Help! Tohatsu 70 HP M70A2 Overheating

Petedacook

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Hello,

I'm new to boating and the forums. Like the title says, I have a Tohatsu M70A2, 70 HP 2 stroke, that is overheating. I do not know where to start on a boat motor and I need some help, please.

I have used the boat last weekend Sat- Sun and this weekend Sat-Sun. After about 7 hours of boating on the Potomac this past Sunday, I got the overheating warning sound. Water was coming out of the telltale, so I thought this was an error (I know this was a wrong assumption). We traveled about a football field after the sound began, at a light throttle. Engine still starts, idles and seems fine except for a slight back fire every few revolutions. Not sure if backfire is due to a lean mixture with the oil/gas as I went from 40:1 (previous owner recommend) to 50:1 per the manual. Top plug is lean, bottom plug is rich.

I am getting water out of the telltale. After the boat sitting all night, I went to start it this evening and when I turned the switch to on I found the alarm going off. It could not have been hot at this time because it was not running all night/day. I hooked up the muffs and started it. The hot warning sound continued for the 30 seconds or so I had the engine running. How water and exhaust spewed from a hole in the top back of the shaft (I assume this is normal?). I don't know if this is a blown head gasket or what.


I have checked the manual and the only thing I can find in there is where the impeller is located. What are my trouble shooting steps with this engine? I have worked on cars for years, but I am at a loss when it comes to where to start on troubleshooting an outboard.

Can anyone please give some advice? Tohatsu guru, can you give me the advice I need?

Thanks for any help,

Pete
 

Sea Rider

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Welcome to Iboats,

How old is that M70A2, did PO changed plugs, impeller, thermostat ? A blown gasket will make the engine miss, and probably shut, will depend on the head gasket condition. Is the sensor clean ? You could have a bad ignition coil ?

Happy Boating
 
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Petedacook

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Hello, Thank you so much for your response and help!

How old is that M70A2- Paperwork says it was a 1988 model.

Did PO changed plugs- I am not sure PO changed the plugs. The plugs were dirty and needed to be changed after my adventures the weekend before the weekend before lasrt. I changed the plugs before going out this past weekend. The weekend before they were the PO plugs. I went from champion ql78yc to NGK B8HS-10 which is recommended in the service manual, gaped according to spec. The champion plugs he was running are not in the manual.

did the PO change the impeller, thermostat- I do not see a service record of these items, so I assume no. My assumption is based on anecdotal evidence that the lower unit oil was gross and smelled of rotten eggs when I changed it last week (before going out this past weekend).

A blown gasket will make the engine miss, and probably shut, will depend on the head gasket condition.- Engine runs strong, idles good, but there are some signs I did not note before when hooking up the muffs that leads me to believe something is amiss: water & exhaust comes out of the port on the upper side of the shaft I did not note before when I ran the muffs prior to overheating incident.

Is the sensor clean ? - Where is the sensor? How can I test it?

You could have a bad ignition coil?- It does not exhibit signs of a bad coil. This is an overheating problem, and possible damage caused by overheating. It seems to me a bad coil would cause a misfire, and I am not experiencing a misfire, more of a cross fire if I had to diagnose it. I can post a video if that will help.

My kids were swimming in the potomac and stirring up muck from the bottom while we sat there. If mud/muck getting sucked into the engine were the problem, where would I tear apart the engine to find/remove it? I don't know where such a mess would accumulate.....maybe the cover to the telltale is my best bet to start? I don't know.

At this point my best guess based on my experience with cars:

1. overheating problem:
A. thermostat
B. impeller
C. clogged coolant ports
2. blown head gasket:
A. coughing/spitting out the exhaust every fer revolutions
3. Lean mixture:
A. caused by me changing the plugs (keep in mind the motor ran fine for 14+ hours prior to overheating)
B. lean mixture caused by me changing the fuel oil ratio to book spec 50:1. Again, 14+ hours on it before the overheating incident.


Again, thanks for your feedback and help. This boat has helped me with depression and made me enjoy life for the short time I have owned and operated it. I have such a blast with my kids on this boat, losing it is a major loss for me.

Pete
 

pvanv

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If you get an overheat indicator when cool, it's a false indicator. Check the sensor.
 

