Volvo Penta AQ120 overheating

Hi all, I'm a proud owner of a Shakespeare Shadow speedboat. The boat is equipped with a Volvo Penta AQ120A inboard engine form around 1964. The engine runs perfectly fine, but it starts to overheat when idling.

The strange thing is that the engine merely overheats when the sterndrive is in downward position (its normal position when driving the boat). As soon as I tilt the sterndrive upwards, the temperature starts to drop to its normal operating temperature of 65 degrees Celsius. It gets stranger though: when I start the engine whilst the sterndrive is in upward position, and I then lower the sterndrive to its downward position and shift the engine in forward gear, the cooling system works perfectly fine. As soon as the engine is at idle, it starts to overheat again in a few minutes.
When starting the engine whilst the sterndrive is in downward position, and I then shift the engine in forward gear, it would still overheat.

The thermostat looks a bit rusty but opens up nicely at 60 degrees Celsius. I replaced the impeller and checked some hoses, which all seemed fine. There is no air leak as far as I can tell. I attached an external hose to the impeller house and put the other end of the hose in a bucket of water, which I had to refill every minute whilst the engine was running. The temperature remained at 60 degrees Celsius.

My conclusion is that there must be a blockage in the water inlet on the sterndrive. However, when I blow air through the hose that is connected to the inlet, air bubbles come out of the inlet and I experience minimal resistance when doing it, which indicates that there is no such blockage in the water inlet.

I hope I gave you guys sufficient information to form a possible solution to my problem. Thank you in advance and I'm looking forward to your reactions!

As I recall, the hose from the transom to the drive is weird, i.e. not a straight hose, but has molded curves... my guesses...
1) some one cheaped out and installed a straight hose which kinks is certain positions.
2) correct hose but improperly installed.
3) the "Water neck", that right angle fitting on drive where the raw water hose connects is not properly seated and shifts
position causing a loss of suction.
4) pin hole vacuum leak in the "water neck"

sandkicker said:

As I recall, the hose from the transom to the drive is weird, i.e. not a straight hose, but has molded curves... my guesses...
1) some one cheaped out and installed a straight hose which kinks is certain positions.
2) correct hose but improperly installed.
3) the "Water neck", that right angle fitting on drive where the raw water hose connects is not properly seated and shifts
position causing a loss of suction.
4) pin hole vacuum leak in the "water neck"

Click to expand...

Thanks for your reply! You're right about the hose from the transom to the drive, it has two molded curves and looks a bit like an S-shape. It's kinda hard to inspect the hose since my boat is currently in the water, so it might be best to get it out and see if the hose clamps are well attached and if the hose in general is still in a good shape.

The water neck feels solid, I wasn't able to move or shift the thing.

Yesterday I experienced another strange thing: the engine was running whilst the sterndrive was in downward position (after I started the engine with sterndrive upwards, and then tilted it downwards). I throttled at idle for a few seconds, and then stopped. Since I stopped throttling, the engine started to immediately overheat again. The engine really doesn't make any sense when the sterndrive is in downward position..

..........

Bart_Postmus said:

The strange thing is that the engine merely overheats when the sterndrive is in downward position (its normal position when driving the boat). As soon as I tilt the sterndrive upwards, the temperature starts to drop to its normal operating temperature of 65 degrees Celsius.
Your drive is equipped with a Lift-Out unit only..... this is NOT a trim system.
Be careful when using it! If you were to power up with the drive lifted, you may damage the Lift-Out unit's vice rod.

It gets stranger though: when I start the engine whilst the sterndrive is in upward position,
Try to NOT operate the engine with the drive lifted.

and I then lower the sterndrive to its downward position and shift the engine in forward gear, the cooling system works perfectly fine. As soon as the engine is at idle, it starts to overheat again in a few minutes.
When starting the engine whilst the sterndrive is in downward position, and I then shift the engine in forward gear, it would still overheat.
Inspect the S hose that connects the water neck fitting to the chrome tube on the transom shield. It may be bad and not holding against suction.

The thermostat looks a bit rusty but opens up nicely at 60 degrees Celsius. I replaced the impeller and checked some hoses, which all seemed fine. There is no air leak as far as I can tell. I attached an external hose to the impeller house and put the other end of the hose in a bucket of water, which I had to refill every minute whilst the engine was running. The temperature remained at 60 degrees Celsius.
That test will not render good results for the entire seawater path.
Suggestion:
Tape off the seawater intake at the lower gear unit.
Using a shop vac, pull negative pressure on the seawater pump's suction hose.
Use shave cream on the suspect areas.
If the shave cream disappears, you've found your suction breach.

My conclusion is that there must be a blockage in the water inlet on the sterndrive. However, when I blow air through the hose that is connected to the inlet, air bubbles come out of the inlet and I experience minimal resistance when doing it, which indicates that there is no such blockage in the water inlet.
The smallest suction breach will prevent the seawater pump from pulling in seawater.

I hope I gave you guys sufficient information to form a possible solution to my problem. Thank you in advance and I'm looking forward to your reactions!

Click to expand...

Bob makes some excellent suggestions here.

sandkicker said:

As I recall, the hose from the transom to the drive is weird, i.e. not a straight hose, but has molded curves... my guesses...
1) some one cheaped out and installed a straight hose which kinks is certain positions.
2) correct hose but improperly installed.
3) the "Water neck", that right angle fitting on drive where the raw water hose connects is not properly seated and shifts
position causing a loss of suction.
4) pin hole vacuum leak in the "water neck"

Click to expand...

