MAF testing questions

amit9

Veteran Member
Joined
Oct 7, 2004
Location
Toronto/Scarborough
TDI
None...
Hi:

Lately my fuel economy numbers have not been good at all, so I was trying to look at some of the basic items. I did some searches on the forums on how to test the MAF sensor. This is what I have done so far.

Test #1 - Unplugged the MAF and drove it, car was a bit more responsive right away while keeping the same driving pattern.

Test #2 -
a. With MAF unplugged and car warmed up fully, in neutral, floored the gas pedal, RPM gauge was able to cross the 50 mark in the red line zone. Then shut off the engine, and did test b.
b. Plugged in the MAF, in neutral, floored the gas pedal, RPM gauge was able to reach the 46 mark, just above the first red line, was not able to reach the red line zone.

Test #3 - Requires a VCDS scan. Can someone please help me with this? I am in the East end (Scarborough) and work in Markham, so can meet up around there. I believe this test would be the most conclusive.

Questions -
1. Are these signs of a bad MAF?
2. Is it OK to drive for some time with the MAF unplugged till I source out a replacement?

Thanks in advance.
 

petee_c

Veteran Member
Joined
Jun 2, 2014
Location
Heidelberg, Ontario, Canada
TDI
15 Golf TDI, was 06 Jetta TDI, 15 q7 tdi, was 11 Q7 TDI
I'm not a mechanic, nor do I play one on TV, but:

1. Most times (4 outta 5 times?), a bad or failing MAF should give you a engine code of some sort on an OBDII scan.
On some vehicles (my Boxster is one of them, there should be a range that a MAF generates in terms of voltage... you could try seeing if that may help you out.

2. On a gas car, it was fine to drive without a functioning MAF for abit... there is a stock cold loop that the car runs on i think
 

imo000

Top Post Dawg
Joined
Dec 13, 2005
Location
Cambridge
TDI
2009 M-B ML320 Diesel & '05 Passat TDI Manual 5-Speed
What is the last thing you did to the car before noticing the fuel mileage drop?

At this time you are just guessing what's wrong. Don't drive the car with an unplugged MAF. Does your check engine light works?
 
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amit9

Veteran Member
Joined
Oct 7, 2004
Location
Toronto/Scarborough
TDI
None...
I'm not a mechanic, nor do I play one on TV, but:

1. Most times (4 outta 5 times?), a bad or failing MAF should give you a engine code of some sort on an OBDII scan.
On some vehicles (my Boxster is one of them, there should be a range that a MAF generates in terms of voltage... you could try seeing if that may help you out.

2. On a gas car, it was fine to drive without a functioning MAF for abit... there is a stock cold loop that the car runs on i think
1. I have a cheap "Scangauge" plugged in and no codes relating to a bad/failing MAF were displayed while the MAF was plugged in.
2. Yes that's what I read too in the forums that the ECU will use some "default" setting in case of a bad/unplugged MAF.

What is the last thing you did to the car before noticing the fuel mileage drop?

At this time you are just guessing what's wrong. Don't drive the car with an unplugged MAF. Does your check engine light works?
Two things - have been filling up at a new and cheap but volume diesel pump, and cleaned the MAF with isopropyl alcohol 70%. Yes the CEL does work and yesterday when I had the MAF unplugged it did display a P0102 code. After I plugged the MAF back in and cleared the code, it didn't display the CEL.

The car is at 291K/16 years, so I'm thinking that the MAF may be past it's normal service life?

Thanks.
 

petee_c

Veteran Member
Joined
Jun 2, 2014
Location
Heidelberg, Ontario, Canada
TDI
15 Golf TDI, was 06 Jetta TDI, 15 q7 tdi, was 11 Q7 TDI
1. I have a cheap "Scangauge" plugged in and no codes relating to a bad/failing MAF were displayed while the MAF was plugged in.
2. Yes that's what I read too in the forums that the ECU will use some "default" setting in case of a bad/unplugged MAF.



Two things - have been filling up at a new and cheap but volume diesel pump, and cleaned the MAF with isopropyl alcohol 70%. Yes the CEL does work and yesterday when I had the MAF unplugged it did display a P0102 code. After I plugged the MAF back in and cleared the code, it didn't display the CEL.

The car is at 291K/16 years, so I'm thinking that the MAF may be past it's normal service life?

