Code P0299 "turbo underboost" Resolution Repository

SUNRG · January 26th, 2010, 09:53

If you get a CEL / Emissions Workshop message, have the code read and it comes back with P0299 turbocharger underboost - please post what you did to resolve the problem to this thread.

This way anyone that gets this code in a B5.5 can view this thread and see what the most common causes and resolutions were.

Thanks!

SUNRG · January 26th, 2010, 09:57

*Update: reseating the loose line did not resolve the issue. Emissions Workshop (and I assume P0299) occured Friday again, coinciding with limp mode onset (driving up a slight incline at highway speed) stayed on until Sunday morning when the CEL went away, but my B5.5 went into limp mode three times during a round-trip drive from Roanoke, VA to Johnson City, TN.*

My B5.5 had increasingly been going into limp mode, eventually the CEL / emissions workshop light came on. P0299 was the code.

The apparent cause was a loose clean air line from the N75 valve to the air filter box. It was secure on the air box side but ready to fall off the fitting on the N75 side.

Reseating the line on the N75 valve seems to have resolved the issue. No cell or limp mode since. I will update this post if that changes.

tpitt1946 · January 26th, 2010, 10:16

I've had to replace two N75 valves on my Passat. The most recent one was a week or so ago. I do see the new one is a little different. Hope it lasts.

texascadillac42 · January 26th, 2010, 10:51

I am fighting this problem right now. I tried a spare N75 I had laying around with no luck. It wasn't a new one, but one I had from a spare motor so that's not to say it wasn't faulty as well. I checked all vacuum lines at the N75, checked that the turbo actuator was moving freely with vacuum applied and examined all intercooler hoses.

The car will run fine for several miles, whether its easy driving, or hard acceleration runs. Then all of the sudden (usually after pulling a slight hill at moderate throttle), it will go into limp mode. Code P0299 comes up as boost pressure control range not reached.

If I clear the code/shut the car off and restart it runs great for another couple miles. I am wondering if the MAP sensor is failing?

SUNRG · January 26th, 2010, 10:57

sounds like you've eliminated quite a few possible causes. fwiw - Jeff (RocketChip) said excessive fuel or air filter restriction could trigger P0299.

texascadillac42 · January 26th, 2010, 11:03

Checked the air filter last night, cleaned the snow screen out, and everything looked good there. Could be a fuel filter....

I noticed (and forgot to mention this earlier) that the car seems to surge slightly. Not enough to really affect speed, but you can feel it surge a little with moderate throttle input. Not sure if this is a common symptom or not.

I will put a new fuel filter on tonight or tomorrow. It has roughly 15,000 miles on it, but the car sat for a LONG time before I got it running again, so it may be time to change it again. I have no idea how old the fuel was in the tank, but several hundred gallons have been run through it since the fuel filter was installed.

texascadillac42 · January 26th, 2010, 11:41

Well, after pulling the motor cover to change the fuel filter. I found the problem. I had checked all 3 intercooler hoses, but not the hose that connect the plastic tube at the back of the motor to the intake manifold. It had a nice split in the bottom of it.

Problem solved.

auntulna · January 26th, 2010, 20:21

Mine limps only once in a while when going uphill at 70+, otherwise it's very strong. This would be a maximum boost demand from the computer. Fuel and air filter are recent, so I don't think it's them. It never happened until I got RC2 13K ago.? I will check the hoses again, and then maybe buy a new N75? Hard to know if that would be it.

texascadillac42 · January 26th, 2010, 20:34

It is worth checking all the vacuum lines going to the N75 with a tool such as a mity vac. That way you can see if everything is holding vacuum adequately, that the actuator arm moves freely through its range.... etc.

TheGrove · January 27th, 2010, 01:00

Quote:

Originally Posted by texascadillac42

Well, after pulling the motor cover to change the fuel filter. I found the problem. I had checked all 3 intercooler hoses, but not the hose that connect the plastic tube at the back of the motor to the intake manifold. It had a nice split in the bottom of it.

Problem solved.

That's the exact same place mine split! Glad you found it.

texascadillac42 · January 27th, 2010, 06:32

Ordered from www.1stvwparts.com this morning. $97.81 plus shipping. It was the cheapest I could find. Part number 3B0 145 838A.

SUNRG · February 1st, 2010, 06:17

*Update to post#2 above: Reseating the loose line (running from the airbox to the N75 valve) did not resolve the issue. Emissions Workshop (and I assume P0299) occured Friday again, coinciding with limp mode onset (driving up a slight incline at highway speed) stayed on until Sunday morning when the CEL went away, but my B5.5 went into limp mode three times during a Sunday round-trip drive from Roanoke, VA to Johnson City, TN.*

shri11 · February 4th, 2010, 18:53

Hi all, drive a 2004 Jetta PD, I had been hearing a whistling sort of noise coming from near back bottom of the engine, almost near the exhaust pipe, and today check engine light came on, code P0299. "Faulty wastegate bypass regulator valve" one Probable cause was listed as "Faulty charge air pressure sensor"

Will any defect be visible by raising the car and looking under? thanks Shri Sarathy

TheGrove · February 5th, 2010, 11:14

Probably, I believe one of the main culprits is the hose off of the turbo that rubs on a wiretie until it creates a hole.

auntulna · February 5th, 2010, 16:15

About The Grove's comment, my experience with a hole in the first hose off the turbo was low power all the time and smoke with acceleration. I think a true hole anywhere else along the charge air system will be felt as poor performance in multiple situations.

The underboost code is a lot harder to pin down, maybe N75, maybe vacuum leak, because the car runs fine most of the time. There is a procedure for a turbo boost test at myturbodiesel.com, that I would like to try when I can.