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They do make replacement temperature sending units and they are known to go bad. It’s located at the passenger side cylinder head behind the alternator.
I have a laser heat gun. They are cheap at harbor freight. (Temp senders are cheap too). 1st thing I’d do if the car was mine is to laser the radiator hoses and see what the actual physical temp is, a bad sending unit can “say” it’s 240.
One other thought do you have a 15 or 18lbs pressure cap?
Thanks for the link. I don't know which pressure the cap is. That has not been changed in a couple years. It is whatever is stock.
Originally Posted by ~Josh
Have you verified the temp is physically at 240+?
They do make replacement temperature sending units and they are known to go bad. It’s located at the passenger side cylinder head behind the alternator.
I have a laser heat gun. They are cheap at harbor freight. (Temp senders are cheap too). 1st thing I’d do if the car was mine is to laser the radiator hoses and see what the actual physical temp is, a bad sending unit can “say” it’s 240.
Hmmm. Didn't think of that. I do have an IR heat gun. I will check with that.
I want to take a minute to bump this thread and revisit this issue.
The car has been largely parked much of the summer as I have spent much of the last few months out of state. I'm back now and want to tackle this problem once and for all, so here's where I am.
I can drive the car but what I notice is that any prolonged idling will quickly get the engine very hot, as in 230-240* range, or higher. As long as I can keep air flow through the radiator, the car will stay around 195-205. I'm very happy with that. I drove it the other day and when I got home, the temp was up around 230* so I popped the hood and checked. Both fans were running, but at low speed. This was with the AC on. I was under the impression that AC forces the fans to run in high speed. And I know the fans *can* run in high speed as I have forced them into high speed.
So I believe that, for whatever reason, the CPU, or whatever controls fan speed is not turning fans on high speed. It's obvious that airflow at cruising speed is not a problem as I can easily drive 30+ miles on the highway without overheating. Yet as soon as I get stopped in traffic, temperature begins to shoot up. There is clearly something wrong as this problem is new. I have had this car for a number of years and didn't have any overheating issues, even in the middle of a Vegas summer, until maybe around late last year or early this year.
With that information, what should I be checking next? Ideas?
I want to take a minute to bump this thread and revisit this issue.
The car has been largely parked much of the summer as I have spent much of the last few months out of state. I'm back now and want to tackle this problem once and for all, so here's where I am.
I can drive the car but what I notice is that any prolonged idling will quickly get the engine very hot, as in 230-240* range, or higher. As long as I can keep air flow through the radiator, the car will stay around 195-205. I'm very happy with that. I drove it the other day and when I got home, the temp was up around 230* so I popped the hood and checked. Both fans were running, but at low speed. This was with the AC on. I was under the impression that AC forces the fans to run in high speed. And I know the fans *can* run in high speed as I have forced them into high speed.
So I believe that, for whatever reason, the CPU, or whatever controls fan speed is not turning fans on high speed. It's obvious that airflow at cruising speed is not a problem as I can easily drive 30+ miles on the highway without overheating. Yet as soon as I get stopped in traffic, temperature begins to shoot up. There is clearly something wrong as this problem is new. I have had this car for a number of years and didn't have any overheating issues, even in the middle of a Vegas summer, until maybe around late last year or early this year.
With that information, what should I be checking next? Ideas?
How did you force the fans to run in high speed? Have you checked all the fuses? With the engine running hot and the a/c on you have to check the state of relay's 43, 44 and 45. From what you are describing it looks like relay 43 and 44 or the control for them is no good. Are both fans running in low speed when engine is fully hot and ac is on. If so it would point to relay 43 and 44 not switching when commanded. If 44 and 45 are on with 43 open (off) the right fan would run in high speed and left fan would be off. If relay 45 was not working the right fan would never come on. I think I got that right. The logic is crazy because there are so many different failure modes that it makes my head hurt to look at it. I think the main thing is that you have to get a volt/ohm meter and check all the voltages at the relays to verify relay, command state and continuity of the control wiring.
Edt for clarifications.
Last edited by C5MSG2004Vert; 08-15-2021 at 08:55 PM.
I don’t know how you tell the difference exactly, but how do you know the fans are only low speed?
There is a noticeable difference between high and low speed. Not only can you hear the difference in the fan speed but high speed blows noticeably more air.
