MAF Sensor Diagnosing

1990 Ford Mustang AOD supercharged w/cold air intake, 19 lb/hr. injectors, stock MAF sensor

For a while, I have been dealing with a very rough idle ... everyone keeps asking me what cam is in my car ... no cam just running rough at idle. It stalls once when starting cold ... have changed IAC several times thinking this was the problem which would be typical. However, today, I started the car and immediately unplugged the MAF and it ran good no rough idle! I put the connector back on and it ran like crap ... pulled it off and it ran good. I repeated this over several times until the engine was warm. And, each time, it ran good without the MAF sensor harness connected and each time I connected the harness, it ran terrible. And each time, it was a noticeable change within about 2 seconds. So, is the MAF bad??? Have checked numerous times with code scanner and no MAF codes have ever showed up except if it stalled a few months ago. So, the computer doesn't seem to be having a problem with it ... yes, no???

I have noticed that it runs rough when cold, on restarts and at red lights after sitting for a minute or so. Could the heated element be skewed a bit due to the cold air kit? I see that some companies now sell a MAF sensor for CAI installations but do not know if it is a bad MAF sensor or I need to upgrade to a CAI MAF Sensor. Any thoughts? Below I have posted what i was able to read today with my meter. Possibly someone sees something wrong since my manual doesn't cover this area of testing to well.

Resistance (unhooked)
Terminal A to B: 180,000 ohms
C to D: 4,160 ohms

Key On (hooked up - backprobed)
Terminal A: battery voltage (12 volts)
B: 2 ohms (B Terminal to ground - Key off)
C: ,07 Volts
D: .08 Volts

Engine Running
Terminal A: Alternator Voltage - 14 Volts
B: - - -
C: .20 Volts
D: 1.20 Volts

A= VPWR
B= Ground
C= MAF RTN
D= MAF SIG

Other than this crappy idle, the car runs pretty good and purrs at highway speeds. Everything else has been checked or replaced. This is the first time I have found and been able to duplicate it running bad at idle!!! So, is the MAF bad? Are the values off? Does it need a CAI calibrated MAF sensor for 19 lb/hr. injectors (these are new on the market but maybe this is why they are selling them now). Why isn't the computer showing it as being bad since it runs so terrible. Again, disconnecting it, and an immediate improvement in idle quality is easily seen and heard. I know the computer reverts to the tables when disconnected. And finally, last time I took it through emissions, it stumbled a bit but all emissions numbers were "0" except one number which was virtually zero. But, lately, it seems to burn a lot of gas and smells strong w/o the cats so I am not sure if it is rich or lean! Either way, I have found a problem in the MAF potentially which is causing this. Would like some verification of this problem and input to resolve this problem w/o throwing some more money at it! Thank you for your help!

Okay, it is a BBK cold air intake which is quite larger than the factory box setup. At idle, it may not be reading as accurate due to low airflow ... but at higher RPM's it probably is reading better and more true. So, that is why I think I could have the problem just at idle and off idle. Afterall, isn't this why everyone is saying to try to "clock" the MAF sensor which I did to no avail. It is significantly different running when unplugged (runs great) as opposed to being plugged in. It is the original Ford 21 year old MAF sensor that came with the car. I am the only owner. Can get numbers if truly needed. Everyone asks if its cammed but it runs rough and misses though it could be construed as a "cammed" engine. The supercharger is designed by Kenne Bell as a kit which utilizes the stock 19 lb/hr. injectors and a FMU is supplied which handles the fuel flow under boost conditions utilizing stock injectors and MAF. However, I am not referring to any boost conditions since it is rarely that I am under boost due to the extreme acceleration of this car. Just ain't many spots that I can hold the pedal down for too long since 100 comes real quick. I just checked some readings with my meter quickly to post some numbers that may be meaningful. Do you have an accurate description to check MAF meters that I can follow. My book is confusing on this issue and I believe a few things were left out. I did find out that one of my ohm tests was done wrong but the other is correct between the C & D terminal. Its the C & D terminal that I need to redo as suggested since it is a differential voltage. but the A&B voltage reading was correct and was battery voltage or alternator voltage. A complete descriptive test procedure would be helpful. Having the car run so terribly that it shakes at idle but disconnecting the MAF evens it out tremendously seems like a problem to me. I also notice it acts this way when cold, after restarts, and after the car sits at a light for 20 seconds or so. Yes, I have gone over the "Surging Idle Checklist" too many times. this is where I noted the "clocking of the MAF" to get it positioned better in the idle airflow and running airflow. Seems to me, this is the problem I am experiencing. Only other problem I have found is the vacuum seems to be down 2-3 in Hg. Checked for vacuum problems everywhere as suggested but have not found anything yet other than a little bit at the EGR valve which I think is in all cars due to the way they are made. Swapped one out and no difference was made even though I know there is slight leak between the diaphram rod and housing. No codes have been detected regarding the MAF yet except once or twice when it stalled. And each time I start it cold, it cuts out after 5-10 seconds and on restart stays running, idles down then gets rough. This is the only identifiable problem on this car impacting the running of the vehicle per se. Had it out on I-95 tonight and it ran hard and strong ... got off and came to a light and the rough idle and off-idle was there as usual. How could it run great at idle with the MAF disconnected but crappy with it connected? Doesn't seem to make sense other than something is wrong with the MAF or the CAI is causing this ... thus, the reason that BBK is selling a 19 lb/hr. cold air tuned MAF? just my thoughts. Input and help with this matter greatly appreciated because I have been over this motor too many times to count, and this is the only thing I am finding and it can be duplicated over and over!

The fuel pressure is stock fuel pressure as on any other Mustang with the stock fuel pump which has been replaced. Kenne Bell even told me NOT to put in a larger fuel pump cuz it would heat up the fuel recirculating it back to the tank cuz I wanted to put a larger pump on it for future upgrades. During idle and driving around town, the supercharger does NOT come into play since all of the air is bypassed more or less. It is supposed to drive like a stock 5.0 until the pedal is nearly floored (vacuum drops to about 4-8 in Hg.) and then the bypass valve closes and then it jumps to an immediate 7 pounds of boost instantly. Then, the FMU takes over the chore in conjunction with the stock fuel injectors. I've checked fule pressure and it was 34 psi @ idle (vac. hose attached) and 41-42 psi (vacuum hose disconnected).

Tonight, I measured 1.40 Volts between terminal C & D with a voltmeter at idle while it chugged along ... this seems too high to me? Thought it should be in the 0.6 volt range. I saw a graph somewhere which shows 1.40 Volts equivalent to the airflow at around 30 miles per hour or so. Input? Does anyone have an exact test procedure for the MAF sensors?

Checked MAF volatage readings today between C&D terminals. At idle had around 1 volt and as I opened the throttle, it progressively increased as I opened the throttle. So, I think it is working properly. Will have to go back one day and check ohms at pin 9 & 50 but I think they are in spec. Seems as though it should be a bit lower at idle though. Still sorta perplexed why the engine runs better when the MAF is unplugged? Anyone else experience this?