Johnson 88 overheats at WOT

[Lil' Johnson]

I have a 1988 Johnson 88 SPL. I recently purchased the boat and motor. Before I put the boat in the water, I replaced the water pump with a full kit, and the complete stern bracket assembly with TNT because the original was rusted to the point of no return. The first time out, one of the carbs got trash in it, so I cleaned both carbs. Since then I have used the boat about 6 times, approx 4 hrs each time with no problems. Yesterday I went into the bay and ran aground for about 20 yards. I didn't ever ground the boat or stop moving forward. By the time I realized the motor was slogging mud and I started to trim up the motor, I was through it. Right after that the overtemp horn went off. I ran a short distance(maybe 45 seconds) and beached the boat on the bank of the canal. I let the motor cool for about 10-15 mins after the horn stopped and started it up and headed for the dock.(the motor was hot when I stopped, It was definately overheating) After only going a short distance at full throttle(maybe 3/8 mile) the horn sounded again. I beached the boat again and waited for the horn to stop. The next time I started it up and just idled down the canal, it never got hot. I idled for about 3 miles and never got hot. As soon as i was about 1 mile from dock, I opened it up again. Overheat horn immediately. waited for horn to stop and idled back to dock. After the boat was home, ran it on muffs for 1/2 hour and never even close to hot. What could the mud have done to my motor? The telltale stream is good under all engine loads and RPM's. The motor is and always has been operated in saltwater.

 

[wilde1j]

Re: Johnson 88 overheats at WOT

Drop the LU, take out the thermostats, replacing the covers and force water in the water pump discharge tube/ and or use compressed air to flush all the crap out of the cooling system. Re-assemble and test. Usually the water pump forces the mud and sand out, but not always.

 

[Lil' Johnson]

Re: Johnson 88 overheats at WOT

If there was some crap still in the motor, would I see any coming out anywhere? All of the discharges on the motor let out clean water. Should I leave the T-stats out? It never gets very cold here, especially during boating season.(water temps in the 80's)

 

[wilde1j]

Re: Johnson 88 overheats at WOT

I wouldn't leave the stats out ... there there for a reason, and if they weren't needed, the mfgr wouldn't have put them there. Most motors run in salt have somewhat or greatly restricted passageways for cooling water due to salt build up. Get some mud and sand packed in there, and it's hot time. You might get lucky and find the thermostats packed with debris. BTW, your motor wants to run ~ 140 to 150? F, not under 100?, which you'd probably get w/o stats. The result is lots of carbon build up ... not good.

 

[Lil' Johnson]

Re: Johnson 88 overheats at WOT

Should I pull the heads and exhaust housing? The head gaskets look pretty rough as it is.

 

[tashasdaddy]

Re: Johnson 88 overheats at WOT

do not run that motor without stats. you've been running in salt water, stats do go bad in salt water. on my salt water boat i just do the impeller and stats each spring. i recently just sold that same motor. great piece of machinery.

 

[Lil' Johnson]

Re: Johnson 88 overheats at WOT

I pulled the t-stat housing off(broke a bolt) and there was not restrictions at all, no debris and the t-stats were already gone. So, I pulled the heads and nothing in the water jackets, clean as could be. Next, I was going to pull the exhaust cover on the back of the powerhead but 5 bolts broke and its still stuck on pretty good. I soaked it with penetrating oil on the broken studs, I don't want to break the cover. But I still don't see any evidence of any mud or anything else restricting the water flow. Any ideas? Also, I guess I have to pull the powerhead to remove the broken bolts as they're nbehind the lower cowling and I can't get to them. I don't know when I will get time to pull it because my schedule is crazy, but I don't want to leave the cylinders open either. Should I bolt the heads back on for now?

 

[Lil' Johnson]

Re: Johnson 88 overheats at WOT

Should I pull the two small round covers for each cylinder off of the heads? Could there be something in there?

 

[SKEETR]

Re: Johnson 88 overheats at WOT

Not too familiar with the 88 but on my 115 the head covers could definitely hold mud in the water channels as well as block the flow of vital coolant.