MAF idle hunting - Tuning help needed

[don911]

87 3.2 - I removed the Autothority MAF about 8 months ago. It seemed to be running fine at the time of removal. The car did seem to "diesel" a little at shut down. Anyway, put it back on today and the idle hunts like crazy. It starts and idles great when cold. As soon as it warms a little, it starts hunting. Cycles from 1100ish down to 200. It didn't die but came close.
Re-installed stock intake and chip and all is fine.

Read lots of threads on idle hunting but didn't find anything MAF related. Anyone "tuned" their MAF?

One thing, when I installed the new chip, I bent one of the pins. I strightened it and it looked ok but I don't know if it could have been damaged. Can bending a pin screw up the chip?

Thanks,

[89911]

Having run a car for 4 years with a MAF and 2 years without, my advice to you is leave it off. After futile efforts to tune one with a chip on the dyno (With Steve Wong's help), the basic problem is that they run too rich at full throttle, killing horsepower. Since switching to the original intake, mid and high throttle are crisp and powerful, like a well tuned engine should be.

[don911]

That's not encouraging. I'd like to keep the MAF. Throttle response feels better with the MAF over the flapper box. Except for the idle hunting, car runs great with the MAF. I've never dyno'd with the MAF installed so I don't know about the HP or A/F.
Does the chip control the A/R ratio at WOT?

Also, the car didn't have the idle hunting issue when the MAF was installed last year. Don't know why I'm having the issue now.

[don911]

?

[89911]

I can't help you with the specifics, only the results of the tunings. The person you need to ask is Steve Wong on this board, especially when it comes to chip, MAF's, and tuning. Best of luck.

[Fast951]

The information about the MAF not working well is incorrect. It boils down to whoever is doing the tuning.
When converting a VAF to a MAF the code on the chip (not just the data values) must be changed. Using the VAF transfer function (where voltage to air is calculated) with a MAF is incorrect. Also each MAF with a particular calibration has a unique transfer function.
Another item that people tend to ignore when converting to MAF is the Intake Air Temp, which is used in the code and needed for the VAF but not for the MAF except for startup. We have achieved great results with the MAF conversion on the 951 and the 911 3.2L take a look at yje dyno chart http://vitesseracing.com/Catalog/911/911.html

As far as the rough idle, check the followings:
- Make sure no vacuum leaks
- Your AFR is way off (it sounds as a overly rich idle).

On your DME there is a Fuel Quality Switch (FQS) that you can access by removing the plastic (or brass) plug from the side of the DME. Most chips I have seen has the followings:
FQS 0 (counterclockwise): 0% fuel
FQS 1 (1 click clockwise): +x% fuel
FQS 2: +y% or -y% fuel
FQS 3: +z% fuel (if FQS is -y%) or -z% fuel (if FQS 2 is +y%)

Change the FQS to add or subtract fuel and report if it helps. Also, with the MAF you might need to adjust the TB idle screw.

There is nothing wrong with using a MAF as long as it's done correctly. If your MAF was working fine, then there is something wrong with your setup (including the MAF sensor itself).

Instead of using a Intake Air Temp, they used some sort of an adjustable resistor, it's in the pigtail harness between pin #1 & #4.. Modifying the resistance allows you to alter the AFR. Try leaning it some..

[Kevin70914]

I just did my 87 3.2 easy fix as folows.

Start and run the engine to temp.
Disconect the plug on the idle compensator valve. (located right in the front with a1/2' air line running from the intake snorkle)
Adjust the idle mix until the idle is smooth at ~850 rpm
(Idle mix is behind a plastic idiot cap ,mine was yellow, just above the compensator valve on the intake. Remove the cap if it hasn't been yet)
Adjust the screw to obtain the specified idle.(1/2 to 1 turn,clockwise if I recall, should fix things)
Reconnect the compensator and you should be good to go.
(I did fine tune my idle mix slightly after dring for a while, ~1/4 turn)
The MAF is great. Better response, sound and MPG. Of course I do tend to run through the gears with more gusto:<)

[Nine9six]

I was experiencing the same problem with an Autothority MAF & chip.
I replaced the o2 sensor and the hunting stopped. I believe the MAF makes the car run on the rich side, as mine would not pass CA smog with the unit installed. I eventually removed the MAF, and sold it to a gentleman that lives in a state devoid of smog laws...He loves it BTW.
My car now gets better gas mileage. Go figure :-)
Best of luck to you.

[don911]

Kevin, thanks. I'll put the MAF back on tomorrow evening and do as you suggest. Hopefully it will solve the hunting issue.

Fast951. Impressive dyno numbers. Are those numbers at the rear wheels?

