Johnson 120hk -86 Overheat at WOT

The engine overheat at WOT, both banks go hot (confirmed with temp gun).

My water pressure gauge maxes out at 30+PSI at RPM above 5000 so the impeller seem to work fine.
I had overheat issue early in this season but I thought I got it solved permanently when I switched side of the diaphragms and at the same time gave them some "massage" to loosen upp a bit..

What puzzles me is that i belive that the maximum pressure clearly went down at wot after I did the massage and switch the first time.
I belive that the pressure went down a bit and maxed at around 20PSI.

To me this means that the t-stats doesnt move away from the inlet when pressure rises, or what do you think?
Is 30+ PSI really normal at wot?..

Starting to loose my patience with this engine

Forsete said:

The engine overheat at WOT, both banks go hot. I thought I got it solved permanently when I switched side of the diaphragms and at the same time gave them some "massage" to loosen up a bit.

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I have no idea what the above means... you'll need to go into detail to explain it fully.

I suggest you drop the lower unit to visually examine the water pump assembly... assume nothing.

Possibly a head gasket is failing slightly, allowing the combustion enter the water passage area which would result in a stalemate.... water coming up, combustion going down, water flow is forced to stop... stalemate!

Is your water pressure gauge accurate ??

joereeves said:

I have no idea what the above means... you'll need to go into detail to explain it fully.

I suggest you drop the lower unit to visually examine the water pump assembly... assume nothing.

Possibly a head gasket is failing slightly, allowing the combustion enter the water passage area which would result in a stalemate.... water coming up, combustion going down, water flow is forced to stop... stalemate!

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Hi Joe,

I switched the STBD side Diaphragm and T-stat to Port side and vise versa, and the "massage" was me bending and rolling the diaphragm rubber quite a lot.

I do have new head gaskets and will examine the pump when I get the boat up for this season, just want to test the most basic stuff that can be done whilst in water.

Do I need to be afraid that bolts will break when I go for the change of head gaskets?..

Okay... I understand your explanation of the switching sides and message thing... appreciate that. You got to admit it did sound a little weird.

The tight head bolts.... have a hand held propane torch handy to heat the aluminum surrounding the bolts to help break them free if needed. Also have a fire extinguisher and/or a garden hose near by... better safe than sorry.

re: I do have new head gaskets

I assume that when you changed the head gaskets you saw that the water passages were all clean? The water pressure reads at the tell tale, that's before it gets to the heads, therefore, it could "read" great but yet no water is getting to the heads. Were your water passages clean all around the heads?

water pressure reads before it gets to the heads ? I don't think so, the tell tale is there to tell you that the water is circulating thru the block.

bill_w said:

water pressure reads before it gets to the heads ? I don't think so, the tell tale is there to tell you that the water is circulating thru the block.

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On my 1990 V4 115 Crossflow bubbleback the water flow diagram shows the water going directly from the water pump to the tell tale outlet at the base of the bubbleback. My J115 telltale was pumping water great, but my heads were hot. I removed the heads and found the passages blocked and the thermostats stuck. The same goes for my 1991 Johnson 6hp, and the same goes for my 1993 Johnson 25hp, all three had the identical same problem. I'm in Florida, either it's the saltwater use or it's the all year use down here (or both), the bottom line is that out of 8 engines I've bought in the last year, 6 had to have the head passages cleaned out, as they were totally clogged.

Many others have posted here that the tell tail only tells you the pump is working.
Maybe you have a different engine than my three?

The pressure gauge is connected to the incoming water pressure line to the Control valve.

I plan to make sure that the water control valve works this weekend.
To rule out that the issue is due that the water behind diaphragms can´t escape through the pisser when the Water control valve re-directs the flow (at RPM above ~3000(?)) -I plan to cut off the incoming water line to the STBD thermostat cover and just connect a hose to it that let the water flow overboard, out of the t-stat covers.. i.e. removing the water pressure feature that push the stats inwards.

