Car revving when cooling fans come on
06-19-2017, 02:12 PM
#1
Teching In
Thread Starter
Join Date: Jul 2015
Posts: 46
Likes: 0
Received 0 Likes
on
0 Posts
Car revving when cooling fans come on I have a 5.3ls swap in my 73 Corvette.
It idles fine when temps are lower, but once the first cooling fan comes on, idle jumps to 1400-1500, and with 2nd fan on at 210, revs to 2400-2500. That being said, if I'm in gear, I can take my foot off the pedal and idle at 70mph. I have to fight that with the brakes to slow down.
Where should I look/what should I have done to try to fix this?
It idles fine when temps are lower, but once the first cooling fan comes on, idle jumps to 1400-1500, and with 2nd fan on at 210, revs to 2400-2500. That being said, if I'm in gear, I can take my foot off the pedal and idle at 70mph. I have to fight that with the brakes to slow down.
Where should I look/what should I have done to try to fix this?
06-20-2017, 12:56 PM
#4
Teching In
Thread Starter
Join Date: Jul 2015
Posts: 46
Likes: 0
Received 0 Likes
on
0 Posts
Trending Topics
06-21-2017, 09:40 AM
#10
Restricted User
In today's world of HD camera on even Walmart cell phones, how did you manage to get a picture that blurry?
Did you unplug the fans to see if it still idled high when they were supposed to be on?
Did you unplug the fans to see if it still idled high when they were supposed to be on?
06-21-2017, 09:48 AM
#11
Teching In
Thread Starter
Join Date: Jul 2015
Posts: 46
Likes: 0
Received 0 Likes
on
0 Posts
I did not get to doing anything with the car last night.
Tonight I WILL test the volts at the battery and unplug the fans to see if any changes. Could a low charging alternator possibly be the culprit?
06-21-2017, 10:04 AM
#12
Restricted User
Well, if you unplug the fans and it starts to run hot without idling high, you can rule out any fan or electrical related issues and start watching the tune to see whats happening.
Log IAC steps and all of the idle channels you can.
Log IAC steps and all of the idle channels you can.
06-21-2017, 01:49 PM
#16
Teching In
Thread Starter
Join Date: Jul 2015
Posts: 46
Likes: 0
Received 0 Likes
on
0 Posts
I can try cardboard after it cools down from testing the 'no fans' theory to ensure it is something with the fans for sure, and not just a weird temp thing with the tune.
06-21-2017, 06:42 PM
#17
Teching In
Thread Starter
Join Date: Jul 2015
Posts: 46
Likes: 0
Received 0 Likes
on
0 Posts
Well, when I unplugged the fans, it did not rev up or "hang" in high throttle. ...but it would definitely overheat if I let it.
voltage was 14.4 with fans unplugged, and 14 once both fans came on. I have not placed cardboard between the radiator and intake yet. In theory, they would be robbing the intake of air instead of giving it air, so I'm not sure that would make it rev up..
voltage was 14.4 with fans unplugged, and 14 once both fans came on. I have not placed cardboard between the radiator and intake yet. In theory, they would be robbing the intake of air instead of giving it air, so I'm not sure that would make it rev up..
06-21-2017, 09:04 PM
#18
This whole scenario just doesn't make a lot of sense. I have to believe the PCM is commanding the high idle for some unknown reason, since there would be no other way the idle could get to 2400rpm. It's not like the fans are pushing the throttle blade open, so it almost has to be the IAC opening for some reason. Maybe the fans are dirtying up the signal to the IAC in some way? What harness do you have? Did you mod it? I would check all your grounds since they can do crazy things when grounds become less than secure, and if that pans out, go back to your tuner and have them look at it. I would think if something that big was amiss, it would be easy to spot in the tune. sorry I couldn't be of any more help than that, but thought I'd toss it out there. Good luck with it.
06-21-2017, 09:10 PM
#19
One more thing, if your fan wiring is in fact intertwined with the rest of the engine harness, you might try separating it, and only have the two signal wires going between the relays and the PCM in there and the rest of it away, that way there would be no way to get any interference.