STILL getting MAF code 66, and now code 98. Is my ECU toast???

MustangLX-5.0 · Jun 26, 2010

So, I thought the code 66 was caused by me, apparenty not. I replaced my MAF sensor and that totally resolved my throttle problem I was having. Recently I got a code 66 again, and a code 98. So I ran a KOEO test and got a code 29 which mainly meant a faulty speed sensor. Replaced speed sensor and reset ECU. Checked for codes on KOEO.. only got code 11 twice.. which is a pass. Started up and let idle for a long while. As soon as it started warming up a bit, it started to stall out. Started back up and it started surging. Ran KOER and I am now getting code 66... again.. and a code 98. When I do the KOEO test again, I only get code 66. Why do I keep getting MAF code 66? MAF sensor checks out fine. And I never got code 98 before. What is causing this? Is the computer toast? All the terminals from the MAF sensor leading to the ECU check out fine. They are all within specs of what they should be reading. Is there any way I can have the ECU tested some place?

jrichker · Jun 26, 2010

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.

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.

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 - Wiring & Engine Info

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

Code 98 - basic computer internal tests not passed before it started processing the request to dump codes. Do a Key On Engine Off test before starting the engine. Wait until you see the 11 flash before pressing any buttons if you have a hand operated scanner. The computer is operating in Limp mode, so fix any codes it dumps prior to doing anything else.

MustangLX-5.0 · Jun 26, 2010

Does anyone know how to actually get to the computer? I can see it when I take the kick panel off, but I can't get to any of the harness pins. Theres a black piece of plastic attached to the rear side of the harness preventing access. And the computer is shoved so far up. Do I have to remove the blower motor or part of the dash?

Haus · Jun 26, 2010

you have to take the kick panel off, and unscrew that black piece that holds the computer in place and just yank it out

MustangLX-5.0 · Jun 26, 2010

Please don't tell me this MAF sensor is frickin bad. I just bought the damn thing from Rock Auto. I got 4.2K ohms between pics D & C & between pins D & B (ground). This was with it unplugged. This is the same as the last sensor I just took out. I'm gonna be pissed if this one is bad.

I done the tests between the MAF harness terminals and the ECU harness. On both pins, it measured 0 ohms, so the connection is good. There was no noticeable corrosion on them. Is the ECU burning out the MAF or something? It was good when I first got it. It had measured upwards of 47K ohms. 12V at pins A & B.

What is going on ?

tmoss · Jun 28, 2010

this is the second time I've seen this symptom mentioned today.......

MustangLX-5.0 · Jun 29, 2010

Would air filter oil on the MAF element lower the resistance readings? I really have my doubts that this is the case because I have not had to clean nor apply oil to the filter in several years. I don't know what else is causing this. On my last sensor, I cleaned it several times with MAF sensor cleaner. It did not make any difference.

The thing is, when I first installed the new sensor, the engine ran great. It drove great with no problems. I drove it around that day without issues. Then the next day, I'm halfway up to the gas station and it starts actin up. Intermittently like it wasn't getting gas, but I know I had gas in it. Then I got the CE light. Later found the VSS was faulty, and also the code 66 again. I am beginning to think the ECU is whacked. There is no other explanation for this issue.

jrichker · Jun 29, 2010

What sort of voltage output are you getting between pins C & D on the MAF sensor? How do they compare with the numbers listed in the code 66 test path?

MustangLX-5.0 · Jun 29, 2010

Idling right now at about 725 RPM, it's showing 0.82V . Just started hesitating and idle was slowing down as I typed that. It stalled out now. I tried to give it gas but it was like it made it worse. As the engine speed increased, the voltage did as wel. When I taapped the throttle hard, the V jumped up near 3V. Right now, after it stalled out, it's showing 0.01V .

Starting it up again now...umm... now it keeps stalling out. It wont start now. Not even when I try to give it gas. Hmm .. no friggin idea what is wrong now. ... ohh looky now. I unplugged the MAF sensor and it started right up without issues. Surging.. but running. Throttle is very responsive.

