Surging AFR

[tig]

Basically, yes. It will still oscillate but only within the small range measured by a narrow-band sensor (this is to aid the catalytic converter performance)

Hmmmm... I have no cat. Thus even more reason I would prefer no oscillations, no?

[turbodan]

It makes total sense that if the AFM adjustment screw adjusts the amount of air that bypasses the flap when it's closed (idle or low-load) it will only affect the AFR at idle or low-load. I now understand the theory.

Yes but I think there was another point he was getting at. If the ECU is seeing a rich mixture at idle because you closed the adjustment screw, then it will be in a state of subtracting fuel to correct for that. Then all of a sudden you get on the gas, it's going to go too lean and give you poor throttle response. The ECU will still be subtracting fuel, until it has time to re-adjust to the changing condition.

That is correct.

Running the bypass screw in, as in turning it clockwise or tightening, will reduce the amount of air allowed to bypass the flap. This will result in a greater amount of flap deflection and more fuel which will result in a rich condition. The oxygen sensor will react to this by subtracting fuel. This fuel correction doesn't move too quickly, so when you step on the throttle the mixture will suddenly be lean due to the correction made at idle. The DME will detect this lean condition and correct for it if the engine runs lean under load for a period of time. As soon as the engine returns to idle again, it will return to a rich condition due to the closed down air bypass. At this point the cycle will repeat.

The point is that any change you make to the AFM calibration will interfere with the oxygen sensor control. If the output voltage from the AFM or MAF isn't right the oxygen sensor will struggle to maintain the correct fuel mixture.

Open loop does not necessarily mean worse mileage. You can make simple adjustments to the AFM spring tension and bypass screw to get the kind of AFR you're looking for.

[turbodan]

Basically, yes. It will still oscillate but only within the small range measured by a narrow-band sensor (this is to aid the catalytic converter performance)

Hmmmm... I have no cat. Thus even more reason I would prefer no oscillations, no?

Oscillations are specifically harmful for catalytic converters. Ideal conditions for the cat would be a rock solid 14.7:1 AFR. Due to the limitations of the narrow-band oxygen sensor the mixture must oscillate somewhat but it should remain quite close to 14.7:1.

[tig]

Basically, yes. It will still oscillate but only within the small range measured by a narrow-band sensor (this is to aid the catalytic converter performance)

Hmmmm... I have no cat. Thus even more reason I would prefer no oscillations, no?

Oscillations are specifically harmful for catalytic converters. Ideal conditions for the cat would be a rock solid 14.7:1 AFR. Due to the limitations of the narrow-band oxygen sensor the mixture must oscillate somewhat but it should remain quite close to 14.7:1.

You writing this makes my day. I my intuition was right. Not completely founded on accurate understanding of the theory, but essentially right. I will go to sleep tonight smarter than when I woke.

[slimdevil27]

With all three variations of MAF, AFM's and chips....you are still running lean with the O2 unplugged.

Our ECMs have a biased O2 signal. If you test the harness side of the O2 sensor with the engine running, you will see .5 volts. If the O2 is dead or unplugged it defaults to .5 or stoich. If everything is working and adjusted correctly unplugging the O2 sensor would give you a reading of 14.7 AFR. The O2 sensor then drives this lower or higher as it ocilates. A good O2 should sweep about every 1-1.5 seconds between high and low. Further it should be able to go below .2 volts and above .8 volts when artificially leaned or enriched. At 2500 rpm is should oscillate between high and low 4-7 times per second (IIRC from my snapon vantage). Because or signal is biased at .5 volts, unplugging the O2 will default the ECM to a perfect middle reading as the input signal. Any adjustments that effect air/fuel ratios should be done with the O2 unplugged and monitoring the O2 with a VOM or using a wideband. Make adjustments until your O2 is averaging .5 at 2500 rpm.

Have you verified that the CTS is working in the correct range? If it was biased towards a hot reading the result would be a constantly lean mixture. That would drive the mixture lean at all engine speeds where a vacuum leak would only be apparent at lower engine speeds. Low fuel pressure would have the same effect, lean at all engine speeds.

Have you tested the fuel pressure? Are you running stock injectors? Does turning on your headlights affect the fuel mixture or idle oscillation? Have you cleaned all the grounds?

What I'm getting at is nothing you have tried has resolved the single issue of being lean with the O2 unplugged. I believe that is the biggest flag that something else isn't right. The only other sensor that will drive the ECM lean (or rich) like that is the CTS. As for mechanical problems that would do the same...it's limited to fuel pressure and injector flow (plugged or wrong size), electrically is would be corrosion, AC leakage from a bad alternator diode, a bad ECM...

You can borrow the ECM out of my 535. I have a fuel pressure tester, a Snapon Vantage Scope..........and a damn engine hoist.

[tig]

With all three variations of MAF, AFM's and chips....you are still running lean with the O2 unplugged.

