How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Michael Your Own Question
Michael
Michael, Master Mechanic
Category: Boat
Satisfied Customers: 257
Experience:  Over 25 years experience working on everything from PWC's to Mega Yachts
6854667
Type Your Boat Question Here...
Michael is online now
A new question is answered every 9 seconds

1991 Mercury 75hp outboard. Overheats at idle in about 5 minutes

Customer Question

1991 Mercury 75hp outboard. Overheats at idle in about 5 minutes with the muffs on. Overheats on the lake as well. Does not overheat when traveling a bit above wake speed. I've changed the impeller and water pump housing and the water tube seal in hopes of correcting the problem. The motor is super clean and I've blown water and air through the unit to ensure there was no blockage. I've verified the temp warning is working with my infrared temp gun. I've also replaced the thermostat and poppet valve as a precaution. When I start the engine on the muffs I get a light stream from the tell-tale but the warning chime comes on in about 5 minutes. Compression is about 120 on each of the 3 cylinders. I'm at a loss on this one! What could it be?
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.
Hello my name is ***** ***** am I speaking with. How hot is it getting according to your IR temp gun?
Customer: replied 1 year ago.
I usually shut it down after the warning chime, the top cylinder heat will read up to 180.
Expert:  Jason replied 1 year ago.
Did you replace the top water tube seal? Is there any smoke or steam coming from the tell tale?
Customer: replied 1 year ago.
Yes I just replaced the water tube seal. After I shut it down I do see some smoke coming from the tell tale as the motor heats up a bit more after shut down.
Expert:  Jason replied 1 year ago.
Its more than likely going to be a bad exhaust cover gasket. That is the metal plate on the side of the engine with all the bolts. When that happens exhaust gasses will get into the water stream, causing air bubbles make of exhaust to get in the water jackets. To confirm, take the hose off the inside of the tell tale fitting, and if you need to extend it, and get an old 1 or 2 liter bottle. Hold it upside down. Stick the tell tale hose up it a little bit. Start and run the engine until it gets good and hot, and shut it off. When the engine is running the water will run into and out of the soda bottle. But, any gasses will stay in it. So after you do this turn the bottle upright and take a good whiff of it. If it smells like a tailpipe in there, then it is going to be a bad exhaust cover gasket. Does that make sense?
Expert:  Jason replied 1 year ago.
I work for tips so I do want to make sure you are happy with my service before you go. If you had a further question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the start and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.Questions do not close out, so if you have to come back later on with follow up questions to the issue you still can even after doing a rating.Thank youJason
Customer: replied 1 year ago.
I will have to set up a bottle to test that theory. I should have milky discharge if the exhaust and water are mixing, but I don't see any evidence of that.
Expert:  Jason replied 1 year ago.
There isn't any oil involved here, so there is nothing to turn to milk. Water and exhaust by themselves will not turn milky, does that make sense?
Customer: replied 1 year ago.
It's a 2 stroke and not all fuel is burnt, like any 2 stroke engine there is unburned fuel. ff it's mixing with water there should be some evidence of mayo. I've seen this on other sites. I will try the bottle test, give it some thought and research, if it turns out to be an exhaust gasket I'd be more than happy to pay for the consult. Make sense?
Expert:  Jason replied 1 year ago.
I understand your thinking but that is not going to be the case with this type of problem. But let me know if that bottle ends up smelling like a tailpipe or not. And we will take it from there.-Jason
Customer: replied 1 year ago.
Thank you Jason, I may need to order 27-430057 if that's the case.
Expert:  Jason replied 1 year ago.
I don't quite follow you, is that a question or a statement?
Customer: replied 1 year ago.
I'm sorry, I didn't think my statement in response to your statement was so confusing. My apologies.
Expert:  Jason replied 1 year ago.
Oh okay, no problem. I wasn't sure if you were asking me if you needed that part number or not.
Customer: replied 1 year ago.
I did the 'bottle test' no fumes from tell tale. Now what?
Expert:  Jason replied 1 year ago.
Do this next test with either the boat in the water. Or with the lower unit completely submerged in a large tube. It will not work on the garden hose. Remove the thermostat and leave the thermostat out, and the cover off so that the hole is open. Start and run the engine. Do you get a real good flow of water up and out of the hole?
