I first noticed some funny things going on about 4 weeks ago wile driving my 79 truck would start to overheat after i first started it in the morning the needle on the gouge would go to 7/8s hot or so and then all of a sudden drop to normal and everything would be ok as long as i drove and it stayed fairly warm in between driving, but after it was parked for a wile and had cooled it would do the same thing again jump up to hot and then drop to normal, i flushed the radiator . purchased a new radiator cap . and i found a leak in the radiator that i fixed by removing the radiator and soldering the tiny hole shut, this worked and now my radiator has no leaks . but it still did the getting hot and then dropping to normal thing, well a couple of days ago when i left my house in the morning the gauge went to almost red hot and then stayed there and wile the gauge showed hot my heater started to blow cold air only and now does the same thing every time i drive it now . I thought is was my thermostat and today i got a new one and installed it this evening wile the thermostat was off i filled the cooling system with antifreeze mix 50 50 to just below the open hole so it would not spill wile i placed the the new thermostat in place and bolted it down , this way i made sure the engine was full of coolant i after installing the thermostat hose then topped of the radiator and took a test drive , at first i thought i had fixed the problem because as the engine was heating up i could feel hot air from my heater but when it reached the normal mark on the temperature gauge allofasudden my heater started blowing cold air again like there was no hot water flowing threw it and my temperature shot up to almost read hot , i had my overflow coolant reservoir empty and i noticed it filled with hot radiator fluid but my truck never showed signs of completely boiling over it never shot out any steam but stayed very hot until i shut it off . So fixing the thermostat did nothing at all . what could cause my truck to overheat but not blow hot air from the heater? I mean not even slightly warm air its cold here in Washington and its blowing ice cold air . Also after letting it cool i looked in the radiator and i still had fluid that i could see, it was missing some that went into the radiator overflow tank but it still had enough fluid in it to circulate trough the engine and my heatercore and it should have cooled the truck just fine , i also have no leaks from the waterpump and the waterpump is making no noise at all everything seems normal the belt is tight and not slipping. Anyone has any ideas
That is a head scratcher, ain’t it? My first thought was to check the viscous fan, since you said it gets hot while stationary, although that doesn’t explain the lack of heating.
Did you bleed the cooling system after each drain and refill? I know you said you filled the system to the bottom of the 'stat housing, but all cooling systems have a high point, and only the better designed ones are self bleeding, I can’t think of a pre 80s car of the top of my head that didn’t take some work to remove airlocks after a drain and refill.
Apologies if I’m teaching grandma to suck eggs here, as you seem fairly competent mechanically, but I would be following the routing of the coolant hoses, looking out for any potential air traps/bleed screws, particularly on the heater pipes. Run the engine, open any bleed screws and see if you get coolant or bubbles. If no bleed screws, run the engine 'til it’s warm, then (carefully!) feel around the hoses looking for a cold spot, indicating an airlock. If you find one, try to manipulate the pipe in such a way as to lower the high point and get the air bubble moving. If this is not possible (short/inaccessible hoses) then you may have to partially detach the hose from it’s fitting, at the top if possible, to let the air escape. I would recommend stopping the engine the first time you pop the hose, as it may stick, then fly off spraying you with hot coolant. Although you will need to run the engine to push the water round and blow all the air out.
If none of that seems feasible, one method I’ve seen used but never held much faith in is to simply run the engine with the expansion tank cap off, and keeping it topped right up. I’ve been told this works because with the cap off the air has ‘somewhere to go’, which I don’t really buy, although I have seen this work on more than one occasion, but only after being run for over 20 mins, and it’s by no means certain to do any good ime.
If you’ve identified the pipes that go to/from the heater core, and one of them is hot but the other cold, the problem is in the heater core, it’s either blocked or airlocked and it’s worth pulling some trim off to see if you can find it and investigate further.
Hope this helps, let us know how you get on.
From your description, I’d ask this question: Could you have installed the thermostat backwards?
