Cooling Fans won’t shut off while running

[Glassmuseum]

I recently had to drain the coolant on my 86 turbo and I’ve tried every method possible to bleed the system. For some reason, my slow speed fans kick on for the first time when the mark is in between the first and second hash and they never shut off while the car is running. The gauge stays around the lower hash after they kick on so it seems like it’s running cool but the slow fans never kick off while the car is running. The fast speed will also erratically run for a couple of mins when the gauge is low as well. I don’t see the gauge rise like it’s getting hot when the fast speed kick on. It will switch back to the low speed after a minute or two. The fans do run for a moment after shutting the car off but eventually they do shut off. I know that is an issue that can happen but my issue is only when the car is running.

Things I’ve done:

• Installed new wahler 80C thermostat and drillled a small hole on outer top edge. The PO had installed some no name thermostat and I was convinced this was the issue. I tested the wahler thermostat in boiling water before installing and it started opening closer to 85-88c. I’ve seen these inconsistencies with other wahler thermostats in the past

• Installed new wahler 85c/95c Thermofan switch. I tested it before I installed it and it seems to open slow fan speed at 90’ish C. I also had an older one and it seemed to test at the same opening rate. So a bit confused on these in inconsistencies

• user Clark’s guide to jumper the slow and fast speed fans on the Thermofan switch connector. All good

• various methods to try and bleed the system. bled the system using a pressure tester, refilled using vacuum filler, revved the car with the bleeder screw open until there were no bubbles and steam.

• I had an older 92C/102C Thermofan switch laying around so I installed it and the fans do shut off, but it only happens twice. So after warming up, the fans kick on when the temp gauge is about 10 o’clock between the 1st and 2nd hash and the fans bring it down to just above the first hash. This happens two times but that’s it. After the second time, the fans continue to run like it does with the 85C Thermofan switch. So I have no idea what’s up

My questions are:

• Could this be because the system still isn’t bled? I know HG issues can be related but I’ve done compression tests and those were all good. I’ve held pressure with a pressure tester fine, held vacuum with coolant refiller, I don’t seem to be losing any coolant. I’ve tried everything I could think of for bleeding the system

• Could the Thermofan and thermostat combination be causing this? Are they fighting each other? I may try the 92c/102c Thermofan switch again to see if it’s better. I’ve seen similar behavior before with my previous 944 NA’s and I’m scratching my head here wondering what to try next

• I’ve also tried to temp the coolant neck next to the bleeder screw to verify the gauge is reading correctly. but the temps are all over the place and I have no idea where I should shoot an IR gun. On the turbo, should I temp the neck right by the bleeder screw on the side or the metal pipe that runs to the radiator? Or should I temp the area by the temp sender? And what temp could be considered too high? If someone has a good picture of exactly where I would appreciate it. Just want to make sure I’m not actually overheating

• Could it be the relay? I’ve done the Clark’s test to jumper the connector for the Thermofan switch and nothing failed

Would appreciate any help or clues for next steps or additional testing. Thanks!

[Tom]

If your thermostat isn't opening until 85-88C and you have an 85/93C thermo-switch, it's possible they are fighting each other. When I tested my 85/93C thermo-switch, I found that it doesn't shut off until the temp got down to 82C. See this thread: https://www.carpokes.com/viewtopic.php?t=4648. I never tested the standard temp thermo-switch but it's possible the turn off point is below your 85-88 thermostat opening point... And heat soak might explain why it initially worked, then didn't.

The free way to confirm that is just to remove the thermostat to see if that allows the coolant to get cool enough to turn off the fans...

[walfreyydo]

You only mention in your post that you tested the thermo fan switch, but you also need to test the relay. The clarks garage walkthrough has a guide on how to test the fan relay (its at the bottom of the cooling fan troubleshooting page).

Clarks doesnt mention it for late model 944s but I also wonder if the AC can cause this. I am not sure if the late cars have their own AC relay or if its integrated into the fan relay itself. Maybe others will know more. Turning on the AC does activate the fans so perhaps this could be a potential cause, or maybe testing of the relay already confirms this function is working normally.

