1988 Johnson VRO overheating alarm - help

[ampickles1992]

Hey guys,

Relatively new to boating, but I know my way around general handyman stuff.
I've recently bought a boat with a 1988 90hp Johnson that starts up very easy, runs like a dream, but about 15 mins into driving, it comes up with the continuous alarm which I believe signals overheating.
The cylinders heads are hot to touch after this alarm, but not enough to burn within 2 seconds. The outboard pumps plenty of water, so I didn't think it was a water pump issue.
I've pulled out the thermostats (I believe there are only 2?) today, and there was a bit of sand in the housing for the thermostats, and clogging the thermostats themselves. After a quick clean, and a few hot-cold baths, I've seen that they're once again moving as they should. The alarm however hasn't gone away.

Is there anything else that could cause this issue? The previous owner said he hadn't used it for quite some time, and that it never used to make the alarm sound. It only started making this noise when he ran it on the muffs.

I just want to rule out that the motor isn't overheating before ruling it out as an electrical issue.


Many thanks,
Andy.

 

[emdsapmgr]

Double check the plastic valve body inside the stat housing. Yours was right about when a change came through. The earlier ones had two small pinholes in that body. Newer ones had aa single oval hole... The purpose of the holes is to permit hot water to bleed through to the stats themselves. Sometimes these holes get plugged up with debris over time. If these holes do plug up, they won't let warm water get to the stats, causing them to open. It's also possible that there is sand behind the cyl head cover. Remove the cover and make sure the water passages inside the head are clear.

 

[racerone]

After multiple overheat warnings you should now do a compression test.

 

[jakedaawg]

I.would suggest all of the above plus an impellar. They can look fine but still be weak. And they are cheap and easy.

 

[ampickles1992]

Based on a few other forums, I've removed the thermostats to determine if the issue was caused by them or the pinholes you were referring to. Unfortunately, the outboard still gave the overheat alarm after a while of running.
After touching the cylinder heads, they were hot, but not scaldingly so.

After the alarm sounded, I killed the engine immediately and allowed it to rest. Right after killing it, I trimmed it up out of the water and removed the rubber hoses from the thermostat housing. I've concluded I'm not getting water flow through one of the water paths in the stat housing.

Any idea what could cause this? Would it be a blockage? Or merely the water pump isn't strong enough to push water through the thermostat housing?

There is still water coming out of the tell tale.

Any more help is appreciated, thanks in advance.

 

[dingbat]

Based on a few other forums, I've removed the thermostats to determine if the issue was caused by them or the pinholes you were referring to. Unfortunately, the outboard still gave the overheat alarm after a while of running.
After touching the cylinder heads, they were hot, but not scaldingly so.

After the alarm sounded, I killed the engine immediately and allowed it to rest. Right after killing it, I trimmed it up out of the water and removed the rubber hoses from the thermostat housing. I've concluded I'm not getting water flow through one of the water paths in the stat housing.

Any idea what could cause this? Would it be a blockage? Or merely the water pump isn't strong enough to push water through the thermostat housing?

There is still water coming out of the tell tale.

Any more help is appreciated, thanks in advance.

Water coming from the tell tale doesn’t mean there is sufficient flow to cool the motor.

The water pump should be changed every 2-3 years tops

 

[racerone]

This was 5 years ago.----Like fixed with new water diverters.

 

[oldboat1]

Sand in the t.stat means sand might be anywhere in the cooling system. I would remove the heads and clean out the passages around the cylinders.

 

[racerone]

I am sure it was fixed about 5 years ago !!!

 

[oldboat1]

The typical advice on overheating is that 160F or more is the danger range. Get an infrared gun and check operating temps at the top of the heads. Ocean running (salt) or river running (sand) has special risks. You might consider removing the lower unit, and pumping hose water up the tube. Remove the t.stats for this to clean the bypasses in the housings. (Motor is not running for this procedure.) You may have blockages or partial blockages in water passages around cylinders, so would remove cylinder heads and clear out areas around cylinders (an old hacksaw blade is good for this, or similar flexible tool.)

 

[racerone]

This thing was fixed 5 years ago or perhaps scrapped.