johnb

does anyone know what to set a MAF adjustment to, after it has been grossly adjusted? i guess the adjusting screw generates a voltage to the ECU, what should this be as a starting point?

turning which way LEANS mixture? clockwise, or anticlockwise?

Bryan

First off, the hot-wire air mass meters don't have a real long life span. If you're experiencing fueling problems and don't have air leaks, your AMM is probably shot.

My car consistently underfuels by a small amount at all rpms and loads. You can wind the engine out to 5K rpm at 90% throttle and it's running a tad lean. Push down the accellerator a tad more and click the full throttle switch (which bypasses the AMM and uses a pre-set fuel curve) and the car jumps right up to slightly rich.

Another way to tell if your AMM is shot is that the adjustment won't do anything, or does very little. On my car, you can turn the adjuster infinitely in either direction and it doesn't do anything. I bought a used one (big mistake - it was just as dead) and the adjustment does a tiny bit but not much.

I'm going to tear my used one apart and see what makes it tick. Hopefully I'll have info on repairing/testing them soon.

BTW, my car has cleaned and balanced injectors and a new fuel filter, and perfect fuel pressure. And the car has about 35K miles on it. But it's an '85 so the AMM is dead from age.

I would guess that Bosch followed the same model as for their plain L-jet system - clockwise=richer. Just guessing though.

Bryan

Mike Schmidt

To adjust the MAF you're supposed to use an exhaust gas analyzer. The car is brought up to normal operating temperature, the O2 sensor is disconnected, and the MAF is adjusted until the exhaust gas analyzer reads what it's supposed to. You can do a pretty good job using either an air/fuel mixture gauge or a volt meter instead of an exhaust gas analyzer. Warm the car to normal operating temperature. Disconnect the O2 sensor from the engine management computer, but have the O2 sensor signal output wire connected to an air/fuel gauge or a digital volt meter. If using the volt meter, the positive lead of the meter goes to the signal output wire of the O2 sensor, and the negative lead goes to any available ground. With the engine idling, adjust the MAF until you get the air/fuel gauge to show about a 14.7:1 ratio. That's five of the ten lights being lit on the more common air/fuel gauges. If using the digital volt meter, adjust the MAF until the meter shows around .5 volts. Then you just reconnect the O2 sensor to the engine management computer. If you try to adjust the MAF without disconnecting the O2 sensor from the computer, it won't have much of an effect unless you get to a setting that's so extreme it's out of the computer's adjusting range. If you try to adjust the MAF with the O2 sensor connected to the computer, the computer will just use the information it gets from the O2 sensor to adjust the mixture, and your adjustment at the MAF will just be compensated as the computer brings the mixture back to where it thinks it should be. When you floor the car, the system goes into open loop mode, and the computer ignores any signal from the O2 sensor. It just uses a preset maps stored in the computer then.

Darren '85S

I had the MAF fail on my 85 last year and believe me it is not subtle. First symptoms were stumbles and hesitation and then it eventually wouldn't run at low rpms without bucking. It ran so rich that all the plugs fowled and it spat carbon out the tail pipe. I thnk the MAF adjustment is only used by the 85 and 86 cars. You can adjust as stated above or take it to a shop that can measure exhuast gas. It is a pain to adjust, i counted thirty turns from one end to the other. On the plus side, if it is off, the o2 sensor will have the brain correct for it (if your o2 sensor is working). I have a hunch that it may make your throttle response a little sluggish since the computer has to make large corrections based on the o2 sensor. Anyone know this for sure?

-Darren

Mike Schmidt

The amount that the computer has to correct won't affect the throttle response, as long as the computer is able to adjust to what it considers the normal mixture. There is the possibility of having something so out of whack that getting a normal mixture would be beyond the computers's adjusting capability. The computer adjusts either richer or leaner to get to the "normal" mixture setting, and then stores that adjusted value in memory. It then only has to do fine tuning around that stored value. Disconnecting the battery erases the stored values from memory, and is why the car needs a short time to adjust again after the battery is reconnected. When first started then, the computer will use default values that are permanently stored in it as it adjusts to actual conditions. Those values are then stored into memory again, and are again fine tuned around as you drive. The only time that the computer should really be making large tuning adjustments to the stored values is after the memory is erased.

Bryan

To clarify - or confuse - a bit:

Johnb's car is a Euro version with LH-Jet (hot wire AMM) FI but no O2 sensor/closed loop control. So AMM adjustment and proper functioning has a big effect on how the engine runs.

John, what I did with my '85 Euro was to install a catalytic converter from a 928S4 (after cutting off the air pipes and welding their holes shut) and then installing a 3-wire O2 sensor and hooking it up to an air/fuel ratio meter. If I had to do it over, I'd use a Bosch 4-wire O2 sensor (power/ground for heater, signal, ground for the sensor itself instead of relying on the exhaust system to ground the sensor). This way I can keep a precise eye on fuel mixture. The catalyst is just a convenient way to hold the O2 sensor in the exhaust stream. It bolts right in to the '84-'86 Euro exhaust system. With this system in place, you'll be able to tune your FI to run exactly right. Once it's dialed in perfect, then you can unhook the meter and switch back to your regular front muffer box and drive off and enjoy.

Bryan

Darren '85S

I don't think the brain has memory and learns until the 87 model year.

Normy

Bryan-

You're running the same setup as me (S2 with added S4 cat), and your idea of adding a fuel-air ratio gauge appeals to me. Which one did you add, and where did you get it?

Normy

Bryan

Hey Normy -

I got a Nordskog, formerly Intellitronix, meter from Summit. Cheap, accurate, works great with a generic Bosch O2. Ordered it from Summit, <a href="http://www.summitracing.com," target="_blank">www.summitracing.com,</a> Summit part number NRD-M7009.

Bryan

Mike Schmidt

There's some information on some of the different gauges available at: <a href="http://www.alltrac.net/tuning/afgauge.html" target="_blank">http://www.alltrac.net/tuning/afgauge.html</a>
I also got the one from Summit.

Mrmerlin

I need to adjust my MAF the car failed emissions with a high CO reading.
actual 23.8
SB 20.0 or less
car is an early 86 , 5 speed

John Speake

I would suspect O2 sensor. When you unplug O2 sensor can you adjust the CO to 1% or so ?

With that very high reading, I doubt you will get it down to anything near correct with the pot.

MAFs can fail where they give much higher output volts than they should (thermal runaway) .

If you can measure MAF voilts at idle, they should be around 2.7v. If it's much higher the MAF is toast.

Mrmerlin

Wally the MAF appears to be new as the PO was chasing running problems and i suspect this was one of the parts that was replaced .
The O2 sensor also appears to be new
the running issue was cured with a new engine harness, now to pass emissions.
I dont have a gas analyzer.
So how do i measure MAF volts??
I measured the OHM setting and it was 497 so i was going to reset things to 380.
this car is an 86 so the MAF has to be set up first

Note this car also has a Ott X pipe and the welded on cats

John Speake

MAF output volts are on pin 5 MAF, if you can slip a wire in there with the MAF connector in place, -ve side of meter to car chassis. (ground).

I don't think turning the pot down to 382 or even zero ohms is going to get you anywhere near 1%.