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Mercury 60 overheat alarm at 3800 RPM or more

F

fishwhenyoucan

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Hello All. I am new to the forum. In years gone by I fished alot and had kids, fishing slacked up some. This year I bought a 1988 Bass Buggy 20' Pontoon at what I thought was a good price and it came with a 1988 Merc 60hp. Motor runs out great, starts easy, compression was off slightly (120, 105, 118) and I assumed it might be from not having been run and after just a bit of running it did even out some. The main problem is when I take it to the lake, if you get above 3800 RPM in just about 15 seconds the water pressure gauge pegs out and the overheat alarm goes off and stays on until you throttle down and then it will go off and water pressure returns to about 10 - 15. I had the motor tuned, carbs rebuilt, impeller and gaskets replaced, foot lube changed and checked for leaks. A few notes, the shop doing the work thinks the motor is raising up at higher RPM and is sucking air causing the pressure issue and the overheat problem. Someone down the line trimmed the transom a little bit to lower the motor. I have been told all kinds of stuff from "it's a short shaft motor" which it is a 20" long shaft, to it needs a poppet valve (shop told me this model does not have thermostat or poppet valve). It has what appears to be smaller than normal prop (numbers on prop - A10 P9). The transom could be lowered some more (1- 2 inches) but the motor does fine at lower RPM and the water from the telltale is a steady stream during all of this. Can anyone help? Right now this is frustrating as this shop said well you just need ro run it below 3900 RPM all the time.
 
Can you see if the prop is cavitating at 3800 RPMs? That creates air filled (bubbles) water at the water intake; not very good for cooling.
 
The model number is B239242 . The shop had taken the boat to the lake also and he stated that he could see the cavitation plate out of the water. I did not notice. When I ran it towards WOT, the boat never lost power, and the engine did not over-rev.
 
If the engine does not have power trim/tilt then try manually trimming the engine by changing the position of the fixed pin so the engine is level. Otherwise the engine needs to be lowered to reduce cavitation.
 
Thanks! The shop I have at thinks that it needs to go down a couple of inches. It does have tilt and trim and that helps but does not solve the issue. Tracker mechanic thinks it has a water exit issue and that is why it pegs the pressure gauge out. Does that sound feasible or would it peg out with it pumping air rather than water? Can the prop cause this and would a hydrofoil help?
 
Air compresses and water doesn't. If the impeller ever disentegrated into chunks there could be a piece of impleller in the water system. I would lower the engine first before doing anything else.
 
Thanks for the help. It is good when you have someone outside the circle comment as I getting all kinds of conflicting info. The transom has been trimmed slightly once before I bought it, so this must have been an issue at some point in the past. The pressure gauge which reads a total of 20 psi max pegging out; does that indicate a motor issue? Someone suggested I change motors but if the problem is that the transom is too high, I will have the same problem right?
 
Post a clear picture of the set-up-----------------Sure sounds like a short shaft motor on a 20" transom.------Happens all the time.
 
Don't have picture here at work, but that was what I started to think also so I measured the engine and it measures 20" from the transom mount to the cavatation plate and not the 15" for the short shaft model. I tried adding weight to the rear of the boat and it added a few hundred RPM (from about 3500 to almost 4000). I read where some guys lowered the engine and blocked off the top two holes of the water intake. I had the impeller replaced and it pumps water out even at the higher RPM with the overheat alarm going off. Could that alarm sensor be bad? Another guy asked about the voltage regulator being bad and making the sensor go off at higher RPM. I am not an expert obviously so any input is greatly appreciated. This problem makes we want to just get rid of the boat.
 
UPDATE: Changed prop and now it goes to about 4500rpm before overheat alarm sounds. Here is what the exhaust manifold cover gasket looks like. Motor will run in tank to about 4800rpm and overheat alarm never sounds. Out on the lake, the overheat alarm goes off at 4500rpm. Mercury-gasket.jpeg We are changing this gasket; flushing the lines, lowering the engine 2"; covering the top 2 intake holes and replacing the telltale water line with a larger one and leaving the small oriface off. Any other ideas?
 
Don't know how old your engine is but mine is less than 12 month old, and mine was the same as yours, come to find out made in japan with poor quality parts ,the problem with mine was lower end gasket in the water cooling system which needs to be replaced with a better quality gasket my engineer said he has replaced loads
 
UPDATE 2: Found a piece of impeller under the manifold gasket. This piece had jacked the gasket up. Hope this is the solution! Anyone else ever had this problem?
 
UPDATE 3: Replaced manifold gasket; flushed entire system, water pressure no longer pegs out, rises with RPM up to about 15# at WOT (5500 RPM); backed boat into lake to ensure depth of motor not an issue; at about 5000 RPM overheat alarm sounded. Water pressure good the entire time, good stream from telltale the entire time; water from telltale warm but not hot and no steam, block still not hot enough to burn you or sizzle water. What in the world could be causing this? This engine has no poppet; no thermostats; new impeller; fresh tune up; autoblend unhooked; pre-mix oil and gas; unkook tan wire from temperature sending unit next to top spark plug and alarm goes off. Even after running with alarm on, engine still seems warm but not excessively hot. My thoughts are that the temperature sending unit is defective. HELP?
 
Does anyone know how to get pieces of the old impeller out of the water cooling system? If the power head has to be removed to be sure to get it all out, is this a major project?
 
...how to get pieces of the old impeller out...
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You back flushed it didn't you.

The temperature alarm is set off by a thermal switch. It is a simple electronic device that after a number of years may have changed its operating range. I looked up SN 0B239242 and found part # 87-96829A1, TEMPERATURE SWITCH ASSY. (260 DEGREES). The switch is N/O or (Normally Open) until the water temp reaches about 260 degrees F. It is a pricey part about $176. I would beg, borrow or steal a laser thermal temperature gun and point it at the location of the switch to see what the cover plate temperature is when the alarm sounds before I replaced it. Maybe you can get a decent used part from eBay.
 
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The more that I think about the switch you may be able to remove it and test it in a pot of hot water. You'll have to use one or two 9 volt batteries in series and a test lamp along w/a thermometer to get the temp. reading the switch closes and lights the test lamp.
 
Thanks and I have a laser thermal gun. However, the pressure is now jumping up to about 25# again and it makes me think thereare more loose pieces floating around and when the motor is under load they get stuck causing the water to clog a bit and it overheats. Do you have any advice for dislodging the pieces; you mentioned backflushing, do you mean dropping the foot and flushing with hose? Once again, I appreciate the help as this is a rean pain in the tail.
 
He's talking about reversing the flush process by flushing through the pee whole hose, you need to drop the lower unit I belive, that way anything that is stuck will come out
 
...shop told me this model does not have thermostat...
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Does it have a thermostat cover? If it does then drop the LU, remove the T'stat cover and flush from there w/a brass adjustable spray hose nozzle. It can create some pressure when adjusted to a pinpoint. Build up some pressure by putting your hand over the water tube and let the engine fiil w/water then remove your hand and repeat. If no T'stat cover then cut one end off an old laundry hose and carefully connect it to the telltale fitting. Connect the other end to a garden hose on full flow. Use duct tape if needed.
 
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Thanks all. Doing that this week. Had one dealer just basically say that these three cylinder two carb mercs aren't very good motors and top cylinder which is where the temp sender is located, always runs hotter. This motor starts and runs great and pushes the pontoon boat at 22 mph which seems okay. Any opinion on these older motors?
 
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