astro overheating not the thermostat

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  #1  
Old 10-03-06, 08:52 PM
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astro overheating not the thermostat

1987 astro started acting wierd. When moving down the road,the temp used to climb to about 140f and stay. It started climbing to about 200f and then would slowly drop to about 170f and this cycle would continue. I replaced thermostat with new 195f (recommendded) and it continued exactly the same. I just replaced with a 160f (approved) and now it cycles the same way only between 110f-125f instead of 170f-200f. I could live with this except that when it gets below freezing this winter, it will not put out enough heat. Both heater hoses are fairly new.(one more note, at idle the temp stays at 110.) I am wondering if it is the fan clutch.
 

Last edited by jefpcon; 10-03-06 at 09:06 PM.
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  #2  
Old 10-04-06, 05:16 AM
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I'm not an auto mechanic, just a do-it-yourselfer with an opinion. So, this is just what I'm thinking.

"When moving down the road,the temp used to climb to about 140f and stay." This doesn't sound right. Seems to me it should have climbed to between 185F and 200F or so, then droppped back when the thermostat opened.

"It started climbing to about 200f and then would slowly drop to about 170f and this cycle would continue." That's sounds reasonable for a 195F thermostat. I think it was okay at this point.

"I replaced thermostat with new 195f (recommended) and it continued exactly the same. " That reinforces my belief that the original thermostat was okay.

"I just replaced with a 160f (approved) and now it cycles the same way only between 110f-125f instead of 170f-200f." Odd to me; would expect it to cycle between say 150F and 170F with this thermostat.

"I could live with this except that when it gets below freezing this winter, it will not put out enough heat. " If you put the 195F thermostat back in you'll get a hotter heater core for the winter.

I'm wondering why you think that the temperature cycling between 170F and 200F is a problem?

Hope this is helpful.
 
  #3  
Old 10-04-06, 05:26 AM
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I think Bigfred is on the right track. What it sounds like is that maybe the original thermostat WAS defective and was not cycling, possibly stuck partway open at a point that would keep the water temp at about 140 deg. When you put the 195 one in it seems to have worked about as expected with opening and closing in repsonse to water temp. Same when you switched to a lower rated (165) one. If you need more heat, I would suggest you put the 195 one back in. Might not hurt, considering the age, to replace the temp sender while you're under the hood to make sure you're getting as accurate a reading as possible.
 
  #4  
Old 10-18-06, 05:33 AM
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it is not the fan causing it or it would be overheating when sitting still and running

so it would be something else


it doesnt really make much sense at all that it overheats going down the road but doesnt put heat out

may be a blockage in the heater system

need to rule that out

is the system holding pressure when its hot ?

squeeze the radiator hose with it hot and running and also a few minutes after shutting it off, it should be hard and pressurized

if not then you have a problem, a leak somewhere internally or externally


also rule out a bad temp guage and/or temp sender unit


get a thermometer and read the temp i the radiator when its hot

210 and below is safe and acceptable



Originally Posted by jefpcon
1987 astro started acting wierd. When moving down the road,the temp used to climb to about 140f and stay. It started climbing to about 200f and then would slowly drop to about 170f and this cycle would continue. I replaced thermostat with new 195f (recommendded) and it continued exactly the same. I just replaced with a 160f (approved) and now it cycles the same way only between 110f-125f instead of 170f-200f. I could live with this except that when it gets below freezing this winter, it will not put out enough heat. Both heater hoses are fairly new.(one more note, at idle the temp stays at 110.) I am wondering if it is the fan clutch.
 

Last edited by fast68; 10-18-06 at 09:01 AM.
  #5  
Old 10-18-06, 08:51 AM
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I had an 86 Astro 4.3 that drove me nuts, it would set at normal when idleing but when driving it would peg the temp guage then drop to normal then after a while it would repeat the process. The thermostat was changed, the sending unit twice, radiator flushed twice, radiator boiled and rodded I then determined the guage was screwed up and I wasn't going to remove the dash to replace it. It had 172,000 when I sold it 2 years ago and I saw it on the street a month ago so I guess it hasn't blown up yet and probably has over 200,000 now. So check to see that it is really overheating before you invest a bounh of money for no good reason. Have a nice day. Geo
 
  #6  
Old 10-18-06, 09:01 AM
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yeah the guage is not always accurat they do go bad and its not rare to happen at all

seen it alot

if it runs fine and the system holds pressure and is not leaking anywhere and if there is no moisture in the oil or anything and if the fan clutch is in good shape meaning that it is grabbing good(check by hand with engine turned off it should have alot of resitance when you try to turn the blade set when its hot) then i probably wouldnt be worried unless there were some real symptoms

cycling between 110 and 125 is really low, i really wonder about the sender unit and/or guage accuracy here


the heater not putting out heat would be a whole other deal unrelated
heatr core blocked or a door in the box not opening, or valve in one of the heater core hoses is stuck- if so equipped(check under the hood for a valve in the heatr hose and a small vacuum hose runs to it) both hoses should be the same temp, if not then theres a problem with the valve or the core is plugged and is junk and needs replaced

heater not putting very good heat out is a real common issue especially in the 80s and mid 90s gm trucks,

good luck
 
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