P0234 Pending code/limp mode

[Cyrik]

Hey everyone!

I installed a new HFC a couple weeks ago and the car is throwing P0234 for overboost (see below). I was flashed JHM Stage 1 beforehand and had no such code until I switched to stock while I was waiting on the new Stage 2 tune. (refer to this thread: https://www.audizine.com/forum/showt...ble-for-tuning)

What's throwing me for a loop is the fact that this code is PENDING (no CEL) and kicks the car into limp mode anyways. I've isolated it a bit over the last week or so: It only throws the code when in a higher gear and when I start at a low RPM. I was on the highway earlier and did a pull in 5th gear to keep up with traffic and ~3kRPM I lost all boost. Pulled off to the next exit, pulled out my cheap scanner to clear codes and not 3 mins later I was able to get the car into limp mode again, same situation. No idea if this is relevant but I also noticed a SIGNIFICANT amount of turbo flutter as soon as I crossed the 3k mark, then would go into limp mode. Something is clearly not right here.

A few things I have yet to try (but plan to w/ parts on the way):

- Replace N75, never replaced after owning for 7 years
- Clean MAF sensor (I've been told this could be the problem, it's been years since last cleaned. Source: https://www.audizine.com/forum/showt...-going-Stage-1)
- New spark plugs, cause it's about that time again.

Any prior experience with this would be appreciated! Thanks in advance.

[Cyrik]

Well it's been a week and I got new plugs, new N75, and cleaned the MAF sensor as I stated in my last post.

NONE of these were the solution. Here is a data log I made reading engine speed, specified boost and actual boost. (for whatever reason Datazap didn't wanna graph the other values... not sure if I'm doing something wrong but the data is there with this link: https://datazap.me/u/blucyrik/boost-...log=0&data=1-3 )



The specified boost and actual do not match up when looking at the log... that's what I need to figure out. And how... at this point I have no idea. I've read every single post on p0234 made on this platform in the last 15 years and all of them either point to a bad N75 or just a bad turbo which worries me. I can't afford a new turbo and it blows my mind that all of this is happening right after I install a HFC...

Something I'd like to note is I was able to throw a full CEL for overboost right after I cleaned the MAF. I haven't been able to replicate it since so maybe my MAF is just bad?? This is really REALLY strange. There's still constant turbo flutter in the mid rpm range and the car kicks into limp mode much for frequently now that it's cold out. I have yet to check the wastegate actuator for play but I seriously doubt that's the problem considering most that have problems with it are usually due to underboost conditions, not overboost.

I'm at a loss for what do do next, check the diverter valve and take off the GFB DV+ I guess? There's only so many things that can cause this problem and I've ruled out most of the reasons for it. The ONLY other thing I can think of is my tune is just shit.

[Cyrik]

Welp, I feel like an idiot for not looking absolutely everything over.

Pulled my MAP sensor (G31) and noticed a bit of dirt around the plastic cage that protects the little strip that measures the pressure (if you don't know what I'm talking about, look up a picture of said sensor)

Sprayed it down with the MAF cleaner I bought when I cleaned the MAF sensor. There was an IMMEDIATE difference in how the car drove when accelerating/entering boost. Before I cleaned it, I had a 50/50 chance of the car kicking into limp mode when I stepped on it. Now it's like 1/10. There is no longer a significant amount of turbo flutter when riding through the mid rpm range. This completely explains why my car kicks into limp mode.

My mistake for overlooking this part even though the rosstech wiki specifically says to check/replace this sensor when looking up P0234. I have never heard or seen a MAP sensor fail before. Add me to the statistic! This is an extremely rare failure but also explains why I never saw the code before I re-flashed the car. The tune just adapted the false readings when I was Stage 1 and when I flashed back to stock the readings from the sensor were off just enough to trigger a code.

OEM Bosch sensor $45 shipped from ECS, will update and confirm this is indeed the problem when I get the new sensor in. I'm still surprised this is the problem but beats taking my turbo apart...

[a4audi4fun]

Mileage and have you had the intake valves cleaned?

[Cyrik]

Mileage and have you had the intake valves cleaned?

Engine has ~140k on it, when I did my swap a few years ago the intake valves didn't look bad at all. I suppose this could also potentially be a problem but I feel like I'd have cold start issues if the intake valves were all carboned up. Noted.

[Cyrik]

Another week, another update.

Got the new MAP sensor in - still no dice.

have you had the intake valves cleaned?

I then spent the weekend doing a carbon clean:





After a test drive last night and this morning the car definitely feels smoother upon acceleration so it's nice that I didn't waste my time with this... The compressor surge is less often however I am STILL getting limp mode!!

I am legitimately lost on what to do next. The only thing I can think of is to adjust the wastegate actuator rod but I'd like to hear y'alls opinion on doing this. I have never messed with it before and to my understanding this causes the turbo to produce less boost overall when shortened, correct?

I am also tempted to hit up JHM tuning to see what they think. None of this makes any sense...

[Older_not_Wiser]

Looking at the log link you sent in the second post, you can see that as the boost comes up it meets, then exceeds the requested and the ECU tells the N75 to regulate and the boost drops slightly and the ECU thinks 'OK, back to allowing boost' and it is on this second boost climb that it overshoots and triggers the limp mode.

