View Full Version : Bigger MAF -> MAF voltage low error?

02-07-2007, 12:58 PM
My -89 SC recently failed yearly emission testing and have been bugged by check engine light appearing at idle. Code is 66 -> MAF voltage low. I measured the MAF voltage at idle and it was way below 1.0v... about 0.5-0.6v if I remember right.
I have 42lb injectors and a C&L 76mm "calibrated" with a proper sampling tube.

Is it correct that EEC goes to using its "other" fuel tables if MAF voltage goes below predefined "low" value? That could explain failed emission tests. (I have chip that is "tuned" by specs of my engine but at inspection time I had chip removed.. )
Idle goes bit rough even with the chip when check engine goes on.
I even got new MAF sensor but it did not help :( I also pressurized intake track last summer, and there was no pressure leaks I could find. Vacuum leaks still possible??

- What is normal MAF voltage at idle?
- Does my larger MAF with 42's sampling tube cause this low voltage? (as sampling tube lowers airflow to 30/42 (injector ratio) -> about 71% of "normal)
- is only cure to rechip my EEC -> lower the "maf low" threshold?

02-07-2007, 03:55 PM
You have not stated whether you are running rich or lean. (high HC?)
Anyway, Your MAF voltage is in the same range as mine, around .51-.53. (with no error code or CE light)
I use to get a CE light when I was running to rich, which destroyed my O2 sensors.
I guess you could adjust the voltage at idle to change A/F say with a MAFextender at MAFterburner.COM
and monitor the voltage of your narrow band O2 sensors, the NB sensor voltage jumps up and down around stoich.

I fine tuned the low injector slope on the chip to get A/F at ide correct, using a wide band O2 sensor for stoich(a/f=14.7 @ .45v, some @ .5v),
and adjusted the high injector slope for the rest of the A/F curve (WOT= say 11.0 to 12, 11 is safer)

I recommend bitting the bulliet and getting a wide band sensor such as LM1 or PLX, to stop the guessing

02-07-2007, 05:35 PM
Yep, it was/is very rich at idle... about 10x what is allowed for cars that have catalytic converters. I'm not sure why o2 sensors do not sense richness and so that EEC would report "left/right bank rich" or similar ?! And it is always a code 66.
o2 sensors are quite fresh, about 13k miles.

02-07-2007, 08:25 PM
I just rechecked my MAF is it .4 to .43 at idle. (but I had adjusted injector slope)

SWAG, because the O2 are are saying rich all the time, the EEC is saying the MAF curve/voltage/AF (66) needs to be adjusted.
Voltage to low, may mean it needs more real AIR/voltage to lean out the A/F to 14.7 stoich.
(again change injector slope or MAF curve)
Also if the injectors slopes are cause of it running to rich, then adaptive learning may be OUT of it's corrective range (needs chip work)

(Do the tail pipes on the parking lot put a black spot on the pavement in idle, RICH)

Does it run lean with the Chip? (no spot on parking lot)
A lean idle (say AF = 15.5+) would cause rough idle. (requires WB O2 sensor)

The car will generally run smoother rich (say A/F 12 to 14.6), but at a rich cost/AF.

02-08-2007, 03:22 AM
Your voltage sound low, do you have a chip that is adjusted to lower "maf low" limit? Here is what I found from one "tuning forum"

When I was trying to cure Code 66. I clocked my MAF with a voltmeter probing C&D on the MAF untill I found the highest voltage I could get. Then set MAF_MIN just below my measured value so it would still turn on the CEL due to falure. You will need to play with this for a bit, lowering it little by little untill the CEL stays off and your idle stabilizes. If set to zero you will nullify the CEL. I also didnt change the high limit......I dont see any reason to.

End result..........your car will idle much better instead running in MAF failure mode

I also lowered the MAF_KOER_MIN by the injector ratio (19/42).... so it wont throw a code when running your KOER tests. Not sure if Pauls software has this with the latest definition file. I used both EECEditor and TunerPro when I was setting mine up.

Your pipe should be ok, just make sure you can run as much straight pipe before and after the meter as you can, I opted for 3" just keep things uniform.

Here is some info I found helpful (these are stock MAf volts):

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 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


My 66 error comes usually when the engine has been warmed up so the airflow is as minimal as it gets to maintain, say 800 rpm. When it is not throwing check engine light exhaust smells "ok", but when it goes to "error mode" it smells rich. I had cats off last summer (long story..) and yep, bumpper near exhaust was soothed black.

I'll measure my maf voltage again to be sure later, now we have -30F (-34c) weather, so my problem diagnostic is "frozen" atm :)

But as there are no hundreds of replys in this thread :) I assume this 66 code is not too common with people who have swapped to bigger injectors and MAFs..

02-08-2007, 07:45 AM
If the EEC thinks the MAF failed (like due to low Voltage), it will run in a very primitive mode where it instead uses the TPS value and some very static tables to determine the load and how much fuel is required. One of the many drawbacks to this failure mode is that it doesn't "know" if you put in larger injectors and a MAF calibrated for them, so it runs too rich. The larger the injectors are from stock, the richer it will be.