In your first scenario, you had the engine heat to a high level and then go to normal, and the heater coil would work. This is a symptom of a faulty thermostat. It doesn’t open in its normal operating range.
In your second scenario, you have no hot air out of your heater coil and a temperature that goes hot and stays hot, indicating that your thermostat did not open at all.
Since you will open up the thermostat housing to check this, you could temporarily leave the thermostat out and see if your engine heats up normally. It will take a bit longer without the thermostat because you will be heating all of the cooling liquid at once, rather than just the engine block and gradually introducing the rest of the fluid. This will tell you if the cooling system/radiator is still all OK.
later, Tom.
Running hot + no heat from heater = insufficient coolant flow. The great majority of the time this is caused by air pockets in the system. Until it is known (not assumed, not suspected, not calculated, but KNOWN) that there is no air in the system, this is the thing to address first. If the symptoms are present with the cooling system full of liquid, a faulty water pump (damaged impeller) is the next suspect.
hammerman, welcome to the Straight Dope. Kindly do us a favor: Use standard punctuation, capitalization, spelling, sentence separation, and paragraph breaks. The way the OP is constructed makes it quite a chore to plow through. I was willing to do it once.
Well, he capitalized Luv and Washington. 
Gary, I’m not sure either of those possibilities can be the case here because there was warm air blowing into the cabin until the engine reached operating temperature.
Heater air vascillating between hot and cold is a typical symptom of an air pocket.
Ah. Is it possible that the OP’s solder job blocked a water line?
Maybe blocked one tube (out of dozens) in the radiator, but very unlikely there’s a problem there.
I’m with Gary T here. The only other thing I could think of is a piece of crap floating int he water galley that sometimes blocks the heater inlet / heater control valve.
On thing you might try.
With the car warming up and the heater on, put your hands on the two heater hoses. One should be warmer than the other. The warmer hose is the supply to the heater, the cooler is the return.
When the heater stops blowing warm, loosen the clamp where the supply hose leaves the engine. See if you get air or water. If the heater is still cold after bleeding the air there, then move to the return hose and loosen it where it joins the block, see if you get air that way.
If you get nothing but coolant both times, it is probably a bad water pump impeller.
you could also have a bad head gasket which is blowing cylinder gases into the cooling system, causing an “air” pocket to form. if the overheating just started out of the blue, this is worth considering.
Thanks you all for the tips so far. About my spelling hey i cant be good at everything. Ok this morning i flushed the engine again i flushed the heater core both incoming and outgoing backwards then i flushed the hoses going to and fro from the heater core from both ends ( i even disconnected the hose coming out of the front of the engine going to the heater core and connected my garden hose to it, i have flushed clogged heater cores on other cars before and when one first turns on the water all kinds of crap comes out of them . Not one piece of stuff flushed out of this one it was clean and i have a lot of water pressure the way i hooked the gardenhose to the hoses in the truck was by using a 3/4 inch plastic hose connector . After doing that as soon as i started the truck i looked in the radiator and saw that the water in the radiator was circulating very well so my waterpump is working good. Then i test drove it with my newly installed brand new thermostat and it did the same thing as before, at first it ran very cold then it warmed up to normal at witch time it started blowing cold air and promptly heated up to almost completely overheating but stayed just below doing so . About 7/8ts of the gauge. So i decided to remove the new thermostat and run the truck without it , After removing the newly installed thermostat it took at least 5 miles before the truck even slightly warmed up maybe 1/8 th of the scale, not being sure i drove it back home and up and down that road 3 times 3x7=21 miles and it never even got to normal temperature it heated to maybe a 1/4 of the scale and stayed there and i had slightly warm air from my heater at least it was not blowing cold air that told me i had a little warm water circulating through the heatercore. I then took it on the freeway and drove it for 30 miles it never even came close to overheating like before with the waterpump installed, It never even got to normal operating temperature it got maybe 3/8ths warm at the most. I even removed the radiator camp and smelled the inside of the radiator for exhaust fumes (encase i have a blown headgasket) it smelled clean not one trace of exhaust. After getting home i tested the new thermostat by putting it in a pan of water and heated it, it opened at 160 degrees like it should . Too weird it overheats with the thermostat and does not even get to normal warm without it, that tells me that the radiator definitely is not clogged. And no i could not have clogged the radiator with solder because i used a soldering iron and just a tiny amount of solder on the hole when soldering one can see how much solder goes on the repair and it does not take much to plug a small hole . Now i am not sure if i should reinstall the thermostat because now at least i can drive it without overheating. My question is will this hurt my engine because it now will run at below normal operating temperature ? 1979 Chevy Luv 1.8 liter. And why would running it with the thermostat make it overheat and not let hot water flow through the heatercore. Sorry about the long winded description but this is really a weird and very uncommon problem.
