MAF Location Causing Idle Issues?

331_mustang · Oct 30, 2007

Has anyone ever heard of the location/position of an MAF meter causing idle problems?

Here's why I ask. If I start my car up (cold or warm), it fires right up and finds a perfect idle. However, once I'm driving around, when I push the clutch in to slow down, it drops the idle to about 1200 rpm and won't settled until the car comes to a complete stop. It's the oddest thing. I swear the engine knows when the car is is stopped versus rolling. At first I thought it was just coincidental timing of the engine struggling to settle down to an idle and the stopping of the vehicle, but I've tested the theory several different ways... as long as I'm moving/coasting, no matter how long/far, it won't settle to normal idle.

So, trying to make sense of this... the only conclusion I could come to is the MAF is picking up air readings while I'm coasting and causing the computer to think the throttle is open, when it's not. Not sure why the TPS doesn't offset that reading, but maybe the MAF has a higher priority than the TPS if the readings disagree?

Anyway... has anyone heard of anything like this before?

My injection setup is as follows:
Pro M 80mm MAF
Anderson Ford Power Pipe (which locates the MAF down way low in the front fender)
75mm Throttle Body
30# Injectors
(Naturally Aspirated 347)

HISSIN50 · Oct 30, 2007

The computer raises your rolling idle (it does know you're rolling via the VSS). This is done so the car doesnt stall if you use the brakes, PS, etc etc.

If you have a tuning means, you should be able to change the function or turn it off.

Good luck.

Blackened302 · Oct 30, 2007

hmmm, thanks for clarifying that, JT.

wonder if the twEECer guys will see this and comment, too. i always thought my car was possessed (hehe, guess it is, by the ecu) and didn't know if it was *supposed to do that.. guess it is.

go-stang5.0 · Oct 31, 2007

I had this very same problem for years...I've noticed lowering the idle via the throttle blade screw tends to lower the "haniging idle" while rolling. I know if I set my idle to 900 it hangs at 1500-1600 rpm. When I lower my idle to 750 it hangs at 1100-1200. Hopefully when i get a tune this problem goes away.

Stanger007 · Oct 31, 2007

This is one of the tougher issues to tackle on the tunes.

Mechanical issues aside, I think the hanging idle is a combination of improper values in two main areas:

First is the TB airflow scalar that represents the amount of air flowing at idle based off of MAF volts. To find this, datalog the MAF voltage at your base idle and average it for the idle RPM. Then the formula is: (MAFVoltage * 2.2046226 / 60) = TB Airflow.

Second is the handful of ISC functions that control how much air passes and how fast the ISC should close (decay) to your desired idle.

Here's a quick lesson on how these functions work:
Decay
- Increasing decay rate speeds up the transition
- Decreasing decay rate slows down the transition
Clips & Pre-Positions
- Decreasing airflow speeds up the transition
- Increasing airflow slows down the transition

My logging of a car with these symptoms indicate that the ISC is told to command a higher duty cycle than it should for longer than it needs to. The hanging idle is next up on the list of things for us to fix with it.

HISSIN is right about the VSS telling the car if it is moving or not. The VSS cutoff to go from the rolling idle strategy to the standstill idle is either 5 or 7mph.

Wes

Blackened302 · Oct 31, 2007

thanks, Wes!

final5-0 · Oct 31, 2007

What JT & Wes said

Grady

331_mustang · Oct 31, 2007

Wow.... ok, that was a lot of good info to digest. I get it, but I am twEECerless. My last tune was done by Larocca, and this issue did not exist after the tune. The chip is still in there, but this build is more cubes (347 Vs 331) and the cam is a different grind.

a) Will a good tune/reburn of the chip resolve this? By the way, I'm looking for someone good to do this since I understand Larocca's is no longer in business.

b) Would the changes made (cam/cubes) have caused this issue to arise?

On that note of tuning/idling, talk to me about the IAC and throttle position stop screw. I opened up the IAC bypass to set the idle slight higher, which it now idles great. Should I have used the throttle stop screw instead? In short, where do you begin the adjustments, and where do progress to in order to get your idle tune after a new engine. First IAC then screw, first screw then IAC?

final5-0 · Oct 31, 2007

331_mustang said:
Wow.... ok, that was a lot of good info to digest. I get it, but I am twEECerless. My last tune was done by Larocca, and this issue did not exist after the tune. The chip is still in there, but this build is more cubes (347 Vs 331) and the cam is a different grind.

a) Will a good tune/reburn of the chip resolve this? By the way, I'm looking for someone good to do this since I understand Larocca's is no longer in business.

b) Would the changes made (cam/cubes) have caused this issue to arise?

On that note of tuning/idling, talk to me about the IAC and throttle position stop screw. I opened up the IAC bypass to set the idle slight higher, which it now idles great. Should I have used the throttle stop screw instead? In short, where do you begin the adjustments, and where do progress to in order to get your idle tune after a new engine. First IAC then screw, first screw then IAC?

Here are my thoughts

The cam change is hurting you more than the additional 19 cubes

You can have an effect on the idle speed with mechanical adjustments
but it is really controlled by the pcm

When you make mechanical adjustments you can only go so far
before
the pcm's adaptive strategy will try and compensate
to make things normal once again

Of the two adjustments you speak of, the tb blade stop screw will usually
have more effect on idle speed than the bleed screw.

You might clock the meter to see if you get a more stable idle as some of
the time, that can help

The chip you now got most likely has been setup with idle enhancements
soooo
Having said that ..............

I'd back out the tb blade stop screw until it no longer touches.
Then go back past the point of contact about 1/2 of a turn

Then start with an adjustment about 1 & 1/4 of a turn out with
the bleed screw.

I'd wait a day or two and to give the pcm time to stabilize things
with those starting point adjustments

You might then go out 1/4 of a turn on the bleed screw and see how
things go if needed.

I would N O T hose around with the TPS at this time as it had a good
idle before.

Good Luck

Grady

331_mustang · Oct 31, 2007

Thanks Grady. Good stuff. We're good experienced engine builders, but not so experienced with the electronics, so this is a big help. Thanks.

final5-0 · Oct 31, 2007

331_mustang said:
Thanks Grady. Good stuff. We're good experienced engine builders, but not so experienced with the electronics, so this is a big help. Thanks.

Always glad to help ... or ... at least ... try to do it

Just general stuff to get you started

There is an official write up somewhere around here that explains everything
in detail but I know not where it might be

Grady

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