All,

Last night after going out for a nice dinner, I was on a freeway entrance ramp headed back home. The car had been running for ~2 minutes before the ramp.

I noticed that Check Gauges light was on in the HUD so I looked down to see a "Coolant Overheat/Overtemp" type message in the DIC.

I quickly reset the DIC and paged through the gauges until I reached the digital readout of the ECT.

The analog needle was pegged at 260 F and the DIC read 254! I freaked out and began to lift on the throttle, just then the coolant temp dropped to 194, then up to 220, then back to 194, 230, 210, 194 etc.... Finally after about 2 minutes of jumping around it seemed to settle at a normal operating temperature.

I drove the car to work today and logged the OBD II engine coolant temps vs. time to hopefully share with you guys. Unfortunately the problem didn’t reoccur.

What could the problem be?

A sticky thermostat?

Failing ECT sensor(s)?

Voodoo?

Please share any and all advice on what I need to do to troubleshoot/ fix this problem.

Thanks,

Chris

 

Bump

 

Check these threads out. They may have the info you need.

http://forums.corvetteforum.com/c5-t...the-place.html


http://forums.corvetteforum.com/c5-t...-behavior.html

 

Those threads look good! Thanks for the help!

Unfortunately no one posted any resolution so I will have to check to be sure.

Thanks for the links! "Coolant" pulled up many other issues...

 

what have you checked? Have you inspected the wiring? Checked 5 volts and ground? Checked the resistance of the sensor?

 

Sounds like a thermostat that isn't opening up when the ECT gets hot--HOWEVER- What can cause this to occur just as you describe is an "air bubble" in your cooling system-----I wouldn't panic--If this is indeed your probem the spike in temperature is most likely the temp of the air bubble and not your coolant !!!---Try to purge your cooling system--
Always make sure that the rad cap or in our case the coolant reseivoir
water level is always the higher than the engine-- best way to purge is to start the engine cold--with cap off--watch as the thermostat setting is reached and verify that the water is flowing( can be tough but seeing some turbulence in resev will indicate flow)-- the air should purge out on its own with the cap off----You may have to unplug your main fan or block the rad incoming air flow to get the temps up--

 

Possible, but if there was air in the system, I would not expect the temperature to drop....instead, I would expect it to be high...and stay that way.

 

I think that depends on the fluid level. If it is well below the sensor, what you say is correct because you measure the temperature of the air/steam mix. But if the fluid level is fractionally below the sensor, you can see the temperature vary as bubbles/fluid pass by it while warming up.

Then, once the entire fluid mass (block and radiator) is heated and expands, the sensor is completely covered by fluid as the fluid level is now higher than the sensor and the temperature settles to a normal looking value.

 

I have not checked anything yet. I need to wait until this weekend to start working on the car.

I am not sure how an air bubble could become trapped in the cars cooling system. Doesn't the cooling system normally purge these automatically through the puke tank?

 

...so is the nature of this forum...you're going to get different answers. The reality is, if you have been driving the car with no cooling issues, and there has not been an recent work done to said cooling system, then air is not the issue. The indicated temps swings are too erratic. Even if you had a slug of air that passed by the sensor, it's not going to respond that fast with an drastic indication change.

 

UPDATE:

Today I noticed similar symptoms to the spike seen the first post. I drove the car home watching oil temps and everything seemed fine.

Once I got home I unloaded the wife and began logging ECT & RPM vs Time.

Take a look at the graph below, you can see in the beginning the spikes indicated and circled. These spikes went away so I shut the car off thinking that was all the interesting data I was going to see. Just after I turned the engine off I noticed the ECT shot up and remained there. I left the logger on and proceeded to record the second bit of circled data.



It is important to note that the car was fully warmed up (oil temps ~180 coolant temps ~200 F) when this data was taken. I assume the thermostat was fully open and coolant was flowing through the system while the engine was running, then after shutting off the engine the water flow stopped, and the temperature of the fluid (coolant, steam etc.) was recorded at 250+ F.

After scratching my head for a bit and waiting for the engine to cool off I checked the coolant level and saw it was 5/8 inch low to the mark on the tank. I removed the coolant cap and filled to the mark on the tank with a 50/50 mix of dexcool and distilled water.

