Imo000

Ted,
The MAF can be adjusted. It was not ment to be adjusted but the cap for the adjustemnt scew can be removed, and the unit can be adjusted if one wishes it to be.

Hilton

The pin for MAF adjustment using the pot built into the MAF isn't connected - at least on LH 2.3 (Haven't looked closely at a wiring diag for LH 2.2). Per John Speake, the potentiometer built into the MAF is actually separate from the MAF itself and doesn't affect the signal the MAF gives out unless the wiring on the car is set up to use it for that purpose.

The only MAF adjustment available is for non-cat cars fitted with the separate potentiometer for the LH (i.e. doesn't use the one on the MAF either), which uses a separate pin to the O2 sensor and hence does nothing on cat cars.

edit: Just went and had a look at the early 32V wiring diagram - pin 6 on the MAF (the built-in pot) is indeed connected, so its only LH 2.3 where it does nothing. You can measure the resistance of the pot across pins 4 and 6 of the MAF - for a "new" MAF it should be around 300 ohms, and is then adjusted higher to correct idle mixture as the MAF ages and loses calibration.

Cosmo Kramer

Yes you are correct 2.2 is indeed connected and adjustable.

Porken is the guru of the 85 - 86 MAF, he even designed a tool to tune it while the car is running. I read in a thread here that the adjustment goes from 0 - 1000 ohms. I am showing 1022, so the adjuster screw must be completely maxed out on the rich side, however I am still showing symptoms of running lean.

Correct me if I am wrong, but does the O2 sensor only adjust the mixture if it is too rich to protect the cats?

Imo000

I'm prety sure the O2 makes sure the engine doesn't lean out too. Do you still have the adjustment cap on the MAF and does it look intact?

dprantl

The O2 sensor will compensate whether the mixture is too lean or too rich. It does have a maximum range though, and if the car is out of that range there is nothing it can do. If you disconnected the O2 sensor and noticed no difference in running, I would try a new one. They are cheap. +1 just get a mustang 3-wire bosch sensor and splice it in.

Dan
'91 928GT S/C 475hp/460lb.ft

Cosmo Kramer

OK, hooked up a spare MAF (thanks Imo000).

Good news: The car must have gained about 20 hp right across the rpm band and runs a lot smoother, pulls a lot harder. Still can't turn those 275's much!

Bad news: Idle is still at 1100.

Unhooked the O2 and idle went to 1200 and it stumbled a bit off idle, this went away once it was connected. O2 appears to be working. The spare MAF tested and was at 782 ohms on the bench. Mine on the bench was 1020, same reading as at the LH.

Hmm, I am starting to think that I need to verify rpm's with a manual tach. When it drive on level the car creeps away slowly as normal, on a small grade it doesn't creep at all.

PorKen

Have you tested the idle/WOT inputs at the LH/EZF plugs?

It shouldn't make any difference, but the MAF CO grounds to pin 6 of the LH plug.

(LH22/EZF pinout thread)

Tampa 928s

A couple of things to check:
That connector first picture I believe goes to your Tps switch, if the switch is not grounding with closed throttle inj could still fire causing a high idle.
Check you throttle cable if was adjusted and is a little off it will keep your butterfly valve open.
A leaking or incorrectly inserted Maf will raise the idle a lot.
Is your Iac working if it is stuck same symptoms.

Cosmo Kramer

I checked it on both and got the same reading. Also indicated 1022 ohms on the bench.

I have checked idle on the EZF but not WOT, should I check that? Should I check both? If WOT was stuck, it would be in open loop and the O2 disconnect shouldn't have made a difference.

I was looking at your pinout diagram and I see an input for an air temperature sensor, is that the one in the airbox? How much does it affect the mixture?

Iac tested on the bench and connector has been repaired in the picture. Throttle cable has some slack.

Could the idle switch be adjusted too far and holding the throttle plate open?

Cosmo Kramer

OK, all tested at EZF and LH, engine still warm after sitting a couple of hours.

MAF: 782 Ohms on LH
WOT switch: Opening to 0 resistance when right to the floor on both
Idle switch: 0 resistance until throttle is opened then infinite on both
Temp sensor: 741 ohms on both

Mike Frye

Ted,

You've got a lot going on here, and I think (like most of us) you've got more than one problem, which complicates things exponentially. My idle problem had me going for months because there were at least 3 causes (vac leak, TPS switch was marginal, and O2 sensor connector was ng).

Have you been through the LH/EZF test plan? I don't want to give you the standard comment about reading the manual, but it shows ways to isolate the TPS input (and that the LH is responding properly to it) by just using a timing gun (idle:10 deg-off idle/cruise:20 deg-WOT:40 deg. give or take).

This is for the stock chips, so if you have aftermarket chips you may have different readings.

I would suggest adjusting your MAF using the dial on the bench and setting it to 382 ohms or so, then put it in and try adjusting the idle from there.

Most likely you've got a situation where something went bad like a vac leak so the idle went up, then someone adjusted the MAF or throttle bypass screw or something when it was warm to bring it down, then it was too low, so they tightened the throttle cable or something....

John Speake

For your LH2.2 equipped 85/86 car there is a proceedure in the WSM for setting the idle CO pot on the MAF (with O2 sensor disconnected which defaults the LH to mid range of the O2 loop) and also setting idle speed with the screw below the X brace. Have you seen that?

Ken's tool serves much the same purpose.

Cosmo Kramer

Put my MAF back in and set to spec 382 ohms. Performed very similar to the spare one, a lot better then when it was maxed out at 1000 ohms. Idle still steady at 1100 neutral, 1000 in drive.

John: I tried setting the idle with the connector jumped and the idle went up as soon as I disabled the ISV (Mike Frye had the same thing happen on his 85). My air bleed screw on the throttle body is tightened all the way in. The idle also went up 100 rpm with the O2 disconnected and ran a little rough off idle.

I am going to get a flexible drive and try to adjust the MAF a bit while it is running and see how it responds.

Can anyone confirm which way is rich and which way is lean? Logic would say less ohms is rich (like the cooling of the MAF wire = less resistance = more air flow = more fuel)

John Speake

Clockwise to richen, etc. 20 turn pot, no end stops. Stop when you reach the linit or repeated turning can damage the pot

PorKen

The whole 0-1K range is not richening, it's more like the fader pot on a radio. ~382 ohms is coded as 'zero'.

From that 'zero' point, turning CW adds/richens, and CCW subtracts/leans from the fueling calculation/mixture.