To me, this is all a function of how the N75, DV and wastegate are interacting. One of them is lazy and not responding as it should or slightly sticking in some way. You say you changed the N75, was it with a genuine part? It also says you are running GFB DV, I seem to recall seeing that the GFB DV has given some people some issues. I would search the 'Zine for GFB issues and solutions and that may provide some direction for you to pursue.

Hope this helps.

[Cyrik]

Looking at the log link you sent in the second post, you can see that as the boost comes up it meets, then exceeds the requested and the ECU tells the N75 to regulate and the boost drops slightly and the ECU thinks 'OK, back to allowing boost' and it is on this second boost climb that it overshoots and triggers the limp mode.

To me, this is all a function of how the N75, DV and wastegate are interacting. One of them is lazy and not responding as it should or slightly sticking in some way. You say you changed the N75, was it with a genuine part? It also says you are running GFB DV, I seem to recall seeing that the GFB DV has given some people some issues. I would search the 'Zine for GFB issues and solutions and that may provide some direction for you to pursue.

Hope this helps.

I think your analysis is correct. Something between the N75, wastegate, and DV is not happy. Did an oil change today and took off the GFB DV+ and took a peek at it. Everything looked ok from a visual standpoint but I took out the spring that sits between the spacer housing and the piston and took it for a spin. it was MUCH harder to trigger limp mode with this (and the car sounds like it has a bird in it when letting off the throttle lmao). So now I'm going to be switching back to the stock DV when I get the part in from ECS. Will report back soon!

EDIT: The new N75 is a Pierburg unit which I'm almost certain is OEM.

[Cyrik]

Yet again, another week, another update.

Got the new stock DV in, still no change. Constant boost cuts/limp mode and lots of flutter when I floor it. I'm done playing parts darts. I'm tempted to put the stock cat back on to confirm the HFC is what's causing all these problems. I will also be flashing back to stock if that's what I end up doing (I REALLY don't want to...) I will be moving towards diagnosing the wastegate as well. That's literally the only other thing that could be causing overboost. I will also be contacting JHM to see what they think/if they have seen anything like this.

Just so I'm not crazy here, Stage 2 requires a HFC/test pipe to run correctly right? I also have an aftermarket/custom intake and a FMIC. Stock downpipe. No fueling mods done. Since the car seemed happy with Stage 1 for years, what are the chances I just need a J-bung and Stage 1 again? Anyone have any experience running Stage 1 with a HFC? Maybe I simply don't meet the requirements for Stage 2.

[Cyrik]

I should start a blog given how frequently and consistently I update this

Contacted JHM earlier this week regarding my issue. Their response was it's almost certainly a hardware issue and my combo of aftermarket parts should be just fine for Stage 2.

Flashed back to Stage 1 since I know boost levels are slightly lower if I'm not mistaken, same problem.

My brother was in town and was free yesterday so we did logs with graphs as I did a pull with him on the laptop. We got constant boost spikes (actual boost was waaaay over specified). So in a last ditch effort I picked up an OEM Hitachi MAF Sensor from Autozone to rule out the possibility of it being the MAF. No changes. Returned it 2 hours later lol.

At this point there was only one thing left to check... The turbo assembly, more specifically the wastegate. Took my Amazon boroscope and put it through the extra port on the CTS high flow cat. The wastegate flap on the inside looks normal. I didn't find much play in the actuator arm at all when I used a screwdriver to "pry" against the locking tab on the actuator rod underneath the car. To clarify I was able to get the arm to move and open the wastegate flap, it isn't seized.

I then tried to pressurize the diaphragm for the actuator by taking the line from the N75 to the diaphragm off and hooking it up to a bike pump and lo and behold... I couldn't get the actuator arm to move! I think it has a hole in it and it's toast.

Good news is I think I FINALLY found my issue! Bad news is I gotta take off the turbo to confirm and potentially do a fix. A buddy of mine who went to a K04 graciously said I could have his old K03 which was working fine before he upgraded. Guess I know what next weekend is going to look like will update again with pictures and possibly a video when I have the turbo off!

Sent from my Pixel 6 using Audizine Forum mobile app

[Cyrik]

Welp, I got good news and bad news.

Good news first! Swapped my turbo over the last couple days...



...with my buddy's old turbo since he went to K04



Confirmed the wastegate flap is tight on the new unit, moved everything over with the ECS turbo hardware kit they sell for $100, and sure enough the car runs with no leaks!!! (I HATE banjo bolts though!!!) I was extremely satisfied to not have any studs break, only 3 studs actually came out on the bottom. Replaced with new lock nuts that came with the ECS kit and everything worked. Had to fight for access on a couple of those lower studs/nuts that go towards the front of the car. Managed to get it with swivel extensions and a lot of patience.

The bad news... P0299. Car is slow as hell. I didn't move over my N75 or new DV (I used my DV+ again because of my stubbornness...) so I will be in process of swapping these over as I'm sure the DV+ (or the old N75) is causing problems again.

Calling it good on this thread as this has been 2 months in the making. Wastegate actuator diaphragm is for sure blown with my old unit. Crazy rare failure imo. I could have just moved the wastegate actuator from the new unit to the old one, but didn't bother in case something else is up with my old turbo. Case closed.

EDIT: Replaced N75 with the one I purchased rather than using the one that was already on the new turbo. 100% fixed and car boosts properly again.