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Been pulling a P1171 code for a few months now on my second 99 C70 HPT auto, car is mostly stock, with the exception of 2.5" catback that I installed about 10 months ago. On my first C70 that I had a few years back, started pulling the same code, and a replacement MAF solved the problem. Now running into the same issue on this car, and a new MAF (brand new Bosch, correct part number) did not solve the problem. After clearing the code, car runs great and makes full boost (roughly 10 psi) for about 20 miles, then the code comes back up. At this point, max boost falls to about 6-7 PSI, and comes on a bit slower. However, car still runs "ok", smooth idle, smooth building of power, no hiccups or other codes appear.

My thought is that I have a vacuum leak somewhere after the MAF, either in the intake tube, or possibly in one of the lines going to it. So yesterday, I removed the hose that comes from the evap purge valve and goes to both the intake tube and the intake manifold (up between the runners), and capped both holes with vacuum plugs. I figured if the problem lies in one of the check valves (there are two of them) or in the evap purge valve itself, this would eliminate them as possible causes. No luck, still pulling a P1171 after about 15-20 miles. My next step is to pull the intake tube and see if there are any cracks in it that might be letting air in after the MAF.

Any P80 owners out there have this problem, and if so, how did you end up fixing it? Everything I can find online says it's the MAF, or possibly the upstream o2 sensor, although I doubt it's the o2 causing the problem. Any help would be greatly appreciated.

Thanks
 

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This is probably way too late a response, but it sounds like what my V70 was doing - replace all the vacuum lines to the turbo/TCV/intake. Several of mine were hard and crusty, but none seemed bad.. but replacing them did fix the boost. (I used IPD silicone hoses)
Mine didn't throw a code, though.
Does it pass the glove test? (get the engine warm and put a nitrile glove in place of the oil cap - it should not inflate)
 
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