Loss of Power/No Turbo (limp mode)

Loss of Power/No Turbo (limp mode) Please Help

My 2001 Jetta TDI lost it's turbo power which has happened to me before but last time it was the turbo actuator and when I replaced it everything was good. This time around I'm not so sure.

I had my car sitting in my driveway for about a week in fair weather conditions. The next time I drove it, I immediately noticed a loss of power that was similar to when my turbo wasn't working properly.

At first it wasn't throwing any codes, but after a few days the CEL finally came on. The codes are P0102 (Mass or Volume Air Flow A Circuit Low Input) and P1556 (Charge Pressure Negative Deviation). The first thing I did was take a part my MAF and clean it using MAF cleaner and replaced the air filter. This didn't do the trick, so I bought a new Bosch MAF off Amazon and put that in. Still nothing, so I figure my MAF is good.

Next, I went through all my vacuum lines and replaced them one by one making sure there could be no way I was loosing vacuum, but after replacing them I still had no turbo. I read around online and understood the N75 and N18 could be swapped to test if I have bad N75. I swapped the two around and still had nothing to show for it. I ended up buying a new N75 valve anyways and swapped it in but still had no luck.

I cleaned my MAP sensor, which had some oil residue on it, but this didn't work either. To be safe, I bought a new MAP sensor and replaced it but still had no luck.

To rule out that my turbo is bad, I took the vacuum line off of the EGR valve and hooked it to the lone stem on the right side of the N75 which gave me my turbo back. However, I read that this can cause over boost so I switched it back to the stock setup. I'd really like to have it work as intended.

I'm at the point where I have no idea what else it could be. I've read some articles about the brake booster having issues with losing vacuum but wouldn't I have noticed a change in the feel of braking?

I believe most here will agree you need to run logs in VCDS, so we can be of more help to you. Check fuse #34 by pulling it out, not just looking at it in the fuse block. I would also pull the MAF plug, turn on the key and check the voltage at the plug to see if there is a wiring issue at your MAF.

My suggestion: find the "diagnosing limp mode" thread linked to in member Canadian Grizzlie's signature and follow it step-by-step... if you don't use science to track this down you'll run the risk of throwing $$$$ and parts at it for a long time.

Nevada_TDI said:

I believe most here will agree you need to run logs in VCDS, so we can be of more help to you. Check fuse #34 by pulling it out, not just looking at it in the fuse block. I would also pull the MAF plug, turn on the key and check the voltage at the plug to see if there is a wiring issue at your MAF.

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I pulled Fuse 34 out and it's fine. I forgot to mention that I have already tested the plugs running up to MAP and MAF using my multimeter to ensure there was voltage to the sensors.

Unfortunately I do not have or know someone with a VCDS.

I am with Vince, read the thread and don't just throw parts at it.
At idle what is the DC voltage going to the N75 valve?

I had almost the same problem and it was because the connectors at the intercooler were sloppy and causing a boost leak

The link to diagnosing your problem is in my signature... (the same link that Vince was talking about)

Nevada_TDI said:

I am with Vince, read the thread and don't just throw parts at it.
At idle what is the DC voltage going to the N75 valve?

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The signal is a PWM, not voltage, in essence it is turning on and off rapidly.

flee, thank you I know that it is PWM, but at a given frequency you will have readable voltage; I should have said AC voltage, not DC voltage, IIRC.

No one has suggested it yet...whats your vacuum pump doing? Hook directly to a MightyVac or similar and see what your pump health is...

Nevada_TDI said:

flee, thank you I know that it is PWM, but at a given frequency you will have readable voltage; I should have said AC voltage, not DC voltage, IIRC.

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You're right that a voltage will be readable but it will read differently on different VOMs.

I have previously checked the voltages on the MAP, MAF and N75 to ensure they were actually receiving some type of electrical signal. Is there a certain value I should be looking for regarding the cable going to the N75?

I will also check the connectors on the intercooler.

I know my vacuum pump is producing vacuum but I'm not sure how strong. When I check the vacuum of it, should I hook the gauge straight up to the pump or should I Tee it in somewhere near it? What value should I be looking for?

Tee, believe it's around 20"/Hg

Where are you in Canadian Grizzly's limp mode sticky?
Maybe carefully inspect the brake booster hose if you haven't replaced that yet...

May have missed it, you must have checked the turbo actuator. It should hold vacuum. It should just start to move at 3.5 to 5 "/Hg. When the engine is started it should move over.

Sorry I haven't updated this in a while. The problem ended up being the small black and white check valve. It was busted and was impossible to tell. I swapped it out and all is good. Thank you everyone for your help.