[Fast951]

don911, all of the dyno numbers are at the rear wheel. The 911 3.2 runs were made back to back VAF then MAF.

[don911]

Fast951, Do you take Autothority MAF's in trade?

Is VAF the stock intake? Was this done on a stock 3.2L? with stock exhaust? Using a 15% loss estimation for flywheel HP, it looks like the first run (VAF) had total HP of 226. I think a stock 3.2L was estimated at 217HP but don't know for sure.

Regardless of HP numbers, a 11% HP gain between the VAF and MAF in back to back runs is impressive.

[Fast951]

don911, Sorry no trade for the APE MAF we use a different unit.
The car has headers and was using a aftermarket chip when we dynoed it. The goal of the back to back runs was to show the owner what can be gained by using Vitesse software and MAF.
The dyno does not show the crisp response when using the MAF...

[don911]

I was successful in getting my car to not idle at all now. I tried Kevin's procedure below. Initially I did get the motor to idle smoothly but RPM was close to 1100. Motor was not fully warm so I was going to go out for a spin. As soon as the Idle compensator plug touched the valve connector, the motor died. Even with the plug disconnected, it would not idle. It fires right up but dies right away.
I tried adjusting the potentiometer on the MAF and the idle mixture adjustment but not luck. I'm sure all the adjustments are all F'd up now.
Seems it's all a mixture issue but without a way to see the A/F mixture, I don't know what it's doing when I'm turning screws.

Any suggestions before I take 89911's initial advice and fire sale the MAF?

[Kevin70914]

Sorrry to hear that. The car should idle at~850 w/o the compensator. There has to be something else happening. Maybe O2 sensor or the Compensator valve as they have to be 100% for the system to work.

[don911]

I'll try changing the O2 sensor. Yesterday on the way home from work, the car was hunting a little with the stock intake after a warm start up. I don't know how old the sensor is so it might time. I assume the compensator valve is ok since the motor runs great with the stock air meter.

[don911]

$140 later and new O2 sensor installed. Same problem. I did get it to idle better. It would hold a steady idle for 15 seconds or so and then go through a hunting cycle. Didn't hunt as bad as before but still not what it should be.

Thanks for everyone's help on the issue but I'm defeated. I'm going to take 89911 advice and keep the stock air meter. I really like the better initial response the MAF provides but can't live with the hunting idle.

Anyone looking for a MAF?

[Fast951]

Before you give up, record the voltage out of the MAF. You can measure it off of the 4th MAF wire.. You will have on one side of the connector +12V power wire, then the middle 2 wires, the 4th and last wire is the signal..
The voltage is 0-5V, at idle it's 0.5-0.7Vdc ...

[mfloren]

another thing needs to be added here.

the Autothority mafs use a resistor for the air temperature signal to the dme. this resistor is adjustable (As i have so painfully found out)

aparently the air temperature directly affects (atleast on our 951's) the fuel maps.

After testing the sotck Bosch AFM i gor about 2ohms. I just dialed in the adjustable resistor on the mafs to this value and should find out tomarrow if it worked. (i had similar issues with my 951) if it does not work ill post a corection tomorrow.

[Nashville84Cab]

I have the Autothority MAS as well and found my idle to surge a little until I reach normal temperature. This does not occur with my stock AFM, but I really like the MAS and I don't mind living with that small surge during warm up.

Well here is an old thread of someone discussing about how to adjust the Autothority MAS:

Autothority Chip and Mass Air Meter


Quoted from Purrybonker from that thread: "The small screw is a potentiometer ("pot") to adjust the base resistance level in the MAF sensor. The effect of adjusting the screw is to alter the "base level" for air volume data sent to the DME. The pot is supposed to be set to 2200 k ohm (measured across the signal leads of the MAF harness) - but can be adjusted to smooth out rough idle or low rpm surging.

Just unplug the MAF wiring harness where it plugs into the Porsche factory harness. Place a resistance guage across two of the four pins in the MAF harness (the part still plugged into the MAF) - I think it's the first and third pins. Play around while making small adjustments to the pot screw - you'll eventually figure out which pins to use.

Set to 2200 k ohms. If you adjust the pot while the engine is running you will notice an impact to idle mixture/speed."

Good luck and let us know if this helps solve your A/F mixture.

Thanks,
Darrell

[mfloren]

on my car there was not (slight surgering) it was a sputtering / stalling. effect.

now when I placed my hand over the MAF so that the meter was forced to read more air it smothed out pretty fast. and ran great.

so I can either purchase a air fuel controler from a manufacturer like APEX'I or (not reccomended) duct tape the MAF so that it is metering more air.