truck overheating
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Senior User
Joined: Apr 2003
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truck overheating
ok, heres the deal.... i have a 1988 ford bronco with 5.0L. everything under the hood is new, as of a year ago. well, my truck is overheating. my temp gauge will go up slowly, and steadily. and then it will drop rather quickly. it wont actually 'overheat', but the gauge will flirt with the red zone. i check my level and it was down. so i filled it up and it worked fine, at first. i dont have any leaks. and my truck keeps doin this. so i tried changing my thermostat. no difference. its the weirdest thing. i also flushed my system when i changed my thermostat. any help would be great.
Elder User
Joined: Dec 2002
Posts: 559
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From: South Carolina
My older Bronco does that sometimes because the gauges just suck. Rapid cooling indicated on the gauge means one of two things: A - the thermostat is still faulty B-the gauge is lying to you or C-there is an air pocket - three things....one of three things. 
Get one of the higher cost Stants, or a Motorcraft thermostat. They're made of high quality materials, and they generally have a bleed hole built in. If not, you can drill a very small hole (3/64") in the thermostat's flange, and make sure that the "holy" side of the flange is rotated to the top when the thermostat is installed. It will automatically bleed air from the system before the thermostat opens. Works like a champ!
Get one of the higher cost Stants, or a Motorcraft thermostat. They're made of high quality materials, and they generally have a bleed hole built in. If not, you can drill a very small hole (3/64") in the thermostat's flange, and make sure that the "holy" side of the flange is rotated to the top when the thermostat is installed. It will automatically bleed air from the system before the thermostat opens. Works like a champ!
Postmaster
Joined: Jun 2001
Posts: 2,809
Likes: 3
From: Loveland, CO.
I do not think that you have a problem. A normal sequence of events when starting a cold truck goes like this.
1-Start truck from cold
2 - the engine will run with the T-stat closed until it reaches operating temperatue
3 - The T-stat opens and the temp drops back down to normal.
This reflects in the gauge going far over to the hot side - but not in the red - and then all of the sudden going back down to its normal temperature and staying around there. That is completely normal.
This is more noticeable in cold weather because somehow, I'm not sure how, the t-stats are designed to stay closed for a longer period of time, the colder the weather is. This is why you notice it happening more in winter than in summer, although it also happens in summer. A "bleed" hole lessens this effect.
As long as the truck always goes back down to normal temperature and never overheats, then it is behaving normally and there is nothing to be concerned about.
1-Start truck from cold
2 - the engine will run with the T-stat closed until it reaches operating temperatue
3 - The T-stat opens and the temp drops back down to normal.
This reflects in the gauge going far over to the hot side - but not in the red - and then all of the sudden going back down to its normal temperature and staying around there. That is completely normal.
This is more noticeable in cold weather because somehow, I'm not sure how, the t-stats are designed to stay closed for a longer period of time, the colder the weather is. This is why you notice it happening more in winter than in summer, although it also happens in summer. A "bleed" hole lessens this effect.
As long as the truck always goes back down to normal temperature and never overheats, then it is behaving normally and there is nothing to be concerned about.
Senior User
Joined: Mar 2004
Posts: 267
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From: Hanscom AFB, MA
Although I agree that as long as it doesn't overheat, then things will be alright, I think that you ought to check into it to see what the actual temp is.
BTW, the thermostat does not see what the weather or outside temp is and does not change its temperature cycling with different outside temps. It is merely a valve that is effected by the temp of the coolant only.
BTW, the thermostat does not see what the weather or outside temp is and does not change its temperature cycling with different outside temps. It is merely a valve that is effected by the temp of the coolant only.
Hotshot
Joined: Jul 2002
Posts: 14,285
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From: Bend,OR
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Postmaster
Joined: Jun 2001
Posts: 2,809
Likes: 3
From: Loveland, CO.
Originally Posted by Blue'87GT
BTW, the thermostat does not see what the weather or outside temp is and does not change its temperature cycling with different outside temps. It is merely a valve that is effected by the temp of the coolant only.
But whatever, it does stay closed longer when it is very cold outside.
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Junior User
Joined: Sep 2003
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Nelly,
You mentioned performing a flush of your cooling system... How long ago, when you refilled did you "burp" the system?
My guess is that there could be an air pocket or bubble ( very common after a flush/t-stat change, etc ) in the cooling system which is causing a problem. After a flush, I try to make the radiator filler neck the HIGHEST point in the cooling system ( either by jacking it up or parking it on a sloped driveway ) and then I fill it to the top of the radiator with the proper mixture. I then start up the engine and allow the thermostat to open ( which you will notice by the coolant moving and also the level dropping in the radiator ), and slowly top it off for a few minutes until the fluid level remains at the top of the radiator. I do this procedure for a few days to make sure the system is completely "burped" of air.
