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Author Topic:   Johnson 35-HP Thermostat Replacement

Essence posted 06-18-2010 06:47 PM ET (US)
The 1998 Johnson 35-HP outboard attached to my 40th Anniversary 13-footer has a problem. It has worked exceptionally well for me over the last five years.
The heat alarm sounded and the red light lit up on the dashboard after 10 minutes of very slow speed doing harbor work and hauling dinks. The discharge stream seemed normal, discharged water wasn't hot, cowling wasn't warm.
After running faster for about three minutes at 1,200-RPM the alarm stopped sounding, then the light went off shortly after. The motor wouldn't go above 2,500-RPM or so and shuddered at those RPM. It appears the motor was in its reduced speed mode. It got me back to my mooring at 1,500-RPM without sounding again and I got back to work without lifting the cowling to investigate any further.
Seems to me that there is a thermostat [problem] and that I should start my investigation there. My mechanic suggests a new [thermostat] and said that it is straight forward replacement. The location is above the top spark plug; the fixture takes the same size socket as the plug.
Is it that easy? Anything to look out for or any advice to offer? Can I clean the current thermostat and re-use it until I pick up the replacement in the beginning of next week?
Thanks in advance.
John

jimh posted 06-18-2010 09:50 PM ET (US)
[Moved to REPAIRS/MODS.]
jimh posted 06-18-2010 10:37 PM ET (US)
Does the engine operate mainly in saltwater?
Essence posted 06-19-2010 10:34 AM ET (US)
The little Whaler is run almost exclusively in salt water. She is trailer-sailed and since the bottom isn't painted she doesn't stay in the water more than a few days at a time. The engine is flushed for 10-15 minutes with muffs after each haulout; she spends 7-10 days in the water each month. I winterize her and the local Whaler doctor (Paul Eby at Ipswich Outboard in Ipswich, MA) commissions her each spring.
John
BobL posted 06-22-2010 10:27 PM ET (US)
Thermostats in salt water get salted up very easily over time. Simply flushing the thermostats does not remove all of the salt. Change the thermostat. They are cheap and changing them regularly is good insurance against the damage that can be caused by an overheating event.
Plotman posted 06-22-2010 10:59 PM ET (US)
I don't know about your specific engine, but on all the Johnson outboards I have had from 20hp to 70hp, the thermostat housing came off with 4 bolts. Maybe a 2 minute job to replace if you don't mess up the gasket, a 10 minute job if you have to replace the gasket. I clean it out once a year, more frequently if needed.
I always keep a spare thermostat and gasket on hand, as well as a full set of plugs. You can buy them at NAPA.
Martino posted 06-23-2010 07:28 AM ET (US)
Not to rain on this parade, but I would start with the waterpump first. I have water pressure gauges on my engines. On the gauges, you can see a major drop in pressure after the impellers are 2 years old. Can't see much difference in the tell-tale.
Essence posted 06-23-2010 08:07 AM ET (US)
The 13/16 stub for the socket wrench on the plate covering the thermostat peeled off in the socket when I tried to break it (gently but firmly) for access to the thermostat. It looks like the plastic fatigued - perhaps because it got too hot? This socket hub protruded from a larger hub which takes a 1.5" socket. After finding a box wrench large enough to do the job I was able to get the plate off without breaking anything else. I removed the spring to get at the front of the thermostat assembly which has O rings and gaskets before the actual thermostat. In the assembly right in front of the thermostat I found a scrap sliver of black plastic about .5" long, very narrow and flexible; the thermostat looks like it needs replacing as it is corroded and worn. The combination of the plastic sliver (no idea how it could have gotten into this well-protected area) and the worn thermostat look like they are responsible for the overheating alarm.
I'll get the replacement thermostat and the new plate I ordered from Ipswich Outboard today and do the installation. From what I gathered in talking with the mechanics this cover plate for the thermostat assembly is unique to the 1998 motors and is not used anywhere else in the product line.
If this replacement doesn't solve the problem I'll move on to the water pump. I am getting a good discharge from the cooling system - if the water pump is the problem should I be seeing this strong flow or would the flow appear weakened and less robust?
Thanks again.
Tohsgib posted 06-23-2010 11:33 AM ET (US)
every engine is different with the amount of flow coming from the indicator. I have engines that almost dribble and engines that will clean the concrete if in the driveway.
