This is a friend's '01 Jetta TDI. Keeps going into limp mode, sometimes with a check engine light, sometimes without. Has been this way for a long time. Last spring, I replaced all the vacuum lines while replacing the head gasket, timing belt, and a few other bits in hopes of fixing this. Also replaced the N75 at that time. Still goes in limp, and does it more often than not at this point. I decided to try some more troubleshooting today. In doing some driving, I found that the ECU would show two different codes after going "limp". They were: 16618 - Boost Pressure Regulation P0234 - 35-00 - Limit Exceeded (Overboost Condition) 16492 - Manifold / Barometric Pressure Sensor (G71) / (F96) P0108 - 35-00 - Signal too High They were showing up sort of alternatively, maybe a couple times one of them and a couple times the other. Or a couple one, then the other, then back to the one. Once, both popped up at the same time. I was romping on it going up hills during this testing. It only took a couple seconds of heavy acceleration to cause limp to come up after having shut down the car and clearing the codes. I then pulled the MAP sensor from the intercooler. I noted a oily coating on it, so I cleaned it off with air flow sensor cleaner. No change on reinstallation. I'm thinking the problem might be the MAP sensor, but my friend doesn't want to throw money at the car hoping something fixes it. Any other thoughts?