Nissan 3.5 hp overheating

[dtfl74]

I have an old (1992) Nissan NS3.5A that is overheating. I replaced the impeller which was obviously in need of replacement, and when running the motor in a tank water initially came out of the peehole for a couple of minutes, but after awhile water stopped coming out. It's not just a problem with the peehole - the engine itself is overheating to the point that the motor shuts down within a coupla minutes. So something is clogging the water passages in the cooling jacket, not just the telltale.

I ran compressed air backward through the telltale and bubbles came out the lower unit water inlet, so it's not completely blocked, but something in there seems to be blocking most of the water flow.

Can I just run a wire through the telltale opening to clear out any obstructions? How far do I have to run the wire into the powerhead to clear the water passages?

Or do I have to take the powerhead apart and remove the cooling jacket (sounds like a big pain!), which is what the Nissan service manual seems to suggest (although it has very little specific information on this particular motor)?

If a wire can do it, what type and size of wire is suggested, that's both heavy enough to ream it out but flexible enough to go through the cooling jacket water passage?

 

[pvanv]

Re: Nissan 3.5 hp overheating

If the old impelled disintegrated, some bits could be clogging the cooling passages. You would drop the LU and see if you can flush or backflush the cooling system, either with air, or a garden hose. Failing that, you would pull the power head and inspect the cooling passages. In a worst case, you might even have to pull the head off, but a clog in the head passages would probably not stop the telltale in and of itself. You probably have a chunk of broken-off impeller blade jammed in the water tube where it enters the base of the block.

 

[TOHATSU GURU]

Re: Nissan 3.5 hp overheating

What Paul said plus: When you replaced the impeller, was the housing pitted where it rides? If it was, it could wear out the new impeller in a matter of seconds.

 

[dtfl74]

Re: Nissan 3.5 hp overheating

Thanks, Paul, for your input. The old impeller didn't disintegrate, it was just very old and stiff such that the vanes had no spring left in them, so did not expand to fill the pump housing as the vanes rotated.

I pulled the lower end this morning, verified that there were no clogs or debris in the lower end water passage. I also flushed some water via a siphon thru the tell-tale (motor not running) which came through the lower end of the water tube OK. It seemed like plenty of flow considering it didn't have much pressure behind it (just a 1/4-in siphon hose). Therefore, if the impeller were pumping water OK, you'd think there would at least be water coming out the telltalewith the motor running, even at idle speed.

I checked the inside of the impeller casing. It's not pitted, but the upper portion seems slightly roughened compared to the lower portion of the case, but not enough to where it would prevent any flow thru the pump. I checked the new impeller vanes and there's no visible damage, roughening, scoring, etc. at all after running for maybe a total of 15 minutes, mostly at low rpm but high enough that water should still pump.

When I run the motor in the tank, no water comes out the telltale, but after it warms up after a minute or so steam (visible water vapor) shoots out the telltale, with maybe a few drops of water, then I shut it down. So neither the the power head nor the telltale is getting any cooling water flow.

Should I try replacing the water pump housing? I hate to just keep replacing parts until I stumble onto something, which eventually could get pretty expensive!:grumpy:

If the old impelled disintegrated, some bits could be clogging the cooling passages. You would drop the LU and see if you can flush or backflush the cooling system, either with air, or a garden hose. Failing that, you would pull the power head and inspect the cooling passages. In a worst case, you might even have to pull the head off, but a clog in the head passages would probably not stop the telltale in and of itself. You probably have a chunk of broken-off impeller blade jammed in the water tube where it enters the base of the block.

 

[dtfl74]

Re: Nissan 3.5 hp overheating

Thanks, Elvin ... see my reply to Paul regarding the condition of the impeller ... I'm stumped so far!:mad-new:

What Paul said plus: When you replaced the impeller, was the housing pitted where it rides? If it was, it could wear out the new impeller in a matter of seconds.

