p0234 Engine Overboost Condition

HELP!. Hey, I'm a newbie, grew up around Ford (International Harvester) Diesel. Anyway, I just got my first TDI (2001 Beetle with 56K miles) today and I've already got a MIL. Ran the car by Advanced Auto to get an OBD reading, which was an overboost condition (P0234). Symptoms are...no power boost after several minutes of driving at highway speeds. MIL go away after turning off and letting the car set for 10 minutes or so, but the MIL came back quickly (after punching the accelorator) and the car boost is pretty much non-existant. Any help? Is this a huge issue. I'm pretty sure the intake has never been clean (no service record and from what I've been reading here in the TDIClub, most dealers don't know about how to clean it anyway). What I don't want to do is sink another couple thousand into it. Help and suggestions please.

Sounds like your VNT actuator is stuck. Get some spray white lithium grease and try to free it. An adjustable wrench works perfectly for actuating it manually.

--Matt

I had a similar problem a while ago. It was the solinoid that controled the N75 vanes. It was replaced, $80.

Hope this helps.

You lost me there. I've read the "VNT" in some of the older posts for similar problems, but this is Greek to me. Where do I find a diagram or something that shows me where to start looking for the "VNT" on the car. I assume it has something to do with the Turbo as in a arm that can be moved around, but that's about all I can figure. You're continued help is appreciated.

What is the N75? Seen it in many of the older posts, but again I have no clue or software or manuals yet, but I had planned on getting them, but not if this is going to be a needle in the haystack. Do I need a new Turbo or possibly something as drastic as that? I have 24 hours to take this vehicle back or keep it. Please give me a warm fuzzy. I really want a TDI, but I'm thinking it would have been better to have gotten one brand new and immediately have performed an EGR mod, but I'm not made of money.

Hmm... I'm no mechanic.

VNT is the kind of turbo it is, Variable Nozzle Turbine.

The N75 what actually controls the vanes.

If the N75 isn't working right, the turbo creats too much boost and to protect itself the engine cuts fuel.

Other than a repair manual, I'm not really sure where the turbo is.

I would talk this over with the previous owner and see if they knew about and will fix it.

Thanks, Bob Fout...sorry for not knowing all the accromyns yet, but I appreciate any help I can get. I keep reading "vanes" and I have no clue what that is as well, but I assume they have to do with amount of forced air the turbine pushs.

For VNT check out this Thread

Diagrams, pix, and explanation!

My Y2K bug is buggy!

Thanks, Bob Fout...sorry for not knowing all the accromyns yet, but I appreciate any help I can get. I keep reading "vanes" and I have no clue what that is as well, but I assume they have to do with amount of forced air the turbine pushs.

Click to expand...

Yes, the vanes determine how much exhaust is directed into the turbo, therefor how fast it spins, and how much boost is created.

If you have 24hrs to take it back, by all means take it back!

They'll probably bend over backwards to accomodate you.

I have just encountered a very similary situation, and would appreciate some advice on where to continue. I have an '01 TDI Golf with ~73K original miles. Timing belt + all factory recommended service at 70K has been completed (including a brand new MAF under the 'secret' warranty). On a recent trip to Lake Tahoe (~300 mi one-way), I experienced a 'bumped' loss in power while ascending into a higher elevation and colder atmosphere, trying to maintain 70-75mph on an incline. Naturally after the loss, I was struggling to maintain 55mph in lower gears. I found out that if I turned the engine off, then restarted (on the fly--engine off for less than 10 seconds), I was able to regain normal power from the engine. This happened 2-3 times en route to Lake Tahoe. On the way back, I experienced a loss of power once at higher elevation, and once at lower elevation but on level ground (could not drive >65mph). In all cases, I was able to 'reset' the power loss by restarting the engine.

VAG-COM reads 16618 - Boost Pressure Regulation: Limit Exceeded (Overboost Condition)/P0234 - 35-10 - - - Intermittent. CEL reset itself after I got to Lake Tahoe, but relit on the way back home, and has remained lit since (same code).

I've read a lot about possible causes--MAF, VNT, and N75. I was able to verify normal MAF readings at 3000 rpm via VAG-COM, and since the dealer just replaced my original MAF, I tend to think this is not the problem (additionally, my power is not affected through all RPM ranges). That leaves VNT actuator and N75.

Just before my 70K mile service, I noticed a small decrease in available power/acceleration at higher speeds, but nothing that 'bumped' or was otherwise abnormal. Of course, the stealership's response was change the MAF; but I can't help but think that they swapped something out that had nothing to do with my [developing] problem (and I don't want to take it back to them again).

Does this sound more like a VNT actuator or N75? I don't really baby the car too much, so I can't imagine buildup in the turbo affecting the vanes (especially if simply restarting the engine 'clears' the interference). I don't know enough about the N75's interaction with the system to take action, but am wondering if this is a better place to start troubleshooting? I find it odd that I mainly only encounter problems accelerating uphill. Any advice greatly appreciated...

N75 or one of the vaccum hoses attached to it or the VNT. The turning off/on is a strong indication of the electrical piece being reset and functioning find until it has another "hiccup" or unreasonable reading from one of the vaccum hoses.

Start with the hoses, possibly clean the N75 with cleaner. The N75 is about US$80, I believe, that the hoses can be replaced, with high quality tubing, for 20ish.

Well, I got rid of my 01 Beetle, had it for almost 13 hours, expect for the noted power loss (which by the way only happened after 3000 rpm, but once it happen it stayed that way until I was able to turn off the engine, which I did while still cruising down the highway, and that would reset it until I hit 3Gs on the tach) I loved the vehicle; therefore, I've got a dealer coming by tomorrow (29th, March) to show me an '03 with 15K on it. Wish me luck.

Good luck!
May the fumes be with you!

Before I did anything else, I would always check for carbon buildup in the intake manifold. I would be willing to bet that 75% of the TDI's on the road are driving around with clogged intakes. Low power, Kills th MAF, low Fuel Mileage. Check it if you haven't (The other guy, not you since you took your's back).

VWlaw, what part of the country are you in? I'm sure a member would go over a car with you that you are looking at. At least plug in VAgCom.

Good luck
Dave

Two causes of overboost:

-N75 has an internal leakage that routing vacuum to the VNT vacuum actuator preventing it from returning to the low boost position.

-VNT mechanism is jammed up with carbon physically preventing the vanes from returning to the low boost position. This is perhaps the most plausible scenerio.

The boost code is only going to trigger under high fueling situations where the vanes must be in the lowest possible angle to prevent overboost. Since they cannot return to the low boost position and the engine is under heavy load the excess fueling is sufficient to spool the turbine so that boost pressure exceeds desired ECU level thus triggering a limp mode and CEL to protect the motor from damage.

MAF sensors, Intake plugging have NOTHING to do with overboost error codes.

DB