1987 Mercruiser 120 Overheating problem

[bullelkklr]

This is my first post. I have searched the archives extensively.<br /><br />The boat is a 17' 1987 Doral Citation. 120 HP <br /><br />I forgot the serial numbers to eveything at home today.<br /><br />Symptom: Overheats when running 1/2 to full throttle on plane. Seems to run at normal operating temp when at idle or slightly above.<br /><br />History: The outdrive water pump was replaced last year when it overheated. The boat is shared and I believe that it was run without muffs on it. New exhaust flapper was installed too. The boat is in Montana and I am in Colorado - so I am researching as much as possible on line.<br /><br />It started overheating while on a fishing trip last year with a very heavy boat and I was running it pretty hard. We camp across a large lake in MT and we had all the camping and fishing gear and barley pop with us.<br /><br />I have full manuals for the motor and outdrive.<br /><br />When in MT over easter I ran the boat on muffs in the driveway for 45 minutes with no overheating problems.<br /><br />I have removed the thermistat.<br /><br />While running it I opened the engine and exhaust drains. Water came out of both at good rates, but not forcefully. The engine pitcock water was hot to the touch.<br /><br />I pulled the prop and found 1/2 of the exhaust flapper valve in the lower unit. I removed that and went back to the lake. It ran cooler than on previous trips, but after putting 3 more people in it and running it pretty hard, it started to heat up and the temp got to 190 so I shut it down. Again, at idle 3-4mph, the motor cooled down.<br /><br />I plan to replace the lower unit impeller and the flapper again, but should I check the water pump on the front of the motor? And what should I check for?<br /><br />From the archives I see that the block water passages could be plugged up. It has only seen freshwater, but maintenance has not been the best. Water comes out of the pitcocks though.<br /><br />Thanks for any help and advice.<br /><br />Randy

 

[Sea Ducer]

Re: 1987 Mercruiser 120 Overheating problem

Hello,<br /> I had the same problem on a 350 cc in a StingRay 21' cuddy I owned. The impeller in the outdrive was slipping at the higher RPM's. When I took it out it looked good but upon further inspection the brass inset was loose from the rubber part.Easy fix if thats the problem,and it should be replaced every-other-year anyways. Jim

 

[Laddies]

Re: 1987 Mercruiser 120 Overheating problem

Pull the water intake screens on the sides of the lower unit and see if the old pump impeller blades are still in there

 

[bullelkklr]

Re: 1987 Mercruiser 120 Overheating problem

Thanks - I will try all of those things when I go back. I do plan on replacing the lower impleller no matter what, I am more worried about plugged block water jackets and trying to figure out how to trouble shoot the water heater on the front of the motor.

 

[bullelkklr]

Re: 1987 Mercruiser 120 Overheating problem

If I need to replace the water pump on the front of the motor, can I use a standard chevy 2.5 water pump or do I need to get a mercruiser pump? <br /><br />I know that there are issues with using non-marine parts on boats (kerbooooom!), but the waterpump is sealed - will a car water pump fit? Will it work correctly if it does?

 

[rob935]

Re: 1987 Mercruiser 120 Overheating problem

you will need a merc pump, it does sound like impellar trouble to me so try that first if it persists then look at the pump, pumps usually give a tell tale squeal when they give up though??

 

[bruceb58]

Re: 1987 Mercruiser 120 Overheating problem

I doubt it's the circulating pump. If it is that pump, don't use an automotive pump. A marine pump has a special ceramic seal, stainless steel backing plate. In addition the impellor is usually bronze for corrosion protection. I know you say that you use it in fresh water now. You sure that will always be the case?<br /><br />You can buy a non-mercruiser pump. Sierra makes a good pump that is much cheaper than the mercruiser.<br /><br />I assume you found the other half of the exhaust flapper?<br /><br />How hot do your risers feel? Best to use a temp gun but you should be able to lay your hand on it without being uncomfortable. If it's hot, you don't have good water flow which can be caused by bad impeller or blockage somewhere.<br /><br />You said in your post that it was run without muffs. Was this before or after you replaced the impellar. Only takes a few seconds to wipe it out without water.

 

[Gary H NC]

Re: 1987 Mercruiser 120 Overheating problem

Running with no thermostat can't be that good either..

 

[bullelkklr]

Re: 1987 Mercruiser 120 Overheating problem

It was run without the muffs before the impeller got replaced by a merc mechanic. The mechanic would not replace the flapper because he felt he would strip the bolts or break them off. When I took the manifold appart I broke 1 bolt and had to easy out it out. The manifold was partially chipped so I cleaned it up and JB welded it and put it back together, no leaks (the crack was on the outer portion of the exhaust manifold).<br /><br />The boat will never be used in salt water. It has a short lifespan as next year I am buying a Lund with an outboard.<br /><br />I don't know whether the 1/2 flapper I found was part of the old one, or part of the new one that I replaced. I will look for more pieces when I get up there to replace the impeller. Hopefully that works.<br /><br />The risers were not that hot while on the muffs at the house...I didn't check them on the lake.<br /><br />I plan to put the thermistat back in after I figure out what is getting her hot.<br /><br />Thanks for the help guys - I appreciate it and will let you know what I find when I pull the lower unit apart.

