Nissan 9.8 overheating

[Surfingmule]

Hi all,
I'm new to the forum. I just aquired a 1998 Nissan 9.8 2 stroke and it seems to run well except that the engine block gets quite hot after running longer than 5 min. I can keep my fingers on it for about 2-3 seconds before i have to pull away. The pee hole has a healthy stream and the water exiting it is cool to the touch. Don't know if it matters but the power head does have some corrosion and paint bubbles at the gasket. Any thoughts...?

Jake

 

[AlanR]

Re: Nissan 9.8 overheating

Just for kicks and giggles what ratio of fuel to oil are you running?

And How do you know its over heating...? Seems to me if its Moving water I wouldnt see why it would over heat...?

 

[pvanv]

Re: Nissan 9.8 overheating

If in fact the motor is running too hot...

You could have some corrosion blockage in the cooling system, especially if the motor has run in salt, and not been flushed regularly -- could even be a stopped-up thermostat. In order to change the stat on that model, you would pull the cylinder head (you would want a new gasket and new thermostat handy for reassembly)... A Factory service manual will describe the torque values and tightening sequence of the head bolts.

The pee tube indicates that the water pump is working and pressurizing the cooling system gallery in the base of the powerhead. No more, no less.

You could try flushing the cooling system using the flushing plug. Failing that, you could flush with a mild acid such as vinegar.

 

[Surfingmule]

Re: Nissan 9.8 overheating

Hey guys,

Thanks for the responses. I am running the required 50:1 oil ratio. I think what I need to first determine is if it is actually getting too hot. My concern stems from reading some threads about this engine and people discussing the idea that if the rear of the power head gets too hot to keep your hand on it, it is overheating. Is that correct?

Not quite sure how to determine if it is overheating without buying a I/R heat sensor. Pee tube is flowing healthy. Rear upper exhaust has some but minimal water flow (mostly exhaust) and that water is cool until I throttle up, then that water gets hot. I am guessing that is the point where the thermostat is opening and letting more water in. On other outboards I've owned, the rear upper exhaust puts out much more water than this engine.

I'd really like to avoid pulling the head for a thermostat replacement. I'm currently giving it a flushing cycle with Salt-Away for the next 6 days. If that stuff works, theoretically I should feel a lower engine temp with my hand. Any ideas to determine if it is actually overheating?.

 

[Surfingmule]

Re: Nissan 9.8 overheating

Also, I did see the flushing plug, but I cannot get the screw off. It is too tight and attempts to turn it with a flat head screwdriver are resulting in it getting stripped.

 

[pvanv]

Re: Nissan 9.8 overheating

Also, I did see the flushing plug, but I cannot get the screw off. It is too tight and attempts to turn it with a flat head screwdriver are resulting in it getting stripped.

You need a correct screwdriver tip. It's surprisingly large. If still frozen, a hammer-activated (such as Vessel brand) impact driver usually works... unless the head slot is so mangled that nothing will bite.

 

[pvanv]

Re: Nissan 9.8 overheating

Hey guys,

Thanks for the responses. I am running the required 50:1 oil ratio. I think what I need to first determine is if it is actually getting too hot. My concern stems from reading some threads about this engine and people discussing the idea that if the rear of the power head gets too hot to keep your hand on it, it is overheating. Is that correct?

Not quite sure how to determine if it is overheating without buying a I/R heat sensor. Pee tube is flowing healthy. Rear upper exhaust has some but minimal water flow (mostly exhaust) and that water is cool until I throttle up, then that water gets hot. I am guessing that is the point where the thermostat is opening and letting more water in. On other outboards I've owned, the rear upper exhaust puts out much more water than this engine.

I'd really like to avoid pulling the head for a thermostat replacement. I'm currently giving it a flushing cycle with Salt-Away for the next 6 days. If that stuff works, theoretically I should feel a lower engine temp with my hand. Any ideas to determine if it is actually overheating?.

You can get a good idea of motor condition, including temperature, by reading the plugs. Start with fresh plugs, put the motor through its paces, stop, remove plugs, and carefully inspect their color and condition. It's an old-school technique, but still very valid.

To get actual degree numbers from the outside of the head... Short of an IR thermometer... Check with an old-time OMC shop... One that remembers how they used to do things 40 years ago... They used to use special crayons that had different melting points. You would need 2 or 3 different crayons (of different ratings). The technique was to make crayon lines on the area you wanted to test for temp. The colder one would melt, indicating you exceeded that temp. The hotter-rated one would not melt, indicating you had not achieved that temp. The actual temp was between the 2 extremes. If everything melted, you were overheating.