Definitely the MAF, but not the MAF...

stomachbuzz

Well-known member
Joined
Apr 3, 2018
Location
Morgantown, WV
TDI
2000 Golf
So I got a car, with some problems. Let's call it a problem car.

Recently bought it with many issues layered over each other. Car was in trans limp mode, no power, etc.
Replaced trans sensors, trans good as new.
Still no power.
Replace all vacuum lines. Tested check valve. All looks good. I tested turbo actuator with MityVac. Did not measure travel, only confirmed correct movement.
Has power sometimes under certain conditions, but 90% no power.

MAF 101 threads combined with VCDS show tell tale signs of a bad MAF.
low power, actual MAF reading is accurate to requested reading up to ~400 then falls off, and car runs better with MAF unplugged
check, check, and check.

I replace MAF ---> ZERO change.

I look more carefully at VCDS, the MAF is fully capable of reporting 700+ (actual MAF readings occasionally spike to 700+), but the harder I press the throttle, the worse the MAF single gets.
When pedal stays at 100%, you can see the MAF signal oscillate in a damped fashion, almost like someone is jiggling the signal wire.
Very evident here:

If the pedal is suddenly floored, then the requested MAF signal (green) stays where it supposed to be, but the actual MAF signal (yellow) half-way spikes once, then it oscillates and declines. The car surges identically to the yellow line.
Very weird.

Sometimes the MAF will work properly, as seen here

Strong boost was present in both of these spikes, and the car pulled well.

The bad MAF signal is infinitely repeatable, and is 90% the norm, as demonstrated here:


and here:

While driving it this ^ time, it was literally ON/OFF with the throttle pedal. 0 or 100.

In VCDS, I also checked the boost data (data block 011). The actual vs requested boost graphs look identical to the MAF graphs.



At this point, with my limited knowledge, I'm confident that the turbo system is working correctly. I believe it is properly following orders based on a bad MAF signal.

The issue is, what would cause the MAF signal to fluctuate so much during constant inputs?
At this point, it almost seems electrical - reminds me of an RC circuit...
Any ideas?

EDIT: I have figured out somewhat of a pattern of when the cars makes power. At low RPM, and low throttle, it seems "okay". If I floor it, power gone as shown in graphs above. But if I gently increase the throttle, the MAF reading will sometimes "catch" and act properly for a while. Also, starting from a stop and flooring it in 1st gear seems to be immune from this issue. As soon as it shifts into 2nd gear, all power lost again.
 
Last edited:

Matt-98AHU

Loose Nut Behind the Wheel Vendor
Joined
Apr 23, 2006
Location
Gresham, OR
TDI
2001 Golf TDI, 2005 Passat wagon, 2004 Touareg V10.
It looks to me you have more of a boost control issue rather than a MAF reporting issue.

Time to go poke around for a broken EGR pipe, vacuum leak, boost leak, dead actuator etc.
 

Figit090

Veteran Member
Joined
Jun 21, 2015
Location
Northern California
TDI
Lifted Unicorn! '03 Jetta GLS TDI Wagon, 5spd, Candy White, Black leather.
I know it's probably not related but it kinda sounds similar to the fuel defeat that happens when you press the brake pedal while applying the throttle. Just brainstorming; is there any reasonable chance a faulty sensor in the braking system could cause this kind of on/off power issue?

Sent from my Moto G (5S) Plus using Tapatalk
 

Matt-98AHU

Loose Nut Behind the Wheel Vendor
Joined
Apr 23, 2006
Location
Gresham, OR
TDI
2001 Golf TDI, 2005 Passat wagon, 2004 Touareg V10.
I will partially retract my previous statement.

A boost leak would still mean the turbo is spinning plenty fast and thus the MAF readings should still be relatively normal.

