Temperature gauge spikes for few seconds - sending unit bad?

[XH29N0G]

Driving two days ago and then yesterday, my temperature gauge spiked on three occasions.  It was stable at 200 and then took about 1/2 second to rise to max reading, stayed there for one to five seconds, and then descended back to 200 slowly (1-2 second to drop). 

None of the other gauges moved.

I do not see how the unit could heat and cool on those timescales so suspect the sending unit. 

Does this make sense?  Is it the way sending units fail? 

If I replace it, are there any special recommendations on sending units?

Thanks.

[Pete in NH]

It could be a sending unit problem but, I think I would suspect an intermittent short circuit somewhere along the wiring path first. It's  easy enough to change out the sending unit as a starting point and try that first. but, I would not be surprised if it was not the issue. It could also be a problem with the gauge itself.

[Nacho-RT74]

I'm agree with Pete about the wiring deal

[XH29N0G]

Pete and Nacho,

Thank you.  Can you explain the basic principle of how the sensor/gauge system in these cars works?    My vision is similar to the gas gauge, but where the resistance is modulated by the sending unit. 

By this model, an intermittent short makes sense as a way to generate a spike and I see your point about it not likely being the sending unit.  I will try the sending unit first since it is (relatively) easy, but are there any obvious places to look that might show me an intermittent short or gauge problem?

Thanks

[firefighter3931]

It could also be an air pocket in the cooling system as well. What happens is that the air pocket will move around and when the pocket exposes the temp sensor the guage will spike for a brief period until the pocket moves and the sensor gets covered by coolant again.

To fix/check for an air pocket is simple ; get the car up on ramps with the engine cool enough to remove the rad cap. Start it up and allow the thermostat to open allowing circulation. Air will rise to the highest point and if there is an air pocket you will see a "burp" and the coolant level will drop. Add coolant as required.  


Ron

[Nacho-RT74]

exactly the same than the Gas tank gauge and sender, modulating ground. Just internally the sender is somehow different ( obviouslly ) but all Senders use same basic principle.

[Nacho-RT74]

air pocket... mmm it could be too... not common it happens but posible.

Aren't Tstats with a small pin hole to allow the air go out ? ( bleed )

[Dino]

It could also be an air pocket in the cooling system as well. What happens is that the air pocket will move around and when the pocket exposes the temp sensor the guage will spike for a brief period until the pocket moves and the sensor gets covered by coolant again.

To fix/check for an air pocket is simple ; get the car up on ramps with the engine cool enough to remove the rad cap. Start it up and allow the thermostat to open allowing circulation. Air will rise to the highest point and if there is an air pocket you will see a "burp" and the coolant level will drop. Add coolant as required.  


Ron

Oh so that's how you burp a radiator.  I had 6 beers with mine the other day waiting for it to happen but I was the only one burping.   

When I had my car running a few days ago I was revving it a bit until I heard some sizzling noises.  I forgot to put the cap back on and made a bit of a mess.  Oops

[XH29N0G]

OK, I'll burp the radiator first (there could be an air bubble - the temperature change seems too much to me, but I will try), then if it repeats, will try the sender, and then continue on down the list. 

Thanks for the suggestions.

[firefighter3931]

OK, I'll burp the radiator first (there could be an air bubble - the temperature change seems too much to me, but I will try), then if it repeats, will try the sender, and then continue on down the list. 

Thanks for the suggestions.

You'd be surprised how much it can spike.  Steam is much hotter than coolant....when an air pocket hits the sending unit you are hitting it with very hot air. I've seen what you've described happen before and I fixed it by bleeding out the cooling system 



Ron

[68charger440]

I had my temp gauge intermittantly spike on me about 10 years ago  just as you describe.  For me the wire from the sensor had rubbed bare and was intermittently shorting on the alternator housing!

[XH29N0G]

Well, I definitely could have a bubble.  The coolant is pretty full, but some was lost a week ago after it heated up in traffic.  I will top it to see what happens.  That would be the best fix.

And - The heating rates could also be very different from what I thought they should be.  I am always finding out that things don't conform to my preconceptions - no matter how much time I spend trying to think things out.

I will also inspect for shorts.

[XH29N0G]

An update.  I filled the radiator (too full at first) so it overflowed, but so far, I have not seen the temperature spikes reappear.  I have driven it for two weeks now, not a whole lot, and it has not been as hot as before, but it looks like the air bubble hypothesis may have been correct.

Thanks.