Originally Posted by C5MSG2004Vert
How did you force the fans to run in high speed? Have you checked all the fuses? With the engine running hot and the a/c on you have to check the state of relay's 43, 44 and 45. From what you are describing it looks like relay 43 or the control for it is no good. Are both fans running in low speed when engine is fully hot and ac is on. If so it would point to relay 43. If 44 is off fan 2 would run in high speed and fan 1 would be off. If relay 45 was not working fan 1 would never come on. I think I got that right. The logic is crazy and it makes my head hurt to look at it. I think the main thing is that you have to get a voltmeter and check all the voltages at the relays to verify relay and command state.
Yes, I pulled the relay and jumped across the contacts to force high speed. As for the relays, I swapped out relays between the Vette and my Suburban. No guarantee the ones swapped were also good, but just an assumption as everything that worked before still works now. I'll have to go out there and check the different relays at operating temps. It just sucks because, well, it's freakin' hot out here.
Had a similar problem, intermittent at first then constant. New fans, relays, burping etc. Fans would come on low speed but for high speed only the right fan ran. Found the ground wire from relay 44 NO contact to G102 open. Right now I have a jumper from 44 NO contact to G102 and car temp has been normal for a month in Florida.
Here is what someone else found.
Last edited by purrvert; 08-14-2021 at 03:18 PM.
Reason: ADD PIC
Hey guys. Need some advice. I noticed a couple weeks ago that in stop and go traffic the temp is getting a lot hotter than I like, running around 225-230 at lights. I don't ever remember this car running that warm before. So today I got into the car to see if I could figure out what was going on. I've confirmed the fan fuses are all good and I swapped out the cooling fan relays as listed in the fuse cover. I also putt 12v directly to each fan to confirm that both fans do, in fact, work. I'm topped off with coolant and have a new T-stat in the car, but even so, I only have the one fan kick on at around 225 (give or take) and kick off at 215. The other fan (bottom, in front of the TCS module) never kicks on. Am I missing something here?
First off any codes stored ??…if you don’t have a scan tool you can read them through the DIC…Yes, these cars run hot so sitting at 230 for a few minutes at a light wouldn’t concern me…I don’t know of any such fan in front of the TCS module..no such TCS module exists BTW…after the car sits overnight I would check your ECT temp vs outside air temp…they should be very close…the ECT should not be reading 230-240 only after a few minutes after starting…if you can find someone with a bi directional scan tool you can check the fan operation without the car running or at the temps needed to turn the fans on…a tuning program can change the temps at which the fans come on and off !!
St. Jude Donor '14-'15-'16-'17-'18-'19-'20-'21-'22-'23
Originally Posted by C5 Diag
First off any codes stored ??…if you don’t have a scan tool you can read them through the DIC…Yes, these cars run hot so sitting at 230 for a few minutes at a light wouldn’t concern me…I don’t know of any such fan in front of the TCS module..no such TCS module exists BTW…after the car sits overnight I would check your ECT temp vs outside air temp…they should be very close…the ECT should not be reading 230-240 only after a few minutes after starting…if you can find someone with a bi directional scan tool you can check the fan operation without the car running or at the temps needed to turn the fans on…a tuning program can change the temps at which the fans come on and off !!
TCS= traction control system or the BPMV and EBCM is how i understood it
I have confirmed both fans come on in both low and high speed. I have even pulled the radiator out and made sure there wasn't something blocking the flow. So I thought the only thing else is that maybe the thermostat has gone south on me. I cut out a block off plate and put a 5/8" hole in it and replaced the t-stat with the block off plate. Even with that, the car is overheating. I mean no t-stat and even at idle with both fans running in high, it's getting up to 240.
My next step is pulling the WP. Can't think of anything else it could be. Any ideas?
With water being a primary means of cooling (heat transfer) with the car sitting still having no T stat will make the car run hotter and not cooler !!
I want to take a minute to bump this thread and revisit this issue.
The car has been largely parked much of the summer as I have spent much of the last few months out of state. I'm back now and want to tackle this problem once and for all, so here's where I am.
I can drive the car but what I notice is that any prolonged idling will quickly get the engine very hot, as in 230-240* range, or higher. As long as I can keep air flow through the radiator, the car will stay around 195-205. I'm very happy with that. I drove it the other day and when I got home, the temp was up around 230* so I popped the hood and checked. Both fans were running, but at low speed. This was with the AC on. I was under the impression that AC forces the fans to run in high speed. And I know the fans *can* run in high speed as I have forced them into high speed.