Customer: replied 1 year ago.
I'll have to figure out a way to rig something up. I'm not on the water and the motor is too big for a conventional garbage can. I can tell you in the past I have ran the motor till hot, pulled the t-stat cover and it appeared dry up there, but that was with the muffs.
Expert:  Jason replied 1 year ago.
just for giggles you can try it on the muffs. The reason it really should be done with the boat in the water is the garden hose is under water, and will assist the water pump. And thats not something you want. This test is to judge the inmpellers ability to pump. Now if you get nothing or very little even on the garden hose then there is definitely an issue. Let me know what happens.
Customer: replied 1 year ago.
Did the test with muffs. No water coming out the t-stat hole or the poppet hole. I had to push on the muffs with my hands to get water to come out of the tell-tale and it was moderate at best. I can see some wetness in the poppet hole but only below the seal. I've changed the impeller twice on this unit, and the pump housing once along with the tube seal. It seems like I'm getting nothing out of that impeller. I hate to ask, but now what?
Expert:  Jason replied 1 year ago.
Well.... The impeller either isn't pumping water. Or it is, and there is a blockage. So the next step is do drop the lower again. And hold a garden hose up against the water tube. It should flow freely up and out of the thermostat opening. If it is not, the powerhead is clogged up with something, and it would need to come off and come apart to see what is clogging it up. But if water flows freely, then I hate to tell you, but there is something up with the impeller installation. Some part of that job didn't go as planned. I can be anything from the impeller, to the pump housing, to forgetting the key to a torn gasket. Does that all make sense?
Customer: replied 1 year ago.
Well, I pulled the lower unit, stuck the garden hose up the water tube and water did come out of the t-state hole and poppet hole. I seriously don't know what could have gone wrong with the impeller installation. The boat was doing this when I bought it. I bought a new impeller thinking that was the cause. Didn't fix it. Talked myself into thinking the impeller housing, water tube seal and anther new impeller was in order. Didn't fix it. I can't think of anything I missed or did wrong.
Customer: replied 1 year ago.
I do have a Clymer manual which I followed along with watching some you tube videos. For the record I've never worked on a boat motor before so maybe I missed something simple an experienced wrencher would get immediately .
Expert:  Jason replied 1 year ago.
Did you installed the vanes of the impeller in the correct direction? You have to turn the driveshaft clockwise to spin the impeller while also pushing down on the pump cover at the same time. Did you do it that way? Did you get all the gaskets correct?
Expert:  Jason replied 1 year ago.
Obviously the problem is in the impeller installation part of the job someplace. The problem is i'm no there to look over your work and see where you are making the mistake.
We have to leave this someplace at somepoint. Did you have any further qeustions for me on the issue?
Customer: replied 1 year ago.
Yes, put the lubed impeller on the shaft with a new key. I rotated the shaft clockwise while pushing down on the new pump housing. There's really not too much involved in replacing an impeller. Based on your last question, it appears this issue is beyond your expertise.
Expert:  Jason replied 1 year ago.
Hmmm.... No. I probably 70 to 100 of these each year, every year. From everything we have covered so far the problem is in the installation of the impeller someplace. Your doing something wrong, I don't know what, but the issue is definitely in the pump.
Customer: replied 1 year ago.
I guess I was expecting something more than the bare minimum. If you can't figure it out it's always easy to blame the owner. Good day.
Expert:  Jason replied 1 year ago.
We've done every test we can. The results of the test point towards the pump being the issue. Did you forget the grommet between the water pipe and the pump output, because that would do it.
Customer: replied 1 year ago.
Are you referring to the slinger or the water tube seal?
Expert:  Jason replied 1 year ago.
Both would do it. If the slinger is missing exhaust gasses can get into the pump.
Customer: replied 1 year ago.
I put a new water tube seal on it when I replaced the impeller for the second time along with a new housing and plate. This particular motor 0D037383 does not require the slinger but I put it on anyway since it came with the kit. Most believe it doesn't do any good nor any harm on this motor.
Expert:  Jason replied 1 year ago.
When you changed the pump did you use the old base plate. Or did you lift that up?
Customer: replied 1 year ago.
I replaced the base plate (but not the base) and both gaskets, one under one over.
Expert:  Jason replied 1 year ago.