 

[dtfl74]

Re: Nissan 3.5 hp overheating

OK, one more bit of troubleshooting just completed. I connected a garden sprayer to the lower end of the water tube and started up the motor, heated water came out the telltale as well as the lower unit discharge, plenty of flow, engine ran great no overheating. So, given that and the fact that I verified that the lower unit water passage isn't blocked, I've determined that my water pump isn't pumping, and that the rest of the cooling system is just fine.

I also checked the rubber seal that fits the water tube into the lower unit water passage - it's not pitted or disintegrated, so it seems that there shouldn't be any leakage there ... but to be safe, is recommended to put some silicon sealant around the outside of the water tube seal?

At this point, I don't see why the water pump isn't pumping.

 

[TOHATSU GURU]

Re: Nissan 3.5 hp overheating

Then:

The housing or an obstruction in the cooling passages of the lower unit.

Caution

Is this an A model that sold new twenty years ago for $500.00. Be careful of the money pit. They were and still are a trouble free model, but twenty years of unknown corrosion on the internals could mean a pin hole in the water system that allows the engine to suck air essentially causing it to loose the ability to maintain pressure. Personally, I would look for an obstruction, if you don't find one, throw in a housing, if that doesn't fix it, it becomes a static display.

 

[dtfl74]

Re: Nissan 3.5 hp overheating

Elvin,

Thanks again. Yes, it's an A model.

Since water flows fine through the entire cooling system when I connected my garden sprayer to the bottom of the water tube (where it would normally fit into the lower unit via a rubber seal), and I've also verified there are no obstructions through the lower unit water passage, the problem is not an obstruction.

There is no part of the cooling system on this motor that is under suction except for the intake to the water pump, which is just a series of openings around the outside perimeter of the water pump housing. From the impeller thru the rest of the system, it's all pressurized.

Therefore I'll take your advice and order a new pump housing and see if that fixes the problem.

Duane

 

[pvanv]

Re: Nissan 3.5 hp overheating

If you want a housing, order 309S650101 WATER PUMP CASE $19.55, from any dealer, including me. Verify that you have the Factory impeller, instead of just an aftermarket unit. Also confirm that the drive pin is installed, and isn't damaged. As Elvin said, bewate dumping too much $$ into the old girl.

 

[Sea Rider]

Re: Nissan 3.5 hp overheating

If horse has hesitation peing issues at non idle rpm, remove lateral plastic cover, the one where peing hose exits, remove black hose and run same diam wire fully in/out several times. This smal tube tends to clog with salt if no or poor flushing is done to engine after use. Re torque drive shaft housing gasket lower bolts too for a tight gasket fit.

Happy Boating

 

[dtfl74]

Re: Nissan 3.5 hp overheating

OK, I finally figured out my problem!

The old water pump impeller didn't pump (no water from the telltale, and the motor overheated). Then when I replaced the water pump impeller and started the motor the first time, water came out the telltale for a moment or so, and then stopped ... and then the motor overheated so I shut it down.

When I removed the lower unit to verify that the water passage wasn't clogged (it wasn't) and replaced the lower unit, again, water came out the telltale for a moment or so, and then stopped ... and then the motor overheated again ... this really had me stumped!

Then the little light came on - the rubber seal that mates the water passage hole on top of the lower unit with the water tube, I remembered, just slides over the water tube ... what if my water pump made so much pressure that it was popping the seal up and along the water tube, allowing the water to bypass the water tube and flow into the drive shaft housing?

So I removed the lower unit again, saw that indeed the water tube seal had slipped upwards! So I replaced the seal and bedded it in the upper water passage hole on the lower unit with sealant ... then bolted the lower unit back on, and guess what! Yep, I had plenty of cooling flow, virtually jetting out of the telltale in a strong stream!

btw - what a crappy design! Nissan should have incorporated a retaining clip or other mechanism (maybe just a tighter fit?) to keep the lower water tube seal from riding up the tube. A two cent solution would have prevented a lot of headache!

Thanks guys for your input on this.

 

[pvanv]

Re: Nissan 3.5 hp overheating

It did fit well... 25 years ago. After a quarter century, might not hurt to have installed a new one.