LUVTruck.com

I have a 1979 LUV that I purchased a couple years back. When I put the engine in, it didn't have a thermostat and I didn't know it. When it got colder outside, I noticed it wasn't heating up properly. So I put a thermostat in and for a couple of months it worked great. It ran in the normal op temp whenever I drove it. But suddenly when I had the heat on it started to overheat and blew all of my coolant out of the overflow bottle. With the regular thermostat it would do the same thing repeatedly with the heat on, but when I had the heat off, it wouldn't do it. Fearing it was the head gasket, I drilled a hole in the tsat, and it worked fine. It didn't heat up to normal op temp, but when I had the heat on it didn't overheat. I replaced the thermostat a couple days ago and I'm still getting the same problem. Any ideas. Any help would be appreciated

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Yellow 74 LUV. Factory a/c
Blue 79 LUV. Factory a/c almost there
Silver 78 LUV. California emissions donor car now gone
White 79 LUV. Factory a/c

My trucks run perfect 60% of the time all the time
Tucson, AZ

No offense meant here, just starting with the easy questions.......Are you sure the tstat is in correctly?
Have you removed the heater core hoses and shot hose water through it? Does it have good flow?

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OK, I'm over it. Where's the beer?

This is a head-scratcher. I'd think having the heater on would make the engine run cooler. Will be watching this thread.

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Tom
'81 Luv diesel 4X4 - daily driver

Since the heater is really just a small radiator it should tend to cool the engine, not heat it. What i bebieve is going here is the beginning of a leaking head gasket, sorry. Air in the coolant is always trouble in that air bubbles collect in the passages like the heater system. Then when heat is called for a bunch of air heads into the engine block, turns to steam, and out comes the water air mix. I hope i'm wrong but i believe it may be head gasket time.

Thank you guys so much for the replies. Rondog, My thermostat is installed correctly, and I just installed the head gasket a week ago and I know it isn't leaking. I did the test for exhaust fumes in the coolant and it came out clean. This problem has blown my mind and I can't put a finger on it. When I drill a small hole in the tstat, it doesn't have the problem. Any other ideas that could cause this? Thanks again for the help guys

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Yellow 74 LUV. Factory a/c
Blue 79 LUV. Factory a/c almost there
Silver 78 LUV. California emissions donor car now gone
White 79 LUV. Factory a/c

My trucks run perfect 60% of the time all the time
Tucson, AZ

You still are dealing with air in the system. Yes, you have eliminated the source by replaceing the head gasket but from refilling the system some air is trapped in there. It may just work itself out in time.
When i drained and refilled a Luv i would refill, start the engine and leave the radiator cap off. Then remove the return from the heater hose, make sure the heater control was on max heat, and rev the engine a bit until water shot out of the heater connection, then reconnect.
This usually got it all out. Drilling a small 1/16 inch bleeder hole in the thermostat is probably a good idea as most thermostats usually came with these allready. Don't know why thay stopped doing that.

Egg thank you so much. I have bled the cooling system after I refilled it with coolant. I guess I will have to just drill a small hole in the tsat. Any other ideas are welcomed.

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Yellow 74 LUV. Factory a/c
Blue 79 LUV. Factory a/c almost there
Silver 78 LUV. California emissions donor car now gone
White 79 LUV. Factory a/c

My trucks run perfect 60% of the time all the time
Tucson, AZ

My best guess s the heater core is blocked. Remove both hoses and see how the flow is with a garden hose.

Is someone poured too much bars leak or anything like it it can cause the heater core to clog.

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Certified pilots, looking down on people since 1903.

Luvrv8 I will try that test. My guess is that the heater core isn't plugged because when I use a tstat with a weep hole in it, the heat is hot. That is when it finally warms up. Thanks I'll try to see if it's plugged or not.

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Yellow 74 LUV. Factory a/c
Blue 79 LUV. Factory a/c almost there
Silver 78 LUV. California emissions donor car now gone
White 79 LUV. Factory a/c

My trucks run perfect 60% of the time all the time
Tucson, AZ

If the heater core were blocked, how would turning on the heater cause the engine to overheat?

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Tom
'81 Luv diesel 4X4 - daily driver

The heater core takes place of the bypass

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Certified pilots, looking down on people since 1903.

But if the core is clogged/blocked, turning on the heat wouldn't change anything. Either way, no coolant flow through the heater core. What am I missing?

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Tom
'81 Luv diesel 4X4 - daily driver

Heat rises to the top, you would get some heat from the hotel water rising to the top. I personally would pull the the hoses off at the engine and make sure water flows from both ends. Also be sure the heater hoses are installed correctly at the heater valve. Here is the tip off, without a t stat you don't need a bypass system, when you added the t stat your problem started.

You question me like I don't know anything, if you don't like my advise then don't do it and I won't post my 3 decades of owning and repairing LUV's.

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Certified pilots, looking down on people since 1903.

Luvrv8. Do you have a picture of what the hoses are supposed to be connected like? When I put the truck together it didn't have an engine and hoses weren't connected and I just guessed. If you could get me a picture that would be awesome

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Yellow 74 LUV. Factory a/c
Blue 79 LUV. Factory a/c almost there
Silver 78 LUV. California emissions donor car now gone
White 79 LUV. Factory a/c

My trucks run perfect 60% of the time all the time
Tucson, AZ

I don't, refer to the factory service manual.

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Certified pilots, looking down on people since 1903.

Okay. My heater hoses are connected properly. When I turn the heater on, if I don't open the heater valve all the way, my truck doesn't overheat at all. But when the heater is set to max heat, it still overheats. Any ideas?

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Yellow 74 LUV. Factory a/c
Blue 79 LUV. Factory a/c almost there
Silver 78 LUV. California emissions donor car now gone
White 79 LUV. Factory a/c

My trucks run perfect 60% of the time all the time
Tucson, AZ

Before I can offer anymore help I would need to see pictures of the heater valve and all heater hoses and where you have them going.

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Certified pilots, looking down on people since 1903.

I ams starting to believe that my heater core is plugged. When I feel the hoses going into the heater core, the intake is hot, and the house running out of the heater core is cool. Any ideas?

_________________
Yellow 74 LUV. Factory a/c
Blue 79 LUV. Factory a/c almost there
Silver 78 LUV. California emissions donor car now gone
White 79 LUV. Factory a/c

My trucks run perfect 60% of the time all the time
Tucson, AZ

I noticed my gauge getting higher and my heater fan seems to have a short somewhere the fan only seems to only turn on/ work when i'm driving not when i sitting in the driveway unless i just pulled up and left it running .. I get heat from my defroster tube but no heat when the heater is on even when warmed up.

so i'll be tearing into it flushing it out... just seeing this post but here s couple pics

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Previously owned 2 Luvs) 75 & 77 Fleet-side
3rd 77 Step fixing up aka The Yellow Submarine to Luv Ms (Sun)Shine=)
98 GMC Gas Guzzler~>The reason 4 my ~>3rdLuvNext2a66MALIBU =)
66 Chevelle The Oldest of all the vehicles I owned

The little vent that drops down when the heat is selected might be stuck. Check on the vent that drops down when the heat is selected

_________________
Yellow 74 LUV. Factory a/c
Blue 79 LUV. Factory a/c almost there
Silver 78 LUV. California emissions donor car now gone
White 79 LUV. Factory a/c

My trucks run perfect 60% of the time all the time
Tucson, AZ