Overheating!! Head gasket? Blockage? Other causes?

[northernmerc]

This is on a 2000 model Johnson 30 2 cylinder two stroke.

After extensive diagnosis, it was determined that the motor was overheating. Overheating causes the overheat sensor to cut back the RPMs until the engine cools. (The motor surges but will not build RPMs again until it cools. It has been determined that this is not a fuel problem and that the engine is overheating.)

The Johnson dealer replaced the water pump, thermostat, and overheat sensor. That has not fixed the problem. It does exactly the same thing - surges and will only idle until motor cools. Water flow out the indicator tube is good. Compression is good.

The dealer seems to be out of ideas, so I'm hoping someone here can help with this.

After doing some research (including looking through the repair manual), several possibilities stand out.
- leaking head gasket that overheats the water and/or allows compression gases to escape into the cooling water jacket.
-blockage or other water flow problem in the water jackets around powerhead. This motor has only been used in fresh water and shows no signs of corrosion.

Can anyone tell us what diagnosis procedures to follow to determine if head gasket needs replacement? Or is this a case of taking it apart and replacing the head gasket and hoping for the best?

And how and where would one look for and fix a blockage in the powerhead area?

 

[Joe Reeves]

Re: Overheating!! Head gasket? Blockage? Other causes?

What are the compression readings?

 

[HybridMX6]

Re: Overheating!! Head gasket? Blockage? Other causes?

Agreed, compression readings would tell ya if it's a head gasket or not. If it runs fine until it gets hot, that is not likely the issue. T-stat could be sticking (new ones do go bad or are bad when installed), water pump could be installed wrong, or the water tube is mis-aligned in the midsection. Could still be a blockage as well. My only suggestion for blockage would be to pull the t-stat, drop the lower unit, and force water into the t-stat area and see if anything comes out (other than water).
How long has it been on the boat? Have you made any changes to weight distribution that would coincide with when this problem started, or tilt/trim angles?

 

[northernmerc]

Re: Overheating!! Head gasket? Blockage? Other causes?

Compression readings are 118 and 120. It's a long shaft motor (correct for this boat) and extends well below the bottom of the boat (16 foot Lund aluminum). The tilt/trim angle seems fine, the boat planes up for the brief time until the motor overheats. It does not have power trim or tilt. The trim pin is set in the middle set of holes (of five possible positions).

The Johnson dealer just replaced the water pump, thermostat, and heat sensor. They felt certain that this would fix the problem, but the problem is just as before. It seems very unlikely that we would have two bad thermostats. And the mechanic that worked on it is very experienced and should know how to properly install a water pump and temp sensor.

That's why I'm wondering if it could it be something in the powerhead area, either a slightly leaking head gasket or a blocked passage in the cooling jacket area.

 

[northernmerc]

Re: Overheating!! Head gasket? Blockage? Other causes?

Anyone?

I'm also wondering if the Johnson 30 has water deflectors in the powerhead. If so, where are they located? Behind the head? Behind the exhaust cover? Any tips for checking and replacing?

 

[jbjennings]

Re: Overheating!! Head gasket? Blockage? Other causes?

It does have water deflectors under the head. I would think it might be possible that the first time it overheated it may have melted the water tube grommet where the water tube goes into the powerhead or possibly a water deflector.
What kind of indicator stream does it throw? Is it a hard stream on idle? Does it throw water out the exhaust relief as well once it gets going wide open? How does the indicator stream feel like? Warm or hot or cold? You might try going to harbor freight and getting a $20 digital laser heat sensor that you can shine on the powerhead and see if you have any particular hot spots that would indicate a water jacket passage being blocked. Have you ran the lower unit in any sand that could have been sucked up into the water inlets in the lower unit? As a last resort, I might take off the lower unit, hook up a water hose using some duct tape and 3/8 fuel line to the water tube and see how it does when you run it on the water hose. You could then at least isolate the problem to the powerhead or the lower unit.
Good luck---cooling problems can be really annoying to have to troubleshoot!:redface:
JBJ

 

[jonesg]

Re: Overheating!! Head gasket? Blockage? Other causes?

Is the engine actually overheating, I don't see any temperature numbers.

If it actually is running hot, then why did the dealer change the heat sensors, thats a bit like shooting the messenger.

If its not actually overheating the problem is in the power pack.

Try this, when it goes into SLOW put a digi thermometer on the heads, if its not running hot disconnect the heat sensors, if it runs good the power pack SLOW circuit is bad, replace pack.

Heat sensors are just triggers, the SLOW circuit resides in the power pack.

 

[northernmerc]

Re: Overheating!! Head gasket? Blockage? Other causes?

Thanks for the replies. I'll try to explain a little more about what's happening.

It has what looks like a normal indicator stream. At idle, the stream is a bit weak, but when the throttle is opened, the stream is good. At half throttle or less, the motor does not overheat, or at least not quickly. At three quarters or full throttle, it overheats in about two minutes.

It actually is overheating. Before the dealer changed the water pump, thermostat, and heat sensor, we tried disconnecting the heat sensor (dealer thought it might be a faulty sensor). Without the sensor connected, the temperature just kept climbing until the motor became very hot and began to lose power. I immediately shut it off and checked it with an infrared sensor. It truly was too hot!

