Redline tested my MAF, could anything else be the cause of bad readings?

[cparke]

I've been struggling with a multiplicative Fuel Trim too rich CEL for several weeks now beginning when I put fuel cleaner into my system. This weekend, I took my 190hp A4 Quattro 2.8L 6V 30V into red RPM territory (6000-7000 RPM's) and logged the VAG-COM measuring blocks on the MAF's readings. It barely entered the 100 g/s territory when it should have been reading 150-180 g/s as I understand it.

Before I order a new MAF, I'd like to consider if there are any other system failures (bad fuel injectors, air system leak, etc.) that could legitimately make the air flow less than what it should be at high levels and also cause multiplicative Fuel Trim too rich conditions to occur (I do not get bad values for additive)?

My thoughts - MAF sits on the air intake right after the airbox, and my secondary air system is passing ECU's flow and leak tests (though I know my air pump is not 100%). I'm not sure how to test primary air intake, but I believe that the primary air flow is driven by engine vacuum? Since the MAF sits on the primary air intake only, how does the secondary air flow get tested?

Knowing all this information, is there any way this problem is not caused, at least partially, by a bad MAF?

Group 32 values: (normally +/-10%)
@idle B1S1 -5.3
@part B1S1 -25.2
@idle B2S1 -4.3
@part B2S1 -16.4

CEL as a result:

17535 - Fuel Trim; Bank 1 (Mult): System too Rich
P1127 - 35-00 - - -


2nd gear, 6000-7000 RPM's, approx. 100 km/hr (60mph) MAF log data from the redline test (MAF in the last column, highest MAF value seen was 115.21 g/s at 6280 RPM on a 190hp engine, MAF values dropping even as the engine accellerates past that):

Measure blocks group 002 (1-second samplings approx.)
Engine speed (RPM)|Engine load|Injection time (ms)|Mass air flow (g/s)
5960 2.30 4.10 55.76
5960 1.70 2.94 40.97
6040 2.45 4.10 59.37
6120 2.45 4.22 59.51
6200 2.40 4.16 59.86
6280 2.00 3.46 50.83
6320 1.90 3.33 48.47
6120 0.50 0.96 12.36
5920 0.50 0.00 12.43
5800 0.50 0.00 12.50
5640 0.55 0.00 12.43
5520 0.55 0.00 12.43
5400 0.60 0.00 12.92
5440 1.65 2.88 35.83
5480 1.70 2.88 37.71
5560 2.00 3.52 44.79
5600 2.25 3.97 50.42
5680 1.95 3.33 45.07
5760 2.30 4.10 53.19
5760 1.75 2.94 40.76
5760 2.05 3.58 48.06
5680 1.45 2.50 33.82
5720 1.90 3.26 43.40
5720 2.20 3.78 50.76
5760 2.65 4.67 61.11
5760 2.90 5.25 65.83
5880 3.25 5.82 76.53
5920 2.45 4.29 58.26
5920 2.30 4.03 54.09
5880 1.40 2.37 33.68
5720 0.85 1.54 20.00
5720 1.45 2.56 33.75
5640 1.75 3.33 38.89
5640 1.85 3.07 42.64
5680 2.45 4.35 55.90
5720 3.35 6.08 77.15
5760 3.10 5.57 70.42
5880 4.30 7.87 101.87
6000 3.25 6.27 77.15
6080 3.30 6.02 80.62
6120 3.35 5.95 82.50
6120 1.95 3.33 47.64
6120 2.55 4.42 63.40
6080 3.15 5.76 76.73
6080 2.45 4.22 59.93
6040 2.70 4.80 65.28
6000 2.95 5.25 71.46
6040 3.95 7.17 95.34
6040 4.00 7.30 98.54
6080 3.75 6.66 91.73
6080 3.35 6.14 80.90
6080 4.35 7.74 107.15
6200 4.60 8.38 114.09
6280 4.60 8.32 115.21
6400 4.25 7.68 109.58
6520 4.00 7.23 104.37
6560 3.20 5.82 83.96
6560 3.10 5.63 82.08
6600 3.05 5.50 80.83
6600 2.55 4.67 68.12
6560 1.90 3.46 50.14
6520 2.00 3.58 52.64
6440 1.50 2.69 39.65
6400 1.90 3.46 49.44
6360 2.15 3.90 54.93
6320 1.75 3.14 44.93
6280 2.10 3.65 52.78
6320 2.45 4.22 61.59
6320 2.40 4.29 60.90
6320 2.40 4.10 60.07
6200 0.50 0.96 12.92
5960 0.50 0.96 12.15

