Recently bought a 1986 GTS hatch which warms up very slow & when it does finally warm up it doesn't seem to reach the proper operating temp. I know the original gauges are not all that accurate but I can't get the needle to even cross the T on the TEMP indication. Furthermore the heater doesn't blow too hot of air, the car runs rich and the idle is high as if its cold (the hotter I get it the more it wants to drop down), & lastly the thermostat does not want to open, no mater how hard I try to get the thing to warm up.

I've already replaced the thermostat as well as the water outlet due to corrosion & I still get the same problem. A super slow warm up and low operating temp... The next thing I was going to try was to remove the therm & run it for a while to check if water is flowing through the system properly and also give it a good flush afterwards then reinstall the therm & see how it works.

It's a pretty technical issue & I'm not sure what else to do, let me know if anyone has any suggestions or experience with this!!

Regardless of the fact that you have replaced the thermostat, you have all
the classic symptoms of a thermostat that stays open.

To check, try this. Engine off and cold. Remove the radiator cap, start the
engine and look inside the radiator neck. If the thermostat is closed, water
will not be circulating... basically it just sits there stagnant and green/pink
As the engine starts to warm up, the thermostat should begin to open slowly,
and the water in the radiator will start to circulate. The radiator top tank
will start to warm up, and so will the top radiator hose. The bottom hose will
stay cool. Your temperature gauge will move to normal and stay there.

Now, if you see water moving in the radiator (and you can make it surge
by blipping the throttle) as soon as you have the engine running, your
thermostat is not closed.... try another thermostat.

How to test your thermostat out of the engine. The valve flap thing should
be closed. Drop the whole thing into a pot of boiling water. The valve should
open about 3/16" - 1/4" once it gets up to the water temperature. If the
valve is already open when you take it out of the engine, it is no good.



The end you see in the pic goes towards the radiator, and the jiggle valve
goes at the top.

Cheers... jondee86

There could be air in the cooling system.
Make sure that when you bleed the cooling system you do it with the heater on the 'hot' setting.

I have already confirmed the therm isn't opening. Before I put the new one in it was shut & after it was fitted I ran the engine with the cap off waiting for the water to start surging & flowing as the therm begins to open up, but it just doesn't happen. The water just sits in the top reservoir of the radiator not moving or surging at all. The top radiator hose and engine warms up to some extent while the bottom home stays cool. (I am in 70°F weather)

There is some rust and corrosion that has dirtied the coolant so I'm wondering if it's collecting around the therm and causing it to stay ever so slightly open?

I do agree that there could be some air in the system that needs to be worked out & I tried bleeding it with the cap off heater on hot & also just going out and driving it but the therm doesn't open so I can't even bleed it properly anyway...

I do think throwing the therm in a pot of boiling water would be a good idea simply for the assurance that it's working.

Thanks for the input so far. Any other suggestions??

Facts are not stacking up ?? If the thermostat fails closed, then the engine is liable
to overheat because no water is sent to the radiator. Just refreshing my memory...
the thermostat is in the return from the radiator as you can see in this pic...



The thermostat gets heated by the water flowing thru the bypass tube from the
top water neck. If the bypass tube gets blocked, then the thermostat will never
get warm enough to open.

Cheers... jondee86

I agree the facts do not stack up. The bottom hose remains cold, there's no flow of coolant, and yet the car seems to be running below the factory operating temp. In theory if the therm doesn't open up at all the car should start overheating, but in fact its the opposite, the engine doesn't seem to be producing enough heat to open the therm. Could possibly be that the operating temp is hot enough (which multiple things indicate otherwise, mentioned in first post) but the bypass is plugged up & not allowing the therm to get warm.

Maybe I should just go pick up an infrared thermometer to confirm the temp of the coolant. I used to have my Dad's machine shop at my fingertips but I'm ten hours away now living in apartments with very limited tools/working space trying to diagnose this damn thing

Question that puzzles me is where does the heat from the engine go if it is not
being rejected thru the radiator ?? A running engine will generate heat, and a
blocked radiator will usually cause engine overheating in warm weather.



the temp sensors for the ECU and gauge are located inthe top water neck on the
USDM AE86. If there is no water flowing thru the radiator, then there is no flow
past the temp sensors in the water neck, save for the tiny amount that gets
thru the jiggle valve. The ECU will think the engine is cold and the temp gauge
will never reach normal. This part I understand

But your engine does not overheat.... that I don't understand

Cheers... jondee86

PS: You can see the bypass tube in the pic. Make sure is it not blocked if you
get to taking the thermostat out.

Thanks for the useful pics & insight. I picked up an infrared thermometer & will do some more work on it soon. I feel like there's a good possibility the bypass pipe may be plugged & will definitely check while the thermostat is out, as well as giving it a good flush. Also going to drop the therm in a pot of water to see if it starts to open around 180 degrees F.

I'll be back with the results!

So I finally fixed the damn thing! Wish I could give you a clear answer to what it was but I'm not too positive. I boiled the thermostat in a pot, measuring it at different stages of heat and it began to open at 180 degrees F and would start to close when it got much below that, so it seemed to work perfectly. I also checked to see if the bypass pipe was plugged by pouring water through the top water neck, checking to see if it flowed freely through the bypass pipe & into the thermostat housing, which it did. Next, I put the it all back together without a thermostat & threw in some radiator flush/cleaner (coolant was fairly dirty but not all that bad). Ran it for a good 20min then flushed the system and finally put it all back together for the last time with the thermostat installed.

Right away the car warmed up much quicker than it did before and bubbled/fizzed in the radiator & started to flow ever so slightly. Threw the cap on & drove it for a while & the thermostat finally was open! I checked the temps & at the therm housing it is right around 180-170 & the cooled water from the bottom radiator hose is about 150. The thermostat now cycles, opening & closing exactly how it's suppose to & keeping a very consistent temp. My gauge now also reads slightly higher than it was previously.

I guess the dirty coolant was somehow hindering the function of the thermostat some how? But it still does not quite explain why it was overcooling. But it definitely runs much hotter than before & the thermostat functions perfectly now, so I guess I'm satisfied

Hope this is of use to others with cooling issues. Thanks jondee86!!