water pump is next.

Re: 7.4L Merc OVerheating

check the waterflow from the sea water pump to see if it is flowing good stream. run it on muffs and remove hose from inlet to thermostat housing and see if water is flowing through oil coolers. if water pump impellar is coming apart it will clog coolers and restrict flow. take outlet hose from sea water pump and back flush oil coolers to see if any debris comes out. If impellar has not been changed in last year i'd advise doing it now. just like fram filter commercial says pay me now or pay me later ,when it fails and you have to be towed in .

Re: 7.4L Merc OVerheating

Thanks for the advice. I've had the boat two years and haven't touched it yet - so I will do the Seawater pump rebuild this weekend. Do I need to change just the Impeller?

Re: 7.4L Merc OVerheating

Make sure you remove the raw water hoses that go from the pump to the oil cooler/power steering coolers to look for parts of previous impeller failures.

When I replaced my raw pump that the previous owner said he replaced I found the remnants from an even earlier pump failure crammed into the oil cooler.