150 Merc Overheating

[flobbest]

OK guys, I?ve consulted all the local experts and no one has an answer for me. Service has a six week backup at the shop. You?re my only hope. I have and overheating problem with my 1989 Mercury 150 HP XR4 outboard. Here?s the deal?.if my water pressure stays around 8 psi (on plane) the motor runs cool, ? on temp gauge. If the water pressure jumps up to 12 psi, which it does whenever it feels like it, it will begin to heat up. Then if I speed up or slow down and the pressure drops back down, it will cool off. So it?s high pressure = heat and low pressure = cool. I?ve changed the water pump (complete) and popet valve diaphram. Also removed thermostats and bypassed telltale. No change with any of above. Compression is 125 psi all.

Any ideas are greatly appreciated. Thanks....Frank.

 

[gss036]

Re: 150 Merc Overheating

Sounds like your poppet is sticking. Change the spring and carrier, maybe even the grommet while you are in there. When the pressure drops back down, that is when the poppet is opening. Should be at about 3000 rpm+-.

 

[ronnieboy]

Re: 150 Merc Overheating

howdy, the seals on your water tube inside the lower unit are leaking exaust into the water stream, you have to install new seals top and bottom or use a blue silicone sealant to reinstall . had the same problem for 3 years before this forum and andy6374 fixed me up,, good luck ron

 

[flobbest]

Re: 150 Merc Overheating

Ron, I had the power head off yesterday to change the base and plate gaskets. The upper water tube seal seemed to be made of some pretty hard plastic, not like a seal material. And it looked to be in good condition so I just put it back together (power head) with new gaskets. Are you refering to siliconing that part (the top) or where it comes out of the water pump or both? Can I pull the water tube out of the top from below without pulling power head again?

A side note...I had a water hose hooked up to my water tube (LU off) and noticed that a lot of water was coming back down the cavity where my driveshaft and water tube are. I thought that odd because it looks to be pretty much sealed up with LU on. I asked a local expert wrench if that was OK to which he relied....no way should any water be coming down that cavity and that I had a blown base gasket for sure. That's why I pulled the PH and changed the gaskets. I saw no damage to the gaskets (aside from tears from dissasembly) and was suspicious about diagnosis. I looked uo through the drive shaft cavity with PH removed and clearly there are water channels that run up into the PH. So I guess it's normal for water to coming down on top of the water pump, shift shaft, etc?

Thanks for the help....Frank

 

[flobbest]

Re: 150 Merc Overheating

Thanks gss, I aggree, it sounds like the popett valve. I had the power head off and was able to manipulate the PV by hand and it seemed fine. I think I'll just change the whole thing.....thanks again...Frank.

 

[ronnieboy]

Re: 150 Merc Overheating

hey dude, the watertube seals are hard based with soft rubber for the seal , cant hurt, just use blue silicone to assemble top, and yes you can remove tube from below, trust me, seal the water tube, good luck ron

 

[flobbest]

Re: 150 Merc Overheating

Just tried to pull the water tube out, no luck. I pulled pretty hard from the bottom with some hefty needle nose pliers (kinda dinked the edge of the tube a bit) but it doesn't seem to want to move. What next...pull harder, or am I doing something wrong??? Thanks ....Frank

 

[ronnieboy]

Re: 150 Merc Overheating

it has to come loose, just pull, do NOT bend or disfigure the tube ends, if it comes out , you will need new seals, or blue silicone, good luck ron

 

[flobbest]

Re: 150 Merc Overheating

OK, I got the water tube out! I clamped a piece of hose over it so I could get a good grip and wala, out it came. It's been in there for 18 years! I cleaned and ran a bead of high temp silicone around both ends and reinstalled. Won't be able to test it until Monday 8-23. I'll let you know what happens. Thanks again.

 

[ronnieboy]

Re: 150 Merc Overheating

good luck, nice when it works, hahaahaha ron

 

[jddenham]

Re: 150 Merc Overheating

So much for the shop's backlog !

Won't be able to test it until Monday 8-23.

 

[flobbest]

Re: 150 Merc Overheating

OK guys here?s the deal. I siliconed the water tube and took off across the lake at 3,000 rpm. The water pressure went up to 12 psi and stayed there and the temp began rising just as before. I went back to the dock, pulled out the poppet valve and cut about 1 ? coils off with a pair of dikes. I?m guessing that would be about 20 % of the tension removed. Took off across the lake again at 3,000 rpm. Water pressure went up to 10 psi then dropped back to about 6 or 7 and the boat ran cool at ? on temp gage. So I guess I?d have to say that the poppet valve was sticking closed and restricting flow to the power head, and that by relieving some of the spring tension, it opened easier. I?m puzzled though that an 18 year old spring could be too strong? Another issue I have now is that just chugging along at 1,000 rpm and 2 psi, it wants to run up to ? on the temp gage which is hotter than normal. I have all new parts ordered for the poppet valve and will install when they arrive. I?ll post results. Thanks for all the help?.Frank.

 

[gss036]

Re: 150 Merc Overheating

Your last sentence that you have all new parts order is the way to go. After you have put in "ALL" new parts, including the grommet, you will have a better idea of where to go if it still overheats. I think maybe, you cut just a tad too much spring and that will result "MAYBE" in the low speed temp heating up a little.
Definately on the right area to solve you problem.

 

[ronnieboy]

Re: 150 Merc Overheating

as long as it dont go over 3/4 temp idling , no problem, i bet it gets hot at fast idle, and that it cools at wot, try and see, ron

 

[flobbest]

Re: 150 Merc Overheating

Conclusion?.finally got the parts in for the poppet valve. Installed all new parts and off to Lake Erie I went today. Boat ran like a top. Still runs up to just over 1/2 on the temp gage when chugging down the channel but on plane it drops back to ?. I hate to complicate the issue but I did slap in a new water pump impeller just for the heck of it. The one I took out only had a few hours on it and was in pristine condition, so I?m pretty sure it had no effect. Looks like it was the poppet valve all along, even though there was no visible sign of a problem with the original. I find it odd that none of the local experts I talked with ever even mentioned the poppet valve, it?s seems like the prime suspect to me ?.now. I talked to one guy who has 50 years (yes I said 50) experience working on and racing outboards, mostly Mercs no less, and he told me I must have something stuck in my cooling system, some debris or something. Kind of makes you wonder sometimes. Anyway, it?s fixed now thanks to you guys and this forum. Thanks again. Frank.