[Nebone]

Change the MAF, no other way around it. The fuel correction happens since the ECU reads 100 grams/sec but ithe engine is actually taking in ~160 grams/second so O2 sensor is detecting a lean condition and the ECU is shooting more fuel in to correct it.

[turboracer]

Just a shot in the dark here, but too rich means too much fuel on bank 1. On obd II cars, bank 1 will be the side with the number one cylinder in it. A maf reading lower than normal will cause the car to run lean, not rich. I don't know why you got such weird readings from your maf log. An air leak in the intake pipe or intake manifold could cause this. A bad ground for the maf or ECU or the engine could also. Also, the obd code is for one side of the engine, not both. A bad meter will usually cause both to go out of whack. My money says leaky injector, especially after you said you put fuel system cleaner in. Looking at fuel trims, you definitely have something odd on bank one that is causing the ecu to pull a hole bunch of fuel for that side.

[turboracer]

nebone22, as i see the fuel trims, the ecu is actually pulling fuel out. - number means taking away fuel. What sets the code he has is the o2 sensor detecting too much fuel in the mixture. It will adjust to - 25 trim then set a too rich code. All obd II cars are the same this way. This would mean that his airflow meter would have to be reading higher than normal to get the condition to exist. Even then, it would exist on both banks not just one.

[Nebone]

But his MAF readings are so low. I had that issue recently and now its back to normal after replacing the MAF. His fuel issue seems to be another problem but the MAF needs to go.

[cparke]

Just a shot in the dark here, but too rich means too much fuel on bank 1. On obd II cars, bank 1 will be the side with the number one cylinder in it. A maf reading lower than normal will cause the car to run lean, not rich. I don't know why you got such weird readings from your maf log. An air leak in the intake pipe or intake manifold could cause this. A bad ground for the maf or ECU or the engine could also. Also, the obd code is for one side of the engine, not both. A bad meter will usually cause both to go out of whack. My money says leaky injector, especially after you said you put fuel system cleaner in. Looking at fuel trims, you definitely have something odd on bank one that is causing the ecu to pull a hole bunch of fuel for that side.

Air leak is something I'd like to check for, but how do I do that? Short of splashing water on the rest of the intake pipe and looking at it closely while revving the engine, I'm not sure what to do. What is the intake manifold? The intake pipe seems to go into the back of the engine and that's the last I can tell of where the fresh air goes...

Bank 2 is going to throw the too rich code very soon at -16.4, since the spec limit is -10. Adding more fuel cleaner has made the problem worse, suggesting to me that as I clean my fuel lines, more fuel is reaching the engine and the richness problem is increasing. Isn't it normal for bank 1 and bank 2 to have some variation is richness/leanness for the car to operate correctly, or should they normally run nearly identical (like my additive values are)?

I'll check the ground on the MAF, but wouldn't the MAF just appear dead (or drop dead intermittently) if ground was bad? But I guess there no escaping the fact that the MAF is becoming prime suspect more and more as these alternatives get eliminated? (not ruling out that several things could be wrong)

[Nebone]

So you can log the car and understand fuel trims but you don't know what the intake manifold is? Its an air collector to which the intake hose connects to. The mass air flow sensor gets dirty and contaminated over time and it DOES get bad readings rather than completely die. If you want progress with your car, replace the MAF. Even go to a junk yard and find one cheap.

[cparke]

Thanks for all your help---yes I am going to buy another MAF soon and see if my readings change at all, at this point that seems like most prudent next approach. I tried cleaning the MAF sensor, no change. The fuel cleaner's effect on making fuel trim more rich may just be a wild goose chase I've been following.

