I cant figure this out

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Bill Summerfield

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I recently had a thermostat problem and i went ahead and pulled the t-stat till i could replace it. when i replced it i thought all was good. But now when i start my truck, the temp gauge stays low untill it warms up, but then it shots all the way to the top. when i pop the hood and go out and let pressure off of the rad cap, it opens the t-stat and runs normal. Im not sure if rad caps go bad or what the problem can be. any help would be appreciated.
 
I don't think the Rad. cap has anything to do with the temp of your engine. Sounds to me like something wrong with either your Thermostat or the sending unit to your gage. Does the coolant shoot out when you relieve the pressure?
 
any chance that you installed the stat upside down????

Also sounds like you may have air in the system...

Todd Z
 
Im deffinitely getting back pressure when i open the cap, and i did install it right side up. Ive taken the time to get the air out of the system but then i have to open the cap to get the t-stat to open up again and air gets back into the system. resevoir is well overfilled too.
 
I thought the same thing Todd but then convinced myself that it wouldn't matter. No matter which way it was it would still open to release the pressure right? Grim, does it work normally after the first initial release of pressure or is the vehicle pretty much parked till you get this fixed?
 
It is very possible to get a bad new T-stat..my brother just recently got 2 new bad ones within 3 days..they were Murray fail opens...he changed brands and solved the problem...
 
it works fine after the first release. sometimes it takes two tries but once i get the pressure down it seems like the t-stat opens and it runs fine. also the heat doesent start working till i release the pressure, thats how i know the t-stat isnt opening on till i take pressure off.
 
Ok you have air in the system, weather it is getting in from a blown head gasket, intake or clogged rad causing the high pressure...



The coolant bypasses the t-stat to allow circulation for heat, IF you have no heat until the stat opens, you have a clog, air in the system or a bad stat...OR worse a bad head or gasket..

Todd Z
 
Grim it really sounds like it's either in upside down (which it isn't due to it working normally after) or it is sticking and it builds up pressure then when you release the pressure, it is "popping" open. It's gettine stuck when it sets for a length of time. Try a new stat and tell the place where you got it the one you have is defective.;)
 
Grim, I would try a new stat first. See if the place will replace the one you have. If they will then do that first. If you still have the same problem, you have narrowed it down. I know it's easy for me to say since I'm not the one doing the work, but that is the easier fix.
 
I would do the cheapest thing first, get a Stant SUPER STAT, not the regular stat... It should cost about 15 bucks.. Change that and start over... make sure there is no air in the system..

IF the same thing happens you may need to do a compression test on the motor, if that is good, have the radiator flow tested..

Todd Z
 
:lol:
 
Since the T-stat should have a bypass hole to allow circulating a little water even before the T-stat is fully opened, it appears to me that the pressure before and after the T-Stat should be the same with the exception of the slight increase on the intake side since it is on the output side of the water pump. The pressure build up is a function of water temperature and not really pump action. It appears that the temp sensor may be sitting in air (and thus a low reading) until the T-stat opens. I think releasing the pressure in the system may be a red herring. try this; remove or open the rad cap so that the cooling system will not get pressurized. If the same symptoms occur, its not a function of pressure. If the water pump was bad ( a loose impeller ) there would be little water flow through the T-Stat before it opens. Since the water is not circulating, it will get hotter than usual because of the low flow. When the T-stat opens convection will help move the hot water through with hotter water following just behind. That is a possible explanation of some of your observations. Just a far stretching thought.
 
To fully get the air out of the system and have no doubts you need to elevate the front end and let it run without the cap. Park up a hill or on ramps or something.
 
I allways let the truck run for a good 15 min with the cap off, Waite till I see the flow indicating the stat opened, top off and squeeze the top hose to release all the air, and over fill the overflow bottle and drive around...

Todd Z

 
You should replace the Rad cap also.



Personally, I prefer a Motorcraft T-Stat. My sister had a 1989 Mercury Tracer. I used a Robertshaw T-Stat and when the engine warmed up, it wouldn't run. Called the dealer and they told me to use a Motorcraft T-Stat and it will be fine. Sure enough, it worked. The Motorcraft T-Stat had an extra valve in it. It was for steam. The Robertshaw didn't. I figure that was the problem.



I haven't used another brand since.





Tom
 
The t-stat can be tested on top of the stove in water. If you have a cooking thermometer, stick it in and turn the heat on. Watch it when it pops to see at which temp it was.



You "should" (but nobody does) do that with all new t-stat, as A LOT of them don't work at the correct temperature.



Any garage can test your radiator cap in 15 minutes. Pick a day their not busy so they don't charge you for the test. If they want to charge you; it's cheaper to just get a new cap!;)
 

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