MCM 120 (1981) 2.5L running hot??

[merc120_81]

Hello All,
I have a MCM 120 Engine S/n 5804180 circa 1981 (2.5L) in a
16'9" Fourwinns marquois 170 (modified V fiberglass hull)
Running in fresh water, water temp close to or above 70 best guess, and ambient air temp 94.
I can putt and idle around the lake all day, and barely see over 155 degrees.
I put it up on plane, running anywhere between 3200-3800, and temp climbs up to 200 -210 then I back it downand let it idle until it comes back to 150 then repeat the cycle.
I have replaced the impleller, housing, and plates under the impeller. (upper housing of the impeller only, not the lower housing, it looked good, and I couldn't seem to pop it out, any tricks there?)
I have also replaced the watercirculating pump on the front of the engine, as well as the thermostat.

checked temps with IR gun, and put a new temp guage sender/wiring.

This past weekend I took apart exhaust elbow, and shutters where where they were supposed to be. The elbow had some corrossion, so I cleared what I could see, and checked that water was passing. Put a garden hose to each inlet on the elbow, and saw water coming out of three passages at the bottome of the elbow, when I blocked off the other inlets on the elbow.

Nothing I seem to do, brings the highspeed temp down.
1) should the temp, never get over 160 how hot is too hot??
2) please see attached pics of elbow, are the thre square notches, the only place I should see water coming out, is there a 4th one, as there are 4 passages from the manifold.


Help. Please..

Thank you.
Attached, pics of elbow ends.

 

[fishrdan]

Re: MCM 120 (1981) 2.5L running hot??

Were there any vanes missing off the old impeller? If yes, you need to track them down as they can cause a restriction and overheating.

For the water pump base, it just prys off. Stick a screw driver under each side, alternate prying on each side and it should wiggle free.

 

[merc120_81]

Re: MCM 120 (1981) 2.5L running hot??

-The old impeller, was cracked a little, looked actually in pretty good shape, no missing vanes etc.

 

[Fordiesel69]

Re: MCM 120 (1981) 2.5L running hot??

We need to know the temp readings of the manifold, the elbow, and the engine block. I you can also shoot the t-stat housing that would be great to.

Also did you pull the hose off the tstat housing and rev the engine in NEU to 3800 to see if the flow continues to be adequate? Often a housing leak or poor fitment can allow crappy flow on plane.

 

[Don S]

Re: MCM 120 (1981) 2.5L running hot??

-The old impeller, was cracked a little, looked actually in pretty good shape, no missing vanes etc.

Replace the impeller, if the vanes are cracked, the impeller is junk. Bet the blade are all bent over too.
Those blades have to flex back and forth, what do you think makes it pump?

 

[merc120_81]

Re: MCM 120 (1981) 2.5L running hot??

The impeller, the upper housing and the gaskets and plates between the upper, and lower housing all orings were replaced for a first step in trying to get it to cool properly.

so no matter if I am running it all out, or puttting along, that engine should not see over 160????

I will shoot the temps again..
I have seen exhaust manifold, exit by cylenders in the mid two hundered range.

If I shoot the tstat housing when temp gauge is around 180 tstat is about 160,

Top of the elbow, can get close to 200, but sides are usually 170 or so, when the temp gauge is running 180/190

I don't let it run but a minute or so over 180/190, as I don't want to melt/fry anything.

Hope that helps, next time on the lake, i will try and run the Rs up, with the impeller output into a bucket. as suggested, unless anybody chimes in thats a bad Idea??

 

[Fordiesel69]

Re: MCM 120 (1981) 2.5L running hot??

The elbow should NOT be running that hot. You need to rod it and the manifold log out with a hunk of wire. There is somthing holding back the flow. 200 hurts bad on the skin. I can hold my hand on my elbow for a few sec without a burn.

You should reomove the entire tstat block & housing and inspect, then pull the end caps off the manifold log. You can use rtv to remount them afterwards. Rod it all out and inspec with bright light to see if any rubber or rust is blocking it. Then the elbow.

 

[merc120_81]

Re: MCM 120 (1981) 2.5L running hot??

Can you please clarify, ROD it out?? Whats RVT? Thanks for the advice.

 

[cr2k]

Re: MCM 120 (1981) 2.5L running hot??

Don't use rtv (silicone) use the proper gaskets.

Rod it out means to take some stiff wire or rods and ram them through your outer jacket to loosen up the rust and scale there and flush it out.

 

[merc120_81]

Re: MCM 120 (1981) 2.5L running hot??

Well two years of battling this problem....FINALLY SOLVED!!!

And the winner is...

Bellows and cooling hose, changed, found 4 pieces of previous owners exploded impeller (when I changed the impeller after I bought the boat, the impeller was completely intact...) any way... pieces were found in the hose between bell housing and transom, and the metal pipe going through the transom... Thank you very much..for all the help.