Sea Rider

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Are you assuming you had an overheat due to overheat sensor triggering ? Does the engine runs good besides the sensor isssue, if so, disconnect sensor and if everything is ok probably is severely rusted, salty or on its way to become kapput. Remove it and have a look, clean head sensor if possible to recuperate it, if not, install a new one.

If wanting that oldie to run as a champ, clean all 3 carbs, soak them in carb dip, remove al plastic and rubber parts, plugs must be gapped tight to 1.0 mm, 50:1 ratio is opt. Will need to synchronize all 3 idle carb screws to achieve a smooth idle operation. Remove thermo and have a look, if severely corroded or salty change it along thermo gasket. Re torque head gasket to 30 NM.

Pull lower leg and insert a wate hose on thermo hole and give a high water pressure flush for 5 minutes, will remove any crude, debris located on any water passages. Latter check impeller cond, change complete WP kit if previos owner was boating on sandy, muddy waters as impeller liner will be excesively abraded.

Thermo is located on the very top of the cylinder head under a small lid, heat sensor is located at left side around middle of head, is retained by a small metal plate stopper and screw. Post findings, that's a good motor to keep and fully enjoy with family.

Happy Boating
 
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Petedacook

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Thanks Sea Rider. This is a big help. I assume it overheated due to overheat sensor trigger going off. The engie runs good, idles great. I have not run it through the RPM range since the overheat sensor went off. The PO had the carbs rebuilt and they seem to be in good shape.

Do you by chance know where I can obtain parts for this motor? Like the impeller or WP kit?

Thank you much for your help.

Oh, just wanted point out that the overheat trigger goes off a few moments after sterting the motor with the muffs on, but then it comes back on after a little while longer.

Pete
 

Petedacook

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Posting for update purposes and anyone with a similar issue.

Parts are a ***** to find. Engine runs fine- missfire resolved after changing the plugs- no more misfire. Heat alert still going off even when cold. Pulled the sending unit and it still went off........bad sending unit assumed, but engine still runs hot to the touch. I run it for a few seconds to see what happens- run with heat alarm removed from unit, etc. still goes off, but also still runs really hot in the head area. Tested the thermostat and it begins to open at 150 F. It should open at 140F. so it needs replacement. I found a thermo and gasket set on Ebay for ~ $50. But that does not resolve the issue of why it is running hot. I suspect impeller. I am trying to locate parts at the moment. Will update once resolved.

I am finding parts slowly but surely............I get the part number from boats.net and search Google for the part number. The results are so good I found an impeller on amazon for $35.00. I may need to make gaskets, but the housing is still a challenge. I found a guy on craigslist parting out everything I need to replace the impeller and housing, but he wants certified check before shipping- no ebay listing or paypal, only certified check. Kind of sketchy. I am pulling the lower unit tomorrow to asses the impeller repair.

This engine runs strong, no need to scrap a 7K+ engine for a few nickel and dime parts. I thought about sending it to a repair shop, but I do not see them putting the effort into finding the parts as I have and they have a 3-4 week wait before they could even look at the motor.
 
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pvanv

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Impellers are available new from Tohatsu. Check head temps with an IR thermometer. 150 degrees is not a bad open point for the stat... 140 is the BEGIN to open temp. You may need to flush corrosion from the cooling passages with salt-away or vinegar.
 

Sea Rider

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Was OP running that engine on salty waters ? Cylinder head is not a part that will be ever cold, heats enough as not to burn your hand if letting staying there for some seconds and that's as long is working inside normal operating temp.

Which was the thermo and temp sensor cond found? salty, rusted ? As suggested by Paul Salt Away will be a nice product to flush engine on a barrel for some minutes. Or remove thermo and pour an entire vinegar bottle inside head, let soak for some minutes and flush away with a water hose connected to the thermo orifice. Check impeller, probably already shot along liner...

Happy Boating
 
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Petedacook

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Update:

Tore off the lower unit tonight. Super simple job. I found everything to be in really good condition. I bought this boat because the PO had service records. I was not let down. Not only is it clean, but it has been well taken care of. The pump housing, and pump insert are in excellent condition. I did not even have to try to save the gasket, it went away from the base clean.

