merc 3.0 heats up normal then temp gauge bounces crazy fast to max and back to 180

[Alanvan69]

MY temp guage seems to be working fine as it reads stable on the flush mount and reads stable for the first few minutes when started but as it gradually warms up to full operating temp it then starts to bounce extremly fast between max and 180. If i turn of the key and turn it right back on to just monitor the guage it reads 180 solid no bounce what so ever. AS soon as its started again the irradict bouncing reappears. On the ear muffs I could not make it do it but went ahead and changed the temp sending unit anyways just in case and all this did was make it do it at 180 instead of aprox 160 not sure what this means at all either. Boat appears to run perfect even with this issue no cut out nothing. I am wondering if a thermostat could do this? Like could it only partial open and quickly close then the pressure quickly reopens it etc thus give me this very fast bouncing issue and only do it when its running? Again the problem only shows when its at operating temp. Please help

PS: Boat is a 2000 blue water 3.0 merc iron duke motor and motor and outdrive literally look new no corrosion anywhere it has been garaged keep its whole life.

Thanks
Alan

 

[Bondo]

Re: merc 3.0 heats up normal then temp gauge bounces crazy fast to max and back to 18

MY temp guage seems to be working fine as it reads stable on the flush mount and reads stable for the first few minutes when started but as it gradually warms up to full operating temp it then starts to bounce extremly fast between max and 180. If i turn of the key and turn it right back on to just monitor the guage it reads 180 solid no bounce what so ever. AS soon as its started again the irradict bouncing reappears. On the ear muffs I could not make it do it but went ahead and changed the temp sending unit anyways just in case and all this did was make it do it at 180 instead of aprox 160 not sure what this means at all either. Boat appears to run perfect even with this issue no cut out nothing. I am wondering if a thermostat could do this? Like could it only partial open and quickly close then the pressure quickly reopens it etc thus give me this very fast bouncing issue and only do it when its running? Again the problem only shows when its at operating temp. Please help

PS: Boat is a 2000 blue water 3.0 merc iron duke motor and motor and outdrive literally look new no corrosion anywhere it has been garaged keep its whole life.

Thanks
Alan

Ayuh,.... Welcome Aboard,... It's overheatin',.... it needs a New Impeller....

The gauge might be a groundin' issue, donno...

'n it Ain't the Iron Duke...

 

[Alanvan69]

Re: merc 3.0 heats up normal then temp gauge bounces crazy fast to max and back to 18

U say its over heatin it needs a new impeller? The u say the guage might be a ground issue? Its not overheatin and why a ground issue only when it gets to temp and no bounce before. This is the clue right there and is what is wrong but what would cause that? Ground would cause it intermittent at any temp even while heating up would it not?

 

[Alanvan69]

Re: merc 3.0 heats up normal then temp gauge bounces crazy fast to max and back to 18

I'm still having same problem bondo ur fix makes no since as u never really gave me a fix. Anyone else have any input? Anyone else have a 3.0 and aprox what temp do u run at? Is it possible that when thermostat opens then ground is broken somehow?

 

[Bondo]

Re: merc 3.0 heats up normal then temp gauge bounces crazy fast to max and back to 18

Ayuh,.... Runnin' 180? is Too Hot,... ya should be seein' 160? or so,.... Change the impeller....

The gauge is electrical,... the motor can't heat, 'n cool that quickly, so check the Grounds at the sending unit, 'n at the gauge...

 

[Alanvan69]

Re: merc 3.0 heats up normal then temp gauge bounces crazy fast to max and back to 18

Ayuh,.... Runnin' 180? is Too Hot,... ya should be seein' 160? or so,.... Change the impeller....

The gauge is electrical,... the motor can't heat, 'n cool that quickly, so check the Grounds at the sending unit, 'n at the gauge...

Actually bondo it was running at 160 stable then bouncing wildly once heated up and only once heated up to 160 before I changed the sending unit. Now it does the same thing but does it at 180. I have checked the ground at the gauge and its tight and there is no corrozion anywhere either on the guage or on the engine. The sending unit i installed was from a car I guess as we got it at the auto parts store.

A ground how can this be explained with no bounce heating up nice and slowly as one would expect and then as soon as it hits its operating temp then the bouncing starts. A ground being bad would be bad all the time unless the ground has something to do with the thermostat when it opens as this and only this is when it starts to happen. SO if this is the case then what ground would be associatted with it when its hot. I agree it sounds electricall but it must have something to do with when it gets hot. Again if i turn off the motor and turn the key off and back on its totally stable now at 180 with the brand new sending unit instead of 160. I have read other post where people are saying there 3.0 merc are running at 180 since brand spanking new. I to would expect closer to 160 and I did read 160 untill we changed the sending unit. I'm not disagreeing with u at all as I think its electrical but how could a ground go a stray only when at operating temp? Is it at all possible that I've read there is ground groments inside the thermostat housing and that when hot do to flow of water they are are now broken?

Thanks
Alan

 

[CheapboatKev]

Re: merc 3.0 heats up normal then temp gauge bounces crazy fast to max and back to 18

Get the proper marine t stat that your 4 banger calls for first of all.

 

[Howard Sterndrive]

Re: merc 3.0 heats up normal then temp gauge bounces crazy fast to max and back to 18

The sending unit i installed was from a car I guess as we got it at the auto parts store.

they aren't universal. The gauge and the sender are a matched set. You need the sender that matches your gauge.
Senders do fail.
Also possible your gauge has failed. There is circuitry in most (resistor and/or cap) that actually dampens the needle motion and prevents them bouncing too wildly.

However, it sounds alot like your sender wire is just simply rubbing metal somewhere between the sender and the gauge.
The symptom is the same for all 3 scenarios.

how can this be explained with no bounce heating up nice and slowly as one would expect and then as soon as it hits its operating temp then the bouncing starts.

it's ohm's law, the sender is NTC (negative temperature coefficient), when it's cold, the resistance of the sender is high. Your incorrect or faulty sender is coincidentally sharing some of the correct range with the correct or functioning sender. Then as it heats up and the resistance of the incorrect sender drops below the range you gauge can deal with, the gauge starts bouncing, same as the gauge will do if you touch the sender wire to ground.

Get the proper sender and pray the gauge survived all that bouncing.

 

[Don S]

Re: merc 3.0 heats up normal then temp gauge bounces crazy fast to max and back to 18

I have read other post where people are saying there 3.0 merc are running at 180 since brand spanking new.

If you look hard enough, you can probably find one that says their 3.0L runs at 190? sense new, but that doesn't make it right.
What year engine do you have? If you have one with a 160? thermostat, they run at 170 to 175 under load, when then sneak up to 180 there is a problem.
What you need to do is get an IR temp gun and aim it right next to the temp sending unit on the engine while running and see what the temperature is actually doing compared to what the gauge reads. Then you will know where to look at for repairs.