Help Diagnosing Cause of Intermittent "Limp Mode"

[Prospect25]

Hi All,
Don’t post here as much as I used to, mostly because I don’t drive my car as much, but still like to troll and see who’s doing what with their rides and to read allroadinvail’s funny ass responses to people’s posts. Anyway, I have an ’02 B6 A4 1.8TQM with an APR tune and bigger injectors. I haven’t had a lot of issues with the car but when I gave it to my buddy to borrow and wrench on a few weeks ago it seems like it came away with the same problem it had when i gave it to him and my pocket feeling a bit lighter.

The reason I had given it to my friend was he has lots of experience working on VAG cars and my car kept going back and forth between full boost and limp mode, all in the same journey. For instance, I’ll be driving up I-70 westbound up the winding hill from Golden to Floyd Hill and the car would be in full boost (around 15 PSI). If I let off the accelerator slightly when coasting or maintaining speed in traffic and then gave it some more gas it would only give me 5-7 PSI. This would happen, and still does, driving the whole way up to the mountains to go skiing. I was told it was throwing codes for a Front O2 sensor and I also had a boost leak, so I bought some parts and let him go to town fixing the boost leak, among other small maintenance items. All said, I spent around $900 just on parts.

I got the car back and within a day of driving it I noticed the same issue again. I told him what was going on and he was stumped, unsure of what the issue could be. I did some digging on Audizine and a few other sites and came up with what I *think* it could be:

1. Faulty N75 Valve – from what I’ve read it doesn’t throw a CEL, as the only one I have now is for the SAI Pump, and since it controls the wastegate would make the turbo think it’s boosting too high and kick it into limp mode as a safe guard
2. Boost leak – just had my guy go through the whole car and fix all potential issues so I doubt it’s this, but stranger things have happened
3. Diverter Valve – Broken DV means all charged air goes directly to atmosphere which means no power. Wouldn’t make sense though as my car can still produce full boost but is intermittent
4. Other – I have no f’ing idea what it could be. Could I just leave the car as is and keep going back and forth from full boost to limp? Or do I stand the chance of making things worse?

I’m hoping some of you may have heard of something like this before and can give me some suggestions. I don’t own a VCDS cable so am unable to run any data logs on any of the blocks. Kind of at a loss as to what it could be at this point and hoping someone can help a guy out.

Look forward to hearing all suggestions and at least one comical post from allroadinvail (I still owe you Bud’s for the center caps that are still presumably in Jace’s garage).

AJ

[jjvwg]

If you are able to get your hands on a cable or meet up with someone who has vagcom, log requested vs actual boost and also your n75 duty cycle. How many miles are on your k03? If you do get those logs, post them up so we can see what your n75 is doing. No underboost code at all correct?

[bluefox280]

I'll second what jjvwg noted about grabbing a VAG-COM (VCDS) and I've been a big proponent of scanning and obtaining true faults before throwing any parts at the situation.
Where in town are you located? Many of us have a VAG-COM [including myself] and can assist.

Keys things to scan when you locate a user: boost (spec vs. actual), N75 duty cycle, fuel trim (short term / long term), and MAF readings.
Since the soft Limp Mode is intermittent, you can almost certainly rule out the diverter valve since if that mechanically fails, you'll have a constant boost problem.
Same with boost leaks - those are easy to find when you pressurized the system and spot check.

- Erik

[Prospect25]

If you are able to get your hands on a cable or meet up with someone who has vagcom, log requested vs actual boost and also your n75 duty cycle. How many miles are on your k03? If you do get those logs, post them up so we can see what your n75 is doing. No underboost code at all correct?

Sounds like I may be able to bribe Erik with cash/beer/liquor/h00ker to help me with scanning.

I'll second what jjvwg noted about grabbing a VAG-COM (VCDS) and I've been a big proponent of scanning and obtaining true faults before throwing any parts at the situation.
Where in town are you located? Many of us have a VAG-COM [including myself] and can assist.

Keys things to scan when you locate a user: boost (spec vs. actual), N75 duty cycle, fuel trim (short term / long term), and MAF readings.
Since the soft Limp Mode is intermittent, you can almost certainly rule out the diverter valve since if that mechanically fails, you'll have a constant boost problem.
Same with boost leaks - those are easy to find when you pressurized the system and spot check.

- Erik

Erik, let me know if you may be available to help with the scan. I didn't think about the MAF, which is an interesting thing to consider. Would throw some cash or whatever else your way to borrow the cable, ride along, and help show me how to do the logs. I've been debating buying my own cable but, as i said in my earlier post, i rarely drive the car and don't have many problems so didn't want to cough up the $350 for one. PM or text/call me (412-608-0336) and maybe we can line something up for an evening soon. I also like to play hookey from work so give me a reason :)

Thanks for the responses so far!

AJ

[boostedAvant]

Pick up your phone and text me. I can scan yo ass.

[Prospect25]

Pick up your phone and text me. I can scan yo ass.