Tom also brings up an important point about the thermostat opening temperature as it relates to the thermo fan temp switch in the radiator as well. If tstat opens later than the switch, it can cause fans to also run continuously. I recommend using the OEM 92/102 temperature switch, as theres no real benefit on going lower. Porsche designed these cars to run at those temps.

[Tom]

You only mention in your post that you tested the thermo fan switch, but you also need to test the relay. The clarks garage walkthrough has a guide on how to test the fan relay (its at the bottom of the cooling fan troubleshooting page).

Clarks doesnt mention it for late model 944s but I also wonder if the AC can cause this. I am not sure if the late cars have their own AC relay or if its integrated into the fan relay itself. Maybe others will know more. Turning on the AC does activate the fans so perhaps this could be a potential cause, or maybe testing of the relay already confirms this function is working normally.

Tom also brings up an important point about the thermostat opening temperature as it relates to the thermo fan temp switch in the radiator as well. If tstat opens later than the switch, it can cause fans to also run continuously. I recommend using the OEM 92/102 temperature switch, as theres no real benefit on going lower. Porsche designed these cars to run at those temps.

All good points. Here was my logic.... I didn't mention the relay because he said the fans are stopping after he turns the car off. My recollection (which isn't what it used to be) is that a stuck relay will keep the fans running non-stop after the key is removed. Now I suppose things could be cooling down enough for the relay to unstick when the engine is off, but if the fans consistently stop after the typical post-shut down cool off, it didn't seem like the most likely culprit. Belongs on the list, but not my top suspect. GM, if you haven't done so yet, remove the connector on the thermo-switch and confirm the fans don't turn on. Assuming they don't, that narrows it down to a much shorter list (bad switch, air pockets, switch/thermostat mismatch, poor cooling capacity in general, etc.).

The a/c pressure switch is connected straight to the fan relay high speed output, so if it is stuck shut the car will behave as if the relay has triggered the high speed fans -- i.e., the fans will run in high speed whenever ignition is on, but not when ignition is off. That didn't seem to match the symptoms.

I'd normally say it's just the thermo-switch but since it is happening with two different switches, probably not.

I recently tested an 85/93 thermo-switch and saw that it needs to get down to 82 to turn off. And since GM says his thermostat didn't open to 85-88, that seems like a recipe for always-on-slow fans.

The random high speed fans is an interesting quirk though, and might point more to air pockets -- or poor cooling capacity, or some kind of coolant circulation issue, etc. The coolant temp in the head might be ok (as shown on the gauge) but the coolant or air pockets near the thermo-switch in the radiator may occasionally become a localized hot spot. Definitely worth re-doubling efforts to vent the system and confirming the coolant is circulating properly. An infra red temp gun on the upper and lower radiator hose might give clues -- the upper should be quite a bit hotter. If the cooling system is not running at peak efficiency due to air pockets or circulation issues (or a ratty old radiator) -- that could exacerbate the thermostat/thermostat-switch overlap too. If the system were running just right, it might get the coolant temps down low enough to turn off the thermostat-switch, but if not cooling well, it may never get there.

[Glassmuseum]

Thank you both for the reply. I’m currently in the process of installing a brand new 92C Thermo switch and re-bleeding the system. I’m just going to stick with the pressure tester and also just good ole crack the bleeder screw after running the car to get air bubbles out method. Also going to add a little bit of water wetter.

@Tom you mentioned temping the upper or lower radiator hose with and IR gun. Would this be the best place to check what the actual temp is (or close to
It)? on the turbo there’s a metal pipe that comes down off the neck where the bleeding screw is located and connects to the upper rubber radiator hose. should I be temping this metal pipe or should I be tempting the actual rubber radiator hoses?

