I've been getting poor gas mileage, so I did the following tests, as suggested by the FAQ, Art Benstein and others. The Idle Air Control valve by the way seems to be operating fine.

The first set of test results were with the idle mixture trim pot set to 350 ohms (likely the factory setting), as measured between green/yellow (pin 2) and yellow (pin 6) on my LH2.2 AMM:

Air Mass Meter (Art's test):
- ignition on, but car not running, voltage on white/red wire (pin 3) was 1.26 volts (my understanding is that this would make the engine run lean). Art suggested replacing the unit if it was less than 1.5 volts. Fanning air into the unit, I couldn't get it above 2.0 volts
- With engine running, it was 2.2 volts (Art suggested it should be high 2's or 3).

Oxygen Sensor measurements With the sensor output disconnected from the ECU (FAQ - Bosch test):
- 2500 RPM, all vacuum hoses connected: 0.035 volts (indicates lean)
- 2500 RPM, vacuum line to the fuel pressure regulator pulled and plugged: 0.82 volts (indicates rich)
- 2500 RPM, vacuum leak created by pulling the hose to the flame trap: 0.025 volts
- 750 RPM, all vacuum hoses connected (sensor still disconnected): 0.05 volts

Idle Mixture Test Point (FAQ - "Setting Base Idle"):
- Voltage at pink wire test point near battery stayed at 13V constantly.


Because I wasn't seeing the expected oscillation between high and low voltages at the idle mixture test point ("Setting Base Idle" test), I decided to adjust the AMM idle mixture screw, until the voltage was high (13V) half the time, and low (I believe about 0 volts) half the time. After this adjustment, the trim pot measured 440 ohms.

The new Oxygen Sensor measurements were now:
- 2500 RPM, all vacuum hoses connected: 0.045 volts
- 2500 RPM, vacuum line to the fuel pressure regulator pulled and plugged: 0.8 volts
- 2500 RPM, vacuum leak created by pulling the hose to the flame trap: 0.030 volts
- 750 RPM, all vacuum hoses conneced (sensor still disconnected): 0.8 volts


Both before and after the AMM adjustment, if the oxygen sensor is hooked up, the voltage at its output oscillates between 0.2 and 0.7 volts. I understand that the ECU's job is to make compensations to acheive this result, so it tends to indicate that the ECU is fine, but doesn't prove that either the oxygen sensor or AMM are within spec.

The only big difference between the two tests, is the oxygen sensor output (when disconnected) at idle. Originally it indicated very lean; with the new setting it indicates very rich. Is the 0.8 reading the expected result in a perfectly operating system?

I realize that my adjustment to the AMM may simply be compensating for an out-of-range oxygen sensor. My big question is, based on this data, can I definitively conclude that either or both the oxygen sensor and air mass meter need replacing?


Sorry for the long post, and thanks in advance for any input.

David

I have an 84 245 automatic, which was geting poor gas mileage when I got it. After much messing around, it turned out to be a old air filter. Looked OK but was just mostly plugged.
Right now that car has the O2 sensor disconnected and gets about 22mpg mixed driving.
My O2 sensor failed on my 745 recently also. In both cases the sympton was poor and slow idle. Try disconnecting the sensor to see if things change mileage wise.
--
744-16v,745-16v,242Turbo.245DLT

David,

Your tests look pretty decent. The amm readings do seem a bit low compared with what I've seen, but my observations have been limited to one (1) LH2.2 car with -007 amm like yours, three LH2.0 cars and a few more -002 AMMs, and a half-dozen or so -016s in LH2.4.

But the main idea is you are achieving closed loop operation as indicated by the mixture test point and oscillating oxygen sensor output. Don't put too much on the oxygen sensor output when it isn't hooked up, because its response to minor variations is not seen - it flips like a light switch right at the ideal mixture, which you couldn't hope to achieve for more than a few seconds without feedback.

