1991 Mercury 75hp overheating at idle even after impeller replaced

[hawkhunter]

Recently bought a '91 Cajun with a 1991 Mercury 75hp outboard. Took it out on the lake, ran great but while going through the no wake zone the overheat alarm sounded. Upon giving it just a little speed the alarm would stop. Apparently just at idle the motor gets hot. Replaced the impeller with a new one. The old impeller was not broken or burnt. Took it back on the water, same problem. This time I replaced the thermostat, poppet valve, and installed a complete impeller kit including the pump housing. Upon starting I got a decent tell-tale for a few seconds, then none and it eventually sounded the overheat alarm again. I monitor the temp with my infrared gauge and shut it down before any damage can occur.

It appears water is not traveling throughout the cooling system like it should. When I pull the thermostat/poppet cover it appears dry except for a little condensation on the t-stat.. The motor itself has only been a freshwater motor and there is no gunk or sediment build up at all. It's a very clean motor. Compression is about 120 on all 3 cylinders. I'm really at a loss here. I've flushed the system, including the tell-tale, with water and air and still nothing. I've verified the thermostat is working properly and since the motor seems fine at speed I assume the poppet is working properly.

I boat on a chain of lakes with many no wake zones so this issue makes the boat unusable. What am I missing? I can't seem to find the answer after hours of searching. I have a Clymer manual but that hasn't provided any answers either. Any help or ideas?

 

[Silvertip]

Did you get the water tubes properly lined up when you installed the lower unit after the impeller replacement?

 

[Dekker500]

Try blowing canned or compressed air up the water tube when the lower unit is removed. There should be no restrictions, and you should be able to feel the 'puff' from the tell-tale hole. If you get less puff than expected, work backwards from the tell-tale hole, removing the tube leading from the power-head to the tell-tale, verifying that everything is clear. If you don't find anything there, I'd suspect some partial blockage between the water intake and the exit at the power-head. At higher RPMs it is cooling enough, but at low RPMs there is not enough pressure to cool it enough. That'd be my suspicion.

 

[hawkhunter]

Did you get the water tubes properly lined up when you installed the lower unit after the impeller replacement?

Sorry for the delayed response, I thought I'd receive an email when somebody responded. Anyway, I think it lined properly , but visibility is limited. All I see is one tube that appears to be brass that connects to the water pump housing via a black rubber connector that's about and inch or maybe an inch and a half long. The tube remains up in the housing when I pull the lower unit out. I can't seem to find any reference to this tube in my Clymer manual, so I don't know if I should or could pull the tube out to inspect the tube.

 

[hawkhunter]

Try blowing canned or compressed air up the water tube when the lower unit is removed. There should be no restrictions, and you should be able to feel the 'puff' from the tell-tale hole. If you get less puff than expected, work backwards from the tell-tale hole, removing the tube leading from the power-head to the tell-tale, verifying that everything is clear. If you don't find anything there, I'd suspect some partial blockage between the water intake and the exit at the power-head. At higher RPMs it is cooling enough, but at low RPMs there is not enough pressure to cool it enough. That'd be my suspicion.

OK thanks for the reply. I'll have to pull the lower unit off again and check. The worst part is waiting for the sealant to dry before testing it!

 

[hawkhunter]

Finally getting back to the boat. Work is seriously interrupting my hobbies! Anyway, I blew air up the tube and it came out the tell-tale in strong fashion. However, upon looking at the water tube seal I noticed a lot play (approx. 1/8 to 1/4") between the tube and seal even though it seated quite well on the pump. The tube appears to be about 1/2 or 5/8" (hard to measure) but the seal ID is 3/4". Is it normal for it to have that much play or do I need a new seal?

 

[hawkhunter]

Replaced water tube seal. Ran engine on muffs. Light stream from tell-tale, overheats at idle in about 5 minutes. Shut her down. I can't think of anything else that could be causing the overheat at idle. Any ideas?

 

[enginepower]

Is it actually overheating? Maybe your overheat sensor is faulty. Get a hold of an infrared thermometer and check block temp

 

[hawkhunter]

Is it actually overheating? Maybe your overheat sensor is faulty. Get a hold of an infrared thermometer and check block temp

I did use an infrared gun to confirm it's overheating. Top of block getting to 180.

 

[hawkhunter]

Is there a chance the exhaust gasket has failed which allows exhaust into the water jacket therefore blocking the entrance of water?

 

[hawkhunter]

I tested the exhaust gasket by running the motor with the tell tale hose stuck up an upside down 2 liter soda bottle. After running the motor a few minutes, there should have been a strong exhaust smell in the bottle after shut down. There was not. I pulled the t-stat cover, removed the t-stat and ran the motor and no water came out of the t-stat hole. Impeller not turning? What would cause the impellor to stop working besides a bad impeller?

 

[enginepower]

You didn't arch the fins the correct way when assembling? Guide pin broke or missing? Tube not sealing? Restriction?

 

[hawkhunter]

You didn't arch the fins the correct way when assembling? Guide pin broke or missing? Tube not sealing? Restriction?

The impeller fins are correct. I placed the impeller on the shaft, lowered the pump housing onto the impeller while turning the shaft clockwise. Guide pin? I'm not sure I know of any guide pin. I replaced the water tube seal with a new one. Does seem like the water tube seal should fit tighter on the water tube, but I guess it's just designed that way. Also, made sure water is flowing into the pump and there are no restrictions any where in the system. I can only believe the impeller is not working properly for some unknown reason. I'm at a loss of what to check next. Here's a youtube video of the impeller: https://youtu.be/PyE9rko3_1Q

 

[enginepower]

Guide pin...drive pin...I saw it there in your video. Don't guess you have a water pressure gauge. If you have a pee stream, your pump is pumping. No water out t-stat when removed sounds like you have a clog somewhere.