And if you guys have any additional tricks on bleeding the system, I’m all ears. I feel like once you see a steady stream of coolant with no air bubbles coming out of the bleeder screw, and you have full heat, the system should be bled correct? this is my understanding but correct me if I’m wrong.

[Tom]

Thank you both for the reply. I’m currently in the process of installing a brand new 92C Thermo switch and re-bleeding the system. I’m just going to stick with the pressure tester and also just good ole crack the bleeder screw after running the car to get air bubbles out method. Also going to add a little bit of water wetter.

@Tom you mentioned temping the upper or lower radiator hose with and IR gun. Would this be the best place to check what the actual temp is (or close to
It)? on the turbo there’s a metal pipe that comes down off the neck where the bleeding screw is located and connects to the upper rubber radiator hose. should I be temping this metal pipe or should I be tempting the actual rubber radiator hoses?

And if you guys have any additional tricks on bleeding the system, I’m all ears. I feel like once you see a steady stream of coolant with no air bubbles coming out of the bleeder screw, and you have full heat, the system should be bled correct? this is my understanding but correct me if I’m wrong.

I'm not a believer in using an IR gun to gauge the actual temperature. There are just too many variable at once undermining whatever reading you are getting. For the hoses, the suggestion was move about confirming the upper hose is a solid 30-50 degrees higher than the lower -- just to make sure coolant is circulating well.

Water wetter is a good idea, and depending on where you live, you can improve cooling capacity by increasing the percentage of distilled water vs actual coolant. I run 25-30% coolant, but we virtually never see freezing temps. The coolant is more of a lubricant than anything in my climate.

And, no, I would not say you can be confident after you get a steady stream out the bleeder. Air can still be trapped somewhere below. I always treat it as a 2-3 day process. Bleed it when hot until no bubbles and take note of the coolant level. Then in the morning, if it's gone down quite a bit, there's probably an air pocket that got smaller when it cooled. Take it out for a drive to move the air pocket around, then bleed it again. And keep doing that until all the air is gone and the coolant level doesn't vary by more than an inch or two between hot and cold. Also be sure the heater is on when you bleed, so air isn't trapped in the heater core/pipes.

Have you pulled the connector off the thermo-switch and confirmed the fans don't run that way? If you remove the connector, and the fans still run, then you are barking up the wrong tree with the switch. Although, my hunch is your switch and thermostat are on the cusp of fighting each other and that reduced cooling capacity is pushing you into the always-on side of that cusp. A 92c switch and improved cooling capacity (water wetter, higher water/coolant ratio, better venting) will hopefully push you to the other side of that cusp where the fans cycle on and off. With a 92c switch, don't be surprised to see the temp gauge climb above half way at stop lights and very low speeds. It should come down to the thermostat rating while cruising and once the fans kick on.

[Glassmuseum]

Thank you both for the reply. I’m currently in the process of installing a brand new 92C Thermo switch and re-bleeding the system. I’m just going to stick with the pressure tester and also just good ole crack the bleeder screw after running the car to get air bubbles out method. Also going to add a little bit of water wetter.

@Tom you mentioned temping the upper or lower radiator hose with and IR gun. Would this be the best place to check what the actual temp is (or close to
It)? on the turbo there’s a metal pipe that comes down off the neck where the bleeding screw is located and connects to the upper rubber radiator hose. should I be temping this metal pipe or should I be tempting the actual rubber radiator hoses?

And if you guys have any additional tricks on bleeding the system, I’m all ears. I feel like once you see a steady stream of coolant with no air bubbles coming out of the bleeder screw, and you have full heat, the system should be bled correct? this is my understanding but correct me if I’m wrong.

I'm not a believer in using an IR gun to gauge the actual temperature. There are just too many variable at once undermining whatever reading you are getting. For the hoses, the suggestion was move about confirming the upper hose is a solid 30-50 degrees higher than the lower -- just to make sure coolant is circulating well.

Water wetter is a good idea, and depending on where you live, you can improve cooling capacity by increasing the percentage of distilled water vs actual coolant. I run 25-30% coolant, but we virtually never see freezing temps. The coolant is more of a lubricant than anything in my climate.

