Boost control question

I just put a boost gauge on my stock 96 passat TDI. Under hard acceleration, boost peaks around 18 psi then immediatly falls off to about 9psi, then back up to 18 etc. It will eventually stabilize around 14psi or so. What could be making it do this and how can I fix it?
Thanks in advance

Sounds normal to me.

The annoying thing is that it keeps dumping the boost...18psi-9psi-18psi-9psi...before it stabilizes. The car surges when it does this which is kind of annoying.

Your N75 valve might be playing up.

BINGO! Take the N75 out, and spray into it with WD40. Let it drip a bit to allow some to get out (the less in there, the better). Anyhow, pop it back in, and see if things have improved. You may have solved your problem for the long term, but if it did help, you might want to squirrel a bit of money away and anticipate a possible N75 purchase.

If the N75 is not the problem, check in the faq under 'limp mode'. One of your hoses might have a small leak. This is just another possibility, but I would suspect the N75 right now.

Just my two cents here, and please correct me if I am wrong, but isn't this also the type of thing that the "Dawes device" is supposed to help with?

Is the N75 the Wastgate Solenoid Valve, the one on the left hand side of the firewall with the red and blue hoses attached? If so, I have noticed it clicking rapidly right after startup from time to time. Is that normal?

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dtreid said:
Is the N75 the Wastgate Solenoid Valve, the one on the left hand side of the firewall with the red and blue hoses attached? If so, I have noticed it clicking rapidly right after startup from time to time. Is that normal?

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Should have a red, blue, and black hose going to it as well as a 2 wire electrical connector. It should be located on the PASSENGER side of the car (assuming this is a US or Canadian vehicle) just aft of the fuel filter.

I would just add that I had the same problem as you. The surging's worse on full load in fifth gear? My N75 clicked when putting 12V across it to test it. It didn't seem to be sticking, it didn't seem to leak, in fact I couldn't find anything wrong with the damn thing, but replacing it definitely cured this problem! Check your red, blue and black hose for splits or leaks, also check the hose from the black hose from the inlet manifold to to the ECU for starters. N75 costs about £33 in UK (about US $55 ish?)

Clicking is perfectly normal. That means the N75 is working. The N75 is probably working, but is sticking internally. That's why I suggested taking it out, and spraying the WD40 in there. If you put it back in and your problems are gone, then the N75 was getting stuck (not an uncommon problem) and you might want to look into getting a new one.

As for the previous question about the Dawes, technically, yes, it is supposed to help with boost control, but not this problem. The Dawes basically is to control overshoot more than anything. This person's problem was boost surge...meaning full/partial boost cycled over and over again. The Dawes takes a car peaking at a high psi (over 20) which settles down to 15 psi, and converts it to a car which peaks at about 18 psi (or whatever you set it to) and settling to 16 or 17. Meaning you get a more constant boost pressure, and this is also easier on your turbo itself, so longer turbo life.