After the "repairs" made by the dealer, I again checked it with an infrared sensor. The sensor cuts the power when it reaches about 170 degrees (hard to know exactly because the infrared sensor reading varies a bit from one part of the head to another).

I don't think that's hot enough to hurt anything. But if I wanted to travel at half speed, I would have bought a 15 hp motor, not a 30 hp. And travelling at full speed for two minutes and then having to wait a minute for the motor to cool gets old fast.

The next thing may be to pull the head and the exhaust cover at the side of the head and look for a blockage. I suppose that it may be possible that a piece of broken impellor or something else is stopping the water flow for one area of the head. I'll also look for that grommet at the head. Can you tell me more about where it is located?

And can anyone tell me what those deflectors look like, where they are, how many there should be?

 

[wilde1j]

Re: Overheating!! Head gasket? Blockage? Other causes?

They are typically cut from 3/8" hose. Never seen one melt, but in old motors, I've seen them swollen and misshaped.

 

[jonesg]

Re: Overheating!! Head gasket? Blockage? Other causes?

Did you do a compression test yet ?

a compression leak will cause it to run lean and overheat.

Just another possibility to eliminate before pulling anything off.

 

[northernmerc]

Re: Overheating!! Head gasket? Blockage? Other causes?

The compression is 120 on one cylinder and 118 on the other. I checked it twice, including after the overheat episode with the overheat sensor disconnected. It didn't change.

 

[wilde1j]

Re: Overheating!! Head gasket? Blockage? Other causes?

Very good numbers there.

 

[jbjennings]

Re: Overheating!! Head gasket? Blockage? Other causes?

Take the lower unit off. If you don't know how to disconnect the shift rod ask before you bang on it. then use a water hose on the water tube to see if it's a partial blockage in the powerhead or a weak water pump. I would not take the head off unless the above test indicated a partial blockage. Identify the problem and save yourself some time and money.
Good luck,
JBJ

 

[northernmerc]

Re: Overheating!! Head gasket? Blockage? Other causes?

Further to the above thread, can someone tell me the required length and location of the three water deflectors used on the Johnson 30 hp two stroke? This is a 521 cc (31.8 cu in) two cylinder motor.

In case the dealer who replaced the water pump did something wrong, I took the water pump apart. Everything looks good in there.

I also pulled the head off and looked for blockages. Compressed air blows well up the water tube from the leg. I also blew air and poked a wire through water passages around the cylinders. Everything looked very clear and clean.

The only thing I found that looks suspicious is the water deflectors. It has three deflectors, as it should, one at the top and two at the bottom. The top one is solid rubber (not a tube) and is quite long. The bottom right hand (starboard) deflector is shorter and also solid rubber. The bottom left hand (port) deflector is also short, about the same length as the other bottom deflector, and is made of a piece of rubber tubing.

My manual is not much help about how long the deflectors should be or the location of the long and short deflectors. Maybe someone who worked on this in the past put the long and short deflectors in the wrong places.

So I need to know exactly how long the deflectors should be and where the long and short deflectors should go. And can I make new deflectors by using pieces of fuel line? If not, what is good to use for this?

 

[Cofe]

Re: Overheating!! Head gasket? Blockage? Other causes?

I had some trouble awhile back. Here is the thread. http://forums.iboats.com/showthread.php?t=346241 Hope this helps

 

[Daviet]

Re: Overheating!! Head gasket? Blockage? Other causes?

Is the head warped? I have seen the case where after an overheat problem the head gasket allowed just enough compression leakage to pressurize the cooling system at higher RPM. At lower RPM the water pump could circulate water but at high RPM the pump could not overpower compression in block. Just a thought.

 

[Cofe]

Re: Overheating!! Head gasket? Blockage? Other causes?

In any overheat situation the head could be warped.

 

[Cofe]

Re: Overheating!! Head gasket? Blockage? Other causes?

In any overheat situation the head could be warped.

With the head off, place a piece of glass on it and you can tell if it is warped.

 

[northernmerc]

Re: Overheating!! Head gasket? Blockage? Other causes?

Thanks for your ideas. The head seems fine. Compression is very good (118 and 120).

Back to the water deflectors. I pulled the head again and checked the length of the deflectors in there and measured the maximum depth of the space where the deflectors fit.

The top deflector is 1 13/16" long, but it is possible to fit a deflector in that spot 2 3/4" long. The bottom starboard side deflector is 1" long, but the space it fits into is 1 7/8" deep. The bottom port side deflector is one inch long and the space it fits into is also one inch long.

In other words, of the three deflectors, only the bottom port side deflector completely blocks the passage. The other two deflectors have an open space of about one inch behind them.

My question is whether the two deflectors should have a space behind them or should they fill the space and seat against the block at their back end?
Should water flow behind two of the deflectors or not?

Since I bought the motor used, I'm thinking some monkey was in there before me and may have messed up on the deflectors. I asked two local boat shops this question and neither could give me an answer, which surprised me. I'm hoping that someone on this forum will know.

 

[Cofe]

Re: Overheating!! Head gasket? Blockage? Other causes?

You're measurements on those deflectors you took out are correct. The top one has space behind it. The bottom starboard side should have a small water passage grove clear in the back. (mine was filled with impeller trash) Make the deflectors the same length as the ones you took out and you shouldn't have a problem.
I had a thermostat problem on mine also.....caused headaches...... the new thermostat I bought didn't work... Test even new thermostats before you install them.