1987 Suzuki DT85 overheats at high RPM

[mkrussow]

I recently acquired an '87 DT85. The motor had a known issue when I bought it - it overheated at higher RPM (3500+). I hoped that with a little effort, I could resolve the issue. Unfortunately, that has not proven to be the case and I'm out of ideas.

The motor is currently installed on a 16' aluminum deep-v hull. The motor is mounted a bit higher than recommended - the cavitation plate is even with the bottom of the hull. I've checked that the cavitation plate remains underwater at all times when the boat is running. I'm running in fresh water.

After running for a few minutes at higher RPM, the high temp alarm will sound. When I throttle back the motor will quickly cool off. The motor runs well at all speeds, and won't overheat at low to moderate RPM.

Here's what I've tried and what I've looked at:
-compression on all three cylinders is good, 140-155psi
-spark plugs appear normal when removed - they are not white - no indication of running lean
-I've circumvented the oil pump and am running mixed fuel
-I've machined the head, cylinder face of the block and exhaust manifold face to insure good, flat surfaces
-All threaded holes have been inserted to insure that I can get good torque
-The head gasket, exhaust manifold gaskets and motor mounting gaskets have all been replaced
-The water pump has been replaced and all the wear surfaces in the pump housing appear to be in good condition
-The poppet valve and spring have been replaced
-The thermostat opens in hot water. I've also run the motor without the t-stat and it still overheats.
-I have not verified that the thermal switch closes at the right temp (to sound the alarm), but the block and head are too hot to touch for long when the alarm sounds so it appears to be working properly
-All of the cooling channels have been investigated for obstructions and none have been found
-There seems to be an adequate volume of water leaving the 'pee hole' and exhaust ports. The water leaving the 'pee hole' is warm.
-The overheating seems to be localized at the top cylinder - its much hotter than the lower two cylinders

I'm out of ideas. The only other thing that I can think of is that the block has a crack somewhere which is allowing exhaust gas to impede the cooling water at higher RPM.

If anyone has any thoughts on what else I could try, I'd sure be grateful. Otherwise, I guess I'll just part this thing out and try to get as much of my money back as I can.

 

[mphelle8vld]

Re: 1987 Suzuki DT85 overheats at high RPM

It seems like it's not getting enough cooling water at speed, is the intake for the water pump free and clear? When the water pump impeller was installed, were the tips of the vanes rotated away from the direction of rotation of the driveshaft?

 

[mkrussow]

Re: 1987 Suzuki DT85 overheats at high RPM

It seems like it's not getting enough cooling water at speed, is the intake for the water pump free and clear? When the water pump impeller was installed, were the tips of the vanes rotated away from the direction of rotation of the driveshaft?

Thanks for your input.

The intake is clear. I've even run the motor with and without the "scoops" (molded plastic parts that screw to the lower unit) to see if it made any difference.

I'm fairly certain the water pump impeller is installed in the correct manner. The keyway on the impeller won't let you install it backwards.

 

[mphelle8vld]

Re: 1987 Suzuki DT85 overheats at high RPM

In addition to the "scoops" there's another intake on the bottom of the cavitation plate. As far as the impeller, the fins, vanes, whatever you want to call them can be bent either way, they need to bend away from the direction of travel of the impeller.

 

[mkrussow]

Re: 1987 Suzuki DT85 overheats at high RPM

Thanks again for the input.

I'll make sure to check the water intake under the cavitation plate - I have not done that.

I'm confident the impeller is installed correctly.

 

[mkrussow]

Re: 1987 Suzuki DT85 overheats at high RPM

In addition to the "scoops" there's another intake on the bottom of the cavitation plate. As far as the impeller, the fins, vanes, whatever you want to call them can be bent either way, they need to bend away from the direction of travel of the impeller.

The intake on the bottom of the caviation plate seems to be clear.

Could a timing issue be at fault here? The motor generally seems to run well, so a timing issue seems like an unlikey candidate, but perhaps its worth a look?

 

[mphelle8vld]

Re: 1987 Suzuki DT85 overheats at high RPM

If the top plug looks the same as 2 and 3, Med. brown with a light sheen of oil on the insulator, I wouldn't think combustion temp is the issue but then checking the timing would be one of the easier things you've done. I'm still stuck on cooling water not reaching the top cyl and head where the sensor is located. Since you inherited the problem is it possible that pieces of a schredded impeller lodged in a cooling passage at some time? Is this a salt water motor?

 

[mkrussow]

Re: 1987 Suzuki DT85 overheats at high RPM

Since the motor seems to run fine, a timing issue seemed like a stretch and your point about the condition of the plugs sure seems to make sense. By the way, this is a fresh water motor.

I've gone through the cooling passages fairly thoroughly and I know a mechanic took a shot at it before me (that should have been my cue to not purchase the motor). Everything seems to be in good order, but I suppose I could have missed something.

I'm going to try hooking up a water pressure gage to the port on the head this weekend. Do you happen to know what kind of pressures I should expect? Perhaps that will tell me something.

Do you know of a good way to check to see if I've got exhaust in the cooling channels when I'm running? There was one area in the exhaust manifold section of the block that had some material broken away. I didn't see any obvious cracks, but perhaps something opens up when the motor is warm.

 

[mphelle8vld]

Re: 1987 Suzuki DT85 overheats at high RPM

I'm not sure the pressure gauge would reveal much, even if the upper area of the cooling jacket filled with exhaust gas, the gauge could not tell the difference between a gas and liquid. If the water port you refer to is as high as the thermostat, I would install a barbed fitting with a length of small diameter clear tubing with a shutoff at the end. Bring it up to speed to where the alarm is sounding and then slowly open the shutoff to see if the upper area of the water jacket is filling with exhaust. Sounds a little crazy but I have to think you're getting to that point. Where is the port located on the head?