Holla! I haven't seen you on here for a while so didn't know if you were out of the game. Erik (bluefox280) is gonna give me a scan tomorrow as i'll be up in his neck of the woods while heading to Fort Collins so that will give us a good idea of what's going on. I'll let you know what we find.

[bluefox280]

Erik (bluefox280) is gonna give me a scan tomorrow... I'll let you know what we find.

AJ, good meeting with you this evening; wanted to post up hard data to get the ball rolling on this to get you back running 100%.

First, here's the known data on the 3rd gear wide-open throttle (WOT) runs we did:
* Measuring Block 002 (MAF flow rate): 138.xx g/s @ 6200 RPM was the peak
* Measuring Block 032 (fuel trim): +18.8% on the multiplicative (mult) adjustment - which means the ECU is seeing a lean condition and running the fuel injectors more to compensate for a proper combustion burn.

After some hefty reading and searching, signs are pointing to vaccum leak(s) on the engine.
Here's some of the common B6 A4 1.8T vacuum line areas to spot check:
* http://www.pelicanparts.com/techarti...cuum_Leaks.htm

And here's a4darkness master thread on the vaccum lines, parts, check valves, etc:
* http://www.audizine.com/forum/showth...-by-a4darkness

I'd say time to go through the engine bay with a fine tooth comb and check all the known areas of concern.

Let me know if you need anything else.

- Erik

[Prospect25]

AJ, good meeting with you this evening; wanted to post up hard data to get the ball rolling on this to get you back running 100%.

First, here's the known data on the 3rd gear wide-open throttle (WOT) runs we did:
* Measuring Block 002 (MAF flow rate): 138.xx g/s @ 6200 RPM was the peak
* Measuring Block 032 (fuel trim): +18.8% on the multiplicative (mult) adjustment - which means the ECU is seeing a lean condition and running the fuel injectors more to compensate for a proper combustion burn.

After some hefty reading and searching, signs are pointing to vaccum leak(s) on the engine.
Here's some of the common B6 A4 1.8T vacuum line areas to spot check:
* http://www.pelicanparts.com/techarti...cuum_Leaks.htm

And here's a4darkness master thread on the vaccum lines, parts, check valves, etc:
* http://www.audizine.com/forum/showth...-by-a4darkness

I'd say time to go through the engine bay with a fine tooth comb and check all the known areas of concern.

Let me know if you need anything else.

- Erik

Erik, it was great meeting you as well! Thank you so much again for scanning my car, logging the data on the third gear pulls as well as for taking the time to help with the research. After driving to and from Fort Collins and running errands around town this weekend I put an additional 200 miles on the car and the CEL hasn't come back, yet.

Now, it's on to diagnosing the vacuum leak. Could a bad SAI be part of the problem too?

What is the best/most logical way to track down a vacuum leak? Anyone want to help this week or this coming Saturday Feb. 27???

[bluefox280]

What is the best/most logical way to track down a vacuum leak?

For visual, smoke testing is probably the most preferred as you can do it without the engine running.
You can purchase a low grade one, or make one... like this:
* https://www.youtube.com/watch?v=2U5kFib1WxE

Key is low pressure, as you DO NOT want to blow out the existing gaskets, etc.

- Erik

[Prospect25]

For visual, smoke testing is probably the most preferred as you can do it without the engine running.
You can purchase a low grade one, or make one... like this:
* https://www.youtube.com/watch?v=2U5kFib1WxE

Key is low pressure, as you DO NOT want to blow out the existing gaskets, etc.

- Erik

I found the A4 Mods write up on how to make their version of the DIY Boost Leak Tool:
http://www.a4mods.com/index.php?page...boostleak.html

I have an APR Turbo Inlet Hose and am wondering if the inner diameter of that hose is different than the stock TIP. The A4 Mods post is recommending 2" PVC but i measured the circumference of the APR TIP and with the help of my TI-83 I found the diameter to be roughly 3.3". Since it is a thicker TIP than stock, does anyone know if the standard 2" is correct? Also, does anyone have one of these already made? My frustration is growing as the problem continues on and my lack of garage/proper tools is really bringing me down.

Thanks in advance,

AJ

[jjvwg]

If your still running the stock MAF, your aftermarket TIP should be the same size as the stock one.

[Prospect25]

If your still running the stock MAF, your aftermarket TIP should be the same size as the stock one.

Boost Leak Tester.jpgFaulty N75 Valve.jpg

Built the boost/vacuum leak tester per A4mods' writeup. Very straightforward and easy to do (holla if you need to borrow!)

Once attached, hooked up the compressor to ~10PSI and heard some hissing very close to the TIP. Upon digging around found the N75 hoses, both from the Turbo to N75 and N75 to Wastegate were ripped just below the hose clamps. Unfortunately no local Audi or VW shop had them in stock, for $40/each no less, so ended up getting some generic silicone hose from the parts store to hold me over in the interim. Had me a four-finger pour of Basil Hayden's after I buttoned it all back up to restore some of my sanity

Ordered two of these High-Temp Silicone Hoses with hose clamps from 034 and will be back under the hood this weekend to see if this fixes me right.

