Nissan 2.5 hp cooling poor only at high rpms

[fortunate2001]

I have a small 2.5 (forward only) outboard. It starts and runs fine, but at anything over half throttle, the tell-tale pee slows to a drip or stops, and the engine gets too hot.

A mechanic I know thought there might be something (like a piece of impeller) in the cooling water path that is clogging flow, especially so when high rpms put more pressure on the blockage. He said I should drop the lower unit and try flushing out the cooling system backwards, using water or perhaps compressed air to clear the blockage.

I am fairly handy and willing to try, but am stumped by the first question: How do I drop the lower unit? Does this mean unbolt at the bottom near the impeller, or up at the top under the cylinder? Since this is a forward only engine, does that make it easier or harder to reassemble... Is there a gasket kit or other supplies I should get before starting?

Any advice would be greatly appreciated.

 

[pvanv]

Re: Nissan 2.5 hp cooling poor only at high rpms

What precise model? 2 or 4 stroke? The early A models have the impeller on the prop shaft -- easy to change without dropping the LU. Pump for the B models is on top of the LU, and is accessed by dropping the LU. Typically some debris can clog the telltale (only) but if so, the motor will still cool OK. That's normally cleared by applying compressed air (or a stiff wire) to the telltale while the motor is running.

These motors do run warm, thankfully.

In any event, get a water pump kit (or at least an impeller), as the impeller is normal scheduled maintenance every couple of years. While you have it apart, compressed air can help you confirm whether the cooling passages are clear.

 

[fortunate2001]

Re: Drop lower unit help - Nissan 2.5 hp cooling poor only at high rpms

Re: Drop lower unit help - Nissan 2.5 hp cooling poor only at high rpms

Thanks for the reply.

The impeller is located just behind the prop, so I guess I have a 2.5A. Sorry I failed to mention this is a 2-stroke, and run in salt water.

I have already changed the impeller twice. I did it the first time, and when the cooling problem remained, I took it to a local shop for a deeper look. Unfortunately they did the same thing (changed impeller), and told me it was all set, but still no flow at higher RPMs. (Since gone out of business).

Without writing a novellette... What is the procedure for dropping the lower unit?

 

[mmccoy555]

Re: Nissan 2.5 hp cooling poor only at high rpms

There are only two 10mm bolts on the lower unit to drop it down. Most of the time the clog will be up in the powerhead. Spray something like Mercury Powertune then compressed air through the **** tube at the top. If there is any salt build up that should dissolve it. almost 100% of the time on a saltwater engine that size the head gets clogged with salt. I would try that first before dropping the lower unit.

 

[fortunate2001]

Re: Nissan 2.5 hp cooling poor only at high rpms

There are only two 10mm bolts on the lower unit to drop it down. Most of the time the clog will be up in the powerhead. Spray something like Mercury Powertune then compressed air through the **** tube at the top. If there is any salt build up that should dissolve it. almost 100% of the time on a saltwater engine that size the head gets clogged with salt. I would try that first before dropping the lower unit.

I loosened those two 10mm bolts, but the LU did not drop at all. (Just loose now to wiggle back and forth). I did not want to tug on it too much. Do I need to disconnect anything in the drive train first? Since this is an always on in FWD config, I figure something has to give...

 

[fortunate2001]

Re: Nissan 2.5 hp cooling poor only at high rpms

Ha ha- never mind, I jiggled it today and the lower unit dropped down into my hand. I think if I had left it any longer I would have found it on the ground...