Thanks.
?winterblend fuel
 

imo000

Top Post Dawg
Joined
Dec 13, 2005
Location
Cambridge
TDI
2009 M-B ML320 Diesel & '05 Passat TDI Manual 5-Speed
?winterblend fuel

That'll do it and the denser air in the winter will decrease the fuel mileage.

I would just drive the car as it is and worry about it when the check engine light comes on. Until then, worry about something else. :)
 

Johhny04

Veteran Member
Joined
Feb 21, 2006
Location
Markham, Ontario
TDI
2003 Jetta TDi
I'm in Markham and can help you with a scan, after work hours (6:30 pm + ). However, i think it might have something to do with winter fuel, since I've also seen a drop in mileage too.
 

derek5120

Veteran Member
Joined
Sep 25, 2014
Location
Arthur Ont.
TDI
2003 jetta GLS
If you can get your hands on the scanner, even a generic that has data you can check the mass airflow reading. At idle you should be 270-380g/s with a working egr, it will be higher with a egr delete. More importantly 3rd gear pull full throttle you should get over 800g/s give to take. If your maf is bad you wont get much higher then that, also clogged intakes will lower you maf readings also.
 

amit9

Veteran Member
Joined
Oct 7, 2004
Location
Toronto/Scarborough
TDI
None...
If you can get your hands on the scanner, even a generic that has data you can check the mass airflow reading. At idle you should be 270-380g/s with a working egr, it will be higher with a egr delete. More importantly 3rd gear pull full throttle you should get over 800g/s give to take. If your maf is bad you wont get much higher then that, also clogged intakes will lower you maf readings also.
Thanks for the inputs, my generic code scanner does not have mass airflow reading parameter option. A forum member has offered to scan. I'll share the results in the thread.
 

Wingnut

Top Post Dawg
Joined
Oct 10, 2002
Location
Toronto & Whitby
TDI
Silver 2003 Jetta Wagon
Hi:

Lately my fuel economy numbers have not been good at all, so I was trying to look at some of the basic items. I did some searches on the forums on how to test the MAF sensor. This is what I have done so far.

Test #1 - Unplugged the MAF and drove it, car was a bit more responsive right away while keeping the same driving pattern.

Test #2 -
a. With MAF unplugged and car warmed up fully, in neutral, floored the gas pedal, RPM gauge was able to cross the 50 mark in the red line zone. Then shut off the engine, and did test b.
b. Plugged in the MAF, in neutral, floored the gas pedal, RPM gauge was able to reach the 46 mark, just above the first red line, was not able to reach the red line zone.

Test #3 - Requires a VCDS scan. Can someone please help me with this? I am in the East end (Scarborough) and work in Markham, so can meet up around there. I believe this test would be the most conclusive.

Questions -
1. Are these signs of a bad MAF?
2. Is it OK to drive for some time with the MAF unplugged till I source out a replacement?

Thanks in advance.
Your first test is a sign your car has a bad MAF. As stated, the range of airflow as measured by the MAF is between 250-850mg/s. When you unplug the MAF, the ECU sets a default value of 550mg/s. A bad MAF may not read higher than 300 or 350mg/s. Since the ECU uses the airflow to determine how much fuel/boost to add, if the MAF reading is low, so will the performance. By unplugging it, the new default value is higher than the bad value, so you get more fuel, and it feels more responsive. A working MAF showing 850mg/s under load will increase the fuel, boost & performance of the car.
 
Last edited:

amit9

Veteran Member
Joined
Oct 7, 2004
Location
Toronto/Scarborough
TDI
None...
Your first test is a sign your car has a bad MAF. As stated, the range of airflow as measured by the MAF is between 250-850mg/s. When you unplug the MAF, the ECU sets a default value of 550mg/s. A bad MAF may not read higher than 300 or 350mg/s. Since the ECU uses the airflow to determine how much fuel/boost to add, if the MAF reading is low, so will the performance. By unplugging it, the new default value is higher than the bad value, so you get more fuel, and it feels more responsive. A working MAF showing 850mg/s under load will increase the fuel, boost & performance of the car.

Thanks for the analysis Nigel. I will be testing it soon with the help of a member with VCDS.

Question - Reading above, so if the MAF is bad, it will decrease/restrict the fuel i.e. less fuel, so less responsive, but shouldn't it increase my fuel economy since it's using less fuel? :confused:
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
...and with less power delivered by the engine you're likely gonna tend to stick your foot in it a bit more... further negating "less fuel". :)
 
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