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Discussion Starter #1
2003 B5.5 1.8t 155k mile wagon. Almost everything around the motor has been replaced. No vac leaks, no pressure leaks. Turbo spins freely. Car runs perfectly. We've had the 171 and 299 codes forever but after replacing A/C condenser, compressor, receiver/dryer and orifice tube, the C0759 popped up (battery was disconnected overnight). Could anyone help me with these questions?

MAF sensor in airbox was replaced but no ECU reset was done, could this be a problem?
What does C0759 mean in this context? Search and Google aren't much help, something about brake booster was all I found.
Anyone know of a knowledgeable technician in the west central Florida area that can check this problem, code a new key and possibly do an ECU upgrade (like APR)?
Can you do the above things with Ross-Tech 99 dollar version?


Thanks for any help. The loss of the little CEL light would make life a lot better.


addendum: Codes were found with a Chinese dongle and the Torque app.
 

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I have never seen a C0759 code for a Passat. The only one found on Google was a GM tire pressure sensor. If the replacement MAF sensor fixed the P0171 and P0299, the codes will stay stored until cleared. Once cleared, they should not come back unless the issue was not fixed.
 

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Discussion Starter #3 (Edited)
P0171 and P0299 have been there since we bought the car. We replaced the MAF sensor, but never had it matched to the ECU. I don't even know that it is necessary, think I read that here.

I have often (after a repair) and 2 days ago cleared the codes via Torque app but P0171 and P0299 always reappear. I guess my question is does the ECU have to be coded to a new MAF?
 

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No, the new MAF does not need to be coded to the ECU. If you still have the codes, there is another problem, most likely a vacuum leak. How did you determine that there were no vacuum leaks or boost leaks?
 

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Discussion Starter #6
Thanks
Every vacuum hose replaced, every valve, tee, everything to the brake booster line has been replaced with silicone. System was smoke tested with pressure, vacuum tested with water spray. No leaks. I can't think of another place to test or look. Doesn't leak down very quickly either. When I use the Torque app, it says I'm getting 18 psi boost when accelerating. That seems high, where does the ECU get that info?
 

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Discussion Starter #7
16555/P0171/000369 - Ross-Tech Wiki
16555/P0171/000369 - Fuel Trim; Bank 1: System Too Lean

16683/P0299/000665 - Ross-Tech Wiki
16683/P0299/000665 - Boost Pressure Regulation: Control Range Not Reached
I know It is saying it's running too lean and the turbo isn't boosting. These are classic symptoms of a leak in the intake tract. The car drives like it has adequate boost, sounds great. IF (and a big if) there is no leak, what sensors are responsible for the ECU to garner this info? Where else to look? And I hope it's a leak because it would be a cheap fix, I just can't find it if it's there.
 

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I'm pretty sure there is no MAP sensor on these cars so I'd guess the ECM would be using the O2 sensors to generate these codes. As mentioned in the Ross-Tech links, the faults could be in the fuel delivery components.

How soon did the codes reappear? It may be possible that the dongle didn't clear them correctly.
 

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Discussion Starter #9
I'm pretty sure there is no MAP sensor on these cars so I'd guess the ECM would be using the O2 sensors to generate these codes. As mentioned in the Ross-Tech links, the faults could be in the fuel delivery components.

How soon did the codes reappear? It may be possible that the dongle didn't clear them correctly.
CEL reappears after about 8 - 10 miles into second or third start. Usually at the same steep on-ramp. I call the airflow sensor in the airbox the MAF, I think it has another acronym by VW. There's no O2 sensor codes, just sayin'. The FPR hasn't been replaced, in my experience that's an all or nothing failure. Is it possible? I'm fairly certain the dongle clears the codes.
I just realized that I could access the Ross-Tech Wiki. Would having the fuel trim data help?
 

