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VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old March 18th, 2006, 18:16   #1
dataiv
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Default Boost Spike and Diagnosis

Hi all,

I have been doing a lot of searching and research on the site, but I still need some assistance. My 99.5 Jetta has 245,000 km on it, mostly highway. At 235,000 or so, the car was going into limp mode when at full throttle in 5th gear (and sometimes in 4th). In December I took the head off, cleaned the intake, took apart the turbo, and found the vanes weren't really stuck, but a bit sticky sometimes. When I put everything back together the vanes were moving freely. It is fine most of the time, but sometimes it goes into limp mode still with the same code:

Code:
VAG-COM Version: Release  311.2-N

Control Module Part Number: 038 906 018 EB 
  Component and/or Version: 1,9l R4 EDC G400SG  2143
           Software Coding: 00002
            Work Shop Code: WSC 00066
1 Fault Found:
17958 - Charge Pressure: Control Deviation
        P1550 - 35-10 - - - Intermittent
I still get a big boost spike, and I don't know if I should try adjusting the linkage on the VNT mechanism. I have checked the N75 and it appears to be fine. Having VAG-COM cycle the vanes in group 11 basic settings mode seems to be fine, and the rod travels as it should.

So, what I would optimally like to solve, is both problems, the boost spike, and the sometimes going into limp mode. I haven't ever been logging the data when it went into limp mode yet, as I haven't been that lucky...

Anyway, any hints for what I should try next and check next would be much appreciated.

Below is a graph of a run I did today at full throttle in 5th gear from 91-126km/h. I would like to go further, but I ran out of room today.


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Last edited by dataiv; October 11th, 2006 at 07:48.
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Old March 18th, 2006, 18:25   #2
dabear95
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At the highest RPM you have recorded, what is the MAF reading? It should be at least 850-900 mg/h at 3,000RPM.

Can you send me the "raw" data log?



Jason
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Old March 18th, 2006, 18:31   #3
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The MAF is on the chart as well. The light blue colour. It is not, as you say, 850-900.

I have another run in 4th gear where RPM goes up to 3500 and the MAF still reports less.

But could that be an MAF problem as opposed to a turbo problem, given that the two follow each other so closely on the graph? I replaced the MAF once before about 140,000 km ago and it felt different when that was the problem last time, plus it never went into limp mode.

Here are four logs that I took today. One is with the IQ setting at 32768 (the default), which gave IQ of 2.0, then one with the IQ at 3.0, and one with the IQ at 4.0. Each setting made a bit of a difference, which was expected.

http://txn.vangeyn.net/LOG-01-011-010-022-3logs.zip

I should also mention, I forgot in the first post, that the car is completely stock.
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Last edited by dataiv; March 18th, 2006 at 18:56.
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Old March 18th, 2006, 19:25   #4
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Yes, I saw that it was on your graph but I did not want to interpret the date incorrectly. My MAF reading was significantly higher than 850-900. Also, your MAF readings at other RPM levels differ greatly from mine. I would try and unplug your MAF and see if it runs better (more power). Easy to check…




Did you check the vacuum line from the N75 to the turbo? I think the N75 and N18 can be switched. That would need to be confirmed by a more experienced member however. Maybe the N75 is "worn" out, if that is even possible.



I have graphed the "longest" run from the files. I think a run in 4th gear from 1700-4000 would be helpful...






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Old March 18th, 2006, 19:28   #5
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This is my cars requested versus actual:




It seems logical that your car is not reacting fast enough. why...


Jason
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Old March 18th, 2006, 19:34   #6
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Yeah, it is definitely strange. I will do a 4th gear run tomorrow because that's easy enough to do (5th gear up to 3500 RPM is really tough given how fast that actually is!).. but it was in 5th gear that it was going into limp mode. Right when I passed 2800 RPM it went into limp mode yesterday, then I restarted and did it again at almost the same RPM again. Then today, it wouldn't go into limp mode. It's very peculiar.

I think you may have gotten the PSI calculation off, because I know my car "requests" more than 14 PSI of boost. It's still easy enough to see, anyway.

What should the MAF values be? I am not exactly sure, but I though 850-900 was about the maximum it ever is.

If I use a mity-vac on the vacuum hose going down to the turbo, it holds vacuum fine, so I don't think that is leaking at all there.