Plotman posted 06-23-2010 01:58 PM ET (US)
The sliver of flexible back plastic sounds like a piece of impeller to me. Change the water pump.
Essence posted 06-24-2010 07:50 AM ET (US)
After installing the new thermostat I ran the engine on the dry with muffs for fifteen minutes at different rpm from 1000 to 2000. The motor ran fine the whole time: no alarms and a strong discharge stream. I'll splash her today or tomorrow and run her under load in the inner harbor to see if the overheating alarm sounds again.
The black sliver looks to be rubber, not plastic, and I agree with the concerns that it might be part of the impeller. However, when I removed the thermostat I saw that one of black rubber gaskets had a chewed area, very slight, on the feathered edge as if that edge had gotten caught slightly in the threads of the screw-in plate as it was tightened and a small portion of that edge might have torn off, hence the sliver. If the overheating continues I'll move on to the water pump.
After the motor is running correctly I'll tackle a problem I have with our 27' Eastern powerboat. Electrical system is somewhat flukey - for instance, when I sound the boat's horns the radar resets itself, and the old Apelco GPS turns itself off when I close the feed valve on the portside fuel tank as part of my shut down procedure. Any thoughts on this one? I'll swing any discussion of this problem over to electrical if a thread starts up
adlert posted 06-24-2010 08:38 AM ET (US)
Given that your 35 HP has already gotten hot I think it wise to re-torque the cylinder head bolts to help prevent future head gasket problems. I think while there, I'd also do a compression check to make sure no engine damage has occurred so far.
I'm making these suggestions partly because I am uncertain as to whether or not the 1998 35 HP engine came equipped with OMC's "Slow" feature. My not being up on those details certainly shouldn't imply that it does not have it or that you have worse problems. However, if your engine is not so equipped, I would be concerned about the slow running and shuddering you described. You could possibly already have head gasket problems. I think it would be prudent to do the compression and torque checks.
Essence posted 06-25-2010 06:04 PM ET (US)
After running the engine under load through a range of speeds for 90 minutes this morning no problems occurred.
When it was running hot the "slow" feature did kick in, limiting rpm to around 2000.
Thanks to everyone who commented and for all the suggestions.

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Author	Topic: Johnson 35-HP Thermostat Replacement
Essence	posted 06-18-2010 06:47 PM ET (US) The 1998 Johnson 35-HP outboard attached to my 40th Anniversary 13-footer has a problem. It has worked exceptionally well for me over the last five years. The heat alarm sounded and the red light lit up on the dashboard after 10 minutes of very slow speed doing harbor work and hauling dinks. The discharge stream seemed normal, discharged water wasn't hot, cowling wasn't warm. After running faster for about three minutes at 1,200-RPM the alarm stopped sounding, then the light went off shortly after. The motor wouldn't go above 2,500-RPM or so and shuddered at those RPM. It appears the motor was in its reduced speed mode. It got me back to my mooring at 1,500-RPM without sounding again and I got back to work without lifting the cowling to investigate any further. Seems to me that there is a thermostat [problem] and that I should start my investigation there. My mechanic suggests a new [thermostat] and said that it is straight forward replacement. The location is above the top spark plug; the fixture takes the same size socket as the plug. Is it that easy? Anything to look out for or any advice to offer? Can I clean the current thermostat and re-use it until I pick up the replacement in the beginning of next week? Thanks in advance. John
jimh	posted 06-18-2010 09:50 PM ET (US) [Moved to REPAIRS/MODS.]
jimh	posted 06-18-2010 10:37 PM ET (US) Does the engine operate mainly in saltwater?
Essence	posted 06-19-2010 10:34 AM ET (US) The little Whaler is run almost exclusively in salt water. She is trailer-sailed and since the bottom isn't painted she doesn't stay in the water more than a few days at a time. The engine is flushed for 10-15 minutes with muffs after each haulout; she spends 7-10 days in the water each month. I winterize her and the local Whaler doctor (Paul Eby at Ipswich Outboard in Ipswich, MA) commissions her each spring. John
BobL	posted 06-22-2010 10:27 PM ET (US) Thermostats in salt water get salted up very easily over time. Simply flushing the thermostats does not remove all of the salt. Change the thermostat. They are cheap and changing them regularly is good insurance against the damage that can be caused by an overheating event.
