F355 Question about MAF resistance. - Ferrari Life
 
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post #1 of 10 Old 10-31-2015, 04:02 PM Thread Starter
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Question about MAF resistance.

I've read old threads both here and on the other Ferrari site about making adjustments to the MAF resistance to control mixture. It has been stated that the factory spec is about 383 ohms and that increasing resistance leans out the mix while decreasing it richens it. The problem I have with that is this. Agreed that changing the resistance will yield a misreading by the ECU. That is, if the resistance is increased the ECU will think there is less air flowing through the MAF than actually is and, therefore, without other intervention the mixture would become lean. But, there is an O2 sensor in the exhaust which will sense this lean condition and should tell the ECU to add a little fuel to get to the design target. Since this "MAF error" would be connstant it would seem to me that this would ultimately result in a change in LT fuel trim (5.2 car) or a change in the Lambda adaptive value ( 2.7 car).


Now that applies to closed loop operation. BUT, LT fuel trim is learned over time and is applied to mixture correction under both closed and open loop operation. So, the net result of altering the MAF resistance should, over the long term, have no affect other than to alter the LTFT or lambda adaptive value. This seems reasonable to me and furthermore is supported by the fact that high or low LTFT can be an indication/result of a bad MAF. Certainly a change in the value of the bias resistance of the MAF could be interpreted as a failing MAF.


Ok, that said, can someone tell me if that is correct or if not, and if not, just how does altering the MAF resistance have any long term effect on mixture?

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post #2 of 10 Old 11-01-2015, 08:53 AM Thread Starter
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0 views and no comments? I guess what I am saying is correct then.

If you are going to tell someone how to remove a bolt you should at least know which way to turn the wrench.
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post #3 of 10 Old 11-01-2015, 08:55 AM
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The ecu works within windows of values, minimum/target/maximum. You can use an adjustable maf to modify the signal output and force the actual operating(target)afr to be closer to one of the thresholds and not run against the 02 sensor values and create confusion for the
computer. Since the maf is tuned to the engine, they are really intended to be used when an engine is flowing more air than the stock maf was tuned to calculate, or when a larger maf was needed to accomodate for the increased flow, or when the fuel system has been modified to flow more, so you can correct the target injection timing to keep the afr within the programmed thresholds.

Its really an impractical way to tune modern efi cars. However on early systems which didn't have tight thresholds or adaptations and were difficult to reprogram, it was the most practical way for the average guy to tune efi for more power.
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post #4 of 10 Old 11-01-2015, 05:11 PM Thread Starter
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Thanks for the comments. However, perhaps I wasn't clear n my question. Let me rephrase it. I see guys who measure the resistance of their MAF and find that it is off from what they believe is correct. They then conclude that their car must be running rich or lean as a consequence. (Actually they are just guessing because they have not done an exhaust gas analysis.) So they open up the MAF and adjust the resistance to what they are told is correct, believing that this will result in correcting the assumed rich or lean condition. My contention is that, as long as things remain with in the operating windows for adaptive values/LTFT, the long term affect will be only to settle to a new value of the LTFT as the O2 sensor will provide feed back as to the actual conditions based on the oxygen content of the exhaust gases. In other words, if the car was running at +5% LTFT and an adjustment was made to the MAF that initially resulted in pushing the mix richer by 3%, over the long term I would expect to see the LTFT drop to 2% and ultimately the engine would run at the same condition (or damn close) that it did before the resistance was changed. I understand that this is somewhat of a simplification of a dynamic system, but as long as things remain within the ECU adjustment windows it's really the feedback from O2 sensor that is controlling the mixture. It's a feedback system and the O2 sensor is providing the feed back for fuel trim.

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post #5 of 10 Old 11-01-2015, 07:18 PM
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John although i have a limited understanding of fuel trims from what i have read our system uses stft to adjust ltft. In open loop the computer defaults to stoick values. As you said the O2 sensors provide the feedback for stft which as I said earlier provide the feedback fot ltft.

I also believe your last statement to be true.
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post #6 of 10 Old 11-02-2015, 02:10 AM Thread Starter
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STFT is all over the place. You can start you engine and monitor STFT and once in closed loop mode any sudden change in throttle position will make STFT jump like a scared rabbit, as it should. But LTFT is an integral parameter that is used to correct for a consistent rich or lean mixture under more or less steady state operating conditions. And once a value is established it is applied even in open loop mode.
I'm not concerned about how LTFT is derived, but I don't see anyway that the MAF being slightly off, or making a small change in its resistance, would have any long term effect because as I have been saying the LTFT should eventually compensate for the change.

If you look up LTFT what you find is stuff like this:

Quote:
Running too rich High negative fuel trim corrections can be caused by MAF sensor problems, .....

Running too lean High positive fuel corrections can be traced to MAF sensor faults, ....
So my interpretation is that the MAF resistance being off, or changed, can certainly be considered as an MAF fault and ECU would respond to it and correct for it, through LTFT or some other parameter, provided the correction remained within the allowable limits.

