Chessie

Greetings -

So I replace the battery in my car a few weeks back. Nothing unusual, car starts and runs fine but I notice right away that the car is idling higher than usual. It settles in at about 925 RPMs , if the tachometer is accurate. Initially I think maybe the battery is not fully charged and the motor is just idling a bit high to try to help make up the change, but after a long highway run, the idle is still at 925 when I come to a stop.

Before changing the battery the car idled at 850 RPMs. This is with the UMW tune. Stock it used to idle at 750 RPMs.

Anyway, I'm at a loss to explain this. Has anybody observed this behavior? No other changes were made to the car between the time the battery went bad and the time I replaced it. Any info you might have would be greatly appreciated.

Cheers -

-c

"02996ttx50

minor vac leak(s) cause idle fluctuation aka "hunting". common variation is up to 1k rpm and back down to 750 at idle.
so, does it 'hunt" and then settle? or is it steady at 950?

try switching on the a/c. it should immediately settle back to 750 ( or in your case 850 if that's how the tune set it ..)

i'm guessing it's a coincidence, having nothing to do with a battery replacement - since that shouldn't affect anything - and that your car has developed a minor vacuum leak. check the y-pipe, i/c hoses ( any/all connections ) and if still in doubt? have the car pressure tested at the turbos, and listen. seek, and you'll probably find lol. its not a big deal.

RngTrtl

ive had a few small vacuum leaks and it caused the idle to increase a 100 rpms or so and after the engine warmed up and I was idling the CEL would come on. I cant recall the code, but basically there is unmetered air being used and it throws the CEL light. OP I am assuming that you are not getting a CEL?

Macster

Try an E-Gas/throttle body calibration operation.

The steps should be in the owners manual.

From memory: With your foot off the gas pedal through this procedure, turn the key to the on position. Leave on for at least 60 seconds. (During this time I can sometimes hear some faint mechanical noises.) Turn the key off for at least 10 seconds. The next time the engine is started the calibration will be complete.

You probably need to set the window limits. Press and hold the window lower button until the window is all the way down. Then press and hold the window raise button until the window is all the way up and for at least 5 seconds after. Done. Repeat for the other window.

wross996tt

Realize, when you removed the battery, the ECU does a soft reset. So Macster's idea is good. Also, the car will naturally re-learn over time.

With all DME systems, the engine must run for several minutes before the engine control module can
relearn the idle speed and mixture adaptation values!

Problem Remedy
After disconnection of the power supply, the idle speed might
change or fluctuate briefly until the idle speed positioner (M
5.2) or the throttle adjusting unit (ME 7.2) is readapted.

The mixture adaptation is also lost.
After the battery is connected.
With the DME ME 7.2, it is
necessary to carry out a learning
and adaptation routine as
described below:
1. Switch the ignition on for 1
minute without starting the
engine. Do not actuate
accelerator pedal.
2. Switch off ignition for at
least 10 seconds.
This completes the adaptation
of the throttle adjusting unit.

"02996ttx50

most common idle high code is p0507 "idle control high" or some such. annoying as ****

Chessie

Gentlemen -

Thank you for the responses.

The idle does not hunt, but it steady at 925 or so. I stays constant regardless of whether AC is running, lights on, etc. I was thinking that a vacuum leak would cause the idle to hunt, but perhaps that is not the case. Also, performance under load seems OK. Car still seems to pull with full power under acceleration.

I should mention that I have tried the e gas reset to no avail. Also, no CEL appearing.

Perhaps it's still worth having things checked out to ensure there is no leak. In the mean time, any other thoughts?

Thanks again for the help -

-c

RngTrtl

just as a heads up when I had the vacuum leak and CEL light throw after the car was warmed up and I had been driving for a while and then if I came to a stop light and let it idle for a minute or so then it would throw the light.

"02996ttx50

... even more fun when you're trying to gauge the oil level and it says "engine must be idling to check oil" lol

Macster

At idle the VarioCam Plus low intake valve lift mode should be active. In this mode my info is engine speed is controlled by the amount of fuel injected. If fuel pressure was elevated this could result in a higher idle speed.

However, I would have to wonder why the DME would not recognize something's going on and reduce the amount of time the injectors are open?

Or recognizing the RPMs are "high" reduce the amount of fuel being injected to bring the idle speed down where it should be?

The behavior might be explained if the intake air temperature or more likely the engine coolant temperature reading was off, lower (or possibly higher -- more on this below) than it actually is. Thus the DME would think the engine is cold (or too hot) and add more fuel and run the RPMs a bit higher.

If you have the equipment -- an OBD2 code reader/data viewer you might monitor intake air temperature and coolant temperature and see if you can spot something up with the two temperature readings.

The only time my 2003 Turbo's idle speed was elevated was after spending some time in stop/go traffic going up highway 50 towards South Lake Tahoe (and beyond). The ambient temperature was in the low 90's (it was July 3rd) and pretty warm. At some point I noticed the engine idle speed was elevated but I do not recall now by how much. A check of the dash found no warning lights and I don't recall any exceptional water temperature or battery voltage readings either.

After I reached the summit and the speed picked up by the time I got down to the lake the idle speed was back to normal.

dprantl

High idle in MAF-equipped cars (with or without idle oscillation) is almost always due to vacuum leaks. Air leaks into the intake tract downstream of the MAF and is not metered. The DME sets the amount of fuel to be injected based on (among other things) the reading from the MAF. Due to there being more air than the MAF measures, after the combustion process the O2 sensors then tell the DME that the mixture is lean. The DME then compensates by raising the amount of fuel to be injected. More air + more fuel = higher engine speed. The DME cannot reduce the amount of time the injectors are open in this case because it will create a lean-running condition.

Newer engine management systems with electronic throttles can be smart enough to realize this process is happening and can adapt the idle throttle opening to compensate for vacuum leaks if the leaks are small enough. When you disconnect the battery long enough, this adaptation data is erased. MAF-less systems that depend on a MAP sensor to meter intake pressure are much more resistant to this kind of condition.

Another pretty common reason for a high idle in newer cars is because the electronic throttle plate is gummed up, but that almost always throws an OBD code.

Dan

Chessie

Thanks for that information - makes sense to me. I'll get her checked out to see if I can find a source of a vacuum leak.

Cheers -

-c