 

[kd7isf]

Re: 1987 Mercruiser 120 Overheating problem

Originally posted by bullelkklr:<br /> ...I do plan on replacing the lower impleller no matter what, I am more worried about plugged block water jackets and trying to figure out how to trouble shoot the water heater on the front of the motor.

WATER HEATER!!!! <br />DO you mean HEAT EXCHANGER? <br />If you do, and you have let water out of the petcocks, YOUR ENGINE SHOULD BE RUNNING HOT BECAUSE YOU JUST LET THE WATER OUT OF THE "RADIATOR".!!!!!!!!<br />If you have a metal thing that has a cap similar to a cars radiator cap, YOU HAVE FRESHWATER COOLING. With the boat COLD, take the radiator cap off and fill to the full mark. Also, 180-190 IS NORMAL TEMP for fresh cooled. You should use 50% coolant like you do in a car too. Flushing yearly is recommended for these systems.<br /><br />Just so you know, those systems TREMENDOUSLY add to the resale value of your boat. )<br /><br />Just an aside, but if you DO have a heat exchanger, AND you have an impeller problem, it will run hotter under load because the coolant isn't getting cooled fast enough. )<br /><br />Hope this helps )

 

[Rhadley]

Re: 1987 Mercruiser 120 Overheating problem

I also have a 120, and after 2 years, I overheated due to a piece of an old outdrive impeller that blocked inlet water somehow. That caused my flapper to fall off. <br /><br />Removing the thermostat can cause issues right there.

 

[bullelkklr]

Re: 1987 Mercruiser 120 Overheating problem

I meant the front "water pump" - not water heater - sorry. It does not have a heat exchanger.<br /><br />Rhadley - how did you find the piece of impeller?<br /><br />Why would removing the thermostat cause it to heat up?

 

[ziggy]

Re: 1987 Mercruiser 120 Overheating problem

Laddies<br />posted April 29, 2006 05:31 AM Pull the water intake screens on the sides of the lower unit and see if the old pump impeller blades are still in there

that'd be one way to find some impeller pieces. i fould some up at the t-stat after i overheated so you might want to remove the hose and have a look there. while it's disconnected. flush water from front to back. and visa versa. i used a bucket to catch the water. ya'd be amazed how many broken impeller pieces i got out. i think i've heard it recomended to disassemble the drive to go get the pieces out. <br />that seemed like a lot of trouble to me so i used that ft to back flush method.<br /><br />somebody else might confirm, but i think the reason it heats up w/o a t-stat is because the water isn't forced to circulate around the water passages proper w/o the t-stat to restrict the flow and forse the water in the proper direction. ie the cooling passages in the block....just a guess on that one....

 

[Rhadley]

Re: 1987 Mercruiser 120 Overheating problem

I found the impeller piece behind the inlet screen of the lower unit, just as ziggy suggested. <br /><br />As to the thermostat, I have read several places that removing it is a problem but I can't tell you specifically why. As I recall, the system is designed with it there and it messes up the proper flow without it.<br /><br />Just curious, does your boat have a bog problem at acceleration, such as pulling up a skiier?

 

[bullelkklr]

Re: 1987 Mercruiser 120 Overheating problem

I pulled the screens off when I pulled the prop - i found half of the flapper just behind the prop - removing that seemed to help about 50%.<br /><br />ANOTHER Question - Do I purchase a kit to repair the impeller - or should I just purchase the impeller. What seals are needed?

 

[bullelkklr]

Re: 1987 Mercruiser 120 Overheating problem

UPDATE and RESOLUTION:<br /><br />I wanted to thank everyone here for their time.<br /><br />I dropeed the lower unit last weekend on a visit. When I pulled the impeller top housing off I found that 1/2 of the lower impeleer housing was filled with dime size rubber parts.<br /><br />When I had a mechanic in Nebraska replace the impeller - he did not check the flapper valve (the boat seriously overheated) as he was afraid that the manifold bolts would break off.<br /><br />So - I pulled the manifold apart (yes, 1 bolt broke and I had to easy out it out). I could not find the flapper valve - it must have been half way into the exhaust.<br /><br />I could have made the repair for the cost of a couple seals and some lower unit lube...but I bought about 100.00$ worth of parts...better safe than sorry.<br /><br />Next time I will purchase the lower unit housing repair kit and the upper housing repair kit - but not the housings.<br /><br />We took FUBAR (boat's name - purchased that way - shoulda knows out for a 3 hour cruise and she runs COOL now!<br /><br />Randy