Warning - Rediculously long winded... I've been thinking about this for a bit

Having owned a Supra 2 months now, I've got beef with some very strange quirks that are probably much more.

Within the city, the temperature needle only moves to about a notch up from the bottom of the scale after say 10 minutes of driving. The guy i bought it off claimed it never worked (he owned her 8 months), and just driving in the city and seeing it barely move long after the auto temp control has decided it may begin to warm the cabin to my desired temperature, I believed him.

For the first month I experienced my heat dissapearing after (what i had assumed was) the car was warmed up (as judged by wonderful hot heat, and a perfect smooth ilde starting at 1100-1500 (depending on how cold the night was) all the way down to 650 with no hiccups).

The hot air fades to cold every time I wait for a light in cold weather (especialy rainy cold weather like Vancouver) and returns after driving again (quicker with boost); however, my idle never raises back up from 650... (suggesting the engine is still warm) it might creep a little to 700 (after the heat has been completely lost and the auto temp shuts the fans off because it knows it's not working and im geting cold air) but the rpm never re-raises anywhere near what its like when cold starting.

When I get on the highway, the temperature needle actualy raises and maintains slightly below horizontal, and i have everlasting heat. Even after driving a further 15 minutes through the city to my house and idling for a couple minutes to cool down the turbo (the needle stays pegged at slightly below horizontal). So this leads me to believe i have a thermostat stuck open, I over cool in the city and seem to meet adequate cooling on the highway.

Now (for me) its a little tricky because of the old gurgling water noises heard in the car that have plagued so many. They exist when I start the car I hear a little burst of bubbling at idle, that doesn't return til I rev a bit then they appear and speed up (as if under pressure, and I'm to understand the system is supposed to be). When rev'd after a fresh start the bubbles dissapear after about 3000 rpm and don't return until the car has sit turned off for a bit. I don't have water in my exaust or oil, and my coolant level appears to be fine.

Now and then I find a bit of wetness on the radiator directly below the small overflow hose on the cap of the reservoir, but I haven't needed to top up the level yet, coolant is present in the reservoir when the car is off, and about halfway when running.

So the bubbling at first meant to me that I just had air in the system and like a few others have an otherwise normal running bubbling supra. BUT- twice now the needle has gone off the scale through the roof when idling. (which is very disconcerting when I want to get a turbo timer and be absent from the car while idling)

The first time, I noticed it right as I was pulling out of a drive-thru with my bag of crap and before I made it to a parking spot, the needle had returned. I only noticed because the idle hiccuped to 550 for a split second (which never happens).

The second time (a week ago) I was idling after returning home from work (the afformetioned highway driving - which I've done about 12 times without any issue since returning from the off-season). Once again the idle sputtered a brief moment and the temp needle was through the roof, so I shut it off for a moment to hear/see if the car was overheating, and sure enough I could hear the coolant begining to simmer in the reservoir, popped the hood and heard some light pulsing hissing from the resivoir radiator cap area. Having plenty of experience with overheating turbo cars (300zx and Biturbo), I turned the car back on checked that the radiator fan was working, turned the auto temp to full, maintained about 1500 rpm and the heat quickly dissapated.

So now I'm thinking with a hot enough car to get a good temp needle showing, my water flow may be inadequate to cool the engine at idle, though its only happened twice in 2 months and I always idle the car before shutting it off.

I've described the bubbling in detail for those who have it and have a normal running car, in hopes that they may descibe it differently, like perhaps theirs occurs briefly at idle and disapears and doesnt return with any throttle assistance, which might suggest I have a broken water pump? (it bubbles a little at startup, stops and doesnt get enough water flow to fully pressurize the system, bubbles again quicker upon first throttle when the rpm raise a little?)

This weekend I hope to give the thermostat a bath and figure it out. Do they usualy become sticky and unpredictable before breaking, or do they usualy just work fine til they break then they're broke?

recap:
- bad needle, but seems to work on highway
- heat dissapears in city at idle if highway speeds are never reached (low flow to heatercore or open thermostat overcooling?)
- 2 rare unrepeatable instances of slight?(i hope) overheating that appear to rectify quickly with 1500-2000 rpm(**any overheating car gets better if the car is running aslong as the radiator fan is working and has waterflow)

So thanks for reading if you made it that far. Any suggestions, thoughts about my reasoning, or tales of experience with any of the above would greatly be appreciated.

Probably needs a thermostat.

There are a few things you can check.

First, replace the thermostat, simply because it's cheap, and very easy to do. Make sure to get the gasket with it as well.

Second, do a leak-down test on your cooling system. Pressurize it to about 15psi, and listen/look for leaks. It's even better if you can find some UV dye, and the correct light with which to view it.

Having your heat be intermittent can be several things. The first thing I would check is the heater valve, located on the passenger's side firewall, next to the charcoal cannister. Here's a pic (click them all to enlarge them):


Look at the arrow in this pic (simply a close-up). Try to use a piece of wire or zip tie to hold this plunger in the UP position:


If you hold that in the UP position, it allows coolant to flow through the heater core. Many times, this valve will go bad, or the diaphragm will crack and function improperly when vacuum is applied to it, thus, it will not open. If it doesn't open, you don't get heat.

Last, check all the connections on the following pipes. This is how the coolant circulates. Also check all pipes for leaks. Even the slightest pinhole will cause your cooling system to freak out.

I've replaced the thermostat.

She doesn't overheat anymore. The heater core doesn't stop blowing heat anymore either.

However, in cold mornings, the temp gauge pretends to warm up pretty quickly, and i get no heat yet, then it drops away and slowly rebuilds, and the heat comes with it.

Looks like I'll be replacing the gauge temp sensor for $40 ordered.

mmm time for a turbo timer now that she doesnt overheat at idle.