DT 150 Overheat Alarm at Idle Only

[nitro_circus]

New member here looking for advice. I have a 1988 DT 150 that has run great since I bought this boat 4 years ago. I ran the motor at least 20 times this year without an issue. Now, out of nowhere, the overheat alarm and lights are going nuts. The very moment I start the motor the alarm sounds. The alarm and lights continue until I get into the throttle a bit..about 25-percent power. Once I do, the problem completely disappears. The motor then runs perfectly at speed, peeing strong along the way. Once I power down to idle again, the alarm and lights go crazy again. Sometimes the right side light shows, sometimes the left light, sometimes both at the same time. Since this problem shows itself the moment I start the motor I've kinda talked myself out of believing it's a true overheating issue. I live on a shallow sand bottom lake and it's not uncommon to troll into trouble. Over the years I have sucked a bit of sand into the motor a time or two, clogging the pee hole. I simply stick a copper wire in an inch or so and it frees itself up. Even dealing with that a few times, the alarm and lights have never gone off before. It has been awhile since I changed the impeller so I started there thinking it might be worn from the occasional beaching which might affect water pressure. No change. The motor is peeing as strong as it ever has.

I really want to avoid blind trial and error so I'm starting with this forum in hopes that you kind folks can help me narrow it down. To recap...

- Alarm/lights begin the moment I start the motor
- Alarm/lights go away when I get into the throttle 25-percent or so
- Peeing very strong
- Boat runs great at speed and problems begin again when I idle down

I don't have a service manual so I've looked on this forum for this same problem, only to find mostly the opposite problem (alarm sounding when getting into the throttle, not at idle only like mine). Any direction would be much appreciated.

[Thrillseeker]

I don't know if the 2 strokes are the same as the 4 strokes, but on the 4's there is a valve on the leg which should be closed at idle to push water through the head, it opens at speed to reduce pressure. It may be stuck open, and if you get sand through the coolant water that can do it.
Remove the covers off the leg and the valve is held in with two bolts halfway down the leg. If you are looking for a manual, give "recrepairinfo" a google and email them.

[nitro_circus]

Is the valve you're talking about the poppet/relief valve? I'm going to Browns Point to try and locate it on a diagram before I tear into it blindly. What can I say...I'm great with car engines but not very experienced troubleshooting outboards (changing an impeller doesn't count). Gotta start somewhere. Thanks.

[Thrillseeker]

The valve relieves pressure in the cooling system at speed, don't know what it's called but it's in the leg.

[nitro_circus]

I went to Browns Point and saw no relief/poppet valve on their diagrams, so I assume there isn't one on this model (someone please tell me if I'm wrong). I put the muffs on and disconnected the cool water sensors one at a time to see if one of them was faulty. The alarm still went off, again with no rhyme or reason (immediately upon startup, sometimes right, sometimes left, sometimes both). Is there a chance the problem is a restricted oil flow sensor? My logic behind that theory is that if it's partially clogged and not enough oil is reaching the carbs at low rpms, the alarm might be sounding because the motor thinks it's burning too hot. Then, when I get into the throttle a bit and the vacuum is greater, it's pulling enough oil through to satisfy the motor. Is that ridiculous or a possibility?

[nitro_circus]

Ok...I guess my theory about the oil flow sensor was wrong. I changed it and it did nothing for me. I'm at a loss. bottom line, there should only be a few things that would cause this problem immediately upon startup, before the motor has a chance to get warm. Help?...Please?...

[boats.net]

Have you checked your oil sensor? (it located on the motor, will be inline with your oil lines)

[nitro_circus]

Time to resurrect an old post. I’m still fighting the same problem because I’m too stubborn to take the sucker to the shop. I HAVE narrowed the problem down, though…

Last season, I had overheat alarms going off immediately after starting the motor which left me thinking the alarm was false (as there was no time for the motor to actually heat up). What I didn’t realize until this spring was the temp sensors are also flow sensors. Both sensors have a flap that pivots upward as water flows through the heads. So I took them out one at a time, left them hanging upside down so the flap would remain in the “open” position and started the motor. When I did that on the right side, the right side warning light went out. When I did the left side, the left side warning light went out. PROGRESS! The alarm is NOT false…I have a flow problem.

Last year, I only changed the impeller, t-stats and oil sensor. So I looked at the water pump housing a couple weeks ago and noticed some grooving. I bought a new one along with a new plate, put it back together and crossed my fingers. I just got back from my first test drive. NOT fixed. Like last year, it goes away with about 25-percent throttle and runs fine. When I power back down to idle, the warning lights light up and the alarm sounds again. And like last year, sometimes it’s the right side, sometimes it’s the left, sometimes it’s both. I do have a strong stream of water coming out tell-tale.

I’ve tried flushing the heads to no avail. With the lower unit off, I looked up the main water tube going to the motor and it looks clean as a whistle. Since I’m dealing with both right and left warning lights (but sometimes one, sometimes the other), it seems my problem is somewhere between the main water tube and wherever the upward-flowing water splits right and left to go to each head.

Once thing I noticed with the lower unit off…the water tube (which remains connected at the motor when you remove the lower unit) has about a quarter-inch up-and-down play. Probably nothing but I thought I’d mention it. Hard to see what’s going on up there.

Looking for fresh advice in light of my water flow discovery. The fish are laughing at me.

[nitro_circus]

So can anyone tell me the best way to access the upper water tube bushings in this bad boy? Looking up the drive shaft housing with the lower unit off, it's waaay up there. Due to the fact that the tube has some up-and-down play and turns freely when I twist it, I have to believe I have a leak there that's causing this problem.

[boats.net]

Unfortunately you will need to reach with your hand or a tool that can grab. Or you can remove the powerhead.

[CAustin]

Did you try the poppet/relief valve? Mine is a 2002 DT150 and i'm sure the design is still pretty close. With your cowling off, facing the rear of the engine, look on the right bank and at the bottom of the cylinder head. It will be a cover (about the size of the t-stat housings) with two bolts. If it's the same, please for the love of God be careful removing the bolts. Mine broke during removal and I had to remove the powerhead to get to them and drill/re-tap. There is a spring with a plastic piece (the valve) attached to it, inside the housing. Mine had some corrosion on it causing pretty much the same problem. Either remove the corrosion or replace the spring and valve. The parts are cheap.

[N2 Fishing]

I have a 1996 DT150S same issue. Does any one know if my year has a poppet valve? I am not able to find it if it does.