Merc 888 Overheating

[ccarson2]

I just put a newly freshened engine in (old shortblock, but new heads and cam) and installed it in the boat. We fired it up today to break in the cam which means we had to run it for 20 mins at 2000-2500 RPMs. We hooked up the muffs with the hose on full blast. There was a good amount of water coming out with the exhaust so I don't know if this is a flow problem, but after about 10 minutes of running at this RPM the engine climbed past 200 on its way to 240. I shut it off after 15 minute when it hit 220.
Tonight I just pulled off some of the hoses and flow tested the manifolds with the garden hose. There doesn't seem to be a restriction, but I don't know if this is a good indicator. The lower hose from the outdrive did have pressure when the engine was running (it was leaking a good amount when the hose clamp was almost tight, had to tighten it up quite a bit)
Could this just be because I was running it at such a high RPM while out of the water? Does the garden hose on the muffs not provide enough water to keep the engine running cool?
I haven't let it idle yet; tomorrow I'll finish breaking in the cam and we'll see if it overheats at idle.
This is a Merc 888, Ford 302 engine (1977) with a raw water cooling system. Also, there was no thermostat when i pulled the assembly apart - is this correct?
Thanks,
Chris

 

[northvanwatertaxi]

Re: Merc 888 Overheating

On this engine there is supposed to be a thermostat, on the block above the circulating pump that opens at 140-160F
I learned there is not enough pressure on a hose and should not be run at over 1000 rpm, you can use a barrel for break in, if you can find one that big.

if your pump is gone better find out soon or the impeller will meltdown and you will need a new pump housing,
I have an 888 fwc, and just had overheating problems too, it was a failing water pump.... i'm just going to launch rather than use my muffs. There are good ones you can get that use a wire that goes thought the leg to hold them in place.

 

[Don S]

Re: Merc 888 Overheating

With a raw water cooled engine, you need a 140° thermostat. Don't run without a thermostat.
Have you checked the raw water pump impeller in the drive? They are only good for a couple of years.

 

[freddyray21]

Re: Merc 888 Overheating

impeller i bet

 

[gscribner54]

Re: Merc 888 Overheating

If the only problem in the cooling system was a missing thermostat, you would not get an overheat - if anything you would get slow warming of the engine because the thermostat shuts off the path of water to the cooling appatus - a radiator in a car, or in a boat out through the manifold, instead of recirculating the hot water thru the engine. I had a problem with a blown thermostat, which did not open to let the hot water out - keeping the hot water in the engine and resulting in an overheat. Thats not your problem it sounds like

I'd worry that something got into the water jacket ports during the rebuild if you can't find anything obvious in the cooling system. Or the water pump on the engine has a problem or obstruction.

I do agree with the other readers - you should run with a thermostat specified for the engine. My 4.3 v6 likes a 142 degree.

p.s I ran my wife's chevrolet around in the desert in Nevada without a thermostat after the mechanic pulled the bad one out (he didn't have a replacement). Made it another 5000 miles without an overheat - wouldn't make a habit out of it though.