Car idle RPM fluctuations - is it a problem? Solved

Questions on how we spend our money and our time - consumer goods and services, home and vehicle, leisure and recreational activities
Post Reply
Topic Author
fsrph
Posts: 1610
Joined: Sun Jul 26, 2009 7:54 pm
Location: Pa.

Car idle RPM fluctuations - is it a problem? Solved

Post by fsrph »

Subaru Forester, 45k miles. Here's the issue. Once the car warms up, RPM's at idle is about 600. That's fine. If I let the engine idle about every 30 seconds the idle jumps to 1000 for maybe 10 seconds then goes back to 600. A couple minutes later it repeats the same cycle. And keeps repeating the cycle.

I'm not doing anything while this happens, not turning on air conditioning, defroster, etc. Car drives fine and check engine light is not on. Anyone know what's causing this and should I get it checked by a mechanic?

Francis
Last edited by fsrph on Wed Mar 03, 2021 12:46 pm, edited 1 time in total.
"Success is getting what you want. Happiness is wanting what you get." | Dale Carnegie
h82goslw
Posts: 493
Joined: Fri Jun 17, 2016 5:44 am

Re: Car idle RPM fluctuations - is it a problem?

Post by h82goslw »

Your car could still be throwing codes, but not one severe enough to cause a check engine light. Might want to go to autozone or some place with a scanner and see if there are any codes in history that could help you.
sk2101
Posts: 589
Joined: Sun Jun 02, 2013 4:54 pm

Re: Car idle RPM fluctuations - is it a problem?

Post by sk2101 »

I suspect a vacuum leak. Check the rubber hoses for any damage.
andypanda
Posts: 2012
Joined: Sun Nov 12, 2017 8:11 pm
Location: Richmond, Virginia

Re: Car idle RPM fluctuations - is it a problem?

Post by andypanda »

Google Subaru idle fluctuations if you haven't and see the variety of questions over the years/decades. Could be vacuum, could be the ECU is in need of a reset or it could be what the dealers say - it's normal.

Okay, one guy said his went from 1,000 to 5,000 and back and THAT is NOT normal.
hayman
Posts: 50
Joined: Tue Nov 13, 2018 4:04 pm

Re: Car idle RPM fluctuations - is it a problem?

Post by hayman »

You say that you aren't turning on the AC, but if the AC is on, the compressor may be cycling, so it could be normal in that case. I would check if the AC compressor is cycling. 400 rpm difference does seem like a lot, but it depends... might still want to check for codes since it's free.
Topic Author
fsrph
Posts: 1610
Joined: Sun Jul 26, 2009 7:54 pm
Location: Pa.

Re: Car idle RPM fluctuations - is it a problem?

Post by fsrph »

Thanks for your replies. I found the answer. Read this online, "The car defroster does use the air conditioning to defog the front windshield. If the a/c system is not functioning then the system will not work as designed."

My car climate control was set to defrost. I don't remember setting it to defrost, must have been set that way for weeks. Took it off defrost and the idle RPM's stay at about 600.

Francis
"Success is getting what you want. Happiness is wanting what you get." | Dale Carnegie
Silverado
Posts: 1650
Joined: Fri Oct 18, 2013 6:07 pm

Re: Car idle RPM fluctuations - is it a problem?

Post by Silverado »

fsrph wrote: Wed Mar 03, 2021 12:41 pm Thanks for your replies. I found the answer. Read this online, "The car defroster does use the air conditioning to defog the front windshield. If the a/c system is not functioning then the system will not work as designed."

My car climate control was set to defrost. I don't remember setting it to defrost, must have been set that way for weeks. Took it off defrost and the idle RPM's stay at about 600.

Francis
Glad you solved it pretty easily. Back when I had a vehicle that had AC, I always yanked the AC relay out during the winter. My experience, living in the midwest, I just needed warm air blowing across the windshield. No dehumidification needed. So that’s my worthless anecdote.
User avatar
dratkinson
Posts: 6116
Joined: Thu Jul 26, 2007 6:23 pm
Location: Centennial CO

Re: Car idle RPM fluctuations - is it a problem?

Post by dratkinson »

Silverado wrote: Wed Mar 03, 2021 2:20 pm
fsrph wrote: Wed Mar 03, 2021 12:41 pm Thanks for your replies. I found the answer. Read this online, "The car defroster does use the air conditioning to defog the front windshield. If the a/c system is not functioning then the system will not work as designed."

My car climate control was set to defrost. I don't remember setting it to defrost, must have been set that way for weeks. Took it off defrost and the idle RPM's stay at about 600.

Francis
Glad you solved it pretty easily. Back when I had a vehicle that had AC, I always yanked the AC relay out during the winter. My experience, living in the midwest, I just needed warm air blowing across the windshield. No dehumidification needed. So that’s my worthless anecdote.
Mine. You want dehumidification (A/C) with defrost? Push the A/C button.

But have learned that an intermittent surging engine can be tamed by turning on A/C. Should put that on my list to fix.
d.r.a., not dr.a. | I'm a novice investor; you are forewarned.
Post Reply