Gokussj5okazu

I've got a bit of a head scratcher here, I'm about to say screw this engine and sell the whole car.

I rebuilt this KA early last year, but didn't end up driving it until December or so. It ran fine, temp stayed right in the middle of the gauge. However, my heat would come and go randomly, with no particular pattern.

About 2 weeks ago we had a very cold night, around 3*. Unfortunately I was running straight water and no antifreeze. Went out the next morning for work and the car started fine. Had no heat at all 1/2 the way to work, but it did finally start. At about 3/4 the way, or 40 miles, the temp gauge started creeping up and my heat disappeared. Finally made it to work, but it was overheating.

Before I left work that day I topped off the coolant. It took probably 1/2 a gallon. Started on my drive home, and after about 10 miles it started overheating. I made it home but had to stop every 5-10 minutes and add more coolant.

Thinking the radiator was cracked, I replaced it. Then I discovered the water pump was leaking from the weep hole, so I replaced it as well.

Drove it to work 2 more days and it wouldn't heat up at all until I was on the highway over 3k rpm for a while. So I replaced the thermostat, and found the old one was stuck partially open.

Got it all back together 2 days ago, and have been fighting it since. I made sure to jack the front end up as high as possible, pulled the bleeder screw, and can say 100% for certain there are no air bubbles.

I can let it idle literally forever and the thermostat never opens. The top hose gets warm, bottom stays ice cold. However if I hold the throttle open at about 3k for 2-3 minutes, the bottom hose slowly starts getting warmer but the gauge never goes above 1/2. So no overheating. Also, I get no bubbles up in the radiator while it's idling, even with the thermostat open. So head gasket's good?

Now here's where it gets weird. I have tested this 3 times now. I can drive it town about 12 miles, gauge stays at half, and I have fantastic heat. I can beat on it as hard as I can, hell I can even run it at 4k there the whole way, no problems.

BUT, as soon as I get to town and slow down it starts overheating. Gauge jumps to the dot before the H, and I start getting coolant bubbling from the overflow. Also, my heat stops completely. Ice cold. Check the coolant, and I'll be about a quart low. EVERY TIME. Fill it up, rinse and repeat.

SO what's going on? I'm thinking head gasket, but I have no symptoms at all except for disappearing coolant.

Kingtal0n

When a car stays cool on the highway but overheats in traffic/stoplights that is a fan related problem.

You seem to indicate that the coolant stops moving (or vanished) when you slow down. it is unlikely that is slows with engine RPM (water pump sounds like it is working),

I recommend,
1. pressure test the coolant system (couple ways to do this) One way is rent a pressure device (it resembles a bicycle pump) and pump it full of air pressure, or you can use an air compressor. The system should swell up to its potential (cap) pressure (say 15psi~) and sit there, gradually bleeding down slowly. That is your opportunity to inspect spark plugs for coolant, scope the bores for coolant, check for coolant leaks, find where it is going (it didn't vanish). Once you narrow down the coolant leak this should get easier.
2. compression test and record the results
3. Visually inspect the radiator with the cap open, engine warming up at idle. As the thermostat opens you should notice the water begin to flow, and the water coming out of the engine (to the top of the radiator) will get warmer and warmer. Once you verify there is no restriction and find the leak you should have this solved.