And, no, I would not say you can be confident after you get a steady stream out the bleeder. Air can still be trapped somewhere below. I always treat it as a 2-3 day process. Bleed it when hot until no bubbles and take note of the coolant level. Then in the morning, if it's gone down quite a bit, there's probably an air pocket that got smaller when it cooled. Take it out for a drive to move the air pocket around, then bleed it again. And keep doing that until all the air is gone and the coolant level doesn't vary by more than an inch or two between hot and cold. Also be sure the heater is on when you bleed, so air isn't trapped in the heater core/pipes.

Have you pulled the connector off the thermo-switch and confirmed the fans don't run that way? If you remove the connector, and the fans still run, then you are barking up the wrong tree with the switch. Although, my hunch is your switch and thermostat are on the cusp of fighting each other and that reduced cooling capacity is pushing you into the always-on side of that cusp. A 92c switch and improved cooling capacity (water wetter, higher water/coolant ratio, better venting) will hopefully push you to the other side of that cusp where the fans cycle on and off. With a 92c switch, don't be surprised to see the temp gauge climb above half way at stop lights and very low speeds. It should come down to the thermostat rating while cruising and once the fans kick on.

Thanks @Tom
I temped the upper and lower radiator hoses and they were roughly 18°C difference that was about 80 and the bottom one was about 62C.

Unfortunately, now I have another new plot twist symptom. The red light on the temperature gauge came on when the fans kicked on the second time but the gauge was reading around the halfway mark or less.. See picture attached. Edit: forgot to mention, I did recently change the temperature sender with a uro unit. I wonder if got a bad sensor. is that idiot light reading the block temperature? I went ahead and shut it off after this happened.I guess at this point I need to just know if I’m overheating or not. I have no idea if I can trust the gauge and now the light coming on really has me wondering. This is new.Any ideas on this? Guessing this still could be air pockets as well in the system or like you said, poor circulation.

And thank you. Noted on bleeding the system. I live in a warmer climate down in Texas so I could definitely use more water. I’ll just top it off with distilled water. And keep at it with the bleeding. I have not tried unplugging the Thermo switch yet, but I will try that next.

[Tom]

That warning light is controlled by the same sensor as the temp needle, but the two can fail separately. The sensor is in the block -- it has two spade connectors. The wider spade creates a resistance (ohm) reading to ground, which the gauge shows as a temp. The narrow spade just shorts to ground when the temp exceeds the upper limit. We have a test procedure to check the sensor in the download section, but in an effort to get it out into the wild, I've reproduced it below as regular image files. This way you can test both the warning light function and get a good sense if the temp needle is being fed accurate info. Unless your harness is shorting to ground somewhere, or the narrow connector came off and is touching the block (and therefore grounding), it looks like that part of your sensor is suspect. That doesn't necessarily mean the temp portion is bad, but warrants testing it for sure, and if the warning light part is bad you'll want a new sensor either way. Here's how to test....

944tempsensor1.jpg (184.2 KiB) Viewed 40 times

944tempsensor2.jpg (178.31 KiB) Viewed 40 times

944tempsensor3.jpg (161.21 KiB) Viewed 40 times

944tempsensor4.jpg (122.94 KiB) Viewed 40 times

944tempsensor5.jpg (175.06 KiB) Viewed 40 times

[Glassmuseum]

Thanks @Tom

I had forgotten to mention that I just replaced the coolant sensor a few days ago with a uro unit. I did a bit of research and it seems like those units can be faulty and trigger the warning light early so I still had the old vdo factory sensor and bench tested it using your guide and it checked out fine. Installed it back into the car and the warning light issue is gone.

I also have a bit of an update on the underlying issue.

• I pulled the Thermo switch connector off and ran the car up to temp and the fans did not kick on. As soon as I put the switch back on though they came on. So that seems to check out with your test.

