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Hi Everyone,

I'm not an auto mechanic, but I am TRYING to learn everything about my car that I can fix or maintain. I have a shop manual and a Haynes repair book for my 1996 Thunderbird 4.6L V8.

My current challenge is the dash temperature gauge maxing out (and tripping the Check Gauges light). The gauge goes to max, and either stays there, or after 5 to 10 minutes drops very quickly to half or two-thirds range. When the needle moves, it fully moves in seconds! I've connected a code scanner to the ECM and the internal engine temp shows between 175 - 215 F while I drive it this way. I don't think this temp is abnormal right? The fan radiator seems to run when it should.

THIS GAUGE ISSUE BEGAN right after a mechanic told me to drive the car home with the coolant reservoir cap loose and let it sit overnight before tightening. He had put some silicone around the plug wells as a VERY short term fix due to a leaky intake manifold causing misfire on cylinder 4. (I've read this is a common misfire issue.)

I quit driving the car when the gauge acted up. I took it to another mechanic shop for new intake manifold, thermostat, plugs, and plug wires. The gauge continues to max out or drop suddenly. The shop thinks it is the gauge itself because they changed the sending unit and it still has the issue. They say they spent hours of unbilled time on it and so I told them to leave it.

Any ideas would be helpful and greatly appreciated.

Thank You,

Racingmustang
 

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I have never seen a temp gauge fail in such a way that it intermittently pegs the gauge. My first thought would be that it is low on coolant. I would hope the shop would check that prior to spending so much time trying to diagnose it, but I have seen more obvious things missed by supposedly trained professional mechanics.
 

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Your mechanic had you leave the cap off so it wouldnt pressurize when it got up to temp and spray out of the leaky intake manifold. Still bad advice, most shops would have recommended you leave the car overnight or have it towed home - not driven, that would have been a liability issue.

As for your gauge issue - that sounds like a bad electrical connection, more than likely in the wiring or the harness connector ends. If you ground the wire that goes to the gauge it will peg out quickly. A low coolant condition wouldnt be so fastand wouldnt drop back down.
 

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Check your coolant level.
 

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Coolant level most likely low or air is in the system. When air is in the system it creates a small pocket or bubble which creates hot spots. So when that bubble passes by your temp sensors, the sensor sees that it's really hot so your guage goes up, once it passes the air bubble, it goes back down.
 

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Discussion Starter #6
Hi,

Thanks to everyone who replied so far. The most recent shop suggested it might be air getting into the system from the leaky manifold. After they installed the new one I made sure the coolant stayed full and it is not leaking.

They also suggested a bad ground somewhere but didn't find it. They reworked and improved my battery wires a lot while chasing this. I'll continue to look for wiring issues.

I'm wondering if my coolant has gotten fouled up. I'm also wondering if some of that silicone might have gotten sucked into a crack in the system before it set up. Is it likely anything would have happened to my water pump or radiator? Still the ECM seems to show normal temperature.

Maybe I will learn to drain and fill the coolant system while I'm at it.

Racingmustang
 

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Coolant level most likely low or air is in the system. When air is in the system it creates a small pocket or bubble which creates hot spots. So when that bubble passes by your temp sensors, the sensor sees that it's really hot so your guage goes up, once it passes the air bubble, it goes back down.
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Its not the air that gets superheated .. its the lack of coolant circulation not being able to draw away heat from the engine that causes hot spots.

This magic bubble theory is great, but there is no super heated gauge pegging bubble passing by the sensor. The coolant is a far more efficient thermal conductor than air, it would quench that bubble quickly, but it would not be able to draw heat from a passage with no circulation.
 

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its the sending unit. I don't know why people aren't saying that, but it has to be that. Air in the system would not present like that, it would more have constant up and down near the normal range. If your system was getting air other issues would be happening to. I feel like corrosion on the connector would cause the gauge to drop into the cold range randomly, not hot range.

Do the sensor, its easy and cheap.
 

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I thought I had read the sending unit was replaced? If not, it boggles the mind that this slipped past the mechanic(s) that have been involved so far.

These cars are not known to be difficult to "burp" the cooling system, and I have driven with the cap purposely loose with no ill side effects.

Al
 

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Coolant level most likely low or air is in the system. When air is in the system it creates a small pocket or bubble which creates hot spots. So when that bubble passes by your temp sensors, the sensor sees that it's really hot so your guage goes up, once it passes the air bubble, it goes back down.
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Its not the air that gets superheated .. its the lack of coolant circulation not being able to draw away heat from the engine that causes hot spots.

This magic bubble theory is great, but there is no super heated gauge pegging bubble passing by the sensor. The coolant is a far more efficient thermal conductor than air, it would quench that bubble quickly, but it would not be able to draw heat from a passage with no circulation.
Thanks for correcting me. I didn't know exactly how to word it so I put it in laymans term.
 

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I didn't see anything about the sending unit being done. You would be amazed the level of stupidity the runs a muck in the fixing car and truck business.
 

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He said they did the sender in the first post, but I agree. I would be suspicious of an intermittent short to ground on the wire from the sender to the gauge. Next time the gauge is pegged, check resistance to ground on that circuit with the sender unplugged. It should be near infinite. If it's only grounded when the sender is plugged in, it could be a intermittently failing sender. All it takes is someone being careless during installation to bend on the plastic part sticking out the top.
 
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