I am working on a friends boat with a 496HO. It appears to be overhearing. With the scan tool hooked up it will jump from 160 to 193 degrees. I also checked temperatures with a heat gun and it is staying consistant when the temp gauge and scan tool is raising 30 degrees. Could this be a bad sensor and if so which one? Thanks in advance.

If the motor is really getting up to 193, the risers will be pretty HOT! My 496HO uusually ran 170-175ish.

What is your water pressure at idle? Should be around 3 psi.

Check the raw water pump, probably has grooved end plates that are dropping water pressure and causing the cooling system to overheat with low pressure and water flow at that low pressure.
If the water pump is bad , order the new stainless replacement from CP Performance, much better pump than Merc and less money too.

Best Regards,
Ray @ Raylar

The pump has just been rebuilt, end plates looked good. Has good wager pressure. The external temp of the manifolds, block, heads and lines does not change when the temp goes up and down. This tells me it just has a bad sensor.

Use a digital read out temp gun to verify. Water pressure 2 psi min. at idle and of course psi rises when throttle advances.

How many hours/seasons on the antifreeze? After 5-7 seasons or 300 hours, the antifreeze needs changed, as it will silt up. I posted about this a few years ago. Also check for water restrictions. Does it overheat on the hose or when running? I've seen them run fine on the hose, but overheat on the water. Check for restrictions in the drive/assembly, and also the T-fitting for fresh water flush.

That idle water pressure seems low. Mine is 5-7 as soon as I get water flow.

There is no restrictions the pressure is good and the anti freeze. I am trying to say I don't think the engine is overheating. I would like to know if the sending unit or sensor could be bad.

Your talking 2 different sensors going bad the same way at the same time. Pretty unlikely. There is one sensor for the analog gauge and one for the ECM(that the scan tool reads).

Bob Lloyd
Full Throttle Marine

I had a problem with one of my 496s overheating but it was under full load 1/2 throttle on up & found that my p/s & oil coolers were partially plugged from picking up junk on the missouri. I started reverse flushing the raw water side from the intercooler back & solved the problem. BTW the merc scan tool would not pick up anything even when running with the tool hooked up & the problem happening at the same time