165 hp MCM overheating

[Qman]

I am trying to troubleshoot an engine overheating issue. I have a 165hp mercrusier that started overheating after 20-25 minutes on the water. I brought it back to the dock and had to add 50/50 coolant to the reservoir. I started it again and within 5 minutes underway, It was overheating again. Any ideas where to start on troubleshooting? One local boat owner said start by taking the lower unit off and check impeller, yes? Thermostat? Any help would be appreciated in advance. Another issue will be trying to ID parts to replace. Thank you

 

[Qman]

Serial number stamped is hard to read. However I believe the mercrusier 165hp SN: 8600974/ maybe a B as first character
Model MCM 165
Cylinder firing 1342
Max WOT RPM 4200-4600
Plug gap .035
Point gap .018-.025
Engine rotation: CCW

Just read the "only for adults".

Thanks

 

[alldodge]

I am trying to troubleshoot an engine overheating issue. I have a 165hp mercrusier that started overheating after 20-25 minutes on the water. I brought it back to the dock and had to add 50/50 coolant to the reservoir. I started it again and within 5 minutes underway, It was overheating again. Any ideas where to start on troubleshooting? One local boat owner said start by taking the lower unit off and check impeller, yes? Thermostat? Any help would be appreciated in advance. Economic issue will be trying to ID parts to replace. Thank you

Howdy

Yep, that is where you start. In my sig is a link to the adults only section, and in there go to the Mercruiser section, then find impeller replacement.

Here is a link to the parts list. Might need the pump as well as the impeller but need to have a look
http://www.mercruiserparts.com/bam/s.../30834/1595/80

 

[merc120_81]

Sounds like possibly a closed cooling system? Check the oil, and look for traces of water/coolant. (Milky) if Milky oil... head gasket?? >> compression test.

I 2nd the Impeller inspection, I chased impeller bits in a used boat I bought for a long time, before I finally found the pieces of the impeller in lodged in the hose between the bell housing and the engine compartment.. not fun..

 

[Qman]

Engine oil looks clean, no visual signs of it being milky white.

 

[stonyloam]

CHANGE THE IMPELLER BEFORE YOU EVEN THINK about taking it out again!!! If the impeller is not tntact, take the front cover off of the exchanger and look for the pieces. Fail to do that and you will be looking at replacing the head gasket!!! Do not inspect the impeller CHANGE IT, and if the steel lining of the impeller housing is not perfect, change the housing too!

 

[Bt Doctur]

You always start at the impeller/water pump housing for a overheat

 

[Qman]

Thank you. I will put the lower unit apart tomorrow morning. How hard is it to take the front cover off the exchanger? Just want to guage how far I should dig into this.

Thanks

 

[kenny nunez]

In addition to checking the heat exchanger also remove the hose that brings water from the drive pump and back flush it when the lower unit is off. At the same time inspect the hose from the bell housing to the gimbal housing. Before reinstalling the lower unit take a flashlight and look inside the upper housing to make sure the plastic water pocket cover is not melted. Then be sure that the sea water after the heat exchanger has no restrictions going out of the exhaust. And as already advised change the pump in the lower unit.

 

[achris]

As above, and I'd also pull the exhaust elbow and check it's water passages aren't clogged.

Unfortunately Merc also used a 6 cylinder GM engine it designated as '165', so when giving a description of your engine you need to also say it's a 4 cylinder '470' style.

Cheers

Chris. ....

 

[stonyloam]

He said he was adding coolant, so assumed a 470. Just a single bolt holding the HE ends on. If the impeller is intact and replacing takes care of the overheat you probably don't need to take the ends off. Checking the exhaust elbow is s good idea, another way to do it is take the aftmost hose off of the exchanger and stick a hose in it. If the water flows out of the outdrive freely with little back pressure you are probably OK. Should plan on changing the impeller every couple of years.

 

[achris]

Serial number stamped is hard to read. However I believe the mercrusier 165hp SN: 8600974/ maybe a B as first character
Model MCM 165
Cylinder firing 1342
Max WOT RPM 4200-4600
Plug gap .035
Point gap .018-.025
Engine rotation: CCW

0B600974. Makes it 1987.

 

[kenny nunez]

If all the above suggestions do not solve the problem plan on pulling the head as these engines are notorious for head gasket leaks.

 

[2fishy4u]

Welcome to hell, I have been there many times with these wonderful engines. Just had this exact same symptom, ended up being the headgasket. And there was no oil in coolant or water in oil. Just gasses going into the cooling system. But the other times it was either the impeller or the heat exchanger. Hope its the easy route.

 

[Qman]

No Title

Ok Gents, I finally had time to replace the impeller. See attached photo of the old melted one. Suggestion for proceeding. I have been working like crazy, so my plans to get this all squared away has been a long process. Thank you for the suggestions.

 

[stonyloam]

I would replace the housing along with the impeller. I would use an OEM housing and impeller. If you can't find all of the impeller pieces ( looks like 1 piece might be missing) look for it under the front cover of the exchanger. Chances are it will be caught by the first set of tubes. Change it every other year or so from now on and you should be OK.

 

[Qman]

I replaced the impeller, all new parts and put it back together. Still overheating. How long would it take me to replace the head gasket? I am guessing that us where I go next?

 

[alldodge]

Have you taken the ends off the heat exchanger yet?

 

[stonyloam]

Do a compression check before you pull the head. It is about a one day job. I am pretty sure you can pull the head with both the manfolds on it, it is a two person job but it would make the job go faster. You can remove the manifolds on the bench (easier) should you need to get head work done. Get the manual (#7) and follow the instructions EXACTLY including retorquing the head after the first warmup.

 

[nola mike]

There's a good procedure somewhere in the manual about testing the hg. Basically you put a clear hose in line with...the coolant res return hose maybe? You look for bubbles when underway. Can't look for it on my phone, but it's in manual#8. I'd think that a leak of that much coolant would be from somewhere else. HE, manifolds, etc.