I would not spend money for an AMM especially for your poor mileage symptom if you think it is running lean. The oxygen sensor could be slow to react -- indicating lean for a longer time than the combustion is really lean, which might result in poor mileage as the injectors try to compensate at cruise speed. Only a scope on the sensor output would show you that, so for your symptom a new oxygen sensor might be worth the try.

How poor is the gas mileage?
--
Art Benstein near Baltimore

Art:

Before I forget, thanks for your excellent web page on the blower motor replacement. With your detailed description and pictures, the job was simple. Without knowing the tricks, it would have been a lot more painful.


You suggest not spending money on an AMM if I think it’s running lean, but I’m not positive that this is an accurate conclusion. It was based on three things:

1) in an old post, you had suggested that a static AMM output voltage lower than 1.5 volts would cause a lean operating condition (mine was 1.26). I guess your point is that if the AMM is putting out a lower than expected voltage, the ECU thinks that the amount of air passing through it is less than the actual amount, and hence injects less fuel than is required for the actual amount of air. If never adjusted, this makes sense. I'm thinking though that the system design could be based on knowing that a set amount of air will be passing through the AMM at the specified idle speed, and the trim pot can therefore calibrate the AMM/ECU combination to this known amount (as determined at the idle mixture test point) and make the two work together correctly. If this is true, then the static AMM output voltage is not such an issue, providing that the trim pot can bring the idle mixture test point output within range.

2) because the disconnected oxygen sensor's output voltage is supposed to drop to 0.2 when a vacuum leak was induced, it infers that it should be above 0.2 before the leak. Since mine was already below 0.2 before the leak, I assumed it was lean. The spec with the leak says 0.2 or below, so I suppose my 0.025 volts is within spec. Ideally I suppose the spec with all vacuum hoses connected should be in the middle of the range, about 0.5, but as you pointed out, it's hard to achieve without feedback.

3) the idle adjust test point was sitting at 13 volts constantly, which I understand indicates a constant lean condition

Before all these tests, I had assumed it must be running rich in order to use too much gas. I figured that if it was running lean, I would get decent gas mileage but just no power. You point out that if the oxygen sensor is slow to register the exhaust has gone from lean to rich, the injectors are injecting too much fuel for a period of time, hence degraded fuel economy. I suppose an oxygen sensor whose static output always indicates that the exhaust is leaner than it actually is, would consistently call for more fuel than required, and make the engine run too rich. If this is the case with mine, it would mean that all my readings suggesting lean are erroneous, and my adjustment on the AMM actually pushed the mixture into a false "normal" mixture, when in fact it may be constantly rich now.

I do have an oscilloscope. How does one use a scope to interpret whether the sensor output is slow to react or not?

As for my actual mileage, I have to confess that my assessment is subjective. My odometer doesn't work! I should pursue the odometer gear repair (assuming that's the issue) so I can accurately measure my fuel economy.

David,

I'm also unsure if it is an accurate conclusion to disregard the AMM. I think it may be an accurate first step, and economically sound decision. Then you may later conclude the oxygen sensor outlasted the AMM - very possible.

I was going to try to describe the output curve of our oxygen sensors to dissuade you from using its static output voltage in the way that you are. But I found David Hunter's comments to be right to the point: if it is in closed loop, the AMM is in range by definition. You have to see this at idle and under load. That pot on the AMM is a very subtle adjustment; if you can equalize the duty cycle with it, you're doing better than most folks.

The oxygen sensor's output waveform is judged by the rise time on lean/rich transition. Contamination by oil fouling, carbon from a rich mixture or, like I had once, tranny fluid on the outside reference port will affect the volume of oxygen passing through its substrate. With a given exhaust pressure, a reduction in volume would seem to lengthen the reaction time to mixture changes, analogous to you building a bookcase around your home's thermostat. Add to that the effect of contamination on the chemical makeup of the solid electrolyte. There are some good waveform pictures and examples on the web, I think you'll find by searching for the term "lazy sensor", "Nernst cell", or such language.