I have no idea either if the N75 does "wear out"... from other people what I've read, it just dies and doesn't work properly at all anymore. Mine at least seems to be working.

I will try with the MAF unplugged just to see, and I will also get a 4th gear run tomorrow and will post that once I have it.

Thanks for the help.
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Old March 18th, 2006, 19:40   #7
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Yeah, the psi is wrong for sure. I just subtracted 1,000 for an atmosphere.

I don't have my MAF log on my desktop; it is on my laptop at work. I know I had readings over 1,000.

You’re not kidding about trying to log in 5th gear, lol. Let me know what happens with the MAF unplugged.



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Old March 19th, 2006, 20:18   #8
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Alright, two new graphs I have below, as well as the link to the raw data logs. With the MAF unplugged, the "actual" value is constantly at 550 mg/R, and the car was doggishly slow then. On the 4th gear run, I could get nowhere near 4000 RPM in the short space I had to do it. It would have taken quite a while. Though, in that run, the boost appears to stay much more constant than when it's plugged in. With it plugged in, it is a lot faster, yet it fluctuates more. I graphed spec & actual MAF and MAP (boost in millibars) along with RPM and speed again. With the MAF plugged in, I got a better run, due in part to the faster acceleration, but I had to do it twice to get up to ~4000 RPM because of traffic conditions.

Anyway, here is the data. Let me know what you think. I have no idea any more. No limp mode today, though yesterday I did adjust the IQ value which was at ~2.0 mg/R up to ~3.0 mg/R. Less smoke on startup for sure, but I was never getting any smoke at WOT even still. The graph I posted yesterday, along with the logs, are all with the IQ already at ~3.0. When it was at ~2.0, it made no difference as far as the fluctuations in boost, etc... I did check that before modifying it.

The logs: http://txn.vangeyn.net/LOG-01-003-011-022-MAF.zip



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Old March 19th, 2006, 20:46   #9
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Sorry just to butt in but I think you simply need a new MAF. At least you should first rectify the discrepancy of the low MAF output before looking further.

Since the MAF peaks are in slightly in advance of the pressure peaks I'm guessing (wildly, I might add) that the MAF (demand-actual) error makes a contribution to the requested pressure that is not part of the logged pressure demand. Just a thought ...
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Old March 20th, 2006, 05:56   #10
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MAF Time


Dieselgeek has a new non-bosch MAF that is cheaper. Although I do not know if many people have tried it.


It's going to be even faster with a new MAF though!

Edit- I just went back and checked my MAF log. I bet a new MAF will make a huge differance for you. It will probably eliminate the underboost...

Jason

Last edited by dabear95; March 20th, 2006 at 07:09.
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Old March 20th, 2006, 11:46   #11
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What makes you really think that it is the MAF that is the problem? How do we know it's not the turbo boost just fluctuating, which would also affect the air mass? I don't want to buy an MAF unless we are pretty sure..

I just wonder how you arrived at that conclusion?

I can get a new MAF from Roseland here in Canada, for $80, which is not bad at all. Last time I replaced the MAF it was still a super expensive part and I paid something like $350 for it. That was back in late 2000 or maybe early 2001.

Has anyone else ever had limp mode from a Charge Pressure: Control Deviation code from a bad MAF? It doesn't seem to make sense.

This is the part listed on the Roseland website:

Bosch, 0 280 217 121
Mass Air Flow Sensor Assembly
( for all ALH Engines, new, not rebuilt )
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Old March 20th, 2006, 16:13   #12
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Note you are reaching peak demanded pressure at times which by simple physics will develop "full" air flow unless your IC or intake manifold is badly clogged. Your MAF should respond accordingly yet it appears to be low.
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Old March 20th, 2006, 17:15   #13
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Dataiv

I will post my MAF graph tomorrow morning to help.

Simple answer, I don't. But I do think your MAF is bad based on the data. Is there anyone close by with a MAF you can borrow? To make sure.


Jason
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Last edited by dabear95; March 20th, 2006 at 17:17.
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Old March 21st, 2006, 08:15   #14
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Dataiv,
Can't find my graph.


Check out TDIMaxima's posts. Post 5 to be exact.

http://forums.tdiclub.com/showthread.php?t=131916


Jason
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Old March 21st, 2006, 11:52   #15
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Well, thanks, I'm ordering a new MAF and hopefully it will solve the problem.. I will post back when I know one way or the other.

David
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