What I would do next is remove the lower. Remove the impeller. And then squeeze a set of muffs against the lower unit and see if water at least comes out of the water hole in the lower plate. If not, at least that will tell us it's clogged below the plate. Was this pumping water before you changed the impeller. Or was it not?
Customer: replied 1 year ago.
The lower is still of the motor so I'll try that. I just bought the boat a month or so ago. I did a compression test which was good and then the seller ran it for a bout 30 seconds in his driveway. Seemed like an honest guy. It drives good at speed but the alarm sounds when going through no wake zone or just sitting at idle. I thought, that's fine, I'll just put a new impeller in it and we'll be good to go............and now I'm here.
Customer: replied 1 year ago.
Not plugged, water comes up through the water hole.
Expert:  Jason replied 1 year ago.
Can you take a video of you installing the impeller and throw it up on youtube so I can see it for mysef?
Customer: replied 1 year ago.
Ok, I'll try to do that.
Expert:  Jason replied 1 year ago.
OKay keep me posted.
Customer: replied 1 year ago.
https://youtu.be/PyE9rko3_1Q
Customer: replied 1 year ago.
video link provided above.
Expert:  Jason replied 1 year ago.
I can't see it from the video, is there any chance the pump cover or base plate is warped?
Customer: replied 1 year ago.
I don't think so, nothing apparent and both are new.
Expert:  Jason replied 1 year ago.
I'm out of ideas. I'm going to call this one quits. If one of the other guys has an idea they will jump in. Good luck.
Customer: replied 1 year ago.
OK
Customer: replied 1 year ago.
SUMMARY:
1991 Mercury 75hp 3cyl outboard. Overheats at idle in just a few minutes, runs strong and does not overheat even at moderate speed. This is what I've done so far. replaced the impeller, plate, housing, water tube seal, thermostat, poppet valve. I did not install the impeller with the fins in the wrong direction. Have verified no obstructions, via flushing with air and water. Have verified the exhaust gasket is not the issue. I have verified the overheating alarm is correct via infrared temp gauge and touch.
Have verified water is entering the pump, however after removing the t-stat/poppet housing no water comes out of either hole with the motor on the muffs. I can get a moderate stream out of the tell-tale when on the muffs, but as the motor warms it reduces to near nil and it overheats. I can only assume there is a problem with the impeller not turning or fault with the pump and /or it surrounding assembly. I'm not boat engine mechanic, so I need an expert who can help me track down this issue.
Expert:  Michael replied 1 year ago.
Hi, my name is ***** ***** this post and Agree with everything Jason said.. He more or less took you through the entire cooling system so there are very few things left that it could be.I may not have the answer but I'm going to take a little different approach to see if we get any results.....Let me start by asking you a few questions;When the alarm starts to sound have you let the motor keep running a bit longer to see if the temp continues to climb?Is it only the top cylinder that is reads 180 degrees? what is the temp on the lower two cylinders?Does the motor appear to have a history of salt water use?Let's start by answering these questions and I'll explain them next.. Mike
Customer: replied 1 year ago.
OK, first let me say I agree with Jayson as well, because I had done those tests already with the exception of the exhaust manifold gasket.If the top reads 180 the middle would read about 140 and the bottom about 120. Not getting water to the top.When I'm on the lake and the alarm sounds I speed up the boat and the alarm stops. I do not rev the boat on the muffs to prevent engine damage.Never been in salt water, water jacket appear nearly spotless when peering down the t-stat and poppet holes. It's amazing how clean the motor is.
Expert:  Michael replied 1 year ago.
again, have you run the motor for a bit longer after the alarm sounds while monitoring it with the temp gun?
I mean without revving it up , just idling at the speed it gets hot at.
i'll explain after you answer
Customer: replied 1 year ago.
Yes, because the I can't really hear the alarm while I'm at the motor, I keep the infrared on the head until it gets to about 160+ on top, walk over to shut off the ignition and that's when I hear the alarm has already sounded. It's really impossible for me to hear it while I'm back at the motor. So to answer your question, yes it runs for a bit after the alarm goes off I'm sure.
Expert:  Michael replied 1 year ago.
