Slightly rougher idle

The idle on my 3.6 seems a little rough, The tach is rock steady but it just seems a tad rougher, Like the old passenger seat is moving a little more Would a cleaning of the MAF be in order? Otherwise car feels great!

Mine had a lumpy idle when I first bought it, and now it doesn't. I can't pinpoint what fixed it, but here's what I did over the first couple of service visits: - New fuel filter. - Throttle body service. - Motorvac the fuel system. - New plugs. Runs like a champ.

How many miles when you did this? I got that going on as well. Runs like dream but seems to bounce just a little bit at idle I got 13K so I was not even thinking plugs, filter, etc... Maybe I am being just to critical, I am somewhat of a perfectionist

Quotes source post: 5296173

It had about 76,000 km on it... 47,000 miles or so. It had seen about 7,000 miles a year of daily-driver-type usage.

Most of the time what you describe is a dirty throttle body. The MAF does nothing at idle (unless it is shorted).

Ok I am not going to sweat it, for now... Thanks BruceP

Try cleaning the throttle body - smoothed out my idle nicely. Although these cars normally are a little rough - as you know!

Throttle body clean is quick and cheap, I'd start there. http://www.renntech.org/forums/index...howtopic=15413 http://euroautoforums.com/forums/ind...showtopic=1351 http://www.popularmechanics.com/how_...e/1272341.html

Quotes source post: 5295943

and there was no dirt on the thing. (Cleaning was an attempt to rectify a hunting idle and other symptoms that ultimately proved to be due to a failing AOS and not a dirty/bad MAF.) I've got nearly 80,000 miles on the current MAF in my car and haven't had it out to clean it once. Running stock air intake and filter too. For some model year vehicles, a variable cam timing selenoid could fail and the symptom was a lumpy idle. Don't know what your car's year is, if it has the electric variable cam timing selenoid, but if it does, and if MAF cleaning or other potential solutions don't help... Sincerely, Macster.

macster, what years had the electric variable cam thing? I have had a idle issue in past. I cleaned the MAF, ICV and throttle body with some small success, but still feel vibration intermittently. My next thing was to replace fuel filter at next oil change, but perhaps that cam thing is issue. What kind of cash is the repair?

the variable cam timing solenoid failure is normaly the contributing factor on later cars..

isn't this vibration thing considered normal? what's tough to gauge here is we're trying to prescribe something to something where we don't have any quantitative figures, just subjective observations about a seat moving more than usual. then people read this and they're like oh man my seats vibrating, i need to go do x and y as i read on rennlist... what i notice is, the seat sometimes vibrates more, sometimes it doesn't vibrate at all. depends on temp, cold start or strip mall visit, etc. i'm not so sure we're going to come to any sort of logical, salient conclusion here. if the vibration makes your voice sound like you're talking into a whirring fan... then consider a real looksy.

Quotes source post: 5296200

What monitors engine idle and regulates idle airflow? I remember that when I placed a screwdriver on the butterfly in the throttle body (2000) I could feel the butterfly constantly moving back and forth. What tells the butterfly how much to open for idle? On the 99's, the idle control valve or idle air bypass valve directed air around the butterfly, what tells the ICV or IABV to open or close at idle?

Quotes source post: 5299783

From the Tech Book (for egas cars): "The throttle position is determined via a potentiometer fitted at the throttle valve. The DME control unit supplies the potentiometer with a regulated voltage (5 V). Depending on the potentiometer position (throttle opening angle), a higher or lower drop in voltage occurs at the potentiometer. The DME control unit uses the voltage drop and the corresponding current flow to determine the smallest opening angle for the throttle. This angle is defined as the idle speed position."

Quotes source post: 5299783

The DME is monitoring engine RPMs and adjusting air intake (either with the stepper motor throttle or the ICV in the case of the mechanical throttle) in order to maintain the idle RPM setting.

So it is just monitoring RPM? How would it know how much fuel for a given amount of airflow? Or, is it a pre-programmed fuel map with correlating airflow/fuel requirements per RPM?

Quotes source post: 5299979

It is programed for open loop, using the values of the MAF. In closed loop, the O2 sensors make final adjustments.

If my old 996 idled better and revved quicker with the MAF disconnected, how is not part of the idle circuit if you will? Does it go into a default mode?

Quotes source post: 5300461

The MAF IS part of the idling formula. The MAF consists of, not only the flow of air, but IAT and engine load calculations.