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Yep, sounds like the dongle is fine. By MAP I was referring to a manifold absolute pressure sensor as opposed to the MAF mass air flow sensor. Some cars have both, B5's only have a MAF AFAIK. I was just taking a stab at what sensors would measure the boost pressure. But....after thinking about it some more, I would guess (!) that it might be a combination of intake air temp, MAF reading, rpm, etc. The engine should know how much air the engine will consume for any given air temp, rpm, MAF reading, etc. Would that be enough to calculate boost? Or does it just calculate the required fuel and then use the O2 sensor to determine how much air was present (and thus the boost). Maybe there are some tuners on the board who know this type of stuff. I don't but I sure like speculating about it ;)
 

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Discussion Starter #11
Yep, sounds like the dongle is fine. By MAP I was referring to a manifold absolute pressure sensor as opposed to the MAF mass air flow sensor. Some cars have both, B5's only have a MAF AFAIK. I was just taking a stab at what sensors would measure the boost pressure. But....after thinking about it some more, I would guess (!) that it might be a combination of intake air temp, MAF reading, rpm, etc. The engine should know how much air the engine will consume for any given air temp, rpm, MAF reading, etc. Would that be enough to calculate boost? Or does it just calculate the required fuel and then use the O2 sensor to determine how much air was present (and thus the boost). Maybe there are some tuners on the board who know this type of stuff. I don't but I sure like speculating about it ;)
This is AWM B5.5.I think I see where you're going with this. I'm curious if I could bench-test the boost and temp sensors that are by the intercooler and throttle body.... It runs 27 pounds of pressure under hard acceleration, that seem a little high? Does the ECU get it's pressure info there? brain hurts, let me think! Please chime in if anyone knows
WAIT- If it thinks the boost is super high, it would think it's lean? More paradoxes than time travel...
 

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27 psi is very high, but if you are including the atmospheric pressure, (roughly 14.5 psi), then the 12.5 psi is fine. The ECU gets the boost pressure from the sensor on top of the intercooler. If it really is 27psi, the engine could be leaning out (as well as self-destructing internally from knock).

I've never vacuum tested with water, how is it done? Just spray and listen for the rpm to change? If so, a hand vacuum pump may work better. Check anything that has a vacuum line to it and make certain it holds vacuum. The smoke test will get most of them, but many items are behind the check valves, so they need to be tested with a pump.

You should also run a test for MAF flow. If it shows a lower flow than actual, it will run lean. A bad MAF will cause slower spool on the turbo as well as rough idle. It may not throw a code right away, mine took several months to throw a code.
 
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The 27 pounds of pressure is an approximate estimate of absolute pressure, boost is normally quoted in PSI above atmosphere,
27 PSI would equate to about 12.5 PSI boost. This is high for stock but allowing for error it is likely about right.
I suggest you do a thorough check for vac leaks. Check all vac lines and what they connect to, also check the entire PCV system.
You could also check the MAF sensor: Ross-Tech-Tip - Diagnosing a VW/Audi MAF Failure
http://www.ross-tech.net/vag-com/advertising/DiagnosingA_MAF_TechTip.pdf


NOTE: I started this post before PZ posted. I guess it just shows that great minds think alike.
 

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Discussion Starter #14
27 psi is very high, but if you are including the atmospheric pressure, (roughly 14.5 psi), then the 12.5 psi is fine. The ECU gets the boost pressure from the sensor on top of the intercooler. If it really is 27psi, the engine could be leaning out (as well as self-destructing internally from knock).

I've never vacuum tested with water, how is it done? Just spray and listen for the rpm to change? If so, a hand vacuum pump may work better. Check anything that has a vacuum line to it and make certain it holds vacuum. The smoke test will get most of them, but many items are behind the check valves, so they need to be tested with a pump.