I will also blow compressed air through the hose from/to STBD t-stat cover down to water control valve to make sure there is good flow out to the pisser (I will do this with the throttle at WOT to be sure the Water control valve is in correct position).

When I get the boat up I will probably replace the head gasket even if the issue go away due to faulty control valve or something else, -just because I already have the seals and to clean out any crap in the water passages.

(FYI: I have perfect temps at neutral)

Klink said:

re: I do have new head gaskets

I assume that when you changed the head gaskets you saw that the water passages were all clean? The water pressure reads at the tell tale, that's before it gets to the heads, therefore, it could "read" great but yet no water is getting to the heads. Were your water passages clean all around the heads?

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Ah, sorry! What I meant with that was that I already have purchased new gaskets to replace the old ones with, sorry that I was so unclear about this =)
My bad!

Update!

I ran the motor without the incoming water tube from water control valve that normally connects to STBD t-stat cover. I plugged this line. i.e. totally removed the pressure pushing the T-stats inwards.
The small amount of water coming from the T-stat covers escaped trough a hose I just had hanging overboard.

Running the motor this way the pressure could barely pass 15 PSI and the heads were really cool, below 90F. I guess the T-stats where so easilly pushed away that it cooled too good.

I then removed the plug I had on the line from the Water control valve to see how the water flow looked like when driving; at neutral and RPM below ~2000 the stream was steady, similar to a tell tale but it newer really went totally away when throttling up, there was a big change but still there was some water flowing, which seems wrong to me since the control valve should totally remove this incoming water to the T-stat covers?

When I reconnected the hose between the water control valve and the STBD side cover again the pressure at high RPM went up again to max out the gauge 30+ PSI with really hot heads again.. measured 195F on the cylinder wall after a Wot session for 30 secs..

Could my issues be due to a leaky water control valve that never really remove incoming water to the T-stat covers??

dont forget the water deflectors...they are cheap and really are maintenance items under some conditions...you will also need to order head gaskets....

The engine does not have any water deflectors and yes, as I stated earlier I do have head gaskets that I will install..

[QUOTECould my issues be due to a leaky water control valve that never really remove incoming water to the T-stat covers??] [/QUOTE]
Yes or misrouted. The water line and valve must be connected to cool correctly or convert to independet cooling by install 70hp cover and stats

Unless I missed something, have you replaced the thermostats with new ones? This is key.

remo williams said:

Unless I missed something, have you replaced the thermostats with new ones? This is key.

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Yes, I have the new ones without the long nose

Got the boat on land and concluded that the control valve is extremely leaky. The internal plastic part is worn out so the inlet water pressure line is never blocked.. meaning that the water pressure pushing the stats in the seat can´t move away at wot, thats probably why the water pressure is so extremely high at WOT.

Temporarily fixed the leak with some electric tape around the tap..

Klink said:

On my 1990 V4 115 Crossflow bubbleback the water flow diagram shows the water going directly from the water pump to the tell tale outlet at the base of the bubbleback. My J115 telltale was pumping water great, but my heads were hot. I removed the heads and found the passages blocked and the thermostats stuck. The same goes for my 1991 Johnson 6hp, and the same goes for my 1993 Johnson 25hp, all three had the identical same problem. I'm in Florida, either it's the saltwater use or it's the all year use down here (or both), the bottom line is that out of 8 engines I've bought in the last year, 6 had to have the head passages cleaned out, as they were totally clogged.

Many others have posted here that the tell tail only tells you the pump is working.
Maybe you have a different engine than my three?

Click to expand...

You're right, I'm wrong. That is a stupid set up for a tell tale, I'd be plumbing into the thermostat hoses for one.

I can now conform that the culprit was the leaky Water Control Valve.
Since it was leaky before it never really shut off the pressure holding the stats against the inlet in the head. That was why the pressure just got higher and higher as I increased the throttle (maxed out the gauge at 30PSI).
The pressure now maxes out at ~20 PSI and then drops back down to 15 PSI as the revs pass about 3500 Rpm.