Mercruiser 260 Overheating Mystery

[OhSnap]

I have a 1987 Four Winns Horizon 190, 19' boat with a GM 350 / Mercruiser 260HP with an Alpha One outdrive. It is not a closed cooling system and has a power steering fluid cooler. Never used in salt water. There are no exhaust flaps, the exhaust has spirals in them to keep the water from coming into the engine. The thermostat is out of the engine, new belts, hoses, water pump in the lower unit, new engine circ pump, removed the risers and they look clear.

The engine overheats at speeds above idle since I bought it last year. I can idle the boat around the lake all day long and it runs cool. But at higher speeds, it overheats within 5-10 minutes and you can smell the exhaust tubes burning. When it overheats, the engine diesels when you shut it off for about 30 seconds, then finally stops. The temp gauge doesn't usually shoot up until you shut it off, let it sit for a few minutes, then turn the key back on. The temp gauge is a little sticky but it works. I have the glass off of it so I can unstick it once in a while.

Today I took it out with an IR temp thermometer and at idle and running just above idle in forward it's fine, we took it a little higher (plowing, no planed out) and it was ok, took it up to full plane and WOT for 4 minutes and the Port side rubber exhaust tube temp shot up to 280 but the starboard side stayed at 145. We shut it off and the enine dieseled then shut off. The dash gauge was at 100.

The temp on all other parts of the engine were between 95-125
The temp on the cast iron part of the risers was 120-130
Hoses from the engine around 100-110

We restarted the engine ran the boat at headway speed again and it ran cool. The engine temps never got hot.

I hung off the back of the boat to watch the water flow from the exhaust and could see the port side had less water flow, almost a spray, the starboard side it was running out like a stream.

We once again ran it at WOT and this time the Starboard side started to get hot (got up to 190), so we slowed it down and the temp dropped.

I have noticed the water comes out from each side differently and the flow changes from one side to the other.

There is a check ball on both sides of the tstat housing on the ports that sends water to the risers. Anyone know what their purpose is? I am wonderig if this is the problem. Can I just take it out?

Any advice would be great!
Thanks!

 

[boat127]

Re: Mercruiser 260 Overheating Mystery

could be a head gasket. can you get a leak down test done? pull the plugs and see if any plugs appear to have been steam cleaned and note the cylinder if one is steam cleaned

 

[cooter2506]

Re: Mercruiser 260 Overheating Mystery

INstall the correct thermostat and try it. I have heard if you run without a t-stat its not good it will unevenly cool the engine. Also when you did the outdrive impeller was the impeller only changed or was the impeller and housing changed?

 

[Bt Doctur]

Re: Mercruiser 260 Overheating Mystery

"removed the risers and they look clear."
Just where did they look clear? The 4 square ports is where the water goes in, what about the holes where the water comes out? 3 or 4 inch risers? Were they ever replaced?

 

[OhSnap]

Re: Mercruiser 260 Overheating Mystery

could be a head gasket. can you get a leak down test done? pull the plugs and see if any plugs appear to have been steam cleaned and note the cylinder if one is steam cleaned

I did a full tune up to the engine but didn't see any funny looking spark plugs, all chocolate brown

 

[OhSnap]

Re: Mercruiser 260 Overheating Mystery

INstall the correct thermostat and try it. I have heard if you run without a t-stat its not good it will unevenly cool the engine. Also when you did the outdrive impeller was the impeller only changed or was the impeller and housing changed?

I ran it with the old stat, the new stat I bought from the Mercruiser dealer nd then no stat

I bought the kit for the impeller, no housing, just gaskets

 

[OhSnap]

Re: Mercruiser 260 Overheating Mystery

"removed the risers and they look clear."
Just where did they look clear? The 4 square ports is where the water goes in, what about the holes where the water comes out? 3 or 4 inch risers? Were they ever replaced?

It was the water ports on the sides that were clear. They had a coating of rust as expected cast iron parts would, but no scaled, flaking rust.

4" risers

Thanks for all the replies! I really appreciate the help!

 

[chillinjc]

Re: Mercruiser 260 Overheating Mystery

so what did you find out? I had the same issue today, including the temp gauge reading 100 and the port side being cool to the touch. If I ran on plane starboard riser heated up and burnt my lower riser boot. at idle water was pouring through the deformed boot. Im going to pull riser next weekend and check manifold (getting married this weekend, so not a good time to do it unless I want to instantly be re-divorced!) new t-stat beginning of the season, new impeller and housing, risers were clean when I installed in may. did you ever find anything?

 

[OhSnap]

Re: Mercruiser 260 Overheating Mystery

so what did you find out? I had the same issue today, including the temp gauge reading 100 and the port side being cool to the touch. If I ran on plane starboard riser heated up and burnt my lower riser boot. at idle water was pouring through the deformed boot. Im going to pull riser next weekend and check manifold (getting married this weekend, so not a good time to do it unless I want to instantly be re-divorced!) new t-stat beginning of the season, new impeller and housing, risers were clean when I installed in may. did you ever find anything?

Never found the problem. I am still idling around the lake. The problem isn't as bad now though... meaning I can go full throttle for about 5 minutes before there is an issue.

Good luck with getting married... better put that repair off a few days.