You could also get a thermometer and see exactly what temp your theremostat is opening up at by starting the vehicle when COLD, removing the radiator cap and inserting the thermometer into the fluid and noting at what temperature the fluid begins moving ( the stat opening up ).
Hope that helps...
You mentioned performing a flush of your cooling system... How long ago, when you refilled did you "burp" the system?
My guess is that there could be an air pocket or bubble ( very common after a flush/t-stat change, etc ) in the cooling system which is causing a problem. After a flush, I try to make the radiator filler neck the HIGHEST point in the cooling system ( either by jacking it up or parking it on a sloped driveway ) and then I fill it to the top of the radiator with the proper mixture. I then start up the engine and allow the thermostat to open ( which you will notice by the coolant moving and also the level dropping in the radiator ), and slowly top it off for a few minutes until the fluid level remains at the top of the radiator. I do this procedure for a few days to make sure the system is completely "burped" of air.
You could also get a thermometer and see exactly what temp your theremostat is opening up at by starting the vehicle when COLD, removing the radiator cap and inserting the thermometer into the fluid and noting at what temperature the fluid begins moving ( the stat opening up ).
Hope that helps...
Senior User
Joined: Mar 2004
Posts: 267
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From: Hanscom AFB, MA
Originally Posted by JBronco
I was told, when I bought a 195 T-stat in Colorado to replace the 160 that I had in the Bronco from Florida, that it was designed to open slower in colder temperatures; and it does. I thought the same thing, how can a thermostat know what the outside temperature is? But it doesn't have to know what the outside temp is, it just has to know what temp IT is; and if it is colder outside, then the thermostat would be colder too. Maybe it depends on how cold it is to start, that the the bi-metal part moves at a slower rate when starting out from very cold temperatures. That could certainly be done.
But whatever, it does stay closed longer when it is very cold outside.
But whatever, it does stay closed longer when it is very cold outside.
J Bronco, I'm not trying to call you out or anything, so please don't be offended by what I said, but the fact remains, the metal doesn't change properties with different temps, and it still would only see what the temp of the coolant is inside of the passage to measure its opening and closing temps. Now, it would operate more slowly as a result of the outside being colder as the motor would take longer to heat up with the cold winter air passing over the oil pan, block etc, but this would not be a result of the thermostat changing with the season.
Postmaster
Joined: Jun 2001
Posts: 2,809
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From: Loveland, CO.
Originally Posted by Blue'87GT
J Bronco, I'm not trying to call you out or anything, so please don't be offended by what I said, but the fact remains, the metal doesn't change properties with different temps, and it still would only see what the temp of the coolant is inside of the passage to measure its opening and closing temps. Now, it would operate more slowly as a result of the outside being colder as the motor would take longer to heat up with the cold winter air passing over the oil pan, block etc, but this would not be a result of the thermostat changing with the season.
No problem, I found it hard to believe anyway. Perhaps it does operate more slowly in the cold.
As far as the metal changing properties, I never thought that's what it would be. I was thinking that maybe they used more than one type of metal; some thermostats for other types of applications (ie. not for cars) use more than one type of metal so that it can react differently in different temperatures.
Last edited by JBronco; Dec 16, 2004 at 09:26 PM.
Tuned
Joined: Mar 2004
Posts: 281
Likes: 1
From: Petersburg, MI
Originally Posted by JBronco
I do not think that you have a problem. A normal sequence of events when starting a cold truck goes like this.
1-Start truck from cold
2 - the engine will run with the T-stat closed until it reaches operating temperatue
3 - The T-stat opens and the temp drops back down to normal.
This reflects in the gauge going far over to the hot side - but not in the red - and then all of the sudden going back down to its normal temperature and staying around there. That is completely normal.
This is more noticeable in cold weather because somehow, I'm not sure how, the t-stats are designed to stay closed for a longer period of time, the colder the weather is. This is why you notice it happening more in winter than in summer, although it also happens in summer. A "bleed" hole lessens this effect.
As long as the truck always goes back down to normal temperature and never overheats, then it is behaving normally and there is nothing to be concerned about.
1-Start truck from cold
2 - the engine will run with the T-stat closed until it reaches operating temperatue
3 - The T-stat opens and the temp drops back down to normal.
This reflects in the gauge going far over to the hot side - but not in the red - and then all of the sudden going back down to its normal temperature and staying around there. That is completely normal.
This is more noticeable in cold weather because somehow, I'm not sure how, the t-stats are designed to stay closed for a longer period of time, the colder the weather is. This is why you notice it happening more in winter than in summer, although it also happens in summer. A "bleed" hole lessens this effect.
As long as the truck always goes back down to normal temperature and never overheats, then it is behaving normally and there is nothing to be concerned about.
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