'90 GTA overheating

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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Are the fans coming on?
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Yep, when I shut it off in the garage I put it back in the run position and the fans spun right up.
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snipped-for-privacy@sympatico.remove.ca says...

First check the obvious. Is the air dam still intact under the nose. 3rd and 4th-gens are "bottom breathers" and rely on the front air dam to direct air into the radiator when the car is moving.
Next I would suspect the thermostat if the cooling fans are working. The fan should turn on around 238 degrees F and just off around 218 degrees F. For whatever reason GM ran the cars with electric cooling fans HOT!
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lol The obvious has already been done, the air dam is intact. :)
The thermostat gasket is seeping anyway so I might as well replace the thermostat while I'm in there. I'm concerned about the gauge barely moving though.
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SBlackfoot wrote:

Not sure about your Bird, but all the GM's I've worked on around that vintage have two temp senders - one for the gauge, and one for the computer. You might have a whacked sending unit/bad gauge/broken wire.
Ray
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It definitely has two senders, one on the intake (for the ECM?) and one on a head (passenger side, for the gauge?). If it keeps acting up I'll check the sender. It's not exactly easily accesible on this thing.
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snipped-for-privacy@sympatico.remove.ca says...

There should be three on that car. One on the intake for the computer. One in the driver's side head for the dash gauge, and one for the cooling fans in the passenger side head.
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Ah, I forgot about that one. So that's the PITA one between #6 and #8. The one on the driver's side should be a lot easier to get at then, thanks.
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Dennis Smith wrote:

1-gauge (on drivers side head) 2-fans (intake) 3-computer (intake). I believe that it was in 87 that the sensors were reduced to 2.... 1-gauge (drivers side head) 2-fans and computer (intake). It may have been a later year than 87 that this was changed, but I am sure that the change was made by '90.
I have an '85 iroc and have always had problems with it overheating. I would assume that the gauge temp sensor or the gauge itself to be bad if you know the coolant is getting warm and the gauge reads low.
My gauge problems were solved by replacing the melted wire were is passes next to the exhaust manifold. Also , in a previous post, someone mentioned having heard of a TPI that would engage the fan on start up, but not when the temp was hot. I also had this problem...solved it with a new temp sensor(intake) that controls the fan.
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Upon slightly closer inspection I cannot find a sensor on my driver's side head. Just the one on the intake and the one on the passenger side head, just like on my lil 4.3L. <shrug>
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Definitely not a good week for you, I seen your post a couple of days ago about your 91 S-15 and now your TA.. Definitely check the air dam like Dennis said they can cause major overheating. I can't remember how the fans are set-up, but I remember working on a car that a fan would turn on at start up but wouldn't turn on when the temp got to the turn on point. Good luck...

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This week isn't much better. I went to start it after refilling the cooling system and... nothing. No crank, no solenoid click, no dimming lights, nothing. I went to try a different key thinking maybe the VATS is playing tricks on me, same result. Fuel pump is priming which pretty much rules out the VATS anyway. So, a quick jab at the starter solenoid with my 24" breaker bar (damn this thing is low) and she cranked over. Slower than usual and kinda funny sounding, but she cranked and seems to run fine. Looks like when she overheated last week and made the coolant boil it must've killed the starter too. Dammit! That looks like a VERY not-fun job. Oh I hate my life.
I should just scrap both of the damned things.
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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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