Thermostat wont open, help!
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New User
Joined: Dec 2007
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Thermostat wont open, help!
Hey all, brand new to the forum, I have a Ford F-150 4x4 shortbed with a 302 in it. I have recently tried putting a new thermostat in, and it wont open up at 180f. I tested both the thermostats I have, and they tested fine. But when I put it in the truck it wont open up, all the coolant in the radiator and hoses stays cold and the block gets hot, and I have had the block up over 180f. Then when I take it back out and let it free flow, it works fine, accept obviously I don't have any heat and I cant get it up to operating temp. Everything flows fine , heater works fine but I cant put the stat in. Someone told me that it could be some sort of vapor lock , like air getting in somewhere and making a pocket behind the stat somewhere and not letting the stat open. Anyone have any insight on this, Im at a dead end. Thanks
Cargo Master
Joined: Apr 2001
Posts: 2,364
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From: Eustis FL
A little tip is to drill a 1/8th inch hole in the flat part of the stat to allow a small amount of coolant to flow even when cold; The far reaches of the block will get cooling this way instead of a hot spot. I read about it in a Hot Rod mag back in the 80's and I have been doing it ever since. I believe I may have even seen a new stat that has this hole already.
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New User
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thermostat wont open
Im positive I put the stat in the right way, it has writing on it that tells you which way to put it in. I even turned it backwards and tried it that way just to make sure. I also heard about drilling a hole in the stat, but is this just a get by fix, if you had to drill a hole in the stat, wouldn't they all come this way. I'm worried that this is pointing to a bigger problem like sucking air through a gasket somewhere, but I dont see how, I just rebuilt the top end.
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Cargo Master
Joined: Apr 2001
Posts: 2,364
Likes: 3
From: Eustis FL
The hole drilled is a performance tip, a hotrodders thing, if you will; Not necessary to a car's performance, but may help longevity over the long haul...
It will also help air come out.
You say you worked on the top end, but a misplaced gasket will not allow the flow you get with the thermostat is out.
Try to park with the front of truck uphill, remove rad cap and run engine at idle until you feel the coolant in the rad warming up and see it moving (thermostat in). It might burp out any air you might have. Also check (this one is obvious) to see that there is no obstruction when the stat would become open (If it is the wrong size stat, for instance).
It will also help air come out.
You say you worked on the top end, but a misplaced gasket will not allow the flow you get with the thermostat is out.
Try to park with the front of truck uphill, remove rad cap and run engine at idle until you feel the coolant in the rad warming up and see it moving (thermostat in). It might burp out any air you might have. Also check (this one is obvious) to see that there is no obstruction when the stat would become open (If it is the wrong size stat, for instance).
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Freshman User
Joined: Nov 2007
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Just gotta ask the dumb question..... did you get a 180 degree stat? Standard temp for this truck in most electronic parts cataloging is 195 (Stant 13009 or 13359, or equivalent), did you specify 180? I believe the 13358/9 has a jiggle valve anyway, to allow air and steam to bypass the stat. I can check that next time I'm at work..... Also, how are you testing both the stats, and how are you measuring block temp?
TomD
TomD
1st Gear
Joined: Mar 2024
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I have the same question.
I have a 2003 F150 and my thermostat isn't opening either. I replace the thermostat and water pump with Motor craft parts. It's not puffing white smoke or running rough. It runs about 15 at idle and start to overheat. The hose from the thermostat to radiator never gets hot.
Moderator & parts slinger
Joined: Feb 2007
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From: S/W Missouri, Polk county
You are in the 73-79 forum. You need to go to the 1997-2003 forum: https://www.ford-trucks.com/forums/forum25/
