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350 MAG MPI Temperature Issue

A

abl1111

Regular Contributor
"I have a 2003, 350 freshwater

"I have a 2003, 350 freshwater cooled motor w/ 280 hours - still under extended warranty. Bravo 3 drive and cooled via SouthBay strainer.

Temp gauge went to HIGH a few weeks back. Stopped motor, called mechanic - he said if no alarm - no problem. Motor runs fine. I run usually at 3200 rpm's...

Coolant was 3/4 gal low. Oil, was 3/4 quart low. Added both - and changed the temp sending unit and the gauge.

Sometimes, she will run perfectly but at some point within 30 minutes, she will read High.

Used infrared at thermostat/sending unit - at 165'ish degrees @ idle. Bypassed harness via a jumper from sender to gauge - still HIGH.

Ran cold hose water over sending unit, temp. gauge lowered a bit from High - then rose when water removed.

I'm all over the place with troubleshooting; bad thermostat, check compression, head gasket, pressure test cooling system, manifold blockage, etc..

What are your thoughts ? Thx"
 
[b]Used infrared at thermostat

Used infrared at thermostat/sending unit - at 165'ish degrees @ idle. Bypassed harness via a jumper from sender to gauge - still HIGH.

Could be your sender is bad.
 
"and changed the temp send

"and changed the temp sending unit and the gauge."
/I missed that part.
//"Still high"????
/////How high?165'ish degrees @ idle.
(are you sure you changed the sender and not the switch?) I just guessing now.
 
"Sender was changed. I don&#3

"Sender was changed. I don't know if there is a switch - what switch ?

I have bypassed everything and run a gauge directly to the sender and it reads HOT!

Unless I am missing something - I'm leaning toward the idea that the engine is running hot.

I'm wondering if it could be the saltwater impeller ? Or, a thermostat ? I am at a loss. BTW: the engine hours are more like 340.."
 
"Sender was changed."

"Sender was changed."

Did you put anything on the threads? Install w/o sealant on threads.
 
The sender came w/ the OEM &#3

The sender came w/ the OEM 'red stuff' on the threads. I left that on and added nothing else.

Original sender must have had that on it too and it worked well for 4 years.

Should I remove it ?
 
"Yes, at the temp sending unit

"Yes, at the temp sending unit area - I used and IR and it read 165 ish degrees.

But, it's hard to believe (2) different gauges, and (2) different sending units are wrong. I had bypassed (1) gauge and sending unit completely from the engine via jumper wires - and the result showed HOT on the gauge.

The manifolds do feel hot to me though.

?"
 
What does the IR gun show for

What does the IR gun show for the manifolds and risers?
Normally the manifolds should run 180-190 and the risers cooler at 90-130 or so.
The risers should feel warm to the touch but you should be able to keep your hand on them.

Rod
 
"Yesterday, I borrowed my mech

"Yesterday, I borrowed my mechanic's Mercury Diagnostic Computer that plugs right into my boat. Talk about a nice gadget ( sounds like what the SmartCraft gauge would do ).

According to the diagnostics, the boat temp is on the money - 163-165 degrees, even at WOT. My gauge must be bad - even the replacement gauge !

To further confirm, I then borrowed a buddy's spare temp gauge, a Faria brand w/ actual temp. degrees, and hooked it up - shows me at 175 degrees from the sending unit.

Infrared thermometer showed same at sending unit. And, at thermostat, 1.5" lower than sender, it was 165-167 degrees. The risers were very cool - I forget what the manifolds were...

I was going to do a raw water pump R/R -- it's been (4) years - and I know I'm over-due, but I'll wait for the Fall pullout. It seems like a bear of a job ! Removing the hoses from the pump hosing looks like a knuckle-breaker ( blood will be spilled ).

I'm going to get another gauge. I would rather have one that has temp #'s on the fact as opposed to "Cool, Medium, Hot" marks.

I 'think' this case is closed.

Just goes to show you, no matter how careful you are, sometimes it's something stupid.

Anyone know the cheapest place to get the Mercruiser shop manual ?

Thank you everybody. You all gave me a lot of ideas to think about. I do hope this was the problem...

"
 
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