Overheating--where should exhaust come out?

[merc 140 pontoon]

1974 Merc 140, engine s/n 388...., stern drive s/n 390....<br /><br />Used to overheat only at high rpm, now overheating at all rpm. Overheats very slowly at idle, slowly at 1500 rpm, very quickly above 3000 rpm.<br /><br />I ran a simple test tonight, and have some questions about the results. There is a short hose from the thermostat housing to the manifold. I pulled it from the thermostat housing and tightened it over the end of my garden hose. I then ran fresh water through the manifold, slowly increasing flow until my water faucet was wide open. (Obviously, the engine was off throughout all of this.) I got pretty good flow--not as much as would come out of the hose alone, but maybe half as much, or a little more. I'm wondering about all the places the water came out, though. There were five places:<br /><br />-- Two holes in bottom of housing next to transom (gimbal housing?). When the stern drive was tilted up about halfway through its normal operating range, most of the water ran strongly in two streams from these holes. When tilted all the way down, water dribbled out of these holes. Should I see water out of these holes? I thought the exhaust was supposed to come out the prop, but a lot of my exhaust always comes out of these holes.<br /><br />-- Small hole on starboard side of gear housing. This hole isn't plugged. I don't know if it should be. I've read that this changed over the years. The stern drive s/n is 390***x. There was good flow (considering the hole is very small) when the stern drive was tilted down all the way.<br /><br />-- Gap between prop and gear housing. Some flow out of here when tilted down all the way. A mechanic has mentioned that my prop ventilation problem (2350 rpm with 13" pitch props, 3500 rpm with 17" pitch prop) may be due to this gap being too large, due to an incorrect spacer in front of the prop. He hasn't seen the drive. Should I see water coming out of this gap?<br /><br />-- Prop exhaust port. Good flow (more than half of total flow) when tilted all the way down.<br /><br />Does it sound right to have the water (normally would be exhaust) coming out of these five places?<br /><br />For a closed cooling system (mine is open), Clymer says the fresh water section should be flushed and cleaned with any high-quality automotive cooling system cleaning solution to remove scale, rust, mineral deposits or other contamination. Is it possible to do something similat for an open cooling system? I'm not sure I'd want to recirculate the dirty solution somehow, but would running it through once help somehow? Seems easier than taking everything apart.<br /><br />My game plan is to take the engine circ pump apart and check it, then go to the manifold and riser. The stern drive water pocket keeps eating at me, though.<br /><br />Dumb question(?) - why isn't there a gasket between the stern drive gear housing and drive shaft housing?<br /><br />Thanks.<br /><br />Steve

 

[rhartSilverdale]

Re: Overheating--where should exhaust come out?

I have a 140 OMC I/O which has similar holes in the outdrive as you have described. I believe that these holes are water relief outlets which drains some of the water during engine idle. The primary water outlet is usually through the exhaust/center of propeller when the engine, gear and prop are running, and a great volumn of water is required to cool the engine and gears. Time to get a manual I think. It usually contain a cooling circuit diagram. Good luck.

 

[loadnet]

Re: Overheating--where should exhaust come out?

Overheating items to check.. and I'm NOT a Mercruiser Mechanic... just another boat owner like yourself.<br /><br />Put airmuffs on the outdrive to feed it water while running the engine at idle and see if you are getting a good amount of water passing thru your system and back out the outdrive exhaust and under the lower unit area... <br /><br />If you are not getting a good flow using the earmuffs... you may need a new impellar or the whole pump itself.<br /><br />Here's something else that can lead you to believe you need a pump.<br /><br />Put the outdrive up in the air and look up underneath the outdrive on the left side for a radiator sized hose. (approx: 5/8 hose) That's your water pick up hose that runs from inside the transom out thru the transom gimbal housing and it connects to a tube which runs down to your impellar pump.<br /><br />Check to make sure it has no cracks in it and the clamp is in good condition - nice and tight.<br /><br />Usually if your manifolds are clogged the engine doesn't normally start to overheat till you get up into the higher RPMs, pushing the boat..<br /><br />Check your thermostat too.. pull it out and replace it if it's all rusted up.<br /><br />Feeding water up top like you did is fine to check water flow thru the manifolds but it's not telling you if your water pump is working or not.<br /><br />The items I've mentioned will give you a place to start.<br /><br />Hope this helps you out some..<br /><br />My email is: customerservice@401hosting.com if you need to reach me directly.<br /><br />PB

 

[merc 140 pontoon]

Re: Overheating--where should exhaust come out?

Thanks for the reply. My Clymer Mercruiser Stern Drive Shop Manual 1964-1985 shows my cooling circuit on page 349. But the diagram ends where the riser elbow meets the boot going to the exhaust elbow. The "best" diagram I've found of the gimbal housing is the Merc exploded view, but I'm not sure if I'm imagining seeing the bypass holes in the diagram (I may even be looking at the wrong part!).<br />

 

[merc 140 pontoon]

Re: Overheating--where should exhaust come out?

