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Why does temp gauge bounce around?

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Old 03-27-2008, 03:25 PM   #1
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Default Why does temp gauge bounce around?

Anyone have any ideas why my temperature gauge would bounce around. If I have a cold engine it will slowly warm up on the gauge. But if I shut the engine off then restart within half an hour, the gauge shows the engine overheating. If I drive around 3-10 minutes, all of a sudden the gauge will pop down instantly to regular temps. The other day it popped from regular to overheating, back down to regular. I replaced the water pump recently and the problem did not go away. The coolant is full. Is it the thermostat?

Its the 5.0 1995.
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Old 03-27-2008, 05:30 PM   #2
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Given the erratic way in which you describe the movements - suggest to investigate the coolant temperature sending unit, the dash gauge, or the wiring in between - my initial bets would be on the sending unit.

However, air in the system can cause similar results - be absolutely sure the system is filled properly, don't just rely on the overflow tank level.

Not sure how things were being done for 1995 - on mine, there are two coolant temperature sending units - one for the dash gauge, the second for the computer. The dash gauge unit is relatively small, with a one-wire lead, reasonably near the upper radiator hose outlet.
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Old 03-27-2008, 10:34 PM   #3
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givin your problem and my recent experiences.... 99.9% points to the thermostat.

procedure. take off old, put on new, drive, and love the best selling truck for 19 years straight.
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Old 03-28-2008, 01:54 PM   #4
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OK. Where is the thermostat? Does the coolant need to be drained to replace it? I guess I should invest in a Haynes manual for my truck, too.
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Old 03-28-2008, 03:04 PM   #5
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Reread the post by wde3477. I'm new to the site, but he is not. All posts by him are usually right on target. No one fix, is the absolute cure-all for all occasions. With just doing the water pump, you could be low on coolant. Check the level in the radiator when cold. Remove cap, coolant should be right there or down an inch. If it is full then start looking for the intermittent electrical problem. Remember, when you do get it fixed, tell us. Enjoy!
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Old 03-28-2008, 07:43 PM   #6
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I would try to replace the sending unit as wde3477 suggested,they are pretty cheap and fairly easy to get to, if that doesnt fix it then i would replace the thermostat.Be careful when changing the thermostat the bolts love to break off,loosen the bolts if they are hard to loosen,turn back and forth with penetrating oil.Usually they snap off when trying to brake loose if they are going to brake off.
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Old 03-28-2008, 09:38 PM   #7
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Thanks for the votes of confidence given above. At the risk of messing all that up by revealing what I really do know, I'll try to further explain -

The reason I don't think that the thermostat is the problem relates to the following clips from the original post. Not to say the thermostat can't be the problem, it's just not on my current top list of suspects.

Quote:
Originally Posted by bellj View Post
all of a sudden the gauge will pop down instantly to regular temps.
If the thermostat were malfunctioning, could understand a rapid fall from a much higher-than-normal temperature to a lower temperature - but nothing close to my interpretation of the speed of 'all of a sudden' and 'instantly'.

Quote:
Originally Posted by bellj View Post
The other day it popped from regular to overheating
Could not envision any way a thermostat could malfunction that would cause a 'pop' in the upward direction.


Well - for those reasons, and then there was this time back when my temperature gauge was doing similar things - it turned out to be the coolant temperature sensor. :-)~
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Old 03-29-2008, 12:44 PM   #8
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idk, i was just saying whats happenned to me on both my old 5.0's and the thermostat fixed it. and he described what the problem was.
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Old 04-17-2008, 03:55 AM   #9
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i have a 1995 f150 5.8L with 198K and had the EXACT same issue!! i changed my thermostat today and the problem was immeadiatly solved! it was the original one too LOL ... it took just over and hour to change it ... easy at home fix

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