Certainly, as ECofV stated, if you disable the O2 sensor so there is no feedback regarding exhaust gas composition, you can tweak the A/F ratio all over the place by messing with the MAF. But that is another issue.

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post #7 of 10 Old 11-02-2015, 10:10 AM
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The way the o2 sensors won't over come the MAF reading, without remapping the ecu, is if the load calculation is based off the MAF readings and not by a separate MAP sensor. In this case, adjusting the MAF resistance changes the load value at a given rpm, which in turn will modify the target AFR.
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post #8 of 10 Old 11-02-2015, 02:18 PM Thread Starter
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Quote:
Originally Posted by ECSofVirginia View Post
The way the o2 sensors won't over come the MAF reading, without remapping the ecu, is if the load calculation is based off the MAF readings and not by a separate MAP sensor. In this case, adjusting the MAF resistance changes the load value at a given rpm, which in turn will modify the target AFR.

Yes I understand that. The change in resistance will give a false reading for the air mass flow and the ECU will set the A/F to a different position (target) on the map. I.E. the ECU thinks the engine is operation at a different load that it really is. But isn't the roll of the O2 sensor to correct for that? It's looking at the exhaust and seeing that the chosen AF isn't correct and telling the ECU to change the fuel delivery.


I mean, the ECU gets reading from the MAF, RPM sensors, Temp sensors, Throttle position sensor and from those it chooses what the fuel delivery is from the engine map. But the roll of the O2 sensor to provide feedback, by looking at the actual composition of the exhaust and telling the ECU, " I know you think you have set the correct conditions, but your wrong. You need to cut/increase (as required) fuel delivery." If not, why would you need LTFT at all? LTFT is basically telling the ECU that for the condition of the sensors and engine the engine map is wrong and the LTFT is the correction to be applied.


I'm really trying to understand where my think is wrong, if it is, or if it is wrong. For example, if everything is to spec and the MAF tells the ECU that X lbs/sec of air are entering the engine then the fuel map would say inject Y lbs/sec of fuel and the AF ratio would be at the target, X/Y and, assuming the engine is in spec, the exhaust gas should be ok and both LT and ST fuel trim should be zero. But if I dick with the MAF so that it tells the ECU that X lbs/sec of air are flowing when the actual air mass flow in (X-z) then it still injects Y lbs/sec of fuel. However the actual AF is now (X-z)/Y and it's rich. So how will the O2 sensor not see that in the exhaust and not tell the ECU it should cut the fuel flow rate, leading to a negative value for LTFT?


The thing is that according to all the diagnostic procedures I have read excessive LTFT, + or -, can be an indication of a bad MAF. And, other than complete failure, an MAF can only go bad by incorrectly reporting the air mass flow to the ECU and the LTFT is what compensates for that. So how any distinction between an actually erratic reading MAF and one that has been intentionally modified can be made by the ECU? It can't know if some potentiometer in the circuit has sifted resistance due to circumstances or intentionally. At this point the only way I guess I could satisfy myself is to mess with my MAFs and we what happens to LTFT, but I'm not about to do that. Well, maybe on my Toyota truck.

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post #9 of 10 Old 11-02-2015, 03:44 PM
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I don't believe you are trying to understand. I have explained the logic and fueling strategy operations of the ECU's pretty plainly.
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post #10 of 10 Old 11-03-2015, 02:31 AM
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Quote:
Originally Posted by john85QV View Post
I mean, the ECU gets reading from the MAF, RPM sensors, Temp sensors, Throttle position sensor and from those it chooses what the fuel delivery is from the engine map. But the roll of the O2 sensor to provide feedback, by looking at the actual composition of the exhaust and telling the ECU, " I know you think you have set the correct conditions, but your wrong. You need to cut/increase (as required) fuel delivery." If not, why would you need LTFT at all? LTFT is basically telling the ECU that for the condition of the sensors and engine the engine map is wrong and the LTFT is the correction to be applied.
The basic assumptions here are correct. However,when unmetered air is entered into the equation, what the MAF is asking for and what the exhaust sees (via feedback and ECU algorithms/maps) are two very different things. Hence, the correction factor comes into play (LTFT). I think this is the disconnect.

Quote:
Originally Posted by john85QV View Post
The thing is that according to all the diagnostic procedures I have read excessive LTFT, + or -, can be an indication of a bad MAF.
Could be, but rarely is...Frankly when I see high/low LTFT, the last thing I'm concerned with is the MAF.


Quote:
Originally Posted by john85QV View Post
And, other than complete failure, an MAF can only go bad by incorrectly reporting the air mass flow to the ECU and the LTFT is what compensates for that.
A "bad" MAF rarely reports a code. I see a fair amount of MAF-related issues on 360. Although the reference resistance and voltage is measured, any meaningful diagnostic work is typically done with a scope. A properly functioning MAF will respond with a "responsive waveform" with the blip of the throttle; a dead one will not. The more common issue with MAFs is contamination rather than a component failure. Some can be cleaned with success, some not.

Perhaps this helps to support what Josh is saying a bit more?

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