mercruiser 260 overheating

I have a 1978 mercruiser 5.7 that has me stumped. I purchased the boat with a bad motor (water in the oil). I have since put a remanufactured engine in it and can't seem to keep it cool. It stays cool at an idle but slowly heats up when the rpms go over about 1800 regardless of whether or not it is on the hose or in the water and is slow to cool back down.
Checked with an IR gun. Thermostat housing slowly creeps up to 180 but the intake manifold near the housing stays at 160 or less. Log style exhaust manifolds seem to take turns running warm, one might be at 140 and the other creeps up to 190-200, then they alternate. The elbows stay cool however, running about 115 degrees when the manifold is cool to 150 when the manifold gets hot (200 degrees).
Has brand new alpha I lower unit with of coarse a new water pump, new thermostat, newer manifolds and elbows that don't seem to have any rust scale or flakes in them. New water pump on engine.
Almost forgot, this is seawater cooled and has only been run in fresh water. It has power steering also.
I am begining to wounder what the direction of flow is, but haven't found anything pertaining to something that old.
Thanks in advance to any replies.

Re: mercruiser 260 overheating

Hi, I'm having the same problem with a marine power 350, runs pefect from idle to 1800 then starts to overheat above that, let me know what you find out, i haven't found a solution yet, thanks and good luck

Re: mercruiser 260 overheating

could be the water transfer hose by the bell housing is kinking or there may be a blockage in the water hose before it gets to the t-stat housing

check the cooler for weeds/debris

Re: mercruiser 260 overheating

If I take the incoming water supply hose off the thermostat housing I would gues at an idle I have close to three gallons a minute coming out, and thats after the power steering pump cooler.

Re: mercruiser 260 overheating

Is it typical for the intake manifold to reach close to 300 degrees near the choke spring bracket?

Re: mercruiser 260 overheating

check the lower hose when your running it and see if it collapses i have had a couple of friends who had the hose collapse cutting off flow as rpms go up ... typically the hoses should have a spring inside to keep this from happening but sometimes they are replaced with ones that dont . easy enough to check have someone watch it while you run it up in rpms and see if it collapses if so get a spring or one that has one .... good luck

Re: mercruiser 260 overheating

johnweeles said:

Is it typical for the intake manifold to reach close to 300 degrees near the choke spring bracket?

Click to expand...

Yes. That's an exhaust passage to warm the intake, to prevent icing.

Re: mercruiser 260 overheating

I checked the hose and there is a spring inside. I looked at the hose while the engine was running and it looked fine. I have also swapped all the hoses with the hoses off the original engine just to rule out the possibility of a hose that may have a blockage in it. I have seen them come apart on the inside with no visible problem on the outside and cut off supply. if I remember right though that may be more common for fuel lines. Thought I'd give it a try anyway.

Re: mercruiser 260 overheating

Thanks wire2, thats good to know! It just seemed to me like that was a little hot for the intake manifold but it make sense.

Re: mercruiser 260 overheating

Automatically.... Replace the outdrive impeller pump and all. Sorry it'll cost you gaskets and o'rings. It happenned to me. That was the only problem fine now...

Re: mercruiser 260 overheating

Well, changed water pump in outdrive last night. Apparently it was run dry so I installed a water pump kit with the housing and all. Things were a little melted but there were no missing peices so I don't think anything went into the cooling system. The problem is better but after a lake test last night it still runs warm. After about 3/4 throttle for 4-5 minutes the temp comes up to about 165. The mainifolds stay under 180 and the risers about 135 at the most. Things are definetly cooler. Oil pan reads about 200. Now when I slow down though the temp gauge creeps up almost to the red zone and takes a few minutes before cooling down.

Re: mercruiser 260 overheating

Now it's propably the thermostat. or if you have a power steering cooler.. Remove the discharge side and supply side and flush that out. I had debie in mine and worked fine after. Stupid things that we never think about.

Re: mercruiser 260 overheating

I changed the thermostat and flushed the oil cooler out but it didn't seem to make a difference, still runs warm. It just seems odd to me that the intake manifold gets as hot as it does. Heck its burning the paint off. My 4.3 liter intake manifold with an omc cobra outdrive seems to stay pretty cool.

Re: mercruiser 260 overheating

It might be possible that you rotated the impeller the wrong way. Also if you pulled the lower unit the o-ring for the water pick-up might have fallen out while installing the drive. Last but not least the rubber o-ring on top of the water pump housing needs to be installed. One of my freinds had forgotten to install it and had the same problem. As far as the intake manifold getting that hot it should not burn the paint like that.

Re: mercruiser 260 overheating

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