Overheating Firebird

First my truck starts crapping out on the freeway, now my car wants to overheat. This has NOT been a good week.
Okay, the GTA. 1990 Trans Am GTA, 305/700R4, TPI. I got home the other day
and noticed that the temp gauge was at around 40C. Hmm... How odd... Parked it in the driveway and heard an unusual bubbling. I popped the hood to find the overflow reservoir about 80% full and BOILING. Great, just great. Must be that cheapie rad cap I recently stuck on there. So, I went to Napa and bought a new rad cap and gave it a shot tonight. Coolant level looked fine; the rad was full and the overflow was just below the "full cold" mark. I let her idle for a good fifteen minutes until the upper rad hose was too hot to hold for more than a second and the temp gauge read just barely over 40C, and the level of the overflow was a good inch over the "full hot" mark.
Soooo....... Any ideas? The thermostat housing gasket looks to be seeping ever so slightly. Not enough to cause a drip or a puddle, but enough to be damp. Pressure leak? Wouldn't it have to leak like a sieve? No drips on the floor, all hoses look dry, the rad, water pump, and heater core are around four years and maybe 6k kms old (The ol girl doesn't get out much), and the thermostat is around two years old. I did notice that the tranny fluid doesn't look very healthy, awfully clear on the dipstick (orangey-red on the skin) and kinda funny smelling. Could just be old, the shop that did it last turned out to be a bunch of clowns (long story). The coolant level was low in the spring, which is why the new cheapie cap went on (It solved a similar losing-coolant issue last time).
And to top it all off I'm stuck going into work on Saturday. Great...
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Do the fans come on when hot? How is it on the hway, or is it only hot in slow traffic or idling? Get an infared thermo and check the radiator for "cold" spots, may be partially clogged? The fans should come on at 220f, maybe the coolant sensor is bad and doesn't tell the pcm the engine is hot thus no fans? A scan will confirm the temp readings from the coolant sensor, which is a different sensor than the dash gage.

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hot
sensor,
Thanks for the response. The fans come on alright. After I shut it off in the garage I put the ignition back in the run position and I could hear them spin up. I'll see if I can borrow the thermometer from work tomorrow and check the rad. I figure the first step is to figure out why the gauge barely moves, so I'll have to check the cold and hot resistance at the temp sending unit. Any idea what they should read? That looks like a pita to get at too, under the TB. The gauge was definitely working recently.
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On Fri, 26 Aug 2005 19:54:05 -0400, "SBlackfoot"

Even though the stat is only two years old it could be a problem. Since you seem to need to replace the gasket anyway, I'd pull the stat and put it in a pot of boiling water on the stove to verify that it opens. FWIW YMMV DFB
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Well I spent a little time this evening trying a few things.I let it warm up until the infared temp gauge read 185F at the thermostat housing. No pressure in the rad, I could take the cap off without getting sprayed (Yeah yeah, I was careful). The temp gauge on the dash still read a constant 40C, and neither fan came on. I shut it off then decided to let it warm up a little more so I fired it back up, and the damn gauge started working and read around 75C. Once it hit 100C (on the dash, around 218F on the thermostat housing) the passenger side fan came on and did a good job of keeping the temp constant for another fifteen minutes or so of idling. This time the rad definitely had pressure (no burns, I said I was careful... lol). The driver's side fan never did come on. It only comes on when it hits like 220F, right? I noticed that the little seeping leak on the thermostat housing looked dry tonight. So, what do I make of this? Sticky thermostat? What about the odd dash gauge reading? Shouldn't the gauge keep working even with a stuck thermostat?
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SBlackfoot wrote:

F = (C + 40) * 9/5, -40 C = (F + 40) * 5/9, -40
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SBlackfoot wrote:

It sounds like you have nothing wrong other then a bad temp sending unit for the dash unit. I'd start there, and get the dash gauge reading properly.
Ian
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I'd still like to figure out why the coolant in the recovery tank was boiling that evening...
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SBlackfoot wrote

Didn't you say in your original post that you changed the rad cap and that fixed it? You didn't actually say that it fixed it, but it appeared that you had no more problems. If you have a rad cap on one of these systems that doesn't hold proper pressure, the coolant will boil over.
Do you have any problem with the temperature when you are at highway speeds? These cars had a plastic deflector under the front lower section that "must" be in place in order for air to flow over the radiator when the car is at higher speeds. This deflector often gets ripped off and then folks start complaining about overheating at highway speeds. This may have nothing to do with what you are experiencing, but it was a problem area on these cars.
Ian
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I changed the rad cap but I'm not sure if that fixed it or not. When I let it idle for a while right after that the overflow level got awfully high. I guess the only way to be sure is to run the damned thing for a while.

My air damn is intact and in surprisingly good shape. The car has never had a overheating problem before this.
I think I'll just go ahead and replace the thermostat, gasket, and coolant. Couldn't hurt. <shrug>
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SBlackfoot wrote:

Not a bad idea. Might I suggest that you buy the original GM thermostat? It's not a bad idea either.
Ian
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The one that's in it was purchased at the same place as the cheapie rad cap. Unless it's a failsafe stick-only-in-open-position thermostat I think I'll go elsewhere. ;)
The coolant was disgusting. The previous owner dumped around six cans of stopleak in there. When I had it flushed after I bought the car the water pump, heater core, and rad all went out over the course of a week in that order. The shop that flushed it said something along the lines of the stopleak had soaked into the waterjackets so even fresh coolant will turn brown, and it still is. At the time I didn't know enough to argue so I took their word for it. Now I think I'll get something to flush it out properly and see if it clears up at all.
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snipped-for-privacy@sympatico.remove.ca says...

Were the block drain plugs removed? There is one on each side of the block down by the oil pan rail.
I normally drain the radiator and block, then close the radiator drain and run a garden hose into the radiator and let the water drain from the block drains for 15 minutes. You'll be surprised what comes out. Since you are doing the thermostat you could put the garden hose into the intake coolant passage and flush it through there too. That will probably do a better job of getting all the stop leak out of the block.
I pulled the drain plugs for the first time on my '84 T/A a few years ago to do a cooling flush and I couldn't believe how much casting sand came out from when the block was made. All that floating around couldn't have been good for the water pump ...
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SBlackfoot wrote: Now I think I'll get something to flush it out properly

The following is just copied so you can see it...
snipped-for-privacy@sympatico.remove.ca says...

You probably have a knock sensor on one side of the block in place of a block plug. Keep in mind that if you do remove the knock sensor to drain coolant, they have a tendency to produce a reduced signal level after removal/replacement and therefore may set a knock sensor code.
If there are two standard block drains then go to town on the complete system flush with no worries. Also consider the following cut and paste info on flushing: It would be a good idea to change your coolant *more than just a drain and fill*. One way to do this is to remove the thermostat and reattach
the thermostat housing, then fill the radiator with a garden hose, remove the upper hose at the radiator, and run the engine. The water won't be restricted because the Tstat is gone, so full water flow from the engine to radiator will occur. As water is filling the radiator, the waterpump will draw it in, through the engine, and blow out the old
coolant with it through the upper hose, which can be directed towards a
large drain bucket. Dispose of the first course of coolant properly. When the water runs clear, you are done. Replace the thermostat with the new gasket you got in your kit.
Toyota MDT in MO
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