496HO overheating
 

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

Can You Spell Strainer!
 

Randy's post is a good one as the scan tool will not show if the raw side of the cooling system is clogged or restricted and the system will only alarm or guardian if the engine actually starts overheating.
I am always kind of amazed how many boaters raw cooling systems are getting clogged or blocked with debris because they do not have a raw water strainer. It should be obvious and I believe every boat that boats in any water with debris must have a raw water strainer and anyone boating without one is just setting them selves up for problems.

C'mon guys get your boats properly equipped and stop punishing your wallet and boating expierences!

Best Regards,
Ray @ Raylar

Just a note to mr Raylar this is the 1st boat with dual water pickups I have ran on this water & it only happened on 1 side but I did buy some strainers I just have to mount them. Hopefully that will be when I pull them out & have you upgrade them to your 600s

I have a 2002 model 496mag ho (ss risers and aluminum exhaust manifolds from the factory). My engine went into guardian mode last week after about 15 minutes or so of boating. Put the scan tool on it and starboard riser temp was 149 degrees and port 179 degrees! Took off the hose at the bottom of each manifold rail (mine was brass from the factory). Completely corroded from dissimilar metals, check ball locked in solid! Found out that Mercury is aware of the problem and has a cast manifold conversion kit with new sensors, manifold gasket, turbulator gasket, and longer hoses, and the brass part now is plastic! Running fine now! I too thought grooved impeller! NOT! I have a SS Hardin Marine impeller now, made no change, it as the damn stoppage in the brass fittings on each side! You can also check where the hose comes in from the transom-mine stopped up there too about 5 years ago! Hope this helps. I have seen this exhaust manifold issue now by the way a dozen times at various mechanics shops! (thankfully I know nobody with an issue because of a sea strainer but always an extra precaution for sure-but it won't stop the sand putting the grooves inside your brass impeller!).

p.s. I flush my engine 24/7 immediately after coming out of water (dawn dish liquid/salt away, whatever). Never ever left in whatever overnight! It will happen!