I think I have found the problem. One prop is bad on the impeller. Ostensibly, this would cause a 1/3 loss in cooling power.

Check the images. One of the impeller fins are broken. While the impeller seems to be in good shape, one of the fins is broken causing a loss in coolant flow apparently enough to trigger high temp sending unit.

I will get another impeller.

The one question I have left is.........the impeller key was broke. How much of a deal breaker is this? In my mind I can fabricate one, but is there an alternative?

Thanks all for the help. Your assistance is not fruitless as you see results in this user.


Pete
 

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pvanv

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You will need to make a key. Impeller is available: Part #: 353650210M
List Price (MSRP): $38.50
 

Sea Rider

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You have checked the impeller in time, if breaking, pieces can go up head and obstruct water passages. Assume there?s no missing impeller tip missing, right ? If not, tiny broken piece could have made its way up head ? Just in case, flush head with hose through thermo opening before assembling back thermo if it's still removed.

Happy Boating
 

Petedacook

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I got the impeller, impeller key, and thermostat and thermo gasket after doing some intense internet searches. I began looking for part numbers rather than motor name & part needed. I got better results this way.

I put the bottom end back together and the thermostat in with new gasket. Fired it up, ran good, throttled it up on the muffs hanging out my garage in a town house, sounded good, ran good, buzz from overheat went away, temperature felt good (I dont have infrared temp gauge).

Cam inside, wife asked me WTf is wrong with me. It's 11:00 at night and I am firing up the neighborhood. I was so into my work, I lost track of time and was inconsiderate to my neighbors.

Any how, I used toothpaste and worn green scrubby to remove grooves in the impeller housing. Put everything together; grease on WP bolts, housing bolts, pump inlet, drive shaft spline, and all looks and sounds great,

Many thanks to Sea Rider. Appreciate the tips and advice along the way. And they were a big help.

Pete
 

Sea Rider

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You are welcome Pete,

Nice to read that engine is working as a champ, hope same when finally running on water along boat. You'll see that polishing with toothpaste and worn green pad rejuvenates WP liner and just about anything that needs polishing.. Sometimes have dawned myself fixing motors without realizing time until the roosters crowed. LOL!!

Happy Boating
 

Petedacook

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Well, I took the boat out Sunday. At low idle, and low RPM, overheat sensor sounds off. At high RPM/full throttle, overheat sensor does not sound off. So, it would seem I have an overheat condition at low RPMS.

I don't think there are any clogs. The only thing I can think of is that there was no O ring or any seal between the water line pipe and where it went into the impeller housing.

Can anyone confirm there should be a seal of some sort where the water line goes into the impeller housing? I am not able to pinpoint a seal looking at the diagrams and maintenance manual.

Thanks,

Pete
 

Sea Rider

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Overheat cond is usually at wot, do you have a strong and steady peeing ? There's a rubber grommet located at upper housing which seals tight against lower water tube, there should be also 3 rubber components right were tube connects to grommet.

Did you clean heat sensor head, must be mint cond. If still triggering, install a new one to work much better along new impeller. Does the engine runs good from 1/4 to full throttle, no misses, hesitation whatsoever ?

Happy Boating
 
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Petedacook

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do you have a strong and steady peeing - Not steady, but it is strong. At low RPM, the pee stream is weaker than it is at higher RPM to full throttle. At full throttle you can actually see the pee steam arc increase, and feel the pressure build under your touch.

There's a rubber grommet located at upper housing which seals tight against lower water tube. - I see the gromment on the upper housing. It looks fine, almost like a rubber washer between the casing and the lower portion of the tube. I can't see the other 3 components. What I see is a rubber washer on the lower portion of the casing. Will post pics.

While I was on the water Sunday, I installed a used (tested working according to seller) heat sensor and no more issues. I installed that on the water. I checked it on land and found the mounting hardware landed it far from the head unit. The heat sensor and bracket are badly worn, obviously ran in salt water. I don't trust the reading from that sensor and suspect motor was overheating in low RPM entire time because it was so far from target location. Basically floating off to the left of the head sensor location.

Engine runs great across all RPM ranges. Frankly, it is a beast. It revs like a dirt bike engine, under pressure or on the muffs. Does not skip a beat and top end is furious. I have the idle set as low as it will go, and it idles and shifts great.

