Logo
FULL SITE MENU

AQ170 Overheating

C

cj270

Member
HI all,

My AQ170A always had an over heating issue up until a few weeks ago when a full systematic check revealed a drastic mistake my mechanic had made which caused the major overheating issue... Let me explain that issue first...

At the rear of the exhaust header, there is the down pipe which bolts onto the header and has a thick gasket between. My mechanic had faithfully cut all the water jacket openings as seen when you take these 2 housings apart. Hence, all the water which was being pumped into the header, was just exiting straight through the water jacket openings into the down pipe and not filling one side of the header, and circulating in the engine and then through the other side of the header and out as it should.

This issue was sorted by installing a gasket of the correct design.

The engine now has NEW external water pump with new rubber impeller.

Water circulates and exits the exhaust port.

The engine runs fine and sticks to 70 degrees water temperature as long as I keep the RPMs to 3500.

If I run it at 4000 or the recommended cruising max RPM of 4500, the temp rises very slowly upto around 176 Fahrenheit. Then... as soon as I pull the throttle back to idle, to slow the boat down, I can see the temp gauge immediately rise to 194. If I then accelerate again and run the boat, the temp will go back down to 70 or 75 within a few minutes.

Head has been taken off and pressure tested, checked for warpage, block checked for warpage, new head gasket, new external water pump.

This is an old AQ170A and was not originally fitted with a water strainer at all.

The only modification I have done is to install a water strainer by using flexible hoses.
I have taken the connection from the sterndrive which come into the boat through the transom plate, and connected it to the inlet of the strainer and given the outlet of the filter to the copper tube which comes into the external water pump.

At first I had an issue where the strainer installed was way too small and was choking down the water supply to the engine. Now I have installed a vetus FTR330 strainer with 1" hose connections.... surely this has to be enough for this application ?

Could the added length of the flexi hoses be slowing down the water to the pump at idle when the sterndrive is not forcing the water through ?

The strainer is installed very close to the transom so I had to make bends with the hoses to prevent them from getting crushed. There must be a total of around 2.5 to 3 feet of hose.

The carbs have been completely dismantled and cleaned and put back. I have not checked the color of the plugs to see if it has been running lean.

Any help to sort this over heating out would be greatly appreciated as I have always had heating issues with this engine since 2008 when I bought it

The engine was imported from the USA and seems to have been rebuilt. I wonder is the bearings, pistons are too tight ?
*
 
You've covered everything but didn't say anything about a thermostat?? Is there one? Why do you think the engine was imported from the US ? The ones using the GM motors were made in the US(again I think) Volvo's of that variety were made in Europe.(I think) Another thing on the rise or down tube is it clogged? It's 40 years old?Another thing,the water goes from the pump right into the oil cooler(oil cooler clogged) and then to the manifold then to the thermostat housing(no termo or clogged). Then out the riser(clogged). Also not sure of the drive, but the pickup and hose, check the hose connection on the mid section of the drive. Never heard of the pistons being too tight?? Would probably sieze.
 
There IS a thermostat BUT.. the valve part of it has been removed. So it is as if the thermostat is always open. The housing and the spring etc is all there. Just the disc which seals the water circulation when closed, has been removed.

A note.. Our ambient temperature is around 90 Fahrenheit so the water temp in the lake must be around the same ?

I say the engine was imported from the US because I imported it from the US. My boat (a 1960's 18 foot Yamaha STR 18) was originally fitted with an AQ170/270 set up but both engine and drive were shot when I bought the boat so when I restored it, I imported an engine and drive from the USA.

The downpipe which bolts directly onto the exhaust manifold (the small "L" shaped elbow casting) is not blocked. I looked at and pressure washed it and the manifolds etc when I took the head off and just fitted it back.

The rest of the downpipe which fits onto the transom plate cant be blocked coz it is one big open pipe and carries the exhaust and water all mixed (right?)

The oil cooler was taken apart and cleaned and re fitted when doing the head job. It was blocked with pieces of rubber which were from the disintegrated water pump impeller when the engine ran on and turned backward once due to being too hot.
Pistons and bearings being too tight may be unheard of in USA but over here it is a very common issue when engines are rebuilt. Our machine shops don't care to leave the necessary clearances so they end up being tight. Not quite tight enough to make it seize but they suffer from over heating .... then again, this engine, if it has been rebuilt, has been done in the USA and we have not touched the bottom end at all. Only removed the head a few times thinking it had blown a gasket but it hadn't.

Apparently, we have used around 3 feet of flexible hose to connect the strainer... I wonder if that is the problem ? I hope to import a later AQ170's copper water tube set complete with original Volvo strainer, discard that strainer but connect my 1" strainer to that. This way, there will be no length added to the suction pipe.

Any other suggestion would be very welcome.