Plotman	posted 06-22-2010 10:59 PM ET (US) I don't know about your specific engine, but on all the Johnson outboards I have had from 20hp to 70hp, the thermostat housing came off with 4 bolts. Maybe a 2 minute job to replace if you don't mess up the gasket, a 10 minute job if you have to replace the gasket. I clean it out once a year, more frequently if needed. I always keep a spare thermostat and gasket on hand, as well as a full set of plugs. You can buy them at NAPA.
Martino	posted 06-23-2010 07:28 AM ET (US) Not to rain on this parade, but I would start with the waterpump first. I have water pressure gauges on my engines. On the gauges, you can see a major drop in pressure after the impellers are 2 years old. Can't see much difference in the tell-tale.
Essence	posted 06-23-2010 08:07 AM ET (US) The 13/16 stub for the socket wrench on the plate covering the thermostat peeled off in the socket when I tried to break it (gently but firmly) for access to the thermostat. It looks like the plastic fatigued - perhaps because it got too hot? This socket hub protruded from a larger hub which takes a 1.5" socket. After finding a box wrench large enough to do the job I was able to get the plate off without breaking anything else. I removed the spring to get at the front of the thermostat assembly which has O rings and gaskets before the actual thermostat. In the assembly right in front of the thermostat I found a scrap sliver of black plastic about .5" long, very narrow and flexible; the thermostat looks like it needs replacing as it is corroded and worn. The combination of the plastic sliver (no idea how it could have gotten into this well-protected area) and the worn thermostat look like they are responsible for the overheating alarm. I'll get the replacement thermostat and the new plate I ordered from Ipswich Outboard today and do the installation. From what I gathered in talking with the mechanics this cover plate for the thermostat assembly is unique to the 1998 motors and is not used anywhere else in the product line. If this replacement doesn't solve the problem I'll move on to the water pump. I am getting a good discharge from the cooling system - if the water pump is the problem should I be seeing this strong flow or would the flow appear weakened and less robust? Thanks again.
Tohsgib	posted 06-23-2010 11:33 AM ET (US) every engine is different with the amount of flow coming from the indicator. I have engines that almost dribble and engines that will clean the concrete if in the driveway.
Plotman	posted 06-23-2010 01:58 PM ET (US) The sliver of flexible back plastic sounds like a piece of impeller to me. Change the water pump.
Essence	posted 06-24-2010 07:50 AM ET (US) After installing the new thermostat I ran the engine on the dry with muffs for fifteen minutes at different rpm from 1000 to 2000. The motor ran fine the whole time: no alarms and a strong discharge stream. I'll splash her today or tomorrow and run her under load in the inner harbor to see if the overheating alarm sounds again. The black sliver looks to be rubber, not plastic, and I agree with the concerns that it might be part of the impeller. However, when I removed the thermostat I saw that one of black rubber gaskets had a chewed area, very slight, on the feathered edge as if that edge had gotten caught slightly in the threads of the screw-in plate as it was tightened and a small portion of that edge might have torn off, hence the sliver. If the overheating continues I'll move on to the water pump. After the motor is running correctly I'll tackle a problem I have with our 27' Eastern powerboat. Electrical system is somewhat flukey - for instance, when I sound the boat's horns the radar resets itself, and the old Apelco GPS turns itself off when I close the feed valve on the portside fuel tank as part of my shut down procedure. Any thoughts on this one? I'll swing any discussion of this problem over to electrical if a thread starts up
adlert	posted 06-24-2010 08:38 AM ET (US) Given that your 35 HP has already gotten hot I think it wise to re-torque the cylinder head bolts to help prevent future head gasket problems. I think while there, I'd also do a compression check to make sure no engine damage has occurred so far. I'm making these suggestions partly because I am uncertain as to whether or not the 1998 35 HP engine came equipped with OMC's "Slow" feature. My not being up on those details certainly shouldn't imply that it does not have it or that you have worse problems. However, if your engine is not so equipped, I would be concerned about the slow running and shuddering you described. You could possibly already have head gasket problems. I think it would be prudent to do the compression and torque checks.
Essence	posted 06-25-2010 06:04 PM ET (US) After running the engine under load through a range of speeds for 90 minutes this morning no problems occurred. When it was running hot the "slow" feature did kick in, limiting rpm to around 2000. Thanks to everyone who commented and for all the suggestions.