DT115 Weak Tell Tale & White Smoke at High RPM

[ScubaSteve]

Quick background: rebuilt the heads and lower unit this past winter. Replaced transmission, water pump, thermostat, plugs and all gaskets in the process.

The past few trips I've been noticing some white smoke or steam at high rpms.
It's pretty minimal on plane but when I come off plane there's a good amount of white smoke from the exhaust. The tell tale is also weaker at high rpms. When I come back down to idle, the smoke dissipates and the water stream becomes strong again.
I pulled the top plug and it's clear that I'm running rich.

The 3 things that come to mind are impeller, thermostat and poppet valve.
I'm fairly confident that the impeller is good since the motor emits a strong tell tale at idle. This has me leaning towards a bad tstat or poppet valve.

Has anybody had this issue with a similar motor?
I'm really hoping it's not the valve because I believe that means I have to reomove the block (again!). Looking forward to hearing from anybody with some knowledge of this issue.

[boats.net]

The pressure control valve(poppet valve) opens when water pressure reaches about 25 psi so it's not uncommon to see a difference in tell tale pressure when control valve opens. Might check compression to see if all cylinders are even to see if head gasket is sealing ok.

[Solarman]

I've had a problem with the exhaust leaking into my water pump intake area, causing similar problems (dt 140's) when going over 2k rpms. Tell tale starts turning to steam and stopped flowing. I found the sub-water pickup tube grommet was allowing exhaust to interfere with water pump pick up (located below water pump). The sub-water tube (1/2" s/s tube) runs from the screen just above prop, through the exhaust chamber (not on all models?), into the chamber that provides water to the pump. If that grommet around the tube is loose, or burnt, then exhaust gases at higher rpms (2k rpms) will interfere with water getting to pump.

I believe the rich color of the plugs indicates lack of air mixing with fuel, or the timing is affected by gear counting coil, or tps/tvs giving wrong signals. Is this the EFI or carb motor?

Good luck, post back when able to let us know what you find out.

[ScubaSteve]

Sorry I'm just responding now.

I performed a compression test and all cylinders are 115-120 psi after the rebuild which is comforting. The smoke has mostly gone away but I still get some faint steam at high rpms while underway. Next chance I get I'll drop the lower and check the water pump housing and water tube seal.

This is the EFI version so no carb. Solarman I believe we went back and forth a while back about the water tube not being present in my lower unit. There is just the single tube that goes up to the motor and not the second one inside the gear casing housing. Seems kind of strange but it is shown as optional in my manual and the parts diagram.

[Solarman]

It sounds like you have exhaust gases interfering with water supply somewhere? At higher rpms exhaust pressure gets extremely high, and if you have any bad grommets/ gaskets separating exhaust and water pas-sages, this would be an area to focus on.

If you do not have the sub-water tube that's running from under the water pump, across the exhaust chamber to just above the prop, you may either need that assy, or you may have a leak at that grommet that seals that area, or somewhere else between exhaust and water chamber?

Do you have any water on plugs after running (usually lower plugs are first to show?)? If any signs of moisture on plugs, that could indicate a bad mating surface between powerhead and midsection? If so, that mating surface must be welded back up, and ground back down to provide a proper seal.

Are you missing any bolts on the exhaust/water jacket plate on the side of motor? If any bolts are broken, or missing, exhaust can push into water pas-sage and interfere at that point.

Are any of the water supply tube grommets loose, or damaged? This tube goes through the exhaust chamber, as i recall?

It is difficult from this side of the monitor to examine what may be causing the problem, but you know the motor better than any one.

The exhaust pressures are higher than water pressure at high rpms, and you will need to find where it is interfering with the water flow. If you have any corrosion, aluminium deterioration, in the exhaust chamber between the powerhead, and the prop (the midsection), these areas would need attention (welding then grinding level/flat).

If the sub-water tube is optional for your motor, then maybe the seal on that plate that blocks that opening is not good? Apparently your casing is the same as those that have the sub-water tube, but it is blanked off? Did they use a grommet?

Good luck. Post back when able to let us know how it's going.