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Hot Running After Heater Valve Replacement


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Having some trouble with a heater valve replacement. Not my car so I don't know the full history; it is my girlfriend's. I have searched some old threads on GS300 overheating, but can't find a duplicate scenario. Full story:

She had it at the mechanic for an oil change, he told her she had a coolant leak and to watch the level, carry some coolant. I didn't like the idea of ignoring the problem so she stopped by my house one night to have me look at it. The heater valve was disintegrating; i literally poked it with a finger modestly and the inlet pipe broke off. Prior to this, the car was running normally. 1994 GS300, 225k. Has been maintained adequately, though I don't know t-belt, pump etc. history.

It took a several days to get a replacement heater valve, and it is the correct Denso unit as the original, though apparently now made of different material. I was very carefuly when removing the crumbling old one to fish out all parts of the fragmenting pipes so no material was dropped into the hoses.

I refilled the radiator and once it warmed up, it went straight towards the red zone on the temp gauge. I shut it down and comtemplated things a bit, wondering if the thermostat could be stuck (seemed unlikely but that's how it behaved). I parked it uphill and tried "burping" it, etc.but no change. I walked to the auto parts store and bought a thermostat and gasket. I forgot to buy a new cap, and we'll come back to that oversight shortly. ;)

I cracked open the lower radiator hose, installed the new gasket, refilled the radiator and MUCH more coolant went in than came out when I opened the lower house. I restarted the car and it seemed to operate normally. My dad suggested it was a vapor lock of air in the system, and this appears to be correct. I boiled the old thermostat on the stove in a pan and it retracted/opened as the water approached boiling. I drove the car around on some short hops in town and it seemed fine. She came by last night to collect it and give me back my SUV - I gave it to her to drive this week since she lives out of town, commutes to work, etc. and I can survive a while on my own, I live in town and close to things, can work from home. I told her to keep my truck so I could drive it some more and make sure all was well, also I wanted to get some snows on it for her. Also I needed to take a long drive today, so that would be a better test. That did not go so well.

I drove about 20 miles; 5 miles stop and go in town then 15 on the interstate and state highway, steady 60 or 65mph. At that point, steam came out strongly from the rf hood/headlight area (expansion/recovery tank). The car went rapidly towards the red so I shut it down and pulled over quickly, there was a safe spot to stop. The expansion tank had a crack along the hood edge, actually kinda crumbly and disntegrating. Apparently the radiator cap failed. A good Samaritan helped me, looked it all over and suggested it was that or a head gasket (jeez I hope not!). He drove me into town and we got a gallon of antifreeze and a new cap. I installed the cap, topped it off with a 60/40-ish mix, let it idle and the temps looked good and revving it did not get it to heat up.

I drove four/five miles on the highway and it ran very hot - about 3/4 of the way up, did not get into the red or near it. I stopped and disconnected the cable to the heater valve and pushed the lever back towards the firewall (open) and this got more heat out and cooled the engine to wear I was comfortable driving it home, though it was clearly running hot. The gauge was maybe 2/3 up at a steady 55mph with the heater cranked full (85F "auto" setting on cc unit). If I drove 35mph it cooled and the gauge was at 1/2 way. Outside temps today 30F so helpful there; unusually cold for here (near Seattle).

So I'm baffled. Some questions:

1) Could it be air-locked again?

2) Could it be clogged somewhere from something?

3) Could my new thermostat have possibly failed (highly unlikely I would think)?

4) Could I have installed the thermostat wrong? (put the spring side into the block; not even sure if you could pit it in backwards, I don't think the inlet housing had room for the spring assembly)

5) The lever on the heater valve never moves apparently, it just seems permanently at 1/3 open. Is it supposed to be servo-controlled from the CC unit somehow?

6) is there a lower valve to the heater circuit?

7) What should be my diagnostic steps next before I surrender and take it to her mechanic?