 

[hawkhunter]

Guide pin...drive pin...I saw it there in your video. Don't guess you have a water pressure gauge. If you have a pee stream, your pump is pumping. No water out t-stat when removed sounds like you have a clog somewhere.

I'm guessing it may be the upper water tube seal, I've tried everything else. (yes I'm still working on it). Any advice on pulling the middle or "upper" section to get at the tube seal?

 

[Bill kubiak]

I just finished a similar project on my 1998 40hp merc,

When you put the new impeller on did you install the impeller key? if you did, did it stay in place? You have to rotate the driveshaft clockwise to get the rubber fins in the pump housing, did they turn in. I see in all the above posts that the impeller key was not mentioned.

When I had my lower unit off I connected a water hose to the copper water tube and added pressure, to see how much water finally comes out the pee tube, on mine it sputtered out and later when I had the cylinder cover off I found a piece of rubber jammed in the opening at the bottom where the water enters around the cylinder domes. Apparently somewhere in the motors past a water pump came apart and part of the impeller found it's way up there to partially block the opening. Now I did the same test before I put the lower unit on and water came out that pee hole under pressure like it should.

 

[Bill kubiak]

OK thanks for the reply. I'll have to pull the lower unit off again and check. The worst part is waiting for the sealant to dry before testing it!

What sealant? Tthere is no need for sealant when just changing a water pump impeller

 

[hawkhunter]

I just finished a similar project on my 1998 40hp merc,

When you put the new impeller on did you install the impeller key? if you did, did it stay in place? You have to rotate the driveshaft clockwise to get the rubber fins in the pump housing, did they turn in. I see in all the above posts that the impeller key was not mentioned.

When I had my lower unit off I connected a water hose to the copper water tube and added pressure, to see how much water finally comes out the pee tube, on mine it sputtered out and later when I had the cylinder cover off I found a piece of rubber jammed in the opening at the bottom where the water enters around the cylinder domes. Apparently somewhere in the motors past a water pump came apart and part of the impeller found it's way up there to partially block the opening. Now I did the same test before I put the lower unit on and water came out that pee hole under pressure like it should.

Thank you for the help, I appreciate it. Yes I did install the impeller key and turned the shaft clockwise as I lowered the pump housing onto the fins of the impeller. I seem to get good flow with air or water if I blast it up the water tube or shoot air up the telltale. The old impeller had not deteriorated so I don't think there's any debris lodged anywhere, but it could be from a former impellor or seal I guess.

My next step would be to pull the mid section which requires pulling the powerhead so I can take it off. I'm not a boat mechanic so I'll have to order a lift eye to pull it so I can access all the bolts needed to remove the midsection. Hate the thought of pulling this thing all apart but it has to be blockage or the upper tube seal, I can't think of anything else it might be.

 

[hawkhunter]

What sealant? Tthere is no need for sealant when just changing a water pump impeller

On this motor you put a bead of silicone sealant right behind the impeller housing to separate the water pump housing and exhaust. I noticed some guys do it and some don't.

 

[Bill kubiak]

No Title

Thank you for the help, I appreciate it. Yes I did install the impeller key and turned the shaft clockwise as I lowered the pump housing onto the fins of the impeller. I seem to get good flow with air or water if I blast it up the water tube or shoot air up the telltale. The old impeller had not deteriorated so I don't think there's any debris lodged anywhere, but it could be from a former impellor or seal I guess.

My next step would be to pull the mid section which requires pulling the powerhead so I can take it off. I'm not a boat mechanic so I'll have to order a lift eye to pull it so I can access all the bolts needed to remove the midsection. Hate the thought of pulling this thing all apart but it has to be blockage or the upper tube seal, I can't think of anything else it might be.

If your motor is similar to mine, all you need at this point is to pull the cylinder head cover which will expose all the cylinder domes. Most mercury motors do not have a removable cylinder head. At the bottom of the domes is a curved opening, this is where the water enters the powerhead going up from the water pump. I think I have a picture I will look. Anyway that curved opening is where I found a chunk of a water pump impeller small enough to come up the water tube but too large to go any further and is partially blocked the water flow into my motor. Also when this cover is removed you will see to the left side while facing the motor some small holes near each dome, these holes are where most of the water enters around the domes, at the top of the opening is where the pee tube is attached, water must pass over all the domes to get up to the pee tube, from there some of the water goes out the tube and the majority of it is directed down to a passageway and into to the exhaust tube. At this point all you need is one new gasket, I managed to snap off four bolts on that cover and had to drill and retap the holes. Another member here had the same issue and the same snapping of bolts.
In the picture you can see the curved opening and also the jury rig I made to blast hi pressure water up and thru the motor, caution the copper water tube is held in place up the mid leg by a rubber dealy up there, too much pressure and the tube will pop off, no big deal, just put it back, you can see where it goes with a bright flashlight.
The last picture is showing the water flow out of that curved hole using my jury rig, if you have a good flow there and still have an issue you may have to remove the exhaust cover and clean all the water passages in there
Let us know how things go.....two of us here just had this issue and we have lots of posts and pictures of what we did