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Overheating Nissan 70C

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cottonman

New member
This is a long read, but please bear with me.

I have been fighting an overheating problem for 2 years now. The following example is a typical scenario of my Nissan 70C outboard overheating:

I am running down the ICW on plane about 4000-4500 rpms, 30mph. I suddenly get an overheating alarm which causes my outboard to return to idle. Temps on all cylinders read 145-150 degrees with an infrared heat sensor and the motor is peeing great. Temp of the water coming out of the cooling water check port is no hotter than typical bathwater. After about 1 minute the alarm quits sounding and I continue on my way. Any time that I am on plane and running above 4,000 rpm's, I can expect this occurrence. Any time that I am trolling, or running below 4,000 rpm's, I never get the alarm.

I have had the impeller, water pump, t-stat, head gaskets, overheating alarm, and about anything else you can think of replaced to solve this problem without success. I have used three different reputable mechanics work on this problem as well as a technician from the Nissan marine manufacturing company.

After the first mechanic replace all the parts for the cooling system without solving the problem, he thought the problem could be salt intrusion. The second mechanic agreed, but thought that it could be a blown head gasket, so he replaced it....still overheating. The third mechanic said the first two did all he knew how to do and that they didn't rip me a new one. He took the boat out several times and got the same results. He agreed that the problem was probably salt intrusion.

I bought this boat used, and I have maintained my outboard and flushed with fresh water after every trip. For the last year I have used a vinegar flush at least on 10 different occasions. I have also pumped the vinegar through the cooling system and removed a lot of sand/mineral/salt build up to no avail.

Last week I downloaded a user manual for the Nissan 70C and have read over it. On the first couple of pages are installation instructions. In bold it states CAUTION: overheating may result due to lack of water inflow from water intake port if the anti-ventilation plate is at a level higher than the bottom of the boat. It also states that there needs to be between 10mm and 30mm difference between the anti ventilation plate and the bottom of the boat...... I think well why not go and check that.

Amazingly, I find that the anti ventilation plate is even with the bottom of the boat when the motor is tilted down at max tilt. When the motor is level as it would be on plane, the anti ventilation plate is slightly higher (1/4-1/2") than the bottom of the boat.

My questions:
Could this overheating problem have been an outboard installation error this entire time?
Would this anti ventilation plate height cause overheating - the manual says it will, but in reality will it?
I have adjustments where I can raise/lower the motor - how hard is it to adjust a 400lb outboard?
Would anyone recommend I do this myself or do I need a professional to move the motor?

thanks for any responses.
 
Yes, if the motor is too high, you could suck air at speed. That might well cause an overheat. That applies to any motor of any make. The "rule of thumb" is to have the plate within an inch of the bottom of the boat when trimmed correctly. It may well have been rigged a little high from day one. The downside to having it too low is excess drag, resulting in less speed. We usually shoot for that 1-inch mark. Can you photograph the transom area at speed? The water should be flowing at the antiventilation plate, not below it.

You will want help if you are going to use alternate holes in the motor clamp bracket change the jack height. You will also want some sealant to close off any holes in the transom, to prevent water intrusion to the transom -- especially if it's wood, which will rot if allowed to waterlog. If in doubt, see your dealer.
 
Thanks for the quick response. My aim is to take the boat out soon and see what is happening at the anti-ventilation plate at speed. I'll try to take some pics and post them. thanks again.
 
When the motor is level as it would be on plane, the anti ventilation plate is slightly higher (1/4-1/2") than the bottom of the boat.

I bet you measured that by trimming the motor ona trailer and measuring. On plane the vent plate will remain in the same relative position to the water when the motor is trimmed at speed, to the point porpoising or ventilation occurs. It's water intake that Nissan is concerned with when they make a blanket statement that requires mounting within a half inch.

Has anyone actually confirmed an over-heat? Checked the sensor/sender? :confused:
 
Curmudgeon,

Thanks for the reply. Yes, I'm measuring on a trailer, and I haven't thought about the vent plate remaining in the same position to the water while traveling, but it makes sense.
For your second point, no one has actually confirmed an over-heat, but one mechanic thought it could be a bad sensor, so it was replaced.

How would I confirm an over-heat? measure water temp coming out of the check port? I don't have a water temp gauge, but I have checked head temps. I purchased an infra-red heat sensor and used it several times when the alarm sounded. Every time the alarm sounded I immediately pulled the cowl and took temp readings of the #1 and #2 heads, and area where the over-heating alarm is located (I couldn't reach #3). All the temps were in a range from 145-150 degrees (maybe a few readings were 152) and the water coming out of the check port is near bathwater temps. My take on this is that the metal temps at the heads should be fairly hot since they are not water cooled and the internal temps should be lower since they are being water cooled....is this right?

thanks for the responses.
 
cottonman said:
Curmudgeon,

Thanks for the reply. Yes, I'm measuring on a trailer, and I haven't thought about the vent plate remaining in the same position to the water while traveling, but it makes sense.
For your second point, no one has actually confirmed an over-heat, but one mechanic thought it could be a bad sensor, so it was replaced.

How would I confirm an over-heat? measure water temp coming out of the check port? I don't have a water temp gauge, but I have checked head temps. I purchased an infra-red heat sensor and used it several times when the alarm sounded. Every time the alarm sounded I immediately pulled the cowl and took temp readings of the #1 and #2 heads, and area where the over-heating alarm is located (I couldn't reach #3). All the temps were in a range from 145-150 degrees (maybe a few readings were 152) and the water coming out of the check port is near bathwater temps. My take on this is that the metal temps at the heads should be fairly hot since they are not water cooled and the internal temps should be lower since they are being water cooled....is this right?

thanks for the responses.
Click to expand...


Hello did you every figure out your problem my 1998 70c is doing the same thing new impeller with no change in alarm
 
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