Hammerman, dude…
Put in some paragraph breaks. Seriously. They’ll make your posts a lot easier for others to read, and thus, you’ll be more likely to get helpful responses.
Running cold is not good for the engine in the long run.
Though I doubt it’s your problem, be sure the thermostat is installed right way round.
With no thermostat installed, it’s trivially easy to get all the air out of the system. With the thermostat in place, it can sometimes be very tricky to do. Now it sounds even more likely that your problem is an air pocket.
I will repeat myself. Apparently this got overlooked before.
Until it is known (not assumed, not suspected, not calculated, but KNOWN) that there is no air in the system, this is the thing to address first.
I had this exact problem with my 924. The car would blow some of the (cold) coolent out the overflow at startup and then get very hot, very quickly. Then at some temperature the leak would seal, and it would run perfectly until the next time it got cold.
New head gasket fixed it.
Your Luv truck either has a blown head gasket, or a cracked head. I’m guessing that your truck has very high mileage on it. I was a Chevy certified mechanic for many years. I have owned and worked on many Chevy Luv trucks in years past. The little Isuzu built 4 cylinder engines Chevy incorporated in these trucks were very good engines, but they were prone with head gasket problem and the aluminum heads would develop cracks over time and mileage. I find it hard to believe that your 79 Luv truck is still holding together. The bodies were known to rust completely down to the point where safety was an issue. You must have taken very good care of yours for it to last this long.
Your Luv truck has a blown head gasket…check!
The little Isuzu built 4 cylinder engines were very good engines…check!
The bodies were known to rust completely…check!
But you forgot the clutch pedal completely breaking off when shifting
Still one of my favorite cars
Memories - My first truck was a 76 Chevy Luv - with a stepside bed on it.
I still regret selling it.
I bought my LUV pickup used for 150 bucks. I drove it for five years, and when the body pretty much self destructed due to rust, sold it to the scrap yard for 75. It was a great little truck. You probably do have a blown head gasket.
I hope you don’t mind my breaking up your last post. It might help other people figure out what’s going on. Yeah, this is a tough problem.
I’m guess I’m siding with Gary on this. You probably have an air pocket somewhere. You might have a blown head gasket (and that would cause an air pocket), but you said that the truck still has the overflow tank. I think that a blown head gasket would push the coolant in the tank out when the truck was running.
You say that the problem started about four weeks ago. Did anything happen in the week or two before that? I’m used to old cars having minor leaks here and there. If you had to add coolant or did a flush, then you may have caused an air pocket that still hasn’t worked it’s way out. If the truck overheated, then that might have caused it.
Also, does your engine compartment looks something like this? The picture may help people figure out what’s wrong.
Ah, you rich folks and your new trucks. 
My first truck was a '72 LUV with over 200,000 miles on it. I bought it with a broken timing chain, for $200 (when I asked how he knew the chain was briken, he said “because, I can see it hanging out of the front of the engine”). Turns out, the chain itself wasn’t broken, but everything else was. It took a long time to find the correct timing chain cover, because they changed engines halfway through the year, and mine was made in the first half. I drove it for years, until the same thing happened again, and I decided that the engine was too worn out to fix (the oil pressure would drop at idle, even with a new pump).