problem with mass air conversion

Hi Heres the problem, i did a mass air conversion to my 87 mustang because i put a diff motor in it with a different cam and what not. Any ways it wont idle at all with the mass air plugged in , it opens the iac to 100 % and it sets a min volt code 66 for the mass air. If you un plug the mass it idles fine. What i think is the mass air is no good but not 100 percent sure. The mass air is not brand new so its very possible its bad but i just wanted to see if anyone else had any possibles. The bigger cam wouldlnt cause this probble would it, its only a b cam. Also i read some where that i might have to put a newer style map sensor on it? :shrug:
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Is The MAP sensor still connected to vacuum? If so, disconnect it and cap the vacuum line on the manifold. The MAP gets vented to the outside air.

Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.



IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

For those who are intimidated by all the wires & connections, see for what a typical hand scanner looks like. Normal retail price is less than $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see– It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.
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).

The MAF output varies with RPM which causes the airflow to increase or decease. 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.

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 and ground.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
ok so i put a new mass air in and it reads right with the iac plugged in. the problem i am having now is the iac goes 100 % open as soon as i plug it in and if you bring the rpms up it will loose all throttle response and die after about 5 seconds. I dont know why it is commanding the iac on to full open position. Also with the iac plugged in it wont run at all it just dies and has no throttle response?????? Has anyone seen this problem. I think it is setting the 66 code due to the iac being unplugged........
Thats the thing i cant get the base idle to set , even with the iac unplugged it seems to let air through. Like i said before i am a ford tech and this is what i do for a living but i never seen this problem before. tp is set to spec with out the iac plugged in it idles at like 600 rpms, and wont really change from that, with the iac plugged in it wont even idle it just opens up all the way. The really weird thing is it wont set a code for the iac being unplugged, so this is my thinking,1 i have a vacuum leak that im not finding maybe from the lower intake or something,or 2 the pcm has a driver problem in the iac circuit. I got the 66 code to go away now the only code i get is 95 wich has to do with the fuel pump monitor but the fuel pump is working fine. This is really weird, oh yeah one other thing is that it will run with out the air horn hooked up also. Does anyone know what kind of vacuum i should have with a little b cam?
hmm it only has increased idle with the iac plugged in? hmm...have you tried a different iac? and where do you get that 100% number from? my understanding was that although the ngs supports data stream, the eec-iv's did not have this capability. you dont have a vacuum leak if it idles without the iac. the iac will not set a code on the first ignition cycle, ie the light doesnt come on if youre just setting base idle. it is possible that you have a short to ground on the iac wiring. you know, constant power, eec-controlled ground. when you say it runs without the intake tube attached, is the mass air plugged in or not?
Yea its plugged in but it will run without it plugged in also. Im just using 100 % as another word for stay all the open cause no you cant see any data on the scanner only read codes. Anyways I have taken the iac apart and cleaned it and made sure its ok, and also it worked right before i did the conversion witch was like four days ago. So i dont see how it could have got a short to ground when the car has been sitting in my shop the whole time. I talked to the guy i got the pcm from and he said that he tested in a car right before he shipped it to me and it ran ok. I have checked and rechecked my wiring and it all looks good. I have checked everything, keep the ideas comin. lol
With the IAC unplugged put your hand over the throttle body and see if it still runs. If so, it is a vacuum leak. I suspect you've got a wiring issue, as in you've got the MAF hooked up wrong. Check the wires with it unplugged with the Ford EVTM and see if everything has what it should. I think maybe you've got the power wire switched. Whoever said you don't have a vacuum leak if the IAC is unplugged and it still idles is evidently high. The IAC is a metered vacuum leak, after all. If you've got the IAC unhooked and adjusting the idle doesn't change anything then something is amiss. Do you have the MAP sensor vacuum port open to the atmosphere? It should be. I'm also a ford tech and you're embarrassing me here.
Thats the thing i cant get the base idle to set , even with the iac unplugged it seems to let air through. Like i said before i am a ford tech and this is what i do for a living but i never seen this problem before. tp is set to spec with out the iac plugged in it idles at like 600 rpms, and wont really change from that, with the iac plugged in it wont even idle it just opens up all the way.
if you can get it to idle at 600 rpm with the iac unplugged then YOU HAVE THE BASE IDLE SET you just need to put it a little higher like 850.

and just for the record, *I* am a ford tech as well. :Zip2:
Why am i embarrassing you? This is the the way i have the mass wired

A closest to the front of the car is vpower. It has 13 volts while running its piggy backed to 12 volt source.... red
b is ground. its good also piggybacked
c is signal or vref. has five volts reference with it unplugged
d is signal return reads .6 at idle which is in spec according to the pced.

i have checked the mass wiring and its good used the evtm to wire it........

And yes the map is open to outside air.
Added it to my existing speed denisity just built a mass air harness and swapped the air mgt pins and added in the pins for the mass to the correct pins I found a bunch of sites on how to do this you know budget mass air conversion.
Which computer are you using? Is it still doing the thing where it loses all throttle response and dies if you rev it up with the IAC plugged in? It sure sounds like something is not wired right with the IAC. Does the MAF output stay at .6V when you plug in the IAC or is it throwing it off somehow? I hate diagnosing over the computer.... Sorry I said you were embarrassing me but man, these guys are supposed to ask US the questions. It's a pride thing, you know. What all changed when you put the new MAF on?
well heres the deal when you unplug the iac the mass volt goes way under .6 when you plug it back in it goes right to .6 and then goes right above it and dies due to the fact the iac is stay wide open. with the new massair no change same problem. I didnt change anything with the iac wiring though? Oh its a a9l from a 92 manual 5.0.
oh ok i see.
it seems our own stangnet has an article on how to do this. there is a wire relating to the fuel pump relay you have to move or you get the code 95.

the only commonality i can see between the IAC and the MAF is that they both share VPWR (12 volts). please measure this wire with various things plugged in and unplugged and tell us what you get.
When you say the IAC is wide open what do you mean? When an IAC is wide open the vehicle idles at like 2500 RPM. Do you mean it is shutting all the way and dying? If it idles higher with the IAC plugged in then the MAF reading should be higher with it plugged in. More air coming in, higher MAF voltage. It kind of sounds like it's flooding out. You can't get PIDS on this car right?
i measured vpower in several spots and got 12v or more while running. What i mean by the iac is wide open is it opens all the up and the car wont idle at all. when you plug it in you can here it start sucking air really loud and you can put your finger over it and feel that it is wide open......