Effects of a low MAF reading

[Steve Alexander]

I have a Passat 1.8T 20v Turbo Sport. When I bought it from my friend he said that the emission light had been on for quite a while, but it never caused any problems and it always got through the MOT emission tests.
With the guidance of this forum I set about trying to rectify the problem and using Vagcom the error code identified that the car was running lean on bank 1. Using the Vagcom measuring block 002 I got a reading of about 2.3 g/s at idle, which appears to be within the acceptable tolerance. However I read that a good check was to run in a single gear (e.g. 2nd) from about 2500 rpm up to about 6500 rpm and log the readings. I tried this and the log typically read 800 rpm = 2.72 g/s, 2560 rpm = 30.5 g/s, 3200 rpm = 40.69 g/s, 4640 rpm = 55.22 g/s, 5280 rpm = 60.67 g/s (which is the peak value), 6200rpm = 53.94 g/s, 6720 rpm = 55.03 g/s. The reading then steadily fell as I eased off the accelerator until it settled again around 2.72 g/s at 800 rpm. From what I have read this would appear to be a relatively low reading at the highest rpm. Does my MAF appear to be faulty, even though I am getting readings, and would this lead to the error code and the recurring emission light. I cleared the error code and the warning light did go out but soon returned after a couple of days. I also used block 32 to look at the Lambda learnt values and the idle speed (additive) read -1.3% but the multiplicative read 25%. These reading were obtained within a few hours of resetting to 0%.
With experience, would knowledgeable members feel a new MAF would resolve my problem. Many thanks.

[smokey0123]

What was the fault code?

[Steve Alexander]

I would have to dig out my notes to confirm, but it either 16555 or 17536, I'll have to confirm.

Just confirmed: 17536 - Fuel Trim Bank 1. (Multi): System Too Lean P1128-35--

[smokey0123]

Check the voltage of the lambda sensor (Bank1-Sensor1) in measuring blocks and make sure the voltage looks right it should swich quickly in between 0.2 volts to about 0.8. Do you have the emmisions print out from the last mot? It could be running rich.

[Steve Alexander]

In the measuring block 33 - Lambda regulation - before catalyst.
Zone 1 value, regulation before catalyst, varies from about 0.1% to 4.7% and Zone 2 value, probe voltage before catalyst, varies 1.26 volts to 1.33 volts.

In measuring block 37- Lambda regulation - Delta
Zone 2 value the value, probe voltage after catalyst, varies 0.71 to 0.75 volts

Not sure what this actually means, and it was my first attempt at taking measurements, so could be repeated if necessary.

[smokey0123]

The voltage on bank1 sensor 1 seems very high, can you run the test again with the MAF disconected? (The readings from the MAF are low as you thought).

[Steve Alexander]

I will try re-running the test without the MAF, as you suggest. Unfortunately I will be unable to do this until the weekend, so I apologise now for slow feedback.

With regard to the lambda probe voltage reading, for the lambda probe before catalyst. This was measured using Measuring block 33 with the reading in zone 2. According to the documentation that I have, it states that the specification should be between 0.00 volts and 5.00 volts, so I wasn't initially concerned by this reading as it seemed within spec.

[Steve Alexander]

Smokey 0123

You asked if I had a print out from my last MOT for Exhaust Emission Test.


FAST IDLE TEST
CO Limit <= 0.30 Actual = 0.20 (% vol)
HC Limit <= 200 Actual = 56 (ppm vol)
Lambda Limit 0.97 - 1.03 Actual = 0.993

NATURAL IDLE TEST
CO Limit <= 0.5 Actual = 0.09 (% vol)

Hope this means something to you, it doesn't mean much to me apart from the fact that the values are below or within the limits.

[smokey0123]

The emissions reselts look like they were taken when the car wasnt fully warmed up. They are not to far out but very slightly rich. Check the reading of the coolant temp sensor and compare it to the gauge on the dash and if that looks close change the MAF.

[Steve Alexander]

Smokey0123

Just to let you know that I replaced the MAF after your suggested test.
It had done the job and the car is running much smoother and the emission light is now staying off.
I have run the car for a week now and the learnt lambda readings are now -3% compared with +25% before I changed the MAF.

Many thanks for you suggestions.

Steve