So I believe that, for whatever reason, the CPU, or whatever controls fan speed is not turning fans on high speed. It's obvious that airflow at cruising speed is not a problem as I can easily drive 30+ miles on the highway without overheating. Yet as soon as I get stopped in traffic, temperature begins to shoot up. There is clearly something wrong as this problem is new. I have had this car for a number of years and didn't have any overheating issues, even in the middle of a Vegas summer, until maybe around late last year or early this year.
With that information, what should I be checking next? Ideas?
You probably can check that the PCM is grounding the “control” sides of fan relay 2 and 3 through the “high speed fan relay control” by using a 12 volt test light connected to battery positive…if you can find some thin copper safety wire and wrap it around pins 85 and reinstall the relay either cooling fan 2 or 3 relays when the temp at which the high speed fans come on the test light should illuminate signifying the PCM is grounding the 2 relays !!
Had a similar problem, intermittent at first then constant. New fans, relays, burping etc. Fans would come on low speed but for high speed only the right fan ran. Found the ground wire from relay 44 NO contact to G102 open. Right now I have a jumper from 44 NO contact to G102 and car temp has been normal for a month in Florida.
Here is what someone else found.
Yikes! That concerns me. Is that on the bottom side of the fuse block? The block feels like it just clips into place. I've been trying to work it loose so I can check for loose or damaged wiring.
Originally Posted by BigGun
Is the front air dam or spoiler intact? The car will slowly overheat at idle without it.
Yes. Been checked. All gaps sealed.
Originally Posted by C5 Diag
First off any codes stored ??…if you don’t have a scan tool you can read them through the DIC…Yes, these cars run hot so sitting at 230 for a few minutes at a light wouldn’t concern me…I don’t know of any such fan in front of the TCS module..no such TCS module exists BTW…after the car sits overnight I would check your ECT temp vs outside air temp…they should be very close…the ECT should not be reading 230-240 only after a few minutes after starting…if you can find someone with a bi directional scan tool you can check the fan operation without the car running or at the temps needed to turn the fans on…a tuning program can change the temps at which the fans come on and off !!
It seems that around 245 or so it will throw a code for engine temp. I forget the code off the top of my head, but I've pulled it before. I do have a scan tool that will allow for live data. I've shot the head with an IR while the engine was running hot and my IR reading is within 5* of the display temperature, so I don't think this is a sensor problem. Once again, if I get the car on the open road, I can drive for miles and the temp is stable right around 200*. That tells me it's not water, T-stat or water pump. The fact that I only get the overheating when stuck in traffic tells me it has to be something with the fans/controller. But I might be getting tunnel vision on the problem, hence why I'm asking here. To see if somebody else is seeing something I'm missing.
Originally Posted by C5 Diag
With water being a primary means of cooling (heat transfer) with the car sitting still having no T stat will make the car run hotter and not cooler !!
That's why I had restrictor orifices in there. I get that you have to slow down the water flow to transfer heat.
Originally Posted by C5 Diag
You probably can check that the PCM is grounding the “control” sides of fan relay 2 and 3 through the “high speed fan relay control” by using a 12 volt test light connected to battery positive…if you can find some thin copper safety wire and wrap it around pins 85 and reinstall the relay either cooling fan 2 or 3 relays when the temp at which the high speed fans come on the test light should illuminate signifying the PCM is grounding the 2 relays !!
Since I know I can force the fans to run in high, I think I'm going to connect and run a temporary switch into the cabin so I can manually switch the fans into high speed. If I can do this and my temp drops, then it's obvious that I have a problem with a relay or wiring. At least that way I can narrow down my problem, eliminating the water pump, T-stat, fans or ducting as a possible cause of the problem.
I don’t know how you tell the difference exactly, but how do you know the fans are only low speed?
When the fans are on low speed, it's hard to hear them, if you have headers and non stock mufflers. Also not moving a ton of air on low. However, on my car, at least, when they're on high speed, you can hear them AND feel the air blowing under the car, like if you stand by the drivers door, you can easily feel the air. If you place your hand behind (BEHIND! NOT INTO!!!) the passenger side fan, you'll know.....
I have confirmed both fans come on in both low and high speed. I have even pulled the radiator out and made sure there wasn't something blocking the flow. So I thought the only thing else is that maybe the thermostat has gone south on me. I cut out a block off plate and put a 5/8" hole in it and replaced the t-stat with the block off plate. Even with that, the car is overheating. I mean no t-stat and even at idle with both fans running in high, it's getting up to 240.
My next step is pulling the WP. Can't think of anything else it could be. Any ideas?
I would think a 5/8" diameter hole would be woefully inadequate as far as coolant volume is concerned.......
04-04-2021, 09:50 PM