The water neck fittings are wear items, and require routine replacement.
There is a special beaded gasket underneath the water neck fitting. Bead faces down.
Upper pivot tube bushing may be bad, not allowing the gasket's bead to form a seal.

Bart_Postmus said:

Thanks for your reply! You're right about the hose from the transom to the drive, it has two molded curves and looks a bit like an S-shape.
Yes, commonly referred to as the S hose.

It's kinda hard to inspect the hose since my boat is currently in the water, so it might be best to get it out and see if the hose clamps are well attached and if the hose in general is still in a good shape.
Yes.

The water neck feels solid, I wasn't able to move or shift the thing.
You cannot tell by looking at the exterior of it.
It must be removed in order assess it's condition, and if you have it removed, you may as well replace it.

Yesterday I experienced another strange thing: the engine was running whilst the sterndrive was in downward position (after I started the engine with sterndrive upwards, and then tilted it downwards). I throttled at idle for a few seconds, and then stopped. Since I stopped throttling, the engine started to immediately overheat again. The engine really doesn't make any sense when the sterndrive is in downward position..
Again, this is a Lift-Out unit ONLY..... and is not intended to be a trim or tilt.

Click to expand...

RicardoMarine said:

..........
Bob makes some excellent suggestions here.

The water neck fittings are wear items, and require routine replacement.
There is a special beaded gasket underneath the water neck fitting. Bead faces down.
Upper pivot tube bushing may be bad, not allowing the gasket's bead to form a seal.

Click to expand...

Hi Rick, thank you for your instructions. I'll get the boat out of the water and inspect the S-hose and water neck. Your instructions with the shave cream sounds like a good experiment to begin with.

I am aware that it's a Lift-out only, and not a trim. I was informed by the former owner that it doesn't harm the engine to run it whilst the sterndrive is in upward position, as long as you don't put the engine in forward or reverse, which I never did. He could be wrong of course, you guys know more about it than him and me.

Thank you for your response and i'll try the things you suggested.

Kind Regards,
Bart

Bart_Postmus said:

Hi Rick, thank you for your instructions. I'll get the boat out of the water and inspect the S-hose and water neck. Your instructions with the shave cream sounds like a good experiment to begin with.

I am aware that it's a Lift-out only, and not a trim. I was informed by the former owner that it doesn't harm the engine to run it whilst the sterndrive is in upward position, as long as you don't put the engine in forward or reverse, which I never did. He could be wrong of course, you guys know more about it than him and me.

Click to expand...

Bart, the engine can be operated while the drive is partially up...... just don't rev it high.
As for propelling forward with the drive up a bit.....yes, you can do it, but again, very gently.
As for reversing while the drive is up, that does not work.... the drive will kick up and may become damaged.

Keep in mind that when a stern drive is tilted up, it puts unnecessary stress on the under-side of the drive shaft bellows.
Prolonged storage with the drive UP will lead to premature bellows failure.

In other words, raise the drive no more than need be to clear the ground.
Store the boat with the drive down and aiming straight forward.
Do this, and you will extend the life of the bellows.


.

Good to know Rick, thanks.

I got the boat out of the water today and removed the S-hose and water neck. The S-hose is not damaged at all, but the water neck is not in a great condition whatsoever. The part that is covered by the S-hose is partially broken, which might result in an air leak. We however performed a test where we put positive pressure on the suction hose while the seawater inlet was covered with tape. The S-hose and connectors were covered with soapy water, and we didn't see any air bubbles. it might be possible that a test with negative pressure would result in a different outcome.

We'll replace the water neck in the hope that this will solve our problem. I attached a photo showing the broken water neck. Do you think that this would solve the problem, keeping in mind that we didn't see any air bubbles during the test?

As you’ve just learned, its tough to check for a suction leak with pressure.

Today I bought a new water neck (the REC854031 for the Volvo Penta AQ200-290 sterndrives) and replaced it for the old one. I made sure that the water neck (including rubber gasket) and S-hose were properly installed. Unfortunately, the engine shows the exact same signs as before the replacement of the water neck.

The testing conditions weren't ideal however. We put a big plastic box filled with water underneath the sterndrive (see photo. When we ran the engine the waterline was to its maximum height). During the test, the seawater inlets were beneath the waterline at all times. After two minutes of running, the engine's temperature began to rise again, and I shut down the engine when the temperature exceeded 75 degrees Celsius (normal operating temperature should be 60 - 65 degrees Celsius). After shutting it down, the temperature raised even further to 90 degrees Celsius.

So did we apply an incorrect test or is there still a problem with the sterndrive/engine? Again: the impeller spins, the thermostat opens up at 60 degrees celsius and there is no blockage in the cooling system. I'm really lost at this point, I hope you guys can help me.

You can't do a cooling system test in a closed container of water.... need a flow of COLD water to test...
After a few mins in your test rig, the engine will start to intake hot water... If you ran that test long enough you would literally boil the water in the plastic box.

sandkicker said:

You can't do a cooling system test in a closed container of water.... need a flow of COLD water to test...
After a few mins in your test rig, the engine will start to intake hot water... If you ran that test long enough you would literally boil the water in the plastic box.

Click to expand...

I checked the temperature of the water every minute and it remained cold. Moreover, cold water was constantly flowing in the box since i put a fresh water hose in the box. This can't be the cause of my overheating problem in my opinion.

The engine was running for only a few minutes, and it than started to overheat again.

Bart_Postmus said:

I checked the temperature of the water every minute and it remained cold. Moreover, cold water was constantly flowing in the box since i put a fresh water hose in the box. This can't be the cause of my overheating problem in my opinion.

The engine was running for only a few minutes, and it than started to overheat again.

Click to expand...

Yep, that should work ok...