Ok, this is what I was suspecting.... it is possible that you only have a faulty temp sensor.At idle, the water pressure is low and has to push the water way up to the top of the motor so the lower the idle, the less water pressure you have... if it is not surpassing 180 degrees then you are not in real danger unless it continues to climb. the thermostat should open at 143 degrees and the alarm should sound just over 160 degrees which is a ways from overheating. if you can run the engine in the water with the temp gun focused on the top cylinder and it does not exceed 180 degrees and cools down when the rpm is increased then you don't have anything to worry about. If this is the case then try replacing the temp sensor (which is near at the top cylinder). I'm betting that this will solve the issue.
If in fact the temp climbs higher then I would still consider changing both exhaust cover plate gaskets just to eliminate that possibility, as it is an easy job.
BTW: When there is a restriction in the cooling system such as a pinched or swollen water tube seal, bad poppet, clogged water jackets, etc. , the heating issue is opposite of your scenario... The engine would cool better at idle and start to heat up as the rpm increases as the water can not keep up to sufficiently cool the motor.
Mike
Customer: replied 1 year ago.
The overheat sensor is fine. I ran the motor and temp went to 200 before I shut it down. If it were just the bad sensor, I wouldn't get such high readings from the infrared. I also swapped the existing T-stat with a 123 from 143. I think the only cooling I am getting is due to the garden hose pressure and not the work of the impeller. My only thoughts are the impeller pump is not working properly or the water pumped is leaving the cooling system prior to making it up top. I wish there was some type of a testing connector that would fit on top of the shaft with a drill connection so I could remove the lower unit, hook up the muffs, spin the shaft and verify the pump is doing it's job.
Customer: replied 1 year ago.
Or possibly I'm missing the connection on the water tube seal when I install the lower unit, visibility is not good, but it seems impractical that every time I install the lower unit (must be 10 times by now) I would "miss" the water tube. It does seem like the tube seal should fit tighter on water tube, like it does on the pump outlet, but that must be by design I guess.
Expert:  Michael replied 1 year ago.
Ok, that's why I was asking you if the temp has climbed above 200 degrees... I could not have known that without you telling me. So now we can rule out the sending unit.
Which leaves an insufficient water supply as the main culprit. If the water tube is not sealed properly at each end, you could be loosing a portion of the water that trying to make it to the top. If the pump is operating properly then you need to be looking at the water tube grommets or the power-head base gaskets. if the power-head gasket is blown between the water and exhaust passages, some of the water could be bypassing the block and coming right back out with the exhaust. As I said before, if it were any kind of restriction, the motor would heat up with increased rpm as the water supply would not be sufficient enough. Like you were saying.... it would be very unlikely to miss the water tube 1o times in a row so you're probably getting that right... You said that you replaced the grommet at the top of the water tube right? and you have the grommet in the water pump housing that the water tube fits into?
You could try attaching a water hose to the water tube with the unit removed and turn the water on slightly and see if it leaks out somewhere before it fills the block and starts coming out the exhaust..
Try these suggestions and let me know what you find and we'll take it from there.
Mike
Customer: replied 1 year ago.
I replaced the water tube seal, but like I said there is at least 1/8" of play when the seal slides over the tube. I could have probably left the old one on. I have the grommet (the one visible when looking up the unit with the lower unit off) on order although I inspected it and it didn't seem too bad. It should be here Monday. I have not removed the entire water tube or inspected it past removing the lower unit. I noticed on some videos the tube comes off with the lower unit, but mine hasn't.When the grommet comes in I'll pull the lower unit and test for a leak as you advised. I'm going to put this on ice over the weekend. Thanks for your assistance Mike.
Expert:  Michael replied 1 year ago.
You're welcome, I'm invested in this now and curious as to what it is... I would like to help you resolve it so let me know when you are back on it.Note: There is a grommet in the upper pump housing that the tube fits into, the sleeve that slides over it is just a guide to to help align the tube as you install the gear case. so it's supossed to be loose.The grommet at the top is the more critical one as it has a higher failure rate and if damaged can be the cause of your problem... after that is the power head base gasket and if blown will have the same affect.Have a good weekend, Mike
Customer: replied 1 year ago.
Just a heads up, my part didn't come today, looks like it will be tomorrow before I can continue
Expert:  Michael replied 1 year ago.
ok, thanks for letting me know Mike

Related Boat Questions