I checked the wire harness side of the sensor for proper ground and it was good. I am not sure how to test the sensor yet but that may be next.

Could low coolant cause the problems that I was experiencing? Note: The low coolant message in the DIC has not come on since I have owned the car.


Thanks for any and all help!

 

Not likely that it's the issue. I've had the Low Coolant light pop on and it was because my tank was leaking(replaced) and i never had the temps fly around. If you never had that LOW COOLANT light then your fluid isn't all that low. Mine was leaking at the little tube coming to the tank. I never had issues with it doing what you describe. Wouldn't hurt to pic up a new ECT sensor from NAPA or something and trying it out. I have a spare sitting around(think it was like $7) because when i built my engine i broke off the plastic end for the connector to snap to. I used some Super Glue to reattach it but i kept the sensor just in case it broke back off

 

UPDATE 2:

Well I replaced the sensor last weekend, but last night the problem came back. I have noticed a trend with the erratic readings. I have to drive in the following manner to get the problem to surface.

Drive the car until it is completely warmed up. Park and let it stay for ~1 hour. Start the car back up, and drive for ~2 minutes. Then the coolant temps will be all over the place for 5 minutes, wild swings from the normal ~190F to 250F with only seconds between the 2 extremes.

I talked to a buddy of mine that once worked for GM. He says for the ECT that is in our cars as temperature goes up resistance goes down. He sent me a graph of ECT vs sensor resistance, I checked the resistance of my old and new sensor and they were both in the ballpark for room temp.

So since I am seeing "false?" peaks in coolant temp, that would indicate to me that the resistance is going to 0 ohms. I think this means that somewhere I have a loose connection/ ground.

Anyone have any insight?

 

I am having the same issues. I replaced the sensor, drove it to work the next day, it sat for 8 hours, started it and let it idle for a min while I made a call, about a minute after I left work i was getting the strange coolant temp swings like you described. I got it home and noticed some small air bubbles in the surge tank. I just figured I somehow got a small amount of air in the block while switching sensors. I just started it and let it idle with the cap off and did notice some air in the tank both while i squeezed the upper hose and without squeezing it. It got up to 210ish and didn't see anymore bubbles so I shut it down and will try it again later.

 

Bump/ Update:

I started the car from cold and looked for bubbles in the coolant puke tank and saw none. I did see some slight turbulance, but it was nothing major.

I inspected the connector for the ECT sensor and a very short run of the harness from the connector to the larger part of the wire harness. No obvious cuts melts or abraisions.

I think the major clue of my problem is that it only happens with this drive cycle:

1. Start car and bring up to operating temp ~200F

2. Turn car off and wait 30 min- 1 hour

3. Turn car back on and monitor ECT's.

Since it follows that pattern, I am almost willing to throw out all electrical shorts/ failures, and look for a reason the sensor would be reading temps that inconsistantly.

Even if the sensor was given hot 190F and really hot 230F chunks of fluid (air, steam, coolant) I don't think it would be able to respond to the changes in temperature as quickly as it does.

I was able to record the dash/DIC with my iPhone today, hopefully I can put the vid on YouTube so you guys can see exactly the behavior I am describing.

Anyone have any clues?

Thanks!

 

Here is the video...Shot, edited and posted from the best video equiptment. The iPhone

 

Bumpity Bump...

 

For sure an electrical issue. There is no way the coolant temp sensor would adjust that fast even if the coolant temps spiked. Check the harness and make sure the yellow wire has not rubbed against the alternator bracket.

 

Thanks for the response. I would suspect an electrical issue, however it only happens when driving the car after an initial warm up.

It is somehow linked to the engine coming up to temp, cooling a bit and then coming back up to operating temp. I don't know how that could be electrical.

But I do agree to what you are saying in theory, it would be impossible for the thermistor that is in the ECT sensor to swing that wildly, the system just can't respond that quickly to an actual temp change. I'm stumped.

 

If it's not cooling system related.....then it's ALL electrical....

It's not uncommon for electrical problems to surface after a temperature cycle. During the fluctuation, it would be nice(if you can catch it in time) to measure the resistance to ground, and voltage at the ECT connector.