16485/P0101/000257 - Mass Air Flow Sensor (G70): Implausible Signal

k123

Guest
Feels like I've spent most of my life trying to fix this code so here's a write up of what I checked....needless to say - check the simple stuff first....like I should have done!! :whistle:

Mk4 Ibiza Cupra TDI 160
Fault description:
Juddering or kangarooing, but only really on constant light throttle. She'd do motorways fine but get stuck behind traffic at 40mph on an A road an she'd hesitate and kangaroo and chuck black smoke out the back.

Error code would only trigger on no throttle, coasting downhill. Other than that it wouldn't show for sometimes maybe an hour of driving at different speeds and conditions.

Lift at about 4,000 rpm and the view through the rear screen disappears in bonfire smoke.
Fuel consumption dropped from 350 down to 250 on a tank.

First up, is it really the MAF liek the code suggests? ( I learned the expensive way)
Bosch system so check the electrical readings on the pins of the electrical connector going into the MAF:

1. Temp sensor, not used in this application
2. 12v supply, yellow/black wire from Relay 109
3. Ground, Brown wire from ECU
4. 5v reference input voltage, Red/Green wire from ECU
5. Output voltage, Brown/Blue wire from ECU

I'd bought another MAF which didn't improve things, so checked the readings with the multimeter and all seemed OK so went in search of something else...

Found the following very useful explanation at pistonheads A4 TDI 170 P0299 Underboost on occasions

It's on the wrong engine so I've reposted 'Tame Technicians' wise words here:


P0299 really shouldnt be that hard to fix, it can only be about 4 different things in that engine.

If its got a new EGR valve, N75 and there are no vac leaks, (weather from the know troublesome vac actuator on the manifold flap), or any where else in the vac system, then it is possible the turbo veins sticking and then a turbo is the only fix. But I have to say that is unusual, when turbos stick its usually in the other position and you get overboost rather than under, so dont be sold a new turbo just yet.

I would say 90% of 170PD TDI's I have tested with p0299 has either needed the manifold vac actuator or there has been a massive boost leak somewhere like holes intercooler or split hose (you would surely hear this, loud hissing on boost)

The other 10% the engines wouldnt rev to 4500rpm due to either blocked injectors or low fuel pressure, the poor combustion at very high revs meant there wasnt enough exhaust output to keep the turbo spinning and an underboost code was logged. Often customers would even know the engine didnt rev as they changed up well before this point and the only simpton they reported was intermitant going into limp mode.

Go out and give it a drive trying both pulling from off boost 1200rpm in 3rd 4th through the range, and thrashing it right to the red line in 4th 5th, to see if you can make it play up.

If it fails in the low rpm range is likely a vacuum issue or the software needs updating, If the engine wont pull right the way to the red line and ECU notices an underboost right at the top of the rev range you may have a low fuel pressure or blocked injector issue.


Fault finding:

So it was definitely a boost issue....
VAG.com or VCDS on Engine/003 drive in 3rd at 3000 logging data.
This proved to me boost was down.
MAF actual and requested were matching and crossing paths on the graph. Sometimes higher, sometimes lower than each other but they were trying!
MAP actual and requested we're doing the same, but actual failed to be higher past about 2000 rpm, it was just weak.

Trial and error
Convinced it was the low pressure system I went in search....
Mitivac showed the turbo actuator was moving just fine, started to move at 2Hg and hit the stops by 18Hg (To test, have someone operate the Mitivac for you and watch the turbo from underneath the car at the back of the engine)

The pipe to air filter was clear

There was a hiss when I broke the connector so the vacuum tank was holding pressure
EGR was moving freely under suction

One way valve from engine worked, mityvac could suck but I couldn't blow down it towards the engine when I tried so the one-way bit was working

Anti-shudder / engine shutoff valve (ASV) was doing its job but showed a leak so was replaced (flick the clip off with a screwdriver inside the slot, rotate the actuator and pull through the bracket)

Front bumper off thinking maybe it was a hose somewhere...


Solution found:
After all that and a couple of months testing and scratching my head, the front bumper came off....to reveal a massive hole in the intercooler!! Very obvious but was directly behind the SEAT badge so it couldn't be felt blowing.
Replacement intercooler for less than £300.


Will post a pic of dead intercooler and some useful links if I ever get over 15 posts!!


Hope this helps someone with the same error code. Think I really did check everything before finding the solution so maybe will help others with different problems on the same error code!
 
Jan 28, 2009
703
1
Glasgow
Nice right up...if only everyone went through a process like this before posting "Replaced MAF still same issue...HELPPPP"!?

Most commonly if the MAF fault code shows it is due to a leak somewhere. In fact if you are smoking more than usual, under or over boosting then its almost always a boost leak somewhere in the system or an actuator that needs adjusting or that has failed.

Its ALWAYS good to check the mechanical and vac system first before replacing MAF, turbo, actuator etc. If you are still running your egr and havent taken of the elbow going to it then I think you might find when you do it will be pretty coked with gunk, a good clean will keep the limp mode away and will make the car feel new again :)
 

k123

Guest
Kicked myself when I solved it....so simple! Should have just followed the VCDS Wiki advice really....

Though in my defence I don't really get Dielsels or Turbo's so a new topic for me :D
 

k123

Guest
FMIC Intercooler holed and blowing

With the bumper off it was fairly obvious.....could feel the air blowing as soon as the engine was running. :rofl:


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