Cooling Fans won’t shut off while running

Talk and Tech about turbocharged 924/944/968 cars
Post Reply
Glassmuseum
Posts: 31
Joined: Wed Mar 25, 2026 1:28 pm
Has thanked: 10 times
Been thanked: 3 times
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!

#1

Top
User avatar
Tom
Site Admin
Posts: 8951
Joined: Fri Jun 25, 2021 2:04 pm
Location: Silicon Valley, CA
Has thanked: 936 times
Been thanked: 4016 times
Contact:
Contact 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...

#2

Top
Post Reply