Sorry, I'm just learning all of these things as I go along. The intake manifold is still a confusing item for me. The air hose/pipe runs from the MAF into the intake manifold on the back of the engine then? How can that thing leak? It is very hard to see what that contraption is because it seems like it is upper back part of the engine itself. I changed the vacuum hoses leading to the intake manifold changeover valve, but that isn't the intake manifold itself and I honestly dont understand how that valve works either (other than it opens and closes to control vacuum in the intake manifold?).

[Nebone]

The intake manifold is an air collector/distributor. It is the most upper part of the motor that looks like 6 ribs/pipes connectod together. Air comes in through the filter box into the MAF, then through the intake hose, then to the Throttle Body (part behind the engine that you speak of) then into the Intake Manifold which gets collected and split up (distributed) through those 6 pipes and then into the motor. The MAF measures how much air the motor is taking in so it can use the information determine how much fuel is needed to create an ideal fue/air ratio.

Again, change the MAF. Read

[cparke]

Oh, okay---the whole big thing sticking up in the middle under the hood is the intake manifold. Wow! Can't really miss that (unless you have no idea what you are looking for, as I was). Yes, I guess I know better what the throttle body is now too. But, how does the secondary air injection work in this setup? The MAF doesn't get to read that air flow because it is taken from the airbox, sucked through the pump, and then directly injected into the intake manifold. The engine still gets the secondary air too, doesn't it?

Although it seems many (including myself) are confused about how a bad MAF which gives low readings at redline should make fuel trim lean rather than rich, it seems that this same MAF has caused the too rich code under normal driving conditions for others. I am changing the MAF next, thanks everybody so much for looking at my log and confirming my suspicions that the MAF numbers are way off what they should be under the given engine rpm & load conditions.

[Nebone]

No problem. Update the situation as you get the new MAF installed and logged.

[turboracer]

The easy way to check for air leaks in the intake pipe is to use pressure. You can pressure check the intake pipe and intake manifold with an air compressor and regulator. Use only 3-5 psi. If there is a leak, you will hear it and easily be able to verify with a soapy water solution sprayed on the suspect area. Do a search for boost leak detector. There are write ups all over the Internet.

Generally, 2 bank vehicles will run trims very similar to each other in ideal conditions. As o2 sensors begin to wear out and injectors begin to leak, they will change somewhat from each other. A difference of 10 points is not correct. There is a reason why one side is to rich. Cleaning the fuel lines out with cleaner will not cause you to go rich. There is a pressure regulator in your fuel system that maintains a pressure ratio based on intake manifold pressure vs fuel pressure. The injector always have access to the same volume of fuel in the rail above the injector. The only ways to increase fuel into the engine are, a leaking injector allowing fuel to leak into the intake manifold that was not commanded by the computer, a larger injector that allows more fuel per squirt that the computer commands, the computer commanding a longer squirt, or raising fuel pressure beyond spec for the engine you are working on.

Please post a log when you change the maf. I would like to see the results as well.

P.S. It does sound like you are solving more than one problem here. The results will be interesting.

[cparke]

Ok, I got this replacement MAF sensor on eBay:

http://cgi.ebay.com/ebaymotors/ws/eB...TQ:MOTORS:1123

I changed the sensor alone, not the housing at all (is it really possible for the screen on the housing to get dirty?). While this new MAF is a used part and not specifically tested (but therefore returnable), I figured trying a different MAF, even if it isn't as good as new, should at least give me somewhat different numbers during the redline test to confirm that MAF is the problem. No such luck---same lousy numbers using the new MAF. However, I think the new MAF is also bad, it fails the engine idling value range test much worse than my original one does (my original one is just at or slightly above specified values when idling at the indicated RPM's). So, I'm going to bite the bullet and get a new MAF for $150 now and keep my fingers crossed.

I will also note, I've been getting the following fault code sometimes too, and with the new MAF, this error appeared by itself at first, though later both banks also came up with similar too rich rich fault codes.

17549 - Load Calculation Cross Check: Implausible Value
P1141 - 35-10 - - - Intermittent

Bentley manual seems to suggest that MAF or throttle valve/throttle body would likely be the cause of this code.

