Yet another running hot question

[rhysing]

I have a 91 coupe that has been running way too hot lately.

Just this week, I had the radiator taken out and flushed and cleaned (no debris found as I try to clean the outside regularly). New water pump, new thermostat, new intake manifold gasket, all new hoses installed.

While driving the temp stays somewhere in the 180/200 range, but in stop and go traffic it gets very close to the 260 mark (just before the little warning marks) before the main fan turns on.

Could be that the temp gauge is off (common issue from what I understand), but it is darn scary to watch it get so close before the fan comes on.

Before I buy a new radiator and lower temp fan switch, what other alternatives can I do?

The fan will come on if I turn on the a/c or defroster, but I'm thinking that they should come on much sooner than they do currently.

 

[ecss]

Before I buy a new radiator and lower temp fan switch, what other alternatives can I do?

The fan will come on if I turn on the a/c or defroster, but I'm thinking that they should come on much sooner than they do currently.

How long have you owned the car?

Is this a new problem or has it always been like this?

The analog gauge is not linear and the gauge sender is mounted in the head.

The ECM monitors the ECT (engine coolant temperature sensor) screwed into the front edge of the intake manifold.

You need a scan tool or scanner software to monitor the engine coolant temperature sensor and verify when the fans are turned on. Should be in the low 220's.

 

[rhysing]

I've had the car 2 years. It's always done this, but in the last month the water pump began leaking which led me to fix the things mentioned. Since replacing the water pump and thermostat, it takes longer for the motor to hit that 250 ish mark than it did in the past, but it still does it.

If I run the car in the driveway, the main fan (on left) comes on when the gauge gets near the warning line and the motor cools down relatively quickly. I've never seen the aux fan kick on while just sitting there in the driveway. I can jump the fan, so I know it works, but I don't know if it is being triggered while driving or not or even if it is actually getting hot enough to turn on (if the gauge is indeed off).

I've read on various forums that these gauges can be off quite a bit, so if it is 20 degrees off, my fans would be turning on properly, but that is an unknown for me at this point. I know that they are supposed to come on right around 230.

Are temperature sensors known for going bad? That seems like an easy enough thing to replace.

 

[ecss]

I don't have a 91 service manual but different years had different fan on and off times.

You said the secondary fan comes on when the AC is on which indicates that fan works.
The secondary fan should come on if the temp exceeds around 230F or if the AC high pressure goes above
a certain pressure.

You can also verify the fans are working and can be controlled by the ECM by shorting Pin A to Pin B on the diagnostic connector and turning the ignition On.

You know the primary fan works so both fans must be working.

The ECT sensor is a thermistor which can change calibration.

The only chart I see in the service manual shows at 210 F the resistance is 185 ohms. So 220 F would be less than 185 ohms.

I believe starting in 92 you can press the gauges button on the DIC and it will display
the reading from the ECT sensor on the digital cluster LCD so you don't need a scan tool.

ECM code 14 also sets if the engine temp.ever reaches 266 F for three seconds.

Could also be the sender for the gauge is slightly off which is what you're looking at.

The main reason the Corvette runs hot in stop and go traffic is the air it pulls in is from
the hot cement and asphalt roadway. That's the way it was designed. :ugh It's usually the 84-89 people who complain about the high temps. In 1990 they started using two fans in back of the radiator and the also
changed the angle of the radiator.

 

[rhysing]

I just read that if I hold the up and down arrows on the ac controller for 5 seconds 00 pop on the controller. Scroll up to 16, push the fan button, and a number will show. This number is the engine temp in celcius.

Multiply that number by 1.8 and add 32...to get degrees in farenheit.

My main fan is supposed to come on at 228 and the aux fan at 235.

So I'm off to test this a/c trick and see if it works. Not sure exactly how accurate this will be, but an easy enough trick to try.

 

[rhysing]

Okay, took the car for a nice long drive to get it nice and hot.

Here is what the gauge shows when the fan comes on: You can see my concern.

I did the auto a/c trick and it shows 111 Celsius which would translate to 231 degrees. Now of course I need to find out how accurate the a/c trick is or find someone with a scanner to see if this indeed proves that my gauge is just this far off.

 

[ecss]

So you're saying the fan or fans kicked on at 231F?

The ECM is the only place the ECT sensor is connected. The CCM must be getting the data from the ECM and
then passing it on to the HVAC Programmer.

If that's true I would think the ECM would turn the fans on at the correct temperature. :bash
The temperature the ECM turns the fans on and off are located in the Eprom in the ECM.

 

[rhysing]

Yes, if the a/c trick is accurate, the fans come on at 231 and go off at 217.

Under normal driving, the temp is 188 which should be very close with the 180 thermostat.


Edit to add:

Supposed to be: The primary fan should come on around 228* and the secondary fan comes on 235*

 

[ecss]

Yes, if the a/c trick is accurate, the fans come on at 231 and go off at 217.

Under normal driving, the temp is 188 which should be very close with the 180 thermostat.