But a turbo that is not able to spin up properly in the first place will definitely cause low boost and thus low MAF readings gas it's just simply not pulling in near as much air as it should. So I would focus more on vacuum leaks, dead actuator, stuck N75, seized vane mechanism in the open position or broken EGR pipe or other exhaust leak BEFORE the turbo, thus allowing exhaust pressure to escape that would otherwise be going towards driving the turbo...
 

stomachbuzz

Well-known member
Joined
Apr 3, 2018
Location
Morgantown, WV
TDI
2000 Golf
It looks to me you have more of a boost control issue rather than a MAF reporting issue.
Time to go poke around for a broken EGR pipe, vacuum leak, boost leak, dead actuator etc.
Thanks for your reply. I appreciate you giving me new areas to check.
Do you happen to have a priority list for these issues? I was thinking about EGR myself, but more along the lines of..."doesn't the EGR system introduce air AFTER the MAF....??"
I replaced all vacuum hoses, but I can check the T-fittings and the brake booster hose.
I know it's probably not related but it kinda sounds similar to the fuel defeat that happens when you press the brake pedal while applying the throttle. Just brainstorming; is there any reasonable chance a faulty sensor in the braking system could cause this kind of on/off power issue?

Sent from my Moto G (5S) Plus using Tapatalk
Wow. That's an interesting thought. Pretty good idea as my power loss seems almost 'electronic-like' with how systematic it is. I wonder if there's a fuse for that...
I will partially retract my previous statement.
A boost leak would still mean the turbo is spinning plenty fast and thus the MAF readings should still be relatively normal.
But a turbo that is not able to spin up properly in the first place will definitely cause low boost and thus low MAF readings gas it's just simply not pulling in near as much air as it should. So I would focus more on vacuum leaks, dead actuator, stuck N75, seized vane mechanism in the open position or broken EGR pipe or other exhaust leak BEFORE the turbo, thus allowing exhaust pressure to escape that would otherwise be going towards driving the turbo...
Lol. thanks for revisiting my thread, I appreciate it.
I just can't figure out what would oscillate like this though. It works well, then shuts off, then kinda works again, shuts off, and then stays off. I feel like an air leak somewhere would be constant, not up and down?

:confused:
 

BobnOH

not-a-mechanic
Joined
May 29, 2004
Location
central Ohio
TDI
New Beetle 2003 manual
..........................just can't figure out what would oscillate like this though. It works well, then shuts off, then kinda works again, shuts off, and then stays off. I feel like an air leak somewhere would be constant, not up and down?

:confused:
Some stuff just works that way. I'm not expert with the logs.
I would ignore your feelings, take your data and read back thru the MAF 101. I'm sure it's a beast of puzzle, but the answer should be in there somewhere, likely near the beginning.
 

Figit090

Veteran Member
Joined
Jun 21, 2015
Location
Northern California
TDI
Lifted Unicorn! '03 Jetta GLS TDI Wagon, 5spd, Candy White, Black leather.
Thanks for your reply. I appreciate you giving me new areas to check.
Do you happen to have a priority list for these issues? I was thinking about EGR myself, but more along the lines of..."doesn't the EGR system introduce air AFTER the MAF....??"
I replaced all vacuum hoses, but I can check the T-fittings and the brake booster hose.

Wow. That's an interesting thought. Pretty good idea as my power loss seems almost 'electronic-like' with how systematic it is. I wonder if there's a fuse for that...


Lol. thanks for revisiting my thread, I appreciate it.
I just can't figure out what would oscillate like this though. It works well, then shuts off, then kinda works again, shuts off, and then stays off. I feel like an air leak somewhere would be constant, not up and down?

:confused:
I would give Matt's suggestions priority, I couldn't study your graphs nor am I knowledgeable with diagnosis. Your symptoms just reminded me of the first time I tried to press both throttle and brake, and I was curious if it could be a cause.



Sent from my Moto G (5S) Plus using Tapatalk
 

wonneber

Top Post Dawg
Joined
Oct 12, 2011
Location
Monroe, NY, USA
TDI
2014 Jetta Sportwagon,2003 Jetta 261K Sold but not forgotten
You indicated you replaced the MAF.
Where did you get it from?
Was it OEM Bosch in a Bosch box?
 
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