Our ECMs have a biased O2 signal. If you test the harness side of the O2 sensor with the engine running, you will see .5 volts. If the O2 is dead or unplugged it defaults to .5 or stoich. If everything is working and adjusted correctly unplugging the O2 sensor would give you a reading of 14.7 AFR. The O2 sensor then drives this lower or higher as it ocilates. A good O2 should sweep about every 1-1.5 seconds between high and low. Further it should be able to go below .2 volts and above .8 volts when artificially leaned or enriched. At 2500 rpm is should oscillate between high and low 4-7 times per second (IIRC from my snapon vantage). Because or signal is biased at .5 volts, unplugging the O2 will default the ECM to a perfect middle reading as the input signal. Any adjustments that effect air/fuel ratios should be done with the O2 unplugged and monitoring the O2 with a VOM or using a wideband. Make adjustments until your O2 is averaging .5 at 2500 rpm.

Good info. Thanks. In my case forget about reading the actual O2 sensor, I have a wideband in it's place.

Have you verified that the CTS is working in the correct range? If it was biased towards a hot reading the result would be a constantly lean mixture. That would drive the mixture lean at all engine speeds where a vacuum leak would only be apparent at lower engine speeds. Low fuel pressure would have the same effect, lean at all engine speeds.

Yes, but last summer. I will do it again asap. Great point.

Have you tested the fuel pressure? Are you running stock injectors? Does turning on your headlights affect the fuel mixture or idle oscillation? Have you cleaned all the grounds?

Yes, I have an AFR with a gauge. Yes, stock injectors. No, headlights do not seem to affect mixture. Yes, my grounds have been cleaned multiple times (for other reasons).

What I'm getting at is nothing you have tried has resolved the single issue of being lean with the O2 unplugged. I believe that is the biggest flag that something else isn't right. The only other sensor that will drive the ECM lean (or rich) like that is the CTS. As for mechanical problems that would do the same...it's limited to fuel pressure and injector flow (plugged or wrong size), electrically is would be corrosion, AC leakage from a bad alternator diode, a bad ECM...

I have 3 ECUs. When I changed chips above (and in my dyno runs) I didn't swap the chips in one ECU, but changed ECUs. For now I rule out the ECU.

AC leakage from a bad alternator diode...hmmm...how would one test that? What other symptoms might that cause?

I agree something else isn't right. I appreciate you leaving me some breadcrumbs that might help figure it out. Remember, though, it's all academic as this engine is coming out of this car this weekend and almost every related part is being replaced.

You can borrow the ECM out of my 535. I have a fuel pressure tester, a Snapon Vantage Scope

I'd love to play with that scope!

..........and a damn engine hoist.

[slimdevil27]

Quote:

Have you tested the fuel pressure? Are you running stock injectors? Does turning on your headlights affect the fuel mixture or idle oscillation? Have you cleaned all the grounds?

Yes, I have an AFR with a gauge.

I assume this is an Adjustable Fuel Regulator with a fuel pressure gauge attached.

AC leakage from a bad alternator diode...hmmm...how would one test that? What other symptoms might that cause?

Premature battery failure, headlights that slightly pulse or flicker. On fuel injected engine the overlaying AC voltage can interfere with an AC pulse generating sensor and cause weird symptoms from the ECM. Typically placing a load on the system will negate the AC voltage effects, like turning on your headlights. The test is fairly simple. With a DVOM in the AC voltage position, place the negative probe on the negative battery terminal and the positive on the alternator B+ terminal. With the engine running the AC reading should be less than .3 volts. There are three diodes in the alternator and you are allowed .1 volt AC leakage from each one. Next bring the rpm to 2500 rpm and recheck.

One trick to note is that you can also just disconnect the alternator and run the vehicle from the battery.

Weird symptoms.....I converted a 59 Dodge Coronet with a 413 to TBI. The TBI used a signal from the coil negative to fire the injectors. A bad alternator caused only a slight rough run with the carb....no more that I would expect from an oversized Holley, Edelbrock intake, performance cam and dual exhaust...just a lopey idle. However, when I installed the TBI it would barely run. I double checked everything....it took me about 3 hours to finally nail down that the Alternator was bad. It charged just fine but the AC voltage was screwing up the AC pulse signal from the electronic distributor.....bad enough when only the ignition depended on a clean signal but really bad when both the ignition and the fuel delivery were depending on it.

I think it was an early 90's Nissan pickup....came in for a really rough run. Road tested to verify the problem. Idle was just a little off, but as the RPM rose the engine would run rougher and rougher. On a test drive I went to make a left turn and noticed the engine running smoother every time the turn signal flashed......I turned on the headlights at it all but disappeared.....My boss was incredulous when I got back and told him it needed an alternator...after less than 10 minutes after the work order hit my hand.....he insisted it was there for a rough issue and I insisted it needed an alternator sure enough, AC voltage leakage.

It was a fairly common test back in the day and was part of checking the charging system...most lab scopes had a specific test for diode ripple which pretty much told you the same thing, a bad diode, but on the DC side.

I'm coming over to the city on Friday if you want to play with the scope for a week or two. I can drop it off to you or we could meet somewhere. Shoot me a PM.