Mysterious problem with cooling system

[MotorLicker]

Hi there, I've been lurking on these forums for the better part of 2 months and have found it to be an absolutely invaluable resource for maintenance and diagnosing problems for our beloved tercels. You are all seriously awesome! I vowed to not post unless something went very strange, and I've learned the search bar well (I hope). Also, I'll just get it out of the way - I have the 2wd DLX A/T version, not the 4wd but one day I'll upgrade.

Anyways to the problem,
I've been doing all the usual things found in the faq plus some others but the car is persistently running hot in a weird way. If left to idle in the driveway the T-stat appears to open, fan cycles, thermometer is dead centre over the thermometer symbol/line. If I'm driving at highway speeds, it is also fine and holds dead centre. But, once I hit traffic or slow down, it climbs up to about 3/4 of the way towards red or even a bit higher. Strangely, if I sit in traffic long enough it starts to cool again, but once I get to even 60km/h and then back down to idle/traffic it spikes. It has never touched the white line or had a catastrophic overheat but of course I'm concerned as this is winter in Vancouver and I'm hoping to take her down the coast to LA this summer.

What I've done already:
- Flushed the coolant and changed the t-stat (high quality, OEM) first, about a month ago. It definitely ran cooler after this but only when idling/at highway speeds.
- Set the timing to 10BTDC (ran much smoother, minor difference to overheating problem)
- A month after the first flush I replaced the water pump as it was leaking and while it was open I checked the thermostat on the stove with a thermometer in the water and it worked well - opened @ 180 F
- Decided to replace the radiator with a nice double wall/thick one as the old one was held together by rust
- Replaced upper and lower rad hoses
- Used some prestone radiator flush, flushed for 30 mins, drained and then filled with new coolant (again).
Last 4 points were done yesterday. I've been watching for bubbles to work their way out but I wasn't sure if airlock alone would cause this ongoing problem.

Other info
- The fan works when sitting idle as I mentioned, and if I pull the plug on the sensor it turns on immediately.
- I've noticed that when it's running hot if I pull over the fan will be off and the bottom hose only luke warm which seems like the T-stat. But if I idle long enough it will begin to operate normally after a few minutes.

As I type this out I'm thinking 'god damnit I hope it's not just the T-stat and I wrote all this extra info for nothing'

Any suggestions?

[NWMO]

ML,

I've only been driving my Tercel, the "Rusty Frog", for 10 months or so (17,000 + miles), so I am still learning as well. From what you have shared, this jumps out for me,

I've noticed that when it's running hot if I pull over the fan will be off and the bottom hose only luke warm which seems like the T-stat. But if I idle long enough it will begin to operate normally after a few minutes.

If the temp is noticably above the midway point on the guage, the fan should be running (wherever you are, parked, idling, hwy speed, it should not make a difference). If I were you, I would remove the lead to the fan sensor, which as you stated causes the fan to run continuously and take it for a drive. I'm betting you won't see the high temps at all, if anything it may run cool due to the extra air movement.

If it does continue to run hot, you've at least removed one variable from the equation.

Chris

[ARCHINSTL]

viewtopic.php?f=7&t=6915&hilit=radiator
This may not apply, as you've replaced the radiator, but - maybe a squirrel built a nest in front of the new radiator?
Tom M.

P.S. Welcome to the Club!

[MotorLicker]

Thanks for the replies,
NWMO - I'll try that on my commute home and give an update.

Arch - I had the whole fan assembly/everything out and it looks relatively clean. No squirrel nests that I could see!

Thanks! Glad to be a part of the club

Shaye

[Petros]

normally I would suggest the t-stat is going bad, before they fail they tend to oscillate like that. but if it did it the same way with the old t-stat as well as the new, that is not likely the problem (pf course the new one might be junk too, but I would not automatically assume that, though keep it in mind).