06-30-2008, 03:20 AM
... time has passed and I have started finishing to repair my car (little accident some time ago....) and there is little more info I have found from shopmanual cd..
Normal MAF voltage at idle is about 0.6v and "low voltage" limit is 0.4v so when injectors changed from 30 to 42lbs./hr then idle voltage and C&L + sampling tube is used to fix things idle voltage will drop to around 0.6v *(30/42) = 0.428v, which is quite close to "low voltage" limit.. so there is not much to do without reprogramming the chip.

Anyway, I'll measure voltage again and try to confirm if it is really <0.4v.. or does it loose some voltage in way to EEC.

06-30-2008, 05:01 AM
Contact D.Dalke, I'm sure he can help you...:cool:

XR7 Dave
06-30-2008, 10:25 AM
While I don't recommend running 42's without a chip, there are a few things you can do.

1) You can increase the airflow by opening either the bypass screw (aftermarket TB) or the throttle stop screw. This will usually help you pass emissions also by lowering idle spark at the same time. As long as you don't go too far with it the EEC will still be able to come close to hitting target idle speed. This is a quick fix.

2) You may need to do the above anyway, but you can also get a chip. I can provide you with a baseline file and then you can modify the chip yourself if you need to or you can have me email you updates as required if you'd rather not do it yourself. Cost is reasonable, email me for specifics.

07-22-2008, 09:48 AM
I managed to measure MAF voltage while engines was "cold" and tracked voltage readings while engine was warming to operating temperature.
(voltage was measured from maf-signal -> connector ground -> D-B)

voltage was 0.5-0.6V when engine was cold and idle speed was over 1000rpm but when engine warmed and idle dropped voltage dropped as well... below 0.3V and at that point even opening throttle to point where idle was just below 1000rpm I did not get the MAF voltage above 0.4. And below 0.4v it goes using predefined injector timing table...

So it is not EEC or wires, voltage is really that low.. Anyway it is still possible that engine has a somesort internal vacuum leak as this was not happening "out of box" when I got the car,.. it took over half year it this problem to appear. And 0.3V seems to bit low for just 30 -> 42 injector compensation..

Anyway, I'll measure voltage again from C-D if it makes any difference.. and yes, I just ordered Moates Burn1 and ford adapters so I can make adjustments if needed... It will be nice to see what kind of setting "tuned" my superchip contains compared to stock rom. (if any,.. :) )

XR7 Dave: I'm on very tight budget as I'm no sure if I still keep this car, so first priority is to pass smog test.. but I'll email you soon, as proper base tune could be handy still..

edit:.. same voltage measurements from points C-D..

edit2:... I found small vacuum leak from first connection of supercharger "top" to intercooler pipe.. I dont know if this can matter anything.. but I'll tighten that connection and check voltage again..

07-23-2008, 12:29 PM
Interesting... just tightening that supercharger top "gigantic nut" resulted quite big increase of MAF voltage... now it is 0.40-0.42V at idle. I could newer imagine that small vacuum leak could matter that much!! Idle airflow just seems to be so small that even small things matter.

There is still very very little leak around that connection that I have to take care... and I have to say that finding even that above main leak was bit pain, I just wish I could have somekind of smoke machine to detect leaks..

Anyway, if I can get all, yet undiscovered, vacuumleaks fixed it could allow idle voltage to stay above 0.4V... still, I plan to adjust "alarm" voltage little bit as even normal idle voltage with this C&L MAF is bit close to that error triggering 0.4v..

08-18-2008, 04:21 AM
There must still be some sort of small vacuum leak as when engine was warm after few miles of driving MAF voltage was dipping again under 0.36 (which was the real "alarm voltage"... not 0.4v)
I found 2 small holes (about 1mm) on oil dipstick "pipe", it was disengaged fron support bracket.. I imagine those do not weight much as vacuum leak..

Anyway, bandaid was to lower that alarm voltage..

08-18-2008, 07:30 AM
My -89 SC recently failed yearly emission testing and have been bugged by check engine light appearing at idle. Code is 66 -> MAF voltage low. I measured the MAF voltage at idle and it was way below 1.0v... about 0.5-0.6v if I remember right.

At 800rpm, my maf voltage with a C&L maf housing runs about .43v.

That code can be set multiple ways. Does it get set only when driving? How about Key On Engine On testing?

08-18-2008, 08:36 AM
At 800rpm, my maf voltage with a C&L maf housing runs about .43v.

That code can be set multiple ways. Does it get set only when driving? How about Key On Engine On testing?

Only when engine was idling and warm (after 4-5 minutes of idling), after fixing most vacuum leaks it only appeared when engine was driven few miles.. and only when idling.. I have measured the MAF voltage lately pretty good and check engine behaved pretty much like it is programmed to EEC -> stock "low maf" level was 0.36v and yes, it tricked check engine light just there.
I'm pretty sure I have small vacuum leaks somewhere,.. they do not bother off idle(so small leaks after all..), but I'd like to find then still :)