 

[mkrussow]

Re: 1987 Suzuki DT85 overheats at high RPM

My intent wasn't to use the gage to determine whether or not there was exhaust for the reason you pointed out. I was hoping that it may shed some light on your previous concern - something wedged into a cooling channel somewhere. Your idea to put a shut off at that port to check for exhaust is a good one (the port is located in close proximity to the thermal switch).

I appreciate you taking to the time to offer suggestions. I'll post later this weekend when I have a chance to spend some time on the motor.

 

[mkrussow]

Re: 1987 Suzuki DT85 overheats at high RPM

I got a water pressure gage hooked up. Sitting the in the driveway with the muffs installed, the gage reads 5-10 psi, depending on the RPMs (higher RPM=higher pressure). I pulled the gage off a few times and had a nice, steady stream of water coming out of the line (no exhaust).

I took the boat to the lake and had decent pressure sitting at the dock. However, once the boats gets underway and makes it up on plane, the pressure drops way off - down to 2 or 3 psi. If I pull back to idle and come off plane, the pressure will remain low for a bit and then its like the pump manages to clear itself and will build pressure again. If I run at a speed below planing, the motor seems like its able to keep pressure.

I'd be inclined to think the motor is mounted too high, but the person who had the motor before me had it mounted on a pontoon. I wouldn't think a pontoon would have problems with water intake.

I'm at a loss and once again look forward to any input.

 

[mphelle8vld]

Re: 1987 Suzuki DT85 overheats at high RPM

Did you remove the gauge and check for exhaust when the pressure dropped? I'm thinking that the exhaust would exit easier than water dropping the pressure on the gauge. I had eliminated the intake since you said the cavitation plate stays below the surface, does it when the boat's on plane?

 

[mkrussow]

Re: 1987 Suzuki DT85 overheats at high RPM

I pulled the gage and didn't see any exhaust - just a dribble of water. If there was adequate exhaust to impede the flow of cooling water, I would think that it would have to be at a pressure at least equivalent to the water pressure.

The cavitation plate is always covered with water. Maybe there's cavitation/turbulence beneath the surface and the pump is pulling in entrained air?

Now that I'm 'clued in', there's a fairly obvious decrease in volume from the 'pee hole' at high rpm. I'm learning a lot as I go here.

Reading on-line, behavior like this would seem to indicate some type of problem with the poppet valve, however it has been replaced. Perhaps I've overlooked something there?

 

[mphelle8vld]

Re: 1987 Suzuki DT85 overheats at high RPM

Looking at the bright side, it doesn't appear to have a cracked block, just low pressure and flow at speed. Is the loss of pressure occurring at a certain rpm or related to being on plane/speed? So when you go from idle to WOT, do you still have pressure at high rpms until the boat planes?

 

[mphelle8vld]

Re: 1987 Suzuki DT85 overheats at high RPM

Found something interesting while looking through the parts diagrams. The additional water inlet under the cavitation plate was added in 1983, in 1986 they began offering an optional cover plate for that inlet. I would do another test with that inlet covered in case you're ingesting air at that point with your setup.

 

[mkrussow]

Re: 1987 Suzuki DT85 overheats at high RPM

That is an impressive level of research. Wow. It's a good thought. My prop is also not in perfect condition so I'm sure it's generating some extra cavitation. Maybe it's a combination of factors - prop cavitation, high motor mount, etc.

The loss of pressure seems to be initiated more by boat speed than RPM, but since those two things are typically related it's hard to tell for sure. My boat is fairly light (16' aluminum), so the boat planes very quickly at WOT. If I can keep the boat pushing instead of planing, it seems to maintain water pressure, but that's not very high RPM. Plus, when I'm doing that, the engine's lower unit is covered by lots of water.

I'll try and get the boat out one more time this weekend with that water intake covered and report back. I'll also try to do a little 'massage' work on the prop with a hammer to see if I can work out some of its imperfections.

Thanks again for the input, mphelle8vld. It's great to have the help.

 

[mphelle8vld]

Re: 1987 Suzuki DT85 overheats at high RPM

You have summed it up pretty well, if you get a chance, post a photo of the lower unit/transom. I have a good feeling about the next test.

 

[99yam40]

Re: 1987 Suzuki DT85 overheats at high RPM

85HP on an 16' al boat sounds kind of high HP, what is the boat rated for?

Hope the plate cures this overheating problem

 

[mkrussow]

Re: 1987 Suzuki DT85 overheats at high RPM

The boat is rated for 90hp. It's a deep-v with walk through windshield, etc.

I'm waiting for some t-storms to pass through and will hopefully be able to get out this afternoon. I'd be curious about your opinion on a question related to the motor size. (I'm probably cursing myself even asking this question at this point) The motor is running a SS prop, 17 pitch. WOT is around 5000 rpm. Top speed seems to be in the range of 35-37 mph, which is less than I thought it would be (previous motor was a 50hp and it would run ~28mph WOT).

Assuming mphelle8vld's guidance leads to resolution of my overheating issue, do you think dropping to a lower pitch prop would provide improved performance? The shop manual for this motor indicates it should run in the range of 4800-5500.

 

[99yam40]

Re: 1987 Suzuki DT85 overheats at high RPM

You need to post in the prop forum to get a good answer to that