It would be a shame to re-power this old boat with a different set up as this is how it was originally.
 
All suction hoses must be collapse proof. If you've installed a suction hose that is not collapse proof, you may have issues.


The rear exhaust manifold elbow is often the culprit, and this will usually be at the sea water transfer ports within the manifold and elbow, if the manifold and elbow themselves are OK.


The water neck fitting is often over-looked as well.
However, this part usually causes issues once the drive is up and out of the water during planing speeds.
During planing, the water neck is out of the water.

These water neck fittings are good for about 2-3 years max, and much shorter if in salt water.


Pivot tube upper bushing slop is another concern.
The special beaded gasket (underneath the water neck) will fail if there is excessive pivot tube slop.




.
 
The manifold and elbow were taken off the engine, dismantled and pressure washed and inspected. All ok. Flexi hoses are collapse proof and have run the boat with engine cover off and had mechanic sit by hoses and check at speed. No collapse.

I have taken the outdrive apart and installed all new "O" rings in the tube. I have then taped up the intake grate and hole on the drive and filled water from the tube in the boat, no leaks.
 
cj270 said:
I have taken the outdrive apart and installed all new "O" rings in the tube. I have then taped up the intake grate and hole on the drive and filled water from the tube in the boat, no leaks.
Click to expand...
Unfortunately, it is difficult to reproduce a potential suction breach with pressure. Pressure acts differently than suction does on certain components.

You'll do better with a shop vacuum attached at the engine side, and then use (believe it or not) shaving cream at the various potential areas of suction breach.
If the shave cream disappears, you'll know that a suction breach is at this area.

Sounds strange, but it works.


.
 
Somehow, it doesn't look like the pickup system is letting in air from anywhere.
2 reasons for me to say this...

1) the top of the water strainer has a clear top and you can see inside very clearly. With the old small strainer, we used to see air bubbles continuously coming through the pick up tube into the strainer when the boat was running.
After the new o rings were fitted and the new bigger 1" strainer installed, now it seals completely because when underway, the strainer completely fills with water right upto the lid and there is absolutely no air coming in when the boat is being run and further more, once the boat has been stopped and switched off, the strainer still stays full to the top with water until you loosen the lid. Then you can hear the air go into the strainer and see the water drop through the intake tube into the lake.

2) the temperature rises to 176 Fahrenheit when the boat is run at 4000 to 4500 RPM, then rises immediately to 194 Fahrenheit when the throttle is pulled back and the outdrive drops back into the water and the potential leak points are fully submerged in water... so it just doesn't add up that it is letting in air.
 
Ditto the thermostat.

If you have full sea water supply to the engine, and no suction breach...... then something, somewhere, has to be restricting the flow.

Suggestions:

Measure the depth of your sea water pump impeller cavity (from the cover face surface to the deepest depth). Then also measure the thickness of your impeller. The impeller thickness should exceed the cavity depth by .0XX" when the fit is correct. (perhaps .010" to .013" or so)

Also carefully examine the two impeller "thrust" surfaces. If there's any deep scoring, the impeller may not function at low speeds, but may function at higher speeds.

Take a good look at the impeller "cam". The cam must fold each blade as they contact and leave the cam. This is what creates both the suction and the pressure.


It's not uncommon to see a slight increase in temperature when an engine is shut down. This is because not all heat has yet been removed, and the residual heat increases the temperature of the now static and captive coolant.
However, while it's still running and is brought down to idle RPM, I don't believe that you should be seeing this much temperature increase.


Just an FYI:
This may not be your case, but I've pulled anywhere from small to large amounts of crustaceans from the water intake area of an AQ series V/P out drive before. Little shells that may block sea water flow. I've even found very small ones clear up into the sea water pump impeller area.




.
 
Check inside the outdrive water passages. My AQ170's both overheated after a complete rebuild when up to speed, but ran fine at idle. It took me forever to find the culprit. There is a drain under the water intake on the outdrive. The passage above the drain seems to fill and calcify with minute shells. This cannot be seen, as the area holding the shells is between the intake grate on the outdrive, and the drain hole located on the leading edge of the casting. Water is drawn in at idle from the screened area, no problem. Ramp 'er up to speed, and the hydraulic pressure on the drain hole pushes the shell plug up and out of the cavity, where it blocks the intake inside the outdrive. I had separated one outdrive to replace the center section when I found the plug. What a relief! I used a slender screwdriver to probe the port drive and sure enough, another plug! I was able to break the plug up by working the screwdriver back and forth, then allowing the particles to exit the drain. Cure..? Probe the drain regularly to break up the plug, and your flow will be strong and reliable.

Good Luck!
Arby
 
Arby, no offense intended......... this thread is 3 years old. Perhaps before posting again, take a look at the original posting dates!




.
 
Back
Top