Also, if you haven't, check the heater by measuring its change in resistance immediately after unplugging.

You may be right on track with the AMM for cruising, because it may be squeaking into the closed loop condition only at idle or no load. I think I have an -007 with better output I'd be happy to send you to try. But the only indication of mixture to the system comes from the transitions of the narrowband oxygen sensor.

If you have a wiring diagram, and you will be in the cluster anyway to replace your gear, you might consider running that mixture test signal from the ECU to one of the unused postive referenced idiot lamps in the cluster, like upshift if you have an auto or EGR if you don't have, so you can watch the mixture reactions to driving. The LH2.2 has a good sized transistor buffering the output, so it will handle a lamp as well as an LED, just be sure to keep a handy way to disable it or the blinking light will become annoying.

--
Art Benstein near Baltimore

Art:

I see now your point about not drawing conclusions on static oxygen sensor results. I did find some great examples of dynamic sensor waveforms(http://home.earthlink.net/~mgmcqueary/uses.html) Compared to these graphs, my sensor is very sluggish. My lean to rich transitions are about 0.25 seconds; rich to lean are sometimes 1.0 second for a full transtion. I take it that this is definitive proof that I need a new sensor.

What are the expected results when viewing the idle mixture test point under various driving conditions? Should it stabilize to a 50% duty cycle at various speeds? Is it expected to momentarily track differently on kickdown, etc.?

I thought your idea about having the idle mixture adjustment test point drive an available bulb in the instrument cluster was a great one, but are you sure the LH2.2 ECU will drive this bulb? When I hooked it up while viewing the closed loop transitions at the test point, the transitions stopped with the test point held at a high voltage (13v?). I'm assuming the test point could not sink the current required to keep the test point voltage low. Maybe there's a series resistor on ECU output, or do you think my output has a blown stage? I know the test jig has a 750 ohm resistor in-line, meaning that the maximum current it draws is about 12v/750 ohms = 16 ma. I believe I've read that the bulbs are about 2 watts, so the current would be 2/12 = 166 ma. It's too bad if it doesn't have the current capacity; the implementation was effortless. Simply remove the white/red tach wire from the coil and jump it to the test point with a male-male jumper, then connect the unused other end of the tach wire behind the cluster to the open lug to the left of the speedometer with another male-male jumper. I believe this drives the "choke" indicator which is 3rd from the left side.

Thanks for your offer of an AMM loan; I'm not sure with the border though, it's worth the aggravation (I'm near Toronto). I've posted my odometer experience separately.

Thanks,

David

Hi David,

Again, you are absolutely right. My mistake. I forgot about the series resistor, only focusing on the memory of a discrete transistor that looked like it could dissipate 150mW at least. I'm sure it could switch that 2W (1.2W?) lamp if you had its collector directly available, but it is not, as you found out. Here's a copy of my notes now that I'm home:

Art:

Where do you get all your schematics? (I got your odometer schematic from the other post - thanks!). Do you reverse engineer them by looking at the circuit boards? If so, you must have a lot of patience.

Anyway so I'm looking at oxygen sensors now. FCP Groton has three for my 86 240:
- Bosch Direct Fit - $111.50
- Walker Direct Fit - $72.50
- Walker Universal - $49.95

Do you have any thoughts on whether the cheaper ones are acceptible, or should I bite the bullet and go for the Bosch?

David

As I understand it, the sensors are the same units, but the price difference is accounted for by the need to stock small quantities of specially made wire harnesses. If you can use a crimper and heat shrink tubing, I'd go for the universal.
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Art Benstein near Baltimore

Fix the odometer and know the mileage. How can you know if you accomplished anything without an odo? Sounds as if the LH is working exactly as it should. An oxygen sensor does not go out of range but rather will get "lazy" and change output voltage more slowly with less overall range. The airmass meter's job is to get the mixture about right so that the O2 sensor can then fine tune the mixture. If the system is working closed loop as yours is there is nothing more to do to improve it.
--
David Hunter