Loren said it wasnt, that is what I am questioning......

At idle the air flow does not change - so the MAF voltage should not change. So, disconnecting the MAF will have no effect on idle - the DME will simply see that it is not changing. I stand behind what I said previously - clean your throttle body. On egas cars this is 99% of all idling problems.

Quotes source posts: 5300514, 5300549

See how these are conflicting points to me........ Second, I thought the sensor on the side of the right intake plenum was the IAT? The one you disconnect and the engine lid fan turns on.

Quotes source post: 5300610

That is the engine compartment temperature sensor. There is a different temperature embedded in the MAF to sample intake air temperature.

Quotes source post: 5300549

Are you sure about this? When the load increases at idle (P/S, A/C, etc.) the airflow must increase in order to generate more power to maintain RPM.

Quotes source post: 5300610

If there was not a difference among answers, there would be no need to have a discussion board. Of course the MAF plays a role in the idle of your car. That is why the car is able to idle properly when it is 0F and 100F outside. The DME using the signals from the MAF at idle to calculate airflow and air temperature which helps calculate engine load. Without these parameters, how would the engine know how much fuel to inject? There is a hell of a big difference in the amount of fuel you need at idle for 0F intake temps and 100F intake temps. Timing is also affected by the MAF signal at idle. You can prove this by decreasing the current on the white/black MAF wire and you'll find your idle will stumble in open loop because the mixture will be too lean. There is a middle of the road default mapping for times that the MAF is inoperatable. Works well in average temperatures. This is why a signal range is specified for the MAF at idle. IAT is part of the airflow calculation of the MAF. The sensor for the engine compartment fan has nothing to do with the intake system.

Ok - I think you guys are missing my point. At idle the air flow does not change - correct? The air temp might change slightly. So, if you unplug the MAF - the DME can and does compensate (both readings don't change). If you had a good idle prior to disconnecting the MAF then it will still idle good - if your idle was rough before disconnecting the MAF will still not affect the idle. Try it, I have many times and the idle will not change. After a while you will eventually get a CEL - because the MAF is disconnected. Don't get me wrong the MAF plays an important role in measuring air flow amount and intake air temperature - but these have almost nothing to do with a rough idle. Back to the original problem here - I still stand by my prediction. In 9 out 10 cars with a rough idle cleaning the throttle body will bring the idle back to normal.

Quotes source post: 5300990

That's great info, makes good sense. My 996 bumps down about 30-50 RPM at idle. I will check and clean the throttle body when I get a chance Thanks

Reanimating this old thread.................. The engine stopped yesterday as I came up to a stop sign and easily restarts but won't idle at all. If I drop below 1500rpm it dies. Managed to baby it to Indy about 15 miles and it only threw a CEL half way there. Indy called this morning and says the MAF is dead. After a search on this site this morning, it is conflicting information as to whether the car can idle with or without MAF connected. If my MAF is fried as Indy says, thenI would conclude that your car won't idle at all. Dealer wants to sell new MAF to Indy for over $600.00!!! Pete

MAF from Suncoast is $297 and easy DIY. But, I am prety sure car will idle without MAF. You may do better starting a new thread.

Hi Marc! I would have started a new thread but thought it was relevent enough not to. I can tell you that at least in my case, as soon as i let the rpm drop below 1500 or so, it would shut down. I've ordered a new MAF from the following web site at $282.89 Cdn they also seem to sell in USA. http://www.bimmerspecialist.com/ I hope my Indy was right in diagnosis after reading CEL otherwise I've just bought a MAF for nothing! Pete

I've done MAFs in other similar vintage motronic cars but not 996, the 2000s motronic MBs will start, idle and run with no MAF connected (surefire way of determining whether or not it's the MAF that's stalling you out). I think Cory's post here confirms that the 996 will do the same. https://rennlist.com/forums/5300461-post18.html Worst case put the MAF on the shelf, you'll need it someday!

Hi Loren I own a MY99 C4 with Egas throttle body. I have to face an idle problem. As you always said I cleant the throttle body but default is still present. I didn't found the issue for the moment. That I can add about MAF is when you disconnect it then motor shuts down. Otherwise, I have no error codes on dashboard and the car runs fine. I don't know how to troubleshoot the issue as I think it could be an air leak. Thank you for your help if you have time :-) jLaurent