Idle up and down

[Jay B.]

Hello, I took the Box out for a ride today and I noticed when I stop at a light the idle starts jumping up and down from 800-1200 rpms rapidly and its never done that before. Could this be a idle control issue? Thanks for any info!

[jdraupp]

What year is your box. Any codes being thrown? Could be a variety of things, vacuum leak, maf, o2 sensors, etc. More info about your car will be helpful in diagnosis.

[Jay B.]

Its 2000 S.. I replaced the battery today because the cracking power was very weak so I doubt that has anything to do with it? My car doesn't get driven much maybe ever couple of weeks it comes out. No engine night. The car runs great its just when you stop the idle is up and down weird.

[KevinH1990]

Originally Posted by Jay B.

Its 2000 S.. I replaced the battery today because the cracking power was very weak so I doubt that has anything to do with it?

That is what caused the idle fluctuation. Disconnecting the battery deleted the correct setting from the DME. The DME will eventually relearn the correct setting. To speed it along, try this:

1. Turn the key to the on position, but don't crank the starter for one minute.
2. Turn the key to the off position for 10 seconds.
3. Start the car and take it for a drive.

The fluctuation may not end immediately, but it will be gone in a day or two.

[Jay B.]

Wow, I would have never dreamed just replacing the battery would cause such an issue. Thanks again for the info !!!

[BoxsterSteve]

Originally Posted by Jay B.

Hello, I took the Box out for a ride today and I noticed when I stop at a light the idle starts jumping up and down from 800-1200 rpms rapidly and its never done that before. Could this be a idle control issue? Thanks for any info!

I had the same symptoms... turned out to be a filthy throttle body in my case.
After little bonding time with the car, I had a clean throttle body again. While I was in there, I cleaned the MAF sensor too.
I also did the battery disconnect thing that KevinH outlined, and I got my rock steady idle back.

[Lapister]

Ditto on the Dirty TB I had the same issue erratic idle, and found it all gummed up. Spray cleaned and smooth as can be after.

[Retroman1969]

Mine did this very thing last fall. Turned out to be bad gas. And it took awhile for it to start acting up after the fillup. The closer to empty it got, the worse the irratic idle was. Filling it with fresh gas fixed it immediately.
That may not be it, but it would be the cheapest and simplest thing to try first.

[jdraupp]

Originally Posted by Retroman1969

Mine did this very thing last fall. Turned out to be bad gas. And it took awhile for it to start acting up after the fillup. The closer to empty it got, the worse the irratic idle was. Filling it with fresh gas fixed it immediately.
That may not be it, but it would be the cheapest and simplest thing to try first.

Yep and throw some techron treatment in there on the empty tank just to try another easy fix.

[thom4782]

Classic dirty throttle body symptom

[kk2002s]

I would agree on the dirty TB (I have had the hunting idle, cleaning TB cured it) except it just started after changing the battery so I would first suspect, as mentioned, that learned memory was lost.

[steved0x]

The ecu can (mostly) adapt to a dirty TB, and this is lost when the battery is disconnected. I had the exact symptoms and cleaning the TB cured it:

Throttle body cleaning on 2000 Boxster S

[nieuwhzn]

Steve nailed it. As the MAF sensor is slowly gumming up the ECU will be continuously compensating and the car just happily drives along. You disconnect the battery and it goes back to factory default, I.e. expecting a pristine MAF sensor. In time it will adjust to the dirty MAF again, but better is just to clean the sensor and throttle body.
Don't expect the car to run great immediately after cleaning the MAF, though. The ECU still has to deal with an old sensor, it will take some drive cycles for it to adjust properly.
BTW, if you find a lot of oil in your throttle body thern that could mean your AOS is on its way out. Rather cheap to fix.

[jb92563]

I had a similar issue after an AOS failure.
Disconnected battery, recalibrated gas pedal (eThrottle cars)
Cleaned the TB and the MAF plus some driving time and it all went back to normal.

On cleaning the MAF I discovered that inside the airbox there were bits of paper that looked like a typical product sticker. These bits of paper were fouling the MAF apparently. So check the airbox for contaminants and get a new air filter while your are at it, and remove any stickers you come across.