View Full Version : AFM adjustment for HC

12-12-2015, 08:46 PM
(Was going to put this in naturally aspirated forum but had not seen much traffic there)

I have a 91 GT 5sfe with 300K. Rebuult the motor last year and initially got mpg at spec. Mpg dopped off a bit about 6 months later and resulting in the car barely passing inspection with high HC. This had me chasing vacuum hoses, O2 sensor etc. Eventually I started digging into AFM just to see if there was a problem with injectors or the ECU control.

I ran across the following: http://alflash.com.ua/vf1.htm. That shows how to measure the ECU's interpretation of sensors and how to measure whether the ECU is trying to run lean or rich on a scale of 0-5V. Ran a simple test and disconnected each of the injectors to see what the result was - ECU produced a 3.75V showing a lean condition for which the ECU would run the engine rich (thus opening the remaining injectors longer).

So, under normal conditions, my ECU is showign 2.65V - a slightly richer ECU induced environment - matching the slight dropoff of mpg (29 spec, 25 now - Dallas/Fort Worth). I took the car up to Colorado a couple of months ago and cris/crossed the Continental divide a few times (10,000 ft) running the car above 5000ft for about 4 tanks of gas. MPG shot up to 33/4 mpg during this trip - something I had never achieved before (maybe an odd 30mpg but never over).

Good news is that ECU leaned out the engine given the lower air pressure/volume. But that got me wondering if its possible that my MAP sensor is just getting older and . . . . could I adjust it.

- What is your AFM voltage, what elevation, condition of motor and MAP sensor (2.65V, 600ft, 300K rebuilt at 290K, original MAP)
- Could I adjust the MAP sensor by putting a partial clamp on the vacuum hose or a resister (pentiometer) in the wiring?


12-19-2015, 01:38 AM
I would think that your best places to look for increased fuel consumption would be the spark plugs and wires, cap and rotor, ignition coil, O2 sensor, replace MAP, change fuel filter (oil and air as well) Check flow rate of injectors and send them off to get them cleaned if necessary, catalytic converter.

Essentially, all the basic maintenance things should be eliminated from the equation before you look to the more expensive and/or complicated things.

12-19-2015, 02:29 AM
I had replaced all of those - except the map sensor - as part of the upgrade. My goal with the post was to see how much influence does elevation have on mpg/hc output with an idea that I could influence the ecu afm by manipulating the map.

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02-09-2016, 07:14 AM
(3 months later) - Went to fiddle with the hose to map sensor to try and lean things out/ Hooked up the meter and found the voltage at 2.365 - an indicator that the ECU was leaning things out. Took the car to get inspected - and found out that since it is 25 years or older - smog inspection was no longer needed in the DFW. Yippee. Now I can also get an antique plate for it as well.

In any case, here's something I did try with the map sensor. Tried to slowly pinch the vacuum hose leading to the sensor - to simulate a higher altitude - lower air pressure environment. First time, too fast and caused the car to quit. Redid the test, and found that the voltage dropped from 2.365 to 2.361. It drifted in the correct direction - but not the wide range expected. That has me wondering if I am misreading the voltage - narrow band vs wideband - type of O2 sensor readings.