Tippy

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?

1999Porsche911

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

Tippy

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?

1999Porsche911

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

Tippy

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

Loren

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.

Tippy

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.

Doug Donsbach

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

Doug Donsbach

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.

1999Porsche911

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.

Loren

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.

ls911

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

pszikla

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

Marc Gelefsky

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.

pszikla

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