Edit to add:

Supposed to be: The primary fan should come on around 228* and the secondary fan comes on 235*

I would think when the temp hits 228 F the primary fan would come on.
Why would there be a delay of 3 degrees before the fan turns on if the temps are coming from the ECT sensor?
I suppose there could be a + or - tolerance. So there appears to nothing wrong. :L

 

[rhysing]

That's what I am hoping also. I'll do a quick and dirty code check and see if there are any codes. I'll have to find someone with a scanner to do a real scan just to make sure before I'll be completely confident that the gauge is this far off.

I really appreciate your help with this..

 

[ecss]

Make sure you post back your findings.

 

[rhysing]

I jumped A & B together and got nothing but 12 flashing for a total of six times. (3 short pauses, then very long pause, 3 more short ones).

The car was in the shop Tuesday through Thursday and I know that they had the battery unhooked while some of the repairs were done, so if there were older codes, they haven't popped up since I got the car back.

 

[DonB]

Hi rhysing and other forum members. I was just looking for a solution for the exact same issue on my 1990. My fans cycle as mentioned and my gauges show just how yours do. Just short of the warning band on the gauge. It concerns me a lot.
When the car is moving it isn't an issue, but sit still for a bit and the temp soars. I've only recently bought the car, but it has new radiator and hoses. It never goes to 260, but it will stay just short of the warning strip(as pictured). Once I get going again, it comes down to the middle of the gauge.

It's just a scary situation. This is my first vette and I'm learning it's idiosyncrasies. It was only 65* here today and I'm worried about when it gets much hotter out. I have a mechanical gauge(like a meat thermometer) and will test it with that tomorrow.

 

[ecss]

The computer monitors the engine coolant temperature sensor which is screwed into the front edge
of the intake manifold. The temperature from this sensor is when the fans turn on or off.

The dash cluster gauge gets the temperature from the sender screwed into the passenger side head.
So they won't show the same temperature.

If you have the C68 Auto Climate Control HVAC system you can see the engine coolant temperature the
ECM is looking at by turning the ignition On.

Press and hold the fan up and down arrow buttons at the same time. Hold the buttons until
you see -00 in the display. After that release the two buttons.
Keep pressing the up arrow button until you see -16.
Press the the center auto fan button and the temperature will display in Centigrade in the LCD.
Verify the primary fan turns on around 108.89 C = 228F
Verify the secondary fan turns on around 112.78 C= 235F
These on temps should be close for a 90 and 91.

Watch the video below

https://www.youtube.com/watch?v=lWIrT8tN-a8

 

[Hib Halverson]

The analog gauges in late C4s are so inaccurate they are no better than "idiot lights".

You need a scan tester or use that trick to put ECT up on the HVAC display.

When the coolant temp shows that "high" reading and the HVAC display shows 111°C are the cooling fans running on low?

Check these things:
1) Make sure your front air dam is intact and in good condition
2) Pull the top off the cooling stack and inspect the spaces between the HVAC and the radiator for debris. Remove anything you find.

 

[rhysing]

If you have the C68 Auto Climate Control HVAC system you can see the engine coolant temperature the
ECM is looking at by turning the ignition On.

Press and hold the fan up and down arrow buttons at the same time. Hold the buttons until
you see -00 in the display. After that release the two buttons.
Keep pressing the up arrow button until you see -16.
Press the the center auto fan button and the temperature will display in Centigrade in the LCD.
Verify the fan come on at around 108.89 C = 228F
Verify the fan turns off at around 112.78 C= 235F

That's exactly what I was doing once I learned this trick.

Last week I got a low temp fan switch from Corvette Central and had it installed and wired to both fans (took 1 1/2 hours to install). Now the fans come on at 200 and off at 185. This is without a doubt the best investment I've made.

 

[DonB]

Rhysing, how about a part #. Was it the 244074???? They claim 195/180 on & off respectively. How do you like it's operation and are you using it with the 180 or 160 thermostat? Thanks, Don.

 

[smithdb1960]

Rhysing have you thought of flushing the fins of the radiator? On my 84 it would run hot until I removed the radiator and flushed the air passages.


Sent from my iPhone using Tapatalk

 

[Hawk73ku]

I have a 96 and the gauge is like an idiot light as another poster stated. My gauge is fed directly from a sensor on the passenger side block. The ECM/PCM uses another sensor that is on the face of water pump. But I also have an oil temp gauge that syncs up with coolant temp gauge when engine warms up. If you have an oil temp gauge on your model and it isn't near the same temp as your coolant than one of the gauges is flaky.

 

[Hib Halverson]

In an earlier post I made to this thread I forgot to mention that the 90-96 C4 analog gauges are notoriously inaccurate. In fact, around about 1993, GM started putting a little sticker on the IP that advised new owners of the inaccuracy. Obviously, at this point in time, a lot of those stickers have been removed.

What you need to do is (on a later car) use the "gauges" button to put the ECT up on the digital display under the speed display or on an earlier car, use the HVAC display to show the ECT. Check for the fan ons listed earlier in this tread. If the fans don't come on at or about the listed temperatures, troubleshoot the cooling fan system per the Service Manual.

Also make sure the spaces in the cooling stack between the HVAC condenser and the radiator are clear of debris.
Make sure the front air dam is in place and functional. If not, repair as necessary.
Make sure your radiator cap is functioning properly. If not, replace the cap and inspect the filler neck sealing surfaces and replace the high-fill tank if necessary.