I think this new MAF sensor is shot. Apparently it has to be. I just plugged the MAF sensor back in whie it was running. 3 seconds later it just died.

Ok the sensor is bad obviously. Did I just get a bum sensor, or is something burning it out? I now gotta send this thing back and hopefully get another one. Might have to try the more expensive Ford sensor.

jrichker · Jun 30, 2010

Are you taking the measurements at the MAF or on the computer? If at the computer, there could be some wiring problems.

MustangLX-5.0 · Jun 30, 2010

Measurments taken probing sensor wires on the back of pins C & D.

MustangLX-5.0 · Jul 1, 2010

So I am still not sure if the sensor is bad or not. The sig V is in the range of what it should be when air passes through the MAF tube, yet it stalls out when the sensor is plugged in (after the engine warms up). The actual meaning for code 66 is, MAF Signal voltage below 0.4V. Yet the only time the Sgnal voltage dropped beow 0.4V is when the car stalled out or wasn't running. Would the code come up from stalling out?

While checking resistance between pins C & B on the sensor, should the resistance measure out at 0 ohms? I mean is the ground supposed to be tied to the 5V Ref pin? That dont seem to make any sense.

MustangLX-5.0 · Jul 1, 2010

Different story today. Started it up, and of course it was stalling out right away. Unplugged MAF and it stayed running. I had the meter connected to pins D & C this time and I verified the sensor readings. Swapped probe on C to B. Got same readings. I could blow air down the intake tube and it would make the voltage jump up a couple hundred millivolts. Finally after tweaking with the throttle controls I did get it to start with MAF plugged in. It was eratic and wanted to stall any time I pressed on the throttle. I had to rapidy press the throttle in and out repeatedly to get the throttle to finally respond. Moved some wires around... unplugged and replugged some sensors. Had it stall out. Kept checking the wiring.. then shut key off an restarted. Finally started on its own and stayed running ok. Throttle was somewhat responsive. Let it idle for awhile. Voltage on MAF pins C & D does increase steadily as RPMs go up some. It wasn't eratic either. V would spike to over 3V when I quickly punched the throttle. I can tell the MAF is obviously working. I unplugged the MAF while it was running and it made the engine stall out. Ok, first it hates the MAF sensor plugged in and stalls and runs like crap... now it is the complete oppositte and runs good with it plugged in and starts running sh***y when unplugged. ????

I connected scanner and ran code tests whie engine was running. Got code 45. Typical thermactor fail code. No other codes. No code 98 even. It actually ran the engine idole test without jumping directly to codes first. Ok suspect no code 98 is the reason. Shut engine off. Waited then turned key on, and ran KOEO test. Got back codes 51, 66 and 96. Code 51 is from me unplugging ECT so ignore. Code 66, but I didn't get 66 whille engine was running like the previous time. And 96, secondary fuel pump circuit which I suspect was from the engine stalling out a ton so I am ignoring that. Oh and I also got code 11 twice as I started KOEO. There has to be some intermittent electrical problems goin on here. No idea what or where at. I have carefuly tested all the wiring going to ECU. And there are no problems.

Oh on a last note. For the 4th time already I just happened to find the 1/8th in AC Vac hose had broken yet again. I fixed it shortly after I started messing with the car. I know vac leaks can cause issues, but this is a tiny pinch of a hose end. And I can't see that causing a MAF code 66 or severe stalling.

jrichker · Jul 2, 2010

Looks like computer or wiring problems: if you have another computer handy to swap, try it.

You may want to try and run 4 new wires for the MAF. Ford Fuel Injection will have the MAF connector, pins and other stuff to do that. Check out their products section for pins and pigtails.
See How to solder like a Pro Ford Fuel Injection How To Solder Like a Pro for some excellent adive on soldering & splicing wires.

STILL getting MAF code 66, and now code 98. Is my ECU toast???

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