Limp mode question

Hello TDI Gurus, it has been a while but once again I need some assistance.

The last couple months my 04 jetta TDI has been going into limp mode. Usually I can just restart the car then limp mode would go away. Then recently the car became stuck in limp mode and it would not go away no matter how many times I restarted the car for weeks on end.

I had a friend with a VAGCOM scan the car and got code P0299 Boost Pressure Regulation: control range not reached. I did my homework on some possible causes and today I pulled the car into the garage to attempt to find the issue.

After checking vacuum lines, I hooked my MityVac directly to the Turbo Actuator and discovered the actuator could not hold any vacuum. No matter how many times I pumped the Mityvac the vacuum stayed at 0hg. So I assumed this is why my car was stuck in limp mode. So I put the car back together (had air filter and hard plastic pipe going to EGR off) and when I started the car limp mode was gone and turbo is building boost great again.

This has me confused, if the actuator could not hold any vacuum, how could my limp mode be gone?

Underboost is usually caused by little to no vacuum to actuator, or completely shot actuator. Worst case scenario is the turbo compressor wheel is shot or major boost leak. I don't know what to tell you about the vacuum test you did, but maybe you fixed an unknown vac or charge air leak you didn't know of. Just keep an eye on it.

Its just so odd to me that when I take a Mityvac to the actuator it couldnt hold any pressure but now turbp is working again. I leave for a long 6 month training for employment purposes and really need the car to be ready to go. Drove it pretty far today and Limp mode has yet to return

Maybe the actuator's rubber diaphram is on its way out/hardened and when you used your mityvac, it somehow got it back to the right shape/position temporarily.



From what I can tell on the videos, the actuator is a pretty simple and cheap fix. Just don't forget to calibrate the new actuator.

actuator replacement on a KP39 turbo is living hell from what I have read.

Oh sorry, I was thinking of the VNT-15 on my car.

I have been monitoring my boost gauge lately and it seems that my car will build boost just find but it cant hold boost. According to the gauge, if the needle is not going up its going down, no matter how I drive it. Any suggestions? Not sure if this is just a boost leak or if its related to the actuator not being able to hold vacuum

Boost is usually only needed when power is needed. If you're cruising on the freeway, it'll typically stay round 5psi. If your boost is going down under a hard acceleration, you most likely have a turbo control issue. A boost leak would still show highish boost pressures, but you'd also be smoking a lot more.

According to my boost gauge (if its even accurate) my boost drops to 0 anytime I am maintaining any speed. It will go up as long as I am accelerating

That sounds pretty accurate to me. Technically, these cars can drive round without a turbo at all, there is a stage 0 tune for it. Turds to get up to 60, but it'll maintain it for sure. Like I said, boost is for power, not maintaining speeds.