01 beetle stuck in limp mode

Hello peeps!
My fiance was driving our beetle TDI alh 5spd and she suddenly lost boost. Got a check engine light for low boost sporadic . We managed to limp home and started by replacing all of the vacuum lines. That didn't change anything. So then we tried replacing the n75 valve, no luck. Next we decided to go ahead and replace the turbo. (Had 334,000 miles on it). While we were at it we also replaced the vacuum pump and egr valve and fuel filter. However I was stupid and forgot to prime the filter before trying to start it. It took a minute to start but it fired up with a CEL code. 17656 start of injection timing regulation, control deviation sporadic:
It runs and drives fine except no boost until 3000 rpm. Tried deleting code and it immediately popped back up. Where do I go from here?

Did you test for vacuum at the small nipple on the check valve right past the vac pump?
Should be 25"hg+.

The nipple can get plugged with debris if there was a breach in a hose, the turbo actuator or other component.
I would also check the black and white check valve that leads to the N75 valve.
Those can get plugged as well. And like any good check valve, it should flow in one direction only.

How do the brakes feel? Did you check the hard plastic vacuum tube that goes to the brake booster. The tube will crack at the barb fittings on either end and the first thing that gets affected is the stuff connected to the small vacuum nipple on the check valve.

Thank you for the reply! Yes I ended up replacing that check valve right after the vacuum pump but I have not checked the other check valve leading to the n75! I'll check that now.

Buy VCDS and buy parts based on data

Ok the low boost code has been resolved. Now stuck with 17656 start of injection timing deviation sporadic . Bought vcds and timing is stuck at 255. Removed fuse 34 and there was a noticable change in engine sound but timing stays at 255. This code didn't appear until I stupidly replaced the fuel filter and didn't fill it. So it was very difficult to start after that replacement.

Engine fires right up every time but has no boost until 2500-3000 rpm

Also did ouput test on vcds and heard no change at all

Check the vacuum at the anti-shudder valve at idle and see how much vacuum is there; if you don't have at least 22" at idle there something is leaking vacuum that has not yet showed itself. I had this same problem years ago, and it was the solenoids on the passenger side. The fact that you don't get boost until 2500-3000k RPM is quite likely vacuum caused. As far as the 17656 code goes, since you have VCDS you can individually check each solenoid and see what is happening.
There are a couple of reasons the 17656 code happens, there is air in the well the piston mounts in, the piston is binding up, or there is no voltage going to the solenoid. Fuse 34 is not the control for the Start of Timing Solenoid, but it does power the N75, N18, and Anti-Shudder IIRC.
You are the second person in a week to post about the timing being stuck at 255 degrees; there was a rash of it years ago, and the solution was quite simple, but for the life of me I can't remember the "look there" solution anymore.
Also make sure the case pressure relief valve did not come apart.

Injector #3. Look at the connector and wire for damage. If not, the injector body may need to be replaced. That's the usual cause for timing not changing in VCDS.

Thank you very much for the replies! Injector#3 wire looks perfectly fine unfortunately. So #3 may need to be replaced?

Where in Texas are you? TDIClub member jetta,97 (Marin) is in Plano. He's an excellent TDI mechanic. Better to have someone who knows these cars look at it instead of just throwing parts at it.

Since you don't have boost until 2,500-3,000 RPM, it could be something as simple as a VNT actuator adjustment. You might not have needed a new turbo, just a new actuator...

Looking at the timing if I unplug the injector needle sensor the timing goes to 38. When plugged in it returns to 255. Bad #3?

And I'm in Houston. Just trying to learn more about vw tdis
Also the fins on the turbo wheel were nearly flat. Looks like something had gotten into intake side and ground it down

Have you adjusted the VNT actuator? You will need a MityVac or similar tool. Connect the hose from the MityVac to the barb on the actuator and watch the rod as you pump. The rod should start to move at ~3-5 In-Hg and the lever should be against the stop at ~18 In-Hg. If it is not, adjust the length of the rod. Do not fiddle with the stop screw or lock nut.

What you need is a proper diagnosis. We'll try to help as much as we can here on the interweb, but you'll want to find a good troubleshooting guide (low power/limp mode) here or over at myturbodiesel.com.
Am I reading correctly that you've replaced the turbo and the vanes are already shot?
VW TDI Search Engine

Saz0589 said:

And I'm in Houston. Just trying to learn more about vw tdis
Also the fins on the turbo wheel were nearly flat. Looks like something had gotten into intake side and ground it down

Click to expand...

Do you mean the cold side of the turbo?
If that is the case then you may well have debris in the valves in which case you should look into that first before running the car more. You are liable to make things worse.

Thank you everyone for your help. It turned out to be injector #3 causing the code. I apologize for my lack of knowledge when it comes to these engines. I'm used to older gas vehicles