Temp Unstable
Thread Starter
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New User
Joined: Dec 2006
Posts: 5
Likes: 0
From: Upstate NY
Temp Unstable
Hello to all. Okay guys, this problem has got me baffled. The temp on my 94 F150 is fluxing. At full temp, driving on the highway, with engine under a steady load, the gauge reading goes from between the R/M in NORMAL up to above the L, but dosen't overheat and then back again. While at high temps, heater blows cold air. This is what I have done so far:
1. replaced the water pump
2. replaced the radiator (found out it was 2/3's clogged)
3. replaced thermostat (twice, first one was bad out of box, coinsedence)
4. burped system (I know there is no air in the system)
5. Flushed system ( checked heater hoses for right placement and they are)
After all this, I still have this problem. The truck only has 48,000 miles on it (older getleman bought it new, recently died and I bought it). Does anyone have any ideas that can HELP? I'm at a loss. Thank you for any advice.
1. replaced the water pump
2. replaced the radiator (found out it was 2/3's clogged)
3. replaced thermostat (twice, first one was bad out of box, coinsedence)
4. burped system (I know there is no air in the system)
5. Flushed system ( checked heater hoses for right placement and they are)
After all this, I still have this problem. The truck only has 48,000 miles on it (older getleman bought it new, recently died and I bought it). Does anyone have any ideas that can HELP? I'm at a loss. Thank you for any advice.
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
What engine does your truck have?
I don't have any ideas for you off the top of my head (your first post is a real stumper) but I have some questions. When the heater starts blowing cold air, how cold are we talking and how quickly does it get there? Does the temp gradually fall off or is it sudden? Also, how are you so sure that there is no air in the system?
I don't have any ideas for you off the top of my head (your first post is a real stumper) but I have some questions. When the heater starts blowing cold air, how cold are we talking and how quickly does it get there? Does the temp gradually fall off or is it sudden? Also, how are you so sure that there is no air in the system?
Thread Starter
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New User
Joined: Dec 2006
Posts: 5
Likes: 0
From: Upstate NY
I have the 5.0 engine. When the temp goes up, it starts blowing cold (i.e. no heat)instantly. Now being in upstate New York in the winter this isn't going to work. haha It gets cold up here. As far as the the temp movement, it's gradual. Goes from mid normal to high normal in 30 sec to one min. Burped the system using a flush kit I installed in the heater hose. Thanks for stopping
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
It really sounds like an air pocket in the system. With the truck cool, start the engine with the radiator cap taken off and let the truck warm up without the cap in place. This will bleed the air out of the system. I have a flush fitting in the heater hose too, and I've never had success bleeding the system from that point.
Post Fiend
Joined: May 2004
Posts: 11,289
Likes: 6
From: Lakewood, Ohio
the ford manual says to disconnect the heater hose outlet at the water pumpto bleed or release trapped air in the system.
when the coolant begins to escape, connect the heater hose.
fill the radiator until colant is 1 and 1/2 inch below the cap seal.
install radiator cap.
start engine, warm up, shut off engine,
allow engine to cool, check level, fill as required.
when the coolant begins to escape, connect the heater hose.
fill the radiator until colant is 1 and 1/2 inch below the cap seal.
install radiator cap.
start engine, warm up, shut off engine,
allow engine to cool, check level, fill as required.
Elder User
Joined: Nov 2005
Posts: 834
Likes: 0
Very very interesting problem. Sure sounds like air or something is keeping the coolant from circulating normally. I kind of vaugely remember someone haveing a similar problem and it was the bottom radiator hose partially collapsing. I think everything worked normal at town speeds, but at hiway speeds the hose partially collapsed, and temp went up and down. Can't remember the details.
Please post the solution when you get this one solved.
Good Luck Frank
Please post the solution when you get this one solved.
Good Luck Frank
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Elder User
Joined: Mar 2004
Posts: 580
Likes: 0
From: Chippewa Falls, WI
I have heard that rapid temp changes can some times indicate a blown head gasket. I don't know if that is a possibility on that engine but some smaller cars and light trucks will do it. Just an idea. The only test I know of is to take the radiador cap off and run the engine if there are air bubbles comming up to the filler neck then one or both of your gaskets is bad.
Just something to check out.
Just something to check out.
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Thread Starter
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New User
Joined: Dec 2006
Posts: 5
Likes: 0
From: Upstate NY
Well Guys I found the problem with my truck. Turns out there was a small pin hole in the rubberhose that feeds the temp sensor for the gauge. Swapped all the hoses, because they all looked bad. I just came back from a test drive and everything works. Heat, temp gauge reads normal and steady on the M. Probably didn't need all the new parts, but one thing is for sure I have a brand new cooling system in the truck and shouldn't have anymore problems. On the good side of this I did find the radiator 2/3s clogged, that would have been a bad thing on a cold night on the highway.
This is a great forum and I'm thankful for everyones input. Take Care
Joe from Upstate NY
This is a great forum and I'm thankful for everyones input. Take Care
Joe from Upstate NY