1995 Chevy Cavalier Overheating and fan isn't running

Tiny
ARGHHH
  • MEMBER
  • 1995 CHEVROLET CAVALIER
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 65,000 MILES
Last night on my way home from work, the low coolant light came on. I pulled over right away and the overflow reservoir was completely empty. I filled it up and drove the rest of the way home.

This morning I drove to work, and kept my eye on the heat meter. It got hotter than normal range within 5 miles, and I pulled over. The overflow tank was pouring coolant out from under the cap. I waited for it to cool down and drove home.

I noticed that the radiator fan was never on, even in the red temperatures, so I figured the fan failed, which caused it to overheat, which caused the tank to boil over. This had probably happened before without me noticing, which was why the tank was empty?

So I looked at the cooling fan relay. With the relay out, I can short out the switch terminals and the fan spins, so the fan and switch wiring is good. With the ignition on, there's 12 V on the coil, but the switch wasn't closing. So I bought a replacement relay.

After putting that in, nothing changed. I realized that although one side of the coil is at 12 V, the other side gets pulled up to 12 V as soon as I put the relay in, so I guess the sensor pulls the relay terminal down when it's hot to send current through the coil? I measured resistance from the sensor connector, and it looks like one terminal is grounded.

So I also bought a new sensor, but I haven't put it in yet. I am hesitant because it will let all the coolant run out and I'm not sure how to make sure it's filled back up without air bubbles.

I measured the resistance of the two sensors, though, and I'm not sure the sensor is bad, either. The brand new sensor at room temperature is 3.2 kiloohm. The old sensor in a hot engine is 270 ohm.

Also, while hot, if I plug in the connector to the sensor, I hear something moving briefly near the top of the engine. When I disconnect it, it moves again. So it seems like the sensor is affecting something, which also makes me doubt that it's bad.

So now I'm not sure what to do. If I force the fan on, the temperature doesn't go too high, so I think I can drive it like this temporarily. I filled up the tank with coolant and drove around some while getting parts, and the overflow tank is still mostly full, so I don't think there's a huge leak, if any.

Also, the low coolant light has never gone out, even with coolant in the reservoir, even after I shook it and tapped it to try to get rid of bubbles.

What's my next step? I think maybe the coolant temperature sensor doesn't connect directly to the relay, so I'm not sure if I understand the circuit or where the problem might be. I wish I had a schematic.
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Tuesday, November 11th, 2008 AT 3:23 PM

41 Replies

Tiny
JAMES W.
  • MEMBER
The temp sensor sends a signal to the powertrain control module which starts the fan. You can go ahead and change the sensor as long as you have it. For minimum coolant loss follow this proceedure. With the engine cold, remove the resevior cap to release any pressure and put the cap back on. Have the new sensor ready to install and pull out the old one in one motion and screw in the new one in a second motion. This way you will only lose a cup or two of coolant. Normally I could paste a wiring diagram to this post, but my program is down until Thursday. If you have, or have access to, a fax I can send you a copy. Please advise. Jim
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Tuesday, November 11th, 2008 AT 8:41 PM
Tiny
ARGHHH
  • MEMBER
I bought a week of eautorepair. Net and already found the wiring diagram on there. It is as you say. I was assuming wrong.

The information on the site says it will turn on around 100 degC, which would make the sensor 177 ohms. When I measured it in a somewhat hot engine I got 270 ohms, so that may be the problem after all? It also says "ECT sensor may have shifted out of calibration and should be replaced". I'll try replacing it.

Hah. Good idea.

I'm not so much worried about losing coolant as making sure there's no air in the system, though.

How do I replace this sensor without letting air in? Or how do I bleed the air out after? The system has apparently boiled over previously without my knowledge, so I'm worried there would still be air in the system from that, too.
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Tuesday, November 11th, 2008 AT 9:34 PM
Tiny
JAMES W.
  • MEMBER
If don't have a bleeder port on the thermostat housing, which I'm sure you don't, don't worry about air in the system. When the motor warms up and the thermostat opens, any air will go into the overflow tank.
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Tuesday, November 11th, 2008 AT 10:27 PM
Tiny
ARGHHH
  • MEMBER
Replaced the sensor this morning and drove it around more than 5 miles in 35 degF weather. The temperature did not go much above the middle, but the fan did not come on, either. Maybe I will test the old thermostat in boiling water and see if it matches the resistance from the chart.