Also, from measuring block group 55, these are the values are constantly returned during the redline test:
-1.39 g/s Idle air ctrl -1.39-+1.39g/s
3.33 g/s Learn value -1.39-+1.39g/s

Bentley manual for evaluating group 55 vthat values >1.39 are an indication of "Air passage in area of throttle valve contaminated" and <-1.39 are an indication of "Intake air leak (false air) behind throttle valve" On my car, at idle the idle air ctl. value above is well above +1.39 at all times, but when driving it is usually under +1.39 and may be staying at -1.39. Seems like "air contamination" at idle is another problem. What are they talking about?

I've always been hopeful that changing a simple little part like this sensor could lead to drastic improvements in fuel trim, but no joy so far of course. A boost leak still sounds like a very good suspect explanation to me at this time (if its in the throttle body, suppose that could also explain the bank trim imbalance too?). However, a boost leak pressure test seems to require a separate compressor, and I don't have equipment like that sitting around. Is there a way to do a boost leak test with a mini-VAC tool, or is there any way that folks know to rent tools like this, or do I really need to find a service station that will do the test for me and fix the problem if there is one?

Last, the car does give off a bad smell from under the hood of maybe partially burned fuel when parked in the garage after being driven. And it is leaking a small amount of oil from somewhere (not the pan). Not sure if either of those could be related to my fuel trim problem?

Thanks again for thoughts and advice.

[cparke]

Ok, my brother actually has another car with nearly the same MAF but without fuel trim problems! So I borrowed his MAF this evening to do the redline test on my car with a good MAF, finally! Here are the results:

RPM|Engine load|Injection time|Mass air flow
6080 3.85 6.02 93.68
6160 3.60 5.50 88.61
6200 3.55 5.57 88.40
6200 3.20 4.80 79.65
6200 2.25 3.33 55.34
6160 2.80 4.16 69.23
6160 3.25 4.99 80.76
6200 3.55 5.50 88.33
6320 4.15 6.21 104.72
6360 3.45 5.18 88.12
6280 2.65 3.84 66.39
6240 3.95 6.14 98.40
6240 3.85 6.02 96.18
6240 4.20 6.53 105.00
6240 4.10 6.21 102.57
6240 4.00 6.14 100.62
6280 4.90 7.62 123.06
6360 5.05 7.62 128.61
6360 4.00 6.14 102.64
6360 4.35 6.72 111.32
6360 4.95 7.68 125.83
6440 4.90 7.49 126.39
6520 3.85 5.95 100.14
6520 3.80 5.82 99.58
6560 4.55 6.98 118.47
6440 0.55 0.96 15.21
6160 0.60 0.00 14.79
…
6080 4.00 6.27 97.43
6080 2.65 3.97 64.44
6080 3.35 4.99 81.94
6080 4.50 7.36 108.96
6120 3.60 5.25 90.48
6160 3.65 5.57 89.51
6200 4.10 6.21 101.39
6200 3.30 5.18 82.29
6200 3.70 5.76 91.39
…
6040 4.20 6.46 102.57
6120 2.90 4.16 72.71
6280 5.25 8.06 131.94
6480 5.85 9.02 152.08
6680 4.30 6.59 113.96
6640 0.55 0.96 15.00
6440 2.50 3.39 62.15
6560 4.25 6.66 111.87
6440 0.60 0.96 15.42
6200 0.60 0.96 14.86


The highest MAF value is 152.08, as expected. However, most values are still very low (MAF values seem to vary with load values more than with RPM's). If I didnt have the highest numbers, I'd say this MAF was also bad, but we know that is not true.

Does this log data look normal for doing the redline test, or should I still proceed with getting a leak test done next?

[cparke]

Just learned that I was supposed to be flooring the throttle (WOT) from 2500 RPM up to cut-off to do the redline test correctly, which I wasnt doing. Logs given here are therefore not very good at evaluating the MAF.

Also, highly suggested to turn off all consumers during such a test, especially air conditioning.

[Nebone]

Duh