Code 41 and 91

Haus said:

when i check the ohms of the ground wire. (im guessing my digital multimeter can)

what exactly do i do. i have the red part and the black part of the multimeter.

just not sure how to check the ohms and what goes where and where to check the hego ground.

couldnt find anything on the site. maybe i wasnt loking in the right place

cause i know the orange ground is screwed into my firewall. maybe not enough ground. im gonna try to get it to the intake bolt

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If unsure of something, ask for clarification as soon as confusion is noted. Way back on the first page, we talked about ohming out the ground.

Set your DMM to read Ohms (it kinda looks like an upside down U), on the 100 (or so) ohm scale. Put one meter-lead on the ground-wire's ring terminal and the other lead to the negative battery terminal. It does not matter which lead goes to what. The reading should be around 5 ohms or less.

Have you confirmed that you have proper voltage going to the sensors as well?

If both of those items check out, I would personally disconnect the battery and clear the memory (after you disconnect the battery, push the brake pedal down for 10 seconds. Then reconnect the battery). Run the car and check for codes. You might find that you have an issue with a vac leak, your MAF, or something else that is affecting the air-fuel trim. Such items often don't trigger a CEL.

Haus said:

ok will do.

BTW do i need to have the car started while i check the ohms of the ground?? or no?

thanks for all your help so far. i really appreciate it

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Definitely not. Ideally you have the battery disconnected but simply being sure the key is off is good enough. It's not rocket science. Do note that this doesnt mean that the ground is ok at the sensor (the wire could be broken). I generally do all my testing at the O2 connected instead (so I'm testing closer to the sensor, to take any broken wires from the firewall to the connector into account). If you do this, don't pierce the wires on the O2 sensor pigtail.

0.1 Ohms is excellent, though it's better than most of the grounding pathways I encounter.

I'm still thinking that there's a vac leak or MAF issue (as JR pointed out early on) because you've had parity in the readings and it's done this with a couple sets of O2's IIRC.

The possibility of a MAF problem is a tough one. It would be very easy for a MAF to be just enough off to cause problems and not set a code.

Look for leaks in the air ducting between the MAF and throttle body. Do you have an aftermarket cold air induction kit? Uneven airflow caused by some kits could upset the MAF's output signal.

Wrong MAF sensor in stock MAF body is another possibility. The stock MAF is a 55mm unit with a screen that faces the filter side. The sensor is marked with an arrow to indicate the direction of airflow. The sensor part number is E9ZF-12B579-xx. The xx is a revision number that doesn’t make much difference.
Mismatched MAF – any MAF other than the stock unit is a potential mismatch.
The following list gives OEM MAFs that are a one for one swap with the stock unit.
The 94-95 Mustang 5.0 MAF & sensor is also found on:
1995-94 Mustang 3.8L F2VF-12B579-A2A,
1994-92 Crown Victoria 4.6L F2VF-12B579-A2A,
1995-94 Mustang, Mustang Cobra 5.0L F2VF-12B579-A2A,
1994-92 Town Car 4.6L F2VF-12B579-A2A,
1994-92 Grand Marquis 4.6L F2VF-12B579-A2A,
Evidently the –A1A, -A2A, AA, etc. on the end of the part number is a minor variant that did not change the operating specs. You should be able to ignore it and have everything work good.

If you have a K&N flat panel filter or other filter that requires oiling, excess oil may coat the MAF sensor element and cause problems.

Here’s the code 66 MAF test path: it may provide some useful information about the MAF.

Code 66 MAF below minimum test voltage.
Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing +12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for computer power relay switched +12 volts. Check for 12 volts between the red and black wires on the MAF heater (usually pins A & B). while the connector is plugged into the MAF. This may require the use of a couple of safety pins to probe the MAF connector from the back side of it.

There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater heats the MAF sensor element causing the resistance to increase. The amplifier buffers the MAF output signal and has a resistor that is laser trimmed to provide an output range compatible with the computer's load tables.

The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element, remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).

Changes in RPM causes the airflow to increase or decease, changing the voltage output.. The increase of air across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow. Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF wiring connector and ground. Make your measurement with the MAF disconnected from the wiring harness.


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Engine Information

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

The voltage at idle is a little high.
Does the MAF body have a screen?
Did you verify the part numbers on the MAF sensor?

Haus said:

anything that causes concern for the little High Voltage of the maf at Idle???

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If your idle is higher than the commanded idle setting, this can cause the MAF signal to be a little higher than it otherwise would be.

3 pages?

wow that was a long story,,,i just looked thru ford book on 41/91...sends me straight to fuel control pintest H1,,,check fuel pressure GUYS pintest H has 30 pages im not a typest,,,how much should i give on this? in those links does it have how to ELIMINATE CODE41/91 or ATTEMPT TO GENERATE CODE 41/91,,or a C.B.T. test?

Fuel pressure gauge adapter fittings:
Holley Performance Products Flare to Pipe Fitting 17945NOS AN 4 to 1/16” pipe
Holley Performance Products Female-Male Adapter 16785NOS 1/16” male pipe to 1/8” female pipe

Or
See Auto Meter

I made my own pressure gauge and holder. I bought the NOS or Autometer adapter that you screw into the place on the fuel line where the schrader valve goes. You have to remove the Schrader valve, but save it, since you will reuse it. I ran a piece of SS (stainless steel) braided hose to a 1/8” pipe brass tee fitting that I mounted on the fender well by the MAF. I made a mount bracket out of aluminum angle I got from Home depot and bolted it to the fender well. Then I mounted the brass tee to it with some machine screws and a plate. I sandwiched the brass tee between the aluminum angle and a flat piece of aluminum that I trimmed off the extra aluminum angle. Three screws laid out in a triangle pattern go through both pieces of aluminum to clamp the tee in place. Put the Schrader valve in the spare port of the brass tee. I used a cheap industrial gauge from MSC Direct (Industrial Supply Equipment from MSC Industrial Supply P/N 56468499). It works great and was cheaper than (less than $6) anything Summit had.

If you look through the MSC Direct catalog, you can find any type of gauge you want, including liquid filled. You only need a liquid filled gauge if you mount it directly on the engine. The liquid filling dampens out the vibrations.

You can buy the Autometer Stainless Steel braided hose for like $60. Or a local shop that makes hydraulic hose assemblies can make it for you at a cheaper price. Ordinary low pressure hydraulic hose can be used in place of the Stainless Steel braided hose, the Stainless Steel braided hose just looks nice. I got mine for $4 at a place that sells industrial and military surplus parts of all kinds. The Stainless Steel braided hose goes for about $3.60 a foot and the fittings are probably about $5-$8 each. You can make your own and save some $$$, the shops may have a setup or labor charge to fabricate the hose assembly.













Safety device to prevent major fuel loss in the event of a failure of the hose, fittings or gauge...

If you dont need a permanent gauge, a diagnostic gauge takes about 30 seconds to hook up and costs under 40 bucks in most cases.

It's a must-have tool if you work on EFI cars. Sometimes you need to check the accuracy of a permanent gauge itself.