The other thing that occurred to me was the fan sensor is intermittent, but above about 30 mph, you will not get more cooling if the fan is on or not because of natural air flow from the car moving. If it gets hot in heavy traffic or slow speeds only, and the fan does not come on, that likely the sensor is intermittent. IF it gets hot even at hwy speeds, than it is not the fan or the sensor.

one other item to consider, the temperature gauge sender. either the connection to it is loose and unreliable, or the sensor itself has gone bad. I have had both happen to me on different Tercels. It would cause an unreliable temperature at the gauge. Usually the gauge does not go bad, but anything can happen on a car this old. I think there was one forum member some years ago that replaced the gauge in the instrument cluster and his overheating problem went away (meaning it actually never overheated).

First check the connector at the sender makes good contact. The temperature sender is the one on the front of the head next to the coolant outlet and the valve timing belt cover, (the one on the coolant outlet is for the emissions system).

If you have a spare sender, (or one you can borrow off another toyota) swap it out and than watch the gauge as you drive, looking for the behavior. you can get a used one at a wrecking yard, they are fairly reliable and a good bet. or you can buy a new one, I do not think they cost much.

to test the whole temp system, temporarily install another after market temperature gauge and see if it behaves the same way. this would be the last resort, though a cheap aftermarket gauge does not cost much (perhaps $20-30). if you know the cause, it will be easier to isolate the problem and fix it.

I do not normally advise to start replacing parts, but sometimes swapping out parts is less costly than proper testing and diagnosis.

It is important to find the cause, the head gasket will not tolerate a lot of temp cycling for very long, even if it never gets into the red zone, it still puts a lot of thermal stress on it because of the uneven cooling of the head. It is turn out to be a bad sensor or gauge, than you did not harm any thing at all, but you need to verify it was the bad readings rather than actual temperature fluctuations.

good luck.

[MotorLicker]

Okay so I drove with the fan always on and it weirdly made it run hotter, but more stable. It eventually climbed higher than I'd like so I jumped out and plugged the sensor back in which brought the temperature down after a bit. Again the bottom hose was cold and the top was hot. I think the fan could be a symptom rather than the cause here.

Great, thanks for the reply Petros! I'm going to pick up a new temp sender today and check that out. If it's still erratic I'll go ahead and get the AM gauge and see how that goes. I have a temperature gun as well that I'm going to carry with me for when it's getting hot. I'll point it at the head, tstat housing and hoses and see what that looks like maybe.

Bearing all that, I'll replace the thermostat again.

I will be incredibly relieved if I've been worrying about a ghost for the past 2 months! The previous owner did say it ran hot and that his mechanic said it was fine but the paranoia of owning a 32 year old car outweighs the word of a guy trying to sell me said car.

And yes, a head gasket is one of my greatest fears (even with your very in depth guide to help, Petros) so I'm trying very hard to prevent that

Thanks again,
Shaye

[NWMO]

Shaye,

I'm sorry it wasn't as easy as I had hoped. I will defer to Petros and follow the thread. I assume the antifreeze level is good?

Chris

[Mark]

At first, your symptoms seemed normal to me. When I drive on the highway, lots of heat is created by the engine, but the airflow blowing across the radiator at highway speeds keeps the coolant temp. normal (mid-way on the gauge if the thermostat is working correctly). When I leave the highway and start driving at lower speeds, the engine block is still a bit hotter than normal and the airflow is suddenly less. The needle on the gauge starts creeping up. When it gets to between 1/2 and 2/3, the radiator fan should turn on and bring the temp back down to the 1/2-way mark. From what I understood in your original post, this is what is happening to you? If so I think this is normal.
To verify that your thermostat is working. Start up the car and let it sit at idle. At first, the top hose should get warm and the bottom hose should remain cold. The top hose should get warmer and warmer until the temp gauge needle is almost 1/2 way. Then the thermostat should open and the radiator and bottom hose should suddenly get hot. Let the car continue to sit at idle and the needle on the gauge should continue to rise. The radiator fan should switch on before the needle reaches 2/3 on the gauge. If all these things happen, your cooling system components should be working properly (unless there's an intermittent problem with these components that only happens sometimes).