It seems I can drive it around like this, at least.
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Wednesday, November 12th, 2008 AT 7:01 AM
Tiny
JAMES W.
  • MEMBER
GOOD, keep me posted, Jim
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Wednesday, November 12th, 2008 AT 11:43 AM
Tiny
ARGHHH
  • MEMBER
I tested the old thermistor and the resistance values are correct for boiling water and ice water, so I think the reason it overheated was actually because of air in the system. Everything else has been identical since then, but it hasn't overheated. It probably leaked until empty, but the low coolant light didn't come on immediately, and then when I filled it up, there was still air in the system so it overheated and boiled over? But since then it's been fine. The low coolant light often stays on even when there's coolant in there, so I wouldn't be surprised if it didn't come on when empty. I should probably get a new sensor?
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Sunday, November 16th, 2008 AT 10:49 AM
Tiny
JAMES W.
  • MEMBER
Yes, Iwould advise a new sensor. The perpetual problem with warning lights versus gauges is, a light will tell you what you have just done, ie, over heated your engine, run out of oil etc. And by the time the light comes on, it's too late. A gauge, on the other hand, will tell you what you are about to do, giving you a little warning.
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Sunday, November 16th, 2008 AT 2:09 PM
Tiny
ARGHHH
  • MEMBER
I'm still having problems with the system overheating or mysteriously emptying of coolant.

We've checked the thermostat in a pan of boiling water and the recovery tank pressure cap by pressurizing the tank to 16 psi and both are working.

We've run water through every part of the engine until it comes out clear (was dirty brown with rust and stop-leak), run water both directions through the radiator and it seems to be flowing well.

The radiator hose gets warm after getting up to operating temperature, so it's flowing through the radiator.

I don't see any leaks. The only time I ever see water/coolant coming out of the engine, it's coming out of the recovery tank cap.

The temperature gauge goes up steadily until it gets to operating temperature and then levels off, then after 10 or 15 minutes of driving it overheats, spraying out from under the pressure cap. If I open the bleeder valve at this point air sprays out. If I open the recovery tank cap, water sprays out at first, but then when the spray stops and I finally open it, it's completely empty, like the water was being pushed out by air/steam?

The cooling fan turns on if I turn on the A/C, so the connection to the computer is fine, but I've never seen it come on otherwise, even once when overheating (didn't look other times).

I already replaced the fan relay and temperature sensor, and I tested the old temperature sensor in boiling and ice water, and the resistance was correct with the old one, too.

I really have no idea what's causing the problem. I drove for 400 miles without any problems, but then drove 20 miles with water in the system and it overheated. Maybe the emptying of the tank is just from me opening the cap while it's hot, which lowers the pressure and allows it to boil? That might explain why it only happens sometimes.
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Saturday, December 27th, 2008 AT 12:49 PM
Tiny
ARGHHH
  • MEMBER
Actually the fan is working. Drove it around with cardboard over the radiator and the fan finally kicked on, with the temperature gauge still in the normal range. So that's not it. A little water was coming out the cap of the recovery tank, so maybe that's the problem after all?
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Saturday, December 27th, 2008 AT 1:37 PM
Tiny
JAMES W.
  • MEMBER
Let's try a little experiment. With the engine cold and the cooling system completely full, and radiator cap "off". Start the engine and let it warm up, the whole time you are doing this, watch the fill neck on the radiator for excessive bubbling and churning of the coolant. We're looking for a bad headgasket or a cracked head. Let me know.
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Sunday, December 28th, 2008 AT 1:14 PM
Tiny
ARGHHH
  • MEMBER
I don't know what the fill neck is. This is the kind of system where the recovery tank has a pressure cap, and there's no cap on the radiator itself that I know of.

I flushed it and filled it again yesterday, then drove it for 7 hours and noticed no problems. I stopped after a few hours and bled a little air, added a little more coolant, but that's it. It seemed to be working fine, and I figured I just needed to let the air out better before driving it. I was ready to blame the problems on myself again for leaving air in the system which let it boil over.

But then today I drove 30 minutes to work and parked it, and when I came back a few hours later and twisted the recovery tank cap, it was still under pressure and sprayed out coolant. I opened the bleeder valve and a lot of air came out. I had to add another tank and a half of coolant to fill it back up. Then I drove 30 minutes home, drove up a steep hill (heard gurgling from the heater core, as if there was still air trapped in it), and opened the hood while the engine was still running. I didn't see any coolant leaking out anywhere. I turned off the engine and then heard a spurting noise. I ran over and looked and coolant was pouring out from the recovery tank pressure cap.

So when I turn off the engine, there's a sudden increase in pressurized gas, which then pushes all the coolant out the recovery tank? That's not good, is it?
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Monday, December 29th, 2008 AT 6:05 PM
Tiny
JAMES W.
  • MEMBER
Lets go back to the test I mentioned in my previous response. Again, making sure the system is full as you can get it. With the cap off the reservoir, let it warm up while watching for bubbles at the fill cap. It's sounding more and more like a head gasket problem.
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Tuesday, December 30th, 2008 AT 7:38 PM
Tiny
ARGHHH
  • MEMBER
Ok, I had time to try that today.

At first, the coolant level in the recovery tank rose as the engine ran, but it's not because the coolant is getting hot and expanding; it's because of air entering the system and pushing the coolant out as it runs?