My reply above was for Robert. This is for Cap'n HooknFinger.<br /><br />Thanks for the reply. Three weeks ago I replaced the stern drive water pump (base, impeller, top housing, gaskets). I have good flow out the prop at idle with earmuffs.<br /><br />Put the outdrive up in the air and look up underneath the outdrive on the left side for a radiator sized hose. (approx: 5/8 hose) That's your water pick up hose that runs from inside the transom out thru the transom gimbal housing and it connects to a tube which runs down to your impellar pump. Check to make sure it has no cracks in it and the clamp is in good condition - nice and tight.<br /><br />I will check that. Thanks.<br /><br />Usually if your manifolds are clogged the engine doesn't normally start to overheat till you get up into the higher RPMs, pushing the boat..<br /><br />That was the overheating scenario I had, until Monday. Now it overheats at all rpm. I think I have multiple problems going on. Makes it tough to trouble shoot. For example, I finally cured a problem where the mechanic had wired the Pertronix Ignitor electronic ignition module incorrectly--causing it to shut down. Coupled with my overheating problem, this was very difficult to isolate, but it is fixed now.<br /><br />Check your thermostat too.. pull it out and replace it if it's all rusted up.<br /><br />Thermostat is one year old. Confirmed it is the right one, and tested it on the stove. Also confirmed it was wide open inside the engine housing when overheating (I was careful and used a big towel...).<br /><br />Feeding water up top like you did is fine to check water flow thru the manifolds but it's not telling you if your water pump is working or not.<br /><br />I appear to have good flow from the stern drive to the point where the small rubber hose enters the t-stat housing, and I appear to get good flow through the manifold. My plan is to take the engine circ pump apart next. I suppose I should pull the manifold just to be sure, but I'm worried I'll crack something.<br /><br />Thanks.<br /><br />Steve

 

[Skinsk]

Re: Overheating--where should exhaust come out?

When you replaced the Impeller, did you make sure that the copper pipe was in the drive shaft housing? Also, when you installed the gearcase, did the copper tube line up with the little white sleeve that guides the copper tube into the w/p housing?<br /><br />Unless the circ pump is leaking, or the bearings sound shot, it's probably not the circ pump.<br /><br />Scott.

 

[merc 140 pontoon]

Re: Overheating--where should exhaust come out?

Yes, the copper pipe was there and lined up. I've confirmed good stern drive water pump flow by pulling the feed hose at the t-stat housing. I'm going to pull the circ pump pulley, have a look at the circ pump, then the t-stat housing, look in the block and head passages, then if I have to, the manifold and riser. Arghhhhhhhh.

 

[merc 140 pontoon]

Re: Overheating--where should exhaust come out?

Cap'n HnF - I checked the hoses in the gimbal housing. There are 4 of them that I saw: the large bellows, the small exhaust bellows, the tiny bellows on the starboard side, and the water pickup hose on the port side. All are in very good shape, with no cracking or melting that I can see. All of the clamps that I can see are tight.<br /><br />Steve

 

[Skinsk]

Re: Overheating--where should exhaust come out?

I'd check the thermostat. Maybe it'd been opening just enought to cool the motor at idle, but not far enough for the increased demand at speed. <br />Now maybe it's stuck closed completely..<br /><br />Scott.

 

[merc 140 pontoon]

Re: Overheating--where should exhaust come out?

When it overheated on the lake on Monday, I carefully pulled the t-stat housing, and the t-stat was wide open.<br /><br />I'm going to pull the manifold, riser, t-stat housing, circ pump, and have a look at all the passages. I'm starting to think I have a lot of corrosion clogging things up. Gasket kits should be here later this week, hope to do the work this weekend.<br /><br />Steve

 

[K Hultgre]

Re: Overheating--where should exhaust come out?

I just went through the same exercise with my 70' Merc 140. Something that my manual did not show is the "exhaust flapper" something I discovered while reading this board. According to the image above it should be item 45 located just below the exhaust bellows (item 47). I was able to muscle off the bellows to make sure the flapper did not break and become lodged further downstream creating an obstrucion. The solution for mine was to replace the outdrive impeller and associated housings. Did you turn the drive shaft clockwise (I think) when installing the upper housing (impeller should be cupped towards the direction of shaft rotation - counterclockwise). T-stats are cheap and I have had them bad out of the box on cars I have repaired. Have you tried running it without the T-stat? Good Luck.

 

[K Hultgre]

Re: Overheating--where should exhaust come out?

Dumb question(?) - why isn't there a gasket between the stern drive gear housing and drive shaft housing?<br /><br />On my outdrive the only gasket is for the oil passage between the gear and drive shaft housing. Everything inside there that could get wet are all round shafts with oil seals (except mid shift shaft) to keep the oil in and the water out. Hopefully you replaced the oil passage seal (if yours is so equipped) when you replaced the impeller.

 

[merc 140 pontoon]

Re: Overheating--where should exhaust come out?

Kevin - Thanks for the replies. I will answer each question:<br /><br />I bought a new exhaust shutter last week. Hven't pulled the manifold/riser/boot yet to check it. The mechanic told me that the Merc shutter design is good--when it fails, it breaks off in halves, which blow through. He said the OMC shutter design is bad--it breaks off as a whole piece, and often plugs the exhaust. He also told me to apply silicone to the ends of the shutter's metal mounting rod, because otherwise it'll rattle.<br /><br />Yes, I had a helper rotate the shaft the correct direction while installing the stern drive water pump upper housing. I have confirmed good flow out of the pump.<br /><br />I have run the engine on the lake without the thermostat--it runs about 5 degF cooler at most rpm's (still overheats at high rpm). I have confirmed that the t-stat is working correctly (in water on a stove), and that it is fully open when the engine overheats. My problem isn't with the t-stat.<br /><br />Yes, I installed a new oil passage gasket between the upper and lower drive units. I guess I'm missing something about the design--I thought the gear lube that I pump into the lower hole (until it runs out the vent hole at the top) came in contact with a portion of the "seam" between the top and bottom units. That's why I wondered about a gasket. I guess I'm not understanding the different cavities in the stern drive.<br /><br />Thanks again for the input. I'm learning...<br /><br />Steve

 

[akriverrat]

Re: Overheating--where should exhaust come out?

just wondering if this almost 30 year old engines water jacket is getting plugged with rust and mud/sand/silt?