[Mark]

Oh, and make sure the plastic coolant reservoir is around 1/2-way full.

[MotorLicker]

Antifreeze levels are good, when it's cold the rad is full to the brim and the reservoir is at the full line.

I just popped in the new temperature sender from lordco, the engine was warm when I did it I hope I didn't totally mess up in doing so. I think I let air in the system which caused it to get up to the white line on my drive back to work (less than 2 min drive). I pulled over and let it cool, topped up the fluid and bounced on the bumper to try to get the bubbles out (looked really weird, especially because by chance I was stopped outside of an auto shop). Again, the top hose was hot and the bottom hose cold when this happened. I have it parked at work now, so hopefully the air works itself out before I drive home.

If the gauge still reads the same then I'll have to assume it is the thermostat after all, maybe I've just been in denial.

[MotorLicker]

Mark - the needle is always right at the center when it's idling in a driveway and the thermostat is definitely open in that case, it's when I've been driving at highway speeds and get to traffic that it seems like the thermostat is getting stuck. It will stay at about 3/4 or higher until I sit at idle for an extended period, or get on the highway. I was sure it wasn't the thermostat because I had tested it less than a week ago, but now I'm starting to doubt that

[Petros]

another thought occurred to me, check the function of the radiator cap. it is an important component, it should seat and seal reliably and the spring in it regulates the pressure. if it is weak or unreliable when normal pressure builds it will leak it out, and the coolant will boil and cause a sudden rise in temp (because it can not carry heat away from the hot parts of the engine properly). Usually a visual inspection and compressing the spring (and the retrun valve in the center) is good enough to determine it is working properly. Most auto part stores can test the function for you for free, but if there is any doubt just replace it with the proper new cap. they do not cost much. The cap is usually reliable, but if it does not work properly it will cause all kinds of problems with the cooling systems.

Also, I want to point out that the lower radiator hose SHOULD be cooler than the upper, that means the radiator is working properly. For some reason there is always someone on the forum who will wonder why the lower hose is cooler than the upper and think something wrong. Heated engine coolant comes out the upper hose at about 180-200 deg (when working normally) on its way out the engine and goes into the top of the radiator. It come out the bottom of the radiator much cooler, where it goes into the engine to cool it. if there was little to no temperature difference, it means the radiator is not working.

[MotorLicker]

So on the drive home today the temp went well into the red pretty quickly. The system had sucked a bit of coolant from the reservoir so I topped it up before I left. I thought it was a bit crazy to be overheating so quickly so I pulled over and let it idle while I checked everything. There were no signs of overheating and the engine seemed warm, but regular operating temp. The fan and t-stat weren't engaged and I figured since the only thing I changed was the sender that it couldn't be overheating so severely from that.. Drove for ~1 hour in traffic and the needle stayed centered in the red the whole time. there wasn't a single problem and it ran fine though.

Petros - I changed the radiator cap when I did the first flush on the system. That's good to know about the hoses, I'm new to all this and thought it being cold at the bottom might indicate a lack of circulation/tstat malfunction.

[Petros]

that would be pretty frustrating to have the temp gauge in the red the whole time. I would want to verify proper temperture with a separate gauge, just to be sure. that would spook me even if I knew for sure it was a bogus reading.

Perhaps you got the wrong sender. swap back in your old one see if it goes back to the previous readings.

Yes, grasshopper. I explained it to you so you will have knowledge for next time. Everyone is a beginner at some point in their life. I do not think machinery is natural to the human condition, we have to adapt to it.

[NWMO]

Peter,

I probably missed an earlier post (a month ago), but congrats on 10 yrs as a registered "member". By the way, any references to programs with David Carradine in the lead kind of dates you . I enjoyed the program as well, though I was surely watching re-runs .

Chris