I need some help. Overheat alarm only @ WOT.

[mcgurk34]

The engine is a 1988, 50 HP, 3 cyl. I thought that I solved this problem, but it's back. I get a steady alarm but only at WOT. If I back off the throttle just a little the alarm stops. I've replace the water pump, temp sensor, water jackets are clean. Very strong telltale stream and there is no thermostat on this engine. Carbs have been cleaned and rebuilt. If the idle mixture is rich, approx 2 turns open, I didn't get the alarm but the engine has a rough idle and will foul the plugs. When leaned it out to approx 1 turn open, smooth idle, good performance at any RPM but at WOT, after about 30 sec. the buzzer will sound. What would the idle mixture have to do with WOT operation? Could timing cause this?

 

[andrewkafp]

Re: I need some help. Overheat alarm only @ WOT.

How hot is the stream coming out of the pee hole at WOT ? Is it hot or is the gauge faulty ?<br />Can you touch the stream or is it too hot ? At WOT it should only be warm to very warm.. never boiling hot. Do you have a pressure gauge ? what does it read at WOT ?

 

[Elmer Fudge]

Re: I need some help. Overheat alarm only @ WOT.

It seems that you may have a faulty sensor or a short in the wiring, try following the wires from the temp sender back to the ignition and see if there is any loose wires or anything abnormal. Also if i were you i would definitely install a T/stat and reajust the idle,that should minimize the plugs from fouling.

 

[mcgurk34]

Re: I need some help. Overheat alarm only @ WOT.

The boat doesn't have any gauges so I don't know the actual temp or pressure at WOT. I cannot reach the telltale stream to feel the water temp when running at full speed, but i don't have any sense that the engine is actually overheating. The sensor has been replaced and the wiring looks very good. It takes about 30 to 45 seconds at WOT to trigger the alarm and it will stop in 5 to 10 seconds after reducing the speed. In fact it will run all day with no alarm just short of WOT. (approx 95% of max trottle)I don't think that this engine uses a thermostat. There is no place to install it and my Clymer manual doesn't mention it at all. Still stumped.

 

[CU2NITE]

Re: I need some help. Overheat alarm only @ WOT.

Do you ever start the engine out of the water? Doing this for even a few seconds will damage the impeller causing overheating. You may still be getting a telltale but the water flow is not efficient. I would also follow Unicorns adivce. You can run the motor, slow down put your hand under the telltale and it should still be to hot to hold your hand under if your over heating. Good luck and let us know. <br />Cu<br /><br />-------------------<br />RUNNING THE MOTOR WHILE LISTENING TO THE HORN WILL MAKE YOU MORN!!!

 

[mcgurk34]

Re: I need some help. Overheat alarm only @ WOT.

The motor has never been run without water since I've owned the boat. It was only turned over either in the bay or with the earmuff hose attachment. If the engine is really overheating, what do I do about it? The water pump has already been replaced twice and did not correct the WOT issue. What would be the next step?

 

[TampaTom]

Re: I need some help. Overheat alarm only @ WOT.

You might try lowering the engine a set of holes. Check the prop for nicks. Broken skeg or something on the hull that breaks the water flow. If some air climbs down the lower unit, it will lower your water presure.

 

[KCLOST]

Re: I need some help. Overheat alarm only @ WOT.

When you replaced the water pump, how did you install the impeller into the pump housing... Did you turn the drive shaft clockwise or counterclockwise?<br />Also, what was the part# for the overheat sensor that you recently installed...<br />It should be 87 - 96829A1 for 260 degrees<br />or 87 - 19541A2 for 285 degrees. Which tells me you should be measuring metal temperatures on the head.. If you don't have those sensors rated for that temp. that could explain your problem (assuming everything else is working properly).<br /><br />And what is the serial number for your engine?

 

[quantumleap]

Re: I need some help. Overheat alarm only @ WOT.

Even though you rebuilt the pump, there could be something blocking one of the water passages. Was the old impeller in good shape or broken up? I would snake out the water passages just to eliminate that possibility.

 

[jim dozier]

Re: I need some help. Overheat alarm only @ WOT.

Have you ever pulled the cylinder head? Overheating at WOT only may be a sign of combustion gases entering the coolant stream. If you've never pulled the head maybe time for a new head gasket. If you have already done this, was there any evidence of head warpage? Maybe you need to have the head decked 0.005" at a machine shop and install a new head gasket. I have an 85 hp 3 cylinder Suzuki that had a similar problem and after myriad things were checked and fixed I ultimately retarded the maximum ignition advace 2 degrees and that solved it.