I'm baffled at what is going on here, and I really don't understand the ins-and-outs of this vehicle. I know a head valve can be a suspect here, and this car has mileage on it. It doesn't show any obvious signs of that - no coolant/oil mix in either system, no odor or wet exhaust, etc.

Again, it was working perfectly before the heater valve broke open, and it seemed fine this morning for the first 20 miles until the cap let go. Also that seems to mean higher pressure now than previoulsy; the new cap was clearly stronger than the old one. Both were labeled 0.9, I assume meaning bar or atm. I notice now that with the new cap, the upper radiator hose is bulging a bit at the outlet connection on the head. Maybe I didn't notice that before or it's higher pressure causing it; I ordered a new one and will have it tomorrow afternoon. The other hoses were good - at the heater valve on either side and the lower radiator hose. I'm guessing there's a lower heater outlet hose somewhere I haven't found.

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I just experienced almost this same saga, crumbling overflow tank and all. How was the thermostat seal?

I wound up replacing the head gasket and resurfacing the head. It showed blown head gasket symptoms. The head gasket is metal, so it didn't really "blow," Instead, I think driving hot, even if not pegged, warped the head just enough to allow blow-by into the cooling circuit. I'd like to hear an experienced mechanic's view, but I think running it hot damages the aluminum head pretty easily.

Also, my heater valve was questionable, but I decided to wait until I was sure the problem was fixed. Mistake. It blew on the second drive, with the fitting just crumbling and dumping the new coolant. Another tow home. I just replaced it and hope to have it back on the road tomorrow. We'll see.

In case you're interested, here is my story:


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Thanks for the reply. I did read your post/story before. It sounded a little different since there was no heat, and boiling at idle. I have heat, and it will sit and idle at normal temp. So I think it is different.

But I can't rule out the head gasket; maybe that is it. If so, it warped with very low over-heat, not run in red at all. Which is a shame because it would total this car.

I really, really have liked toyotas, but wow - that heater valve is an insanely bad design and construction. Other Toyota's I have had "in the family" were a little better, a 2003 Camry Solara and a 1998 Land Cruiser. The Solara was bullet proof, but after about 100,000 miles, the Land Cruiser would randomly have coil-over packs fail, leave you with 7 cylinders in the middle of no where. I had to get a code reader and just carry extra coil packs in the back. Also had a factory accessory belt fail afer 20,000 miles and strand me. Really disappointing on an otherwise capable, high-dollar vehicle like that. I guess every vehicle has it's achilles heel. I think if it is indeed a head gasket, the kids will have a pretty lean christmas, and I'll get her a 4 cyl, normally aspirated mid-90s Volvo. I hear they work well. Would be a bummer as these GS cars had some nice virtues.

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I retread your post. You're right, my symptoms were slightly different. One thought would be to flush the cooling system; preferably back flush if possible. There are very small passages (~3 mm) in the head gasket that could easily get clogged and could cause hot spots (??).

We didn't have "no heat" from the heater until after the first "boil over" from the overflow tank. In the course of trying to figure out my problem, I gravity fed water through the heater core (from the valve to the passenger side). Very little water flowed through (i.e. clogged). We hooked up water fro a faucet to in the other direction and gradually turned it up in the reverse direction flowing into a clean bucket. A small amount of debris came out and eventually it flowed through pretty freely. It didn't seem like enough to clog it, but the heater works fine now.

BTW, Rock Auto had the cheapest overflow tank and it fit ok.

I could not see any indication that my head gasket "blew." But I had bubbles in the radiator and rapid overheating after I replaced the thermostat and cap. My son says the gauge never hit the red zone so it apparently didn't overheat. The only other explanation is a warped head. I just got mine all put back together tonight. It ran OK, so we'll see over the next few days.

If you don't have bubbling in your radiator, I would take it somewhere that has a flushing system and get them to back flush it for an extra long time. Not just drain and replace, but flush it through for awhile. I doubt that this would have gotten out the rubber bits from my failed thermostat gasket, but it might work if yours is clogged with something else. Good luck.

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