I ran the engine without a cap on the recovery tank until the coolant had risen to near the top of the tank, and then shut it down. The engine wasn't up to temperature yet. I then bled the air out of the bleeder valve and the coolant level went right back down to where it was. I did not see any bubbles or foam in the coolant tank. I did this two or three more times, with the level rising to overflowing, and then shutting it down and bleeding the air out to get it back to normal level.

It got closer to normal temperature. The radiator hose was warm, so the thermostat was open.

Then each time I turned off the engine, the coolant level would suddenly rise up and overflow the tank. It didn't shoot out or anything, just rose up and overflowed within several seconds. The level of coolant "pulses", too, which might just be from sloshing around somewhere in the system?

Then if I open the bleeder valve, "air" comes out and the level goes back down to what it was before I started it. I tried to smell the air to see if it smelled like gasoline/exhaust, but I couldn't tell anything besides coolant smell.

So then I started it again, and it went up to normal operating temperature (slight above halfway on the gauge, where the temperature usually stabilizes), and the coolant level didn't rise this time. The level stayed pretty constant and I let it run for a while, significantly longer than the previous times as it was warming up. The thermostat was definitely open now.

The level never got too high, so eventually I shut down the engine, and THEN the coolant level rose quickly and big bubbles entered the recovery tank. There were never bubbles any of the previous times. Then I opened the bleeder valve again and approximately the same amount of air came out as other times.

I don't see any oil floating in the coolant, and I don't see any foam in the oil fill tube. We checked the spark plugs several weeks ago and didn't notice anything too strange. The oil level does not seem to rise as the coolant decreases. I have not actually taken out the oil plug to see if there's coolant in the bottom of the pan, though. Need to find a place I can jack it up.
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Friday, January 2nd, 2009 AT 3:49 PM
Tiny
JAMES W.
  • MEMBER
I don't mean to cry wolf, but the air has to be coming from somewhere. With all the bleeding you have done, there should be no air left in the system at all. Keep in mind, the cooling system has a max of 15 lbs pressure where compression is 120 lbs plus. A slight headgasket leak or small crack in the cylinder head will pass pressure into the cooling system, but very little back into a cylinder. A test for this theory is to get a cooling system pressure tester. They are a small unit with a radiator cap attached to a small hand pump with a short hose. With the engine as warm as you can stand it, pull the spark plugs, pressurize the cooling to 15 lbs and let it sit overnight. If the pressure drops within the first hour, pump it up again. Then, in the morning, crank it over to see if any coolant comes out of the plug holes.
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Friday, January 2nd, 2009 AT 8:05 PM
Tiny
ARGHHH
  • MEMBER
Yes, but where? Is the cylinder the only possibility? I've had the head gasket replaced in 2006 already.

What are the long-term effects of this?

Also, why would it not have problems when driving for long distances?
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Saturday, January 3rd, 2009 AT 5:30 AM
Tiny
ARGHHH
  • MEMBER
I can borrow a pressure tester from AutoZone for free, apparently.

I guess I'll try that next.
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Saturday, January 3rd, 2009 AT 2:33 PM
Tiny
JAMES W.
  • MEMBER
Long term affects, if I'm correct about a gasket or head problem, will be very little other than losing coolant and possibly overheating. I say that as long as coolant isn't getting into the oil. Antifreeze just "destroys" the lubrication ability of motor oil.
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Monday, January 5th, 2009 AT 7:39 AM
Tiny
ARGHHH
  • MEMBER
Ok, that's good to hear. Last time I had a head gasket leak it was pretty bad. After the first mechanic lied about fixing it, it eventually broke through into a big leak and I had smoke pouring out the exhaust, etc. This time it doesn't seem so bad.

Instead of wasting $1000 on a repair that only lasts two years, I think I'll try one of the magic snake oil liquids to seal the hole this time. :) I don't think my car's worth a real fix anymore.

But first I'll get a pressure tester and confirm that that's what's actually wrong.

(After driving it around a bit this weekend, I checked this morning and there was only a little air to be bled, and the coolant level hadn't dropped. After driving to work, I saw that it had been pushed out the recovery cap again.)
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Monday, January 5th, 2009 AT 9:04 AM
Tiny
ARGHHH
  • MEMBER
Got it. I'll have to do it tomorrow or Sunday though, and it's going to be cold and snowy Saturday night to Sunday morning.

Why does the engine have to be warm?

Is there any danger involved with this? If lots of coolant leaks into the cylinder can it hydrolock even with the spark plugs out?
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Friday, January 9th, 2009 AT 7:40 PM
Tiny
JAMES W.
  • MEMBER
I like that, "magic snake oil liquid". We used to call it "a miracle in a can". Just keep an eye on the color of your engine oil. If it starts to look like a chocolate malt, you in trouble. Antifreeze in oil will take the bearings out in a heartbeat.
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Friday, January 9th, 2009 AT 7:49 PM

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