You should also run a test for MAF flow. If it shows a lower flow than actual, it will run lean. A bad MAF will cause slower spool on the turbo as well as rough idle. It may not throw a code right away, mine took several months to throw a code.
Spray bottle of water, changes RPM when it hits a hole, cut or tear. Smoke test back in early spring showed that I put the wrong O-ring on the L-pipe from the oil cooler base. The whole PCV system was changed this year. I had an intake manifold leak on cyl 2 and 3(found with a spray water bottle), replaced manifold with proper gasket. Thought that would be the end of the troubles - but NO!!!!

Thanks for the help PZ, at this rate of spending to keep this car on par with a Toyota Yaris I will have to drive it forever - and be buried in it...

Very frustrating.
 

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Discussion Starter #15
The 27 pounds of pressure is an approximate estimate of absolute pressure, boost is normally quoted in PSI above atmosphere,
27 PSI would equate to about 12.5 PSI boost. This is high for stock but allowing for error it is likely about right.
I suggest you do a thorough check for vac leaks. Check all vac lines and what they connect to, also check the entire PCV system.
You could also check the MAF sensor: Ross-Tech-Tip - Diagnosing a VW/Audi MAF Failure
http://www.ross-tech.net/vag-com/advertising/DiagnosingA_MAF_TechTip.pdf


NOTE: I started this post before PZ posted. I guess it just shows that great minds think alike.
I'm about 2 meters above sea level. I will check the Medusa system (vac hoses and such) again this weekend. I'm thinking maybe the hard plastic hose to the brake booster might be leaking but everything works fine. The car runs great. Idles smooth. Accelerates well.
Thank you Tomvw for the link. I went down that road before but didn't have a tutorial like Ross-Tech. I think I see an area on the Torque app that allows trim adjustments, will test it this weekend. The MAF is under 2 years old, it should have a warranty if it is bad. Actually, I'm at the point I'd just rather take it to someone that knows these cars - a fresh set of eyes and all that. All I see here in my area are BMW shops, like they look down at VWs. And the dealer, under whose care I will be so poor as to have the Passat as my final resting place. Seriously.
 

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@OP, sorry about the mis-information...it looks like these cars do have a MAP and I'm glad the experts chimed in on it. Good luck with the search.
 

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Discussion Starter #17 (Edited)
I was driving today and I used the app, it makes me feel guilty to glance at the phone while driving. I took screenshots. I think these results are quite strange, but here we go.

Fuel Trim Bank 1 short Term [06], Latest Raw Value: 16.47%

Air Fuel Ratio (Measured) [ff1249], Latest Raw Value: 14.59:1

This was at an ambient temperature of 97F (36.1C). There was no real change in the A/F ratio from cold start to 96C idle. The A/F went down below 13:1 under acceleration and up above 15:1 upon deceleration, which seems backwards to me. I'm not exactly sure what that means, any ideas?

Found this while idling in the driveway just now:

Fuel Trim Bank 1 Long Term [07], Latest Raw Value: -3.12%
 

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Discussion Starter #19
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Diagnostic Tip:

For a suspected vacuum leak, note the fuel trims at idle and increase engine speed to 2500 RPM and hold. If the STFT immediately decreases and moves to acceptable levels and the LTFT slowly starts to come back down, you have a vacuum leak. After the repair, reset the KAM and start the vehicle. Monitor the fuel trims to make sure they are within the normal ranges. It could take up to 10 miles of driving for an accurate LTFT reading.
Found that online (Eastern Catalytic). 16.47% STFT at idle, -3.12% LTFT at idle; 9.38% STFT at 2500 RPM, 0 LTFT at 2500RPM. Does this indicate a leak? Not a lot of movement...
 

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The STFT does indicate a problem, probably a leak, but I am not all that familiar with fuel trims. When I had my V6, that was one issue I never really had to worry about. I have not even checked mine on my 1.8T and I am sure I should.

The A/F mixture should go rich under boost to prevent pre-ignition (knocking,pinging) under load and it should go lean off throttle. Again, I can't tell if the numbers are correct, but I have seen tuned engines shoot for 10-11:1 under high boost.
 
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