• I’ve been trying to run the car and and drive it and pop the bleeder screw every time and I’m still not getting any bubbles every time. I did have to top off the expansion tank a little bit this morning, though

• unfortunately the same behavior persists so back to square one. The car will get up to temp, fans kick on and they run for longer than I think they should, but they do end up the first two times switching off. Once it hits that third cycle, the fans just run and run and run. I will add one exception though: last night I did get them to kick off a third time, but it took absolutely forever. I had to let the car run for over 10-15 minutes or more and they finally kicked off a third time but I couldn’t get them to kick off again after that or today.

I didn’t want to point out one more thing that has been annoying. When I went to change the thermo switch for the first time, I noticed that the plastic threads were a bit stripped. They seemed fine the first time I changed the switch out, but now after I’ve changed it twice, I’ve had to use Teflon tape to try seal it. It’s currently not leaking, but it was dribbling a little bit. I’m wondering if there’s air getting in there and causing the fan switch issue.

So next steps?

• should I be worrying about a possible small head gasket leak? I went ahead and did another compression test and all four cylinders were between 125 and 128 psi cold. No large discrepancies. Not seeing any white smoke out of the tailpipe, etc.. I did go ahead and buy one of those block tester things just in case

• should I be looking at a new radiator? I’m wondering if the stripped threads on the switch could be causing some issues. I’m also wondering if it really is just an old ratty radiator. I’ve been topping off the expansion tank with distilled water and added some water wetter. I don’t ever really see the gauge get up too much over halfway mark ever. I also noticed that the fans do kick on just above that first hash, which seems a little earl right?. Even if my thermostat is opening around 85 to 88C shouldn’t this 92C Thermo switch be eventually kicking it off? If I’m in the market for a new radiator, does anyone have any recommendations? I’ve done some reading. It looks like CSF and wizard radiators are good replacements

[Tom]

How are you testing to know when the fans are on? In hot Texas weather, it would be perfectly normal for the fans to stay on low for a long time or constantly if the car is not moving. If you are testing in the garage, this may be a non-issue. If you are testing while cruising, how are you even sure they are on? I can usually hear HIGH speed, but it's pretty hard to hear them on low while cruising.

"It’s currently not leaking, but it was dribbling a little bit." dr bob once made a funny comment about oil leaks being called seepage to avoid acknowledging a leak. This sounds like that. I'd try a pressure tester at 14psi and wait 10-15 minutes to see if anything comes out. If even a drop, it needs to be fixed. When coolant comes out, air goes in, and even one drop every 10 minutes will quickly add up and create cooling issues. It may or may not be related to your issue, but you'll want to fix it either way if it's leaking, dribbling, seeping, sweating, percolating, or otherwise getting wet on the outside.

If your thermostat is not opening until (say) 88, and your thermoswitch goes ON at 92, you have a pretty small window for the fans to go off. On the one I tested, the OFF point was 3 degrees below the ON point, which would make it 89 degrees on yours if similar (or possibly even lower if not accurate to the last degree). That would mean your cooling system would need to be working nearly perfectly -- keeping the coolant temp right at the thermostat opening point -- to keep the fans off. Anything less than perfect would cause the fan to run all the time.

I wouldn't get seriously worried about the head gasket yet -- unless you are seeing mocha in the oil cap, or the coolant is spitting out the overflow after a hard drive. This seems more like basic cooling system issues. If it were mine, I'd start by fixing the dribble and confirming it holds pressure with a pressure tester, with zero external moisture (aka leaks). If the fans are still stuck on after that -- when the car is cruising (say 30mph or faster) -- then I'd probably remove that thermostat and run the car without any thermostat (blocking the bypass in the throat), or try a much lower temp t-stat (say 72C). That should allow the fans to turn off while cruising. But if not, then I'd think about replacing the radiator (assuming there's no risk of a bad pump), and personally I'd just get an OEM replacement.

p.s., thanks for yet another data point about URO products being hit or miss, with miss heading up their leader board.