 

[KCLOST]

Re: I need some help. Overheat alarm only @ WOT.

jimd, he doesn't have a cylinder head on that engine, just a cylinder block water jacket cover. No gaskets anywhere to seal water from the cylinders.... And until I get his serial number to confirm, I believe that his overheat sensor should measure metal not water... <br />But a compression check would be recomended in my opinion...

 

[BF]

Re: I need some help. Overheat alarm only @ WOT.

my first thought was along jimd's - exhaust gases entering the water system... but maybe down at the pump. Did you replace the water pump gaskets with the impeller, maybe at WOT there is way for exhaust gases in leg to be sucked into pump. (?) I seem to remember a guy having a problem very similar to this last summer with an I6 (here on iboats)... was an exhaust gasket near the base of the powerhead if I remember right.<br /><br />Brent

 

[Trent]

Re: I need some help. Overheat alarm only @ WOT.

When you did the waterpump impeller you did replace the wear plate? Gaskets? You also sealed the exhaust support tube and tube seal with sealant as required??? See items 10 and 11 web page

 

[Elmer Fudge]

Re: I need some help. Overheat alarm only @ WOT.

Posted by KCLOST: If you don't have those sensors rated for that temp. that could explain your problem (assuming everything else is working properly).<br /><br />

I have to agree with that analysis, it seems logical that's where the answer to this persistent problem may dwell.<br /><br />After a bit of head scratching, how about this, when you push the control handle forward, could it be that as it enters the wot range,it creates a short in the shift box and causes the horn to go off?

 

[mcgurk34]

Re: I need some help. Overheat alarm only @ WOT.

Thanks for all of the responses. You guys are great! Here are some answers to your questions in no particular order. It's a 1988 Mariner 50 HP 3Cyl S/N 0B248535. There is no cyl head on this engine, only water jacket covers. The temp sensor screws into the cyl top cover near #1 spark plug and is part number 87-96829A1 the 260 degree unit. This engine is mounted on a 15 ft Boston Whaler sport and has power tilt/trim. There is an after market hydrofoil mounted to the lower unit by the previous owner. I think it's called a stingray. The water pump was rebuilt (2x) by replacing impeller, all gaskets, O rings and the metal wear plate. I don't remember CW or CCW but the impeller blades are set correctly. The old impeller came out in one piece, no broken blades to get lodged in a water passage somewhere. Keep up the diagnostic's guys, you are bound to hit something I didn't do. I'm still thinking timing might be involved somehow but the timing on this thing is linkage driven and only sets max advance.

 

[andrewkafp]

Re: I need some help. Overheat alarm only @ WOT.

Don't quite understand why you can't have someone drive the boat and you reach over and feel the physical temp of the pee'er. Is the motor that far away from the transom ???<br />The impeller on the 50 is a tiny little thing and the vanes come to a sharp point. I think even if you installed it with the vanes backwards they would self-correct, unlike the long vaned impellers from the big motors. If it's overheating, the pee'er will be have steam coming from the water even at an ambient outside temperature.

 

[KCLOST]

Re: I need some help. Overheat alarm only @ WOT.

Timing shouldn't have anything to do with an overheat condition.... That just doesn't seem possible to me, as optimum fire would lead to the highest heat condition... <br /><br />Ok,,,, I'm reaching here. Are you sure that the sensor lead is not grounded anywhere...<br /><br />And, Are there any water leaks around the spark plugs (from the water jacket) possibly getting to the sensor...(mayby during high pressure conditions it leads and grounds out the sensor)<br /><br />And, You really need to confirm what the actual temp is when running the motor... Run that baby on the muffs and take a temp reading on the tell-tail... Let us know what it is... And if you have a friend, do what Unicorn suggests...

 

[KCLOST]

Re: I need some help. Overheat alarm only @ WOT.

mc,<br /><br />Any luck?

 

[mcgurk34]

Re: I need some help. Overheat alarm only @ WOT.

I won't get to the boat until the weekend. When I do, I'll try to get a measurement of the telltale temp. BTW, There is no steam coming from the pee'er or on the water as Unicorn mentioned and there are no leaks around the spark plugs or anywhere else for that matter. Without gauges I'm working blind here. Every thing else about this engine is normal. Starts easily, and runs great. Compression is approx 130 lbs in all three cyl. At this point I need to confirm if it is overheating or a false alarm. I'll let you know.

 

[mcgurk34]

Re: I need some help. Overheat alarm only @ WOT.

OK, I went to the boat today but I was alone. Ran the boat at WOT until the alarm sounded. Backed off the throttle to neutral and jumped back on to the platform to measure the Pee water temp with a digital probe. Maybe 10 - 15 seconds elasped from the throttle shut down to measurement. The water temp was 110 degree maximum. Bathwater. Is what I'm doing a realistic test?