I tore the bottom end apart and the gromment (discussed above) looks good, and I see the rubber lower water pipe seal seems to also be in good shape.

Side note: I did install a new thermostat and gasket when I did the impeller/insert work. I got the thermo from this seller: http://www.ebay.com/itm/271659362203...%3AMEBIDX%3AIT




I have not torn out the thermo and tested it, but I would assume a new thermo would run at spec.

Thanks Sea Rider!

Pete

Edit:

Pic of upper impeller grommet: http://imgur.com/du2DEcH
Pic of upper impeller grommet:http://imgur.com/tMC5AlC
Pic of impeller housing cooling tube insert: http://imgur.com/jpuHGdJ
Pic of piece of **** replacement heat sensor I installed on the water: http://imgur.com/DPMHRpl
Pic of the old heat sensor: http://imgur.com/J19oWir
Pic of old heat sensor bracket: http://imgur.com/Rtv6q9o
 
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Petedacook

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Since I cannot edit the post.

do you have a strong and steady peeing -
Not steady, but it is strong. At low RPM, the pee stream is weaker than it is at higher RPM to full throttle. At full throttle you can actually see the pee steam arc increase, and feel the pressure build under your touch.

There's a rubber grommet located at upper housing which seals tight against lower water tube. - I see the gromment on the upper housing. It looks fine, almost like a rubber washer between the casing and the lower portion of the tube. I can't see the other 3 components (see pic). What I see is a rubber washer on the lower portion of the casing.

While I was on the water Sunday, I installed a used (tested working according to seller) heat sensor and no more issues. I installed that on the water. I checked it on land and found the mounting hardware landed it far from the head unit. The heat sensor and bracket are badly worn, obviously ran in salt water. I don't trust the reading from that sensor and suspect motor was overheating in low RPM entire time because it was so far from target location. Basically floating off to the left of the head sensor location. I posted a pic of same.

Engine runs great across all RPM ranges. Frankly, it is a beast. It revs like a dirt bike engine, under pressure or on the muffs. Does not skip a beat and top end is furious. I have the idle set as low as it will go, and it idles and shifts great. I keep good plugs in it also. Any fouling and I change them.

I tore the bottom end apart and the gromment (discussed above) looks good, and I see the rubber lower water pipe seal seems to also be in good shape. I do not see any other "3 rubber components" between the washer in pic 1 below "upper impeller grommet" and the pump insert.

Side note: I did install a new thermostat and gasket when I did the impeller/insert work. I got the thermo from this seller: http://www.ebay.com/itm/271659362203...%3AMEBIDX%3AIT

Edit:


I have torn out the new thermo and it seems to operate fine. By 155 it is about fully open.

Thanks Sea Rider!

Pete

Pic of upper impeller grommet: http://imgur.com/du2DEcH
Pic of upper impeller grommet:http://imgur.com/tMC5AlC
Pic of impeller housing cooling tube insert: http://imgur.com/jpuHGdJ
Pic of piece of **** replacement heat sensor I installed on the water: http://imgur.com/DPMHRpl
Pic of the old heat sensor: http://imgur.com/J19oWir
Pic of old heat sensor bracket: http://imgur.com/Rtv6q9o
 
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pvanv

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Yes, the water pipe should have a grommet sealing into the wp housing.
 

Sea Rider

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Sep 20, 2008
Messages
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Pete,

If your peeing port is metal casted on middle leg, poke a wire up-down several times, if with slight wall restriction due to salt layer formation engine should pee much better at idle. If peeing has a strong feel on hand, pump if working spot on.

With respect to mentioned 3 other rubber parts on lower water tube, sorry was looking at the newer 2 strokes M70 parts list version, but the old 70 A2 still has a rubber grommet like near lower end of water tube. Don't know if just centers water tube inside leg or adding a second grommet to better seal tube against upper housing grommet ? Check with a flashlight if can be found somewhere around middle or lower tube portion.

Install a new heat sensor along mounting plate if plate is severely corroded, clean head portion adjacent to sensor as to sense heat range properly. Nice to hear you've got a real beast, congrats, LOL..

Happy Boating
 
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