Doing some further investigation, I read about the N75 Valve and found someone stating that if you blow in one end it should let air through but the other two openings should not. Can anyone explain this further in layman's terms? Erik did some data logs on the block for N75 and requested vs actual were spot on which makes me think the boost leak is the culprit. Is there some other way I can test the N75 (Ohm reading, reading on boost gauge, etc) I can double-triple check it isn't the N75?


To be continued....

[bluefox280]

Upon digging around found the N75 hoses, both from the Turbo to N75 and N75 to Wastegate were ripped just below the hose clamps.

Good find and glad the pressure test was successful in locating some torn hoses.

Doing some further investigation, I read about the N75 Valve and found someone stating that if you blow in one end it should let air through but the other two openings should not. Can anyone explain this further in layman's terms?

The N75 (wastegate frequency valve) is a solenoid valve that is controlling the wastegate on the turbocharger to either make or bleed off boost (so not to create an overboost condition).
The valve has three connection ports: 1 from the intake hose before the turbo, 1 from the intake pipe right after the turbo, 1 from the wastegate actuator.
The long part of the "T" which is connected to the intake tube before the turbo SHOULD NOT leak if you blow air into EITHER of the other 2 nipples; if so, its faulty and the internal seal is damaged.
Here's a diagram of the N75 connection and how the flow occurs:

Is there some other way I can test the N75 (Ohm reading, reading on boost gauge, etc) I can double-triple check it isn't the N75?

Yes, a stock N75 should have a resistance of 25-35 ohms; if above or below that [or even close to either limit] replace it with known good one.

Sounds like you're real close in getting this buttoned-up.

- Erik

[Prospect25]

After placing my order with 034 Motorsport they shipped the item next day and I got the new parts Thursday. Only thing was, the new parts weren't N75 Valve Hoses but instead two pairs of 5mm wheel spacers; what?!? I called the folks at 034 and they were very courteous and professional, admitting they had made a mistake. They ended up overnighting me the correct parts Friday for Saturday delivery, which isn't cheap, so I just want to say thanks to them for doing that.

After finding the culprit boost leak last weekend the disassembly and install of the parts was a breeze. High quality parts from 034 that are sooooo much thicker than the ones from Audi/VW and have the correct inner taper from the Turbo & Wastegate sides to the N75 itself. I also checked the N75 again to see if it was malfunctioning and using the blow/air test, checking the ohms resistance, and heating up hot water and pouring on N75. Everything checked out so I'm pretty certain only the hoses were the issue.

With the new parts installed, I drove up to Morrison for an afternoon of mountain biking. Boost held great to and from and I had no issues in any gear with any boost resistance or indication of safe mode. I drove the car again today, this time up to Evergreen. While driving on US6 West from Denver to Golden the CEL came on. However, going up the I-70 hill from Golden to the Evergreen exit I used all the gears at traffic speed and my car held boost great and again no sign of intermittent limp mode.

I am going to run a scan this week to see what codes it is throwing but overall it seems way better than before. Fingers crossed the codes are for the known SAI issue and the intermittent limp mode was simply a result of torn N75 valves. Thanks again to bluefox280 for the scan, data logs, and information on potential issue (boost leak) and support. I'm getting more convinced over time I should buy a cable of my own, but then how would i get to meet other AZ members in real life?!?!

[bluefox280]

While driving on US6 West from Denver to Golden the CEL came on.

When did it appear? When the car was under load?

I am going to run a scan this week to see what codes it is throwing but overall it seems way better than before.

Let me know if you want to swing by again and get it checked out.

Thanks again to bluefox280 for the scan, data logs, and information on potential issue (boost leak) and support.

You're very welcome - glad to be of help!

- Erik

[Prospect25]

When did it appear? When the car was under load?

Can't remember, but it came back pretty quick and i wasn't paying attention to the dash lights while i was driving

Let me know if you want to swing by again and get it checked out.

boostedAvant was kind enough to give me a scan and a beer and the codes are related to the SAI pump. I think the pump is dead so will have to do more investigative work. To date, no more limp mode issues so i think the N75 hoses solved the problem! I feel so accomplished for fixing it on my own

[Prospect25]

Whelp, the problem has returned. I drove to Vail Friday night with the gf and the problem came back again. Boosted well for a while then cut out. I'm thinking maybe the N75 is acting up as it seems to correlate with engine temp (doesn't do it until the car is at operating temp). For example leading up to the tunnels WB it's all uphill and i was in boost, but only at 7psi as it went into safe mode, and therefore the engine temps were higher (speculating). Then after coming down on the other side of the tunnels the car cools off a bit, thus reducing heat on the N75, and the car starts boosting normally again.

Pretty annoying to have this keep happening. I reckon i could have a boost leak again, won't know until i have access to a compressor again this weekend, but i sort of doubt it. Going to buy a new N75 and see if the problem persists.