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So I have finally gotten bit by this. I have attached OBDII readouts.

1998 boxster, no engine mods, 61,000 miles. The car was running fine until mid-May. Then it sat idle till mid-July. (This is what I get for not driving it.)

Ironically I used a K&N air filter for three years. Never had a problem Took it out in January this year and put a paper one in. (Don't fix what's not broke I guess.)

The battery had pooped (I left the trunk open one day). After recharging the battery I started it up and it ran like crap for a few minutes (no power, would not accelerate, etc) which I assumed was normal as the battery had been disconnected while I recharged it. Took the car out and drove around for an hour without incident.

On the way to the track two days later CEL comes on after having been on the highway for 1.5 hours and just after a fuel stop. Thought maybe I had forgotten to put the gas cap on or it was loose. Check the fuel cap, it is okay. Get to the track, disconnect the battery to clear the CEL and drive on the track for an hour and the CEL comes back.

What I have tried so far:

Checked the fuel cap seal, looks okay.

Checked the oil cap o-ring, looks okay.

Replaced the spark plugs (it was due)

Replaced the fuel filter (it was due)

Dumped a bottle of Techron in the tank. Long shot but I had one lying around.

The O2 sensor readings look okay, at least I think they look okay. Comments?

It looks like the Long Term Fuel trims are maxed out so I am assuming that the car is running lean and the DME cannot add enough fuel, correct?

Tightened clamps and looked for air leaks (sprayed carb cleaner all over the place and did not see anything, any other ways to check for air leaks?).

When the engine is running there is a fairly loud whooshing or hissing noise, I assume this is normal as it is not something you normally hear with the covers in place.

Checked the oil filler tube, seems okay. Anyone know how to disconnect it from the crankcase tube? I undid the clamp but could not get it separated.

Any other tips on looking for air leaks? Where to look?

The mass air flow sensor is reading a little low, 3.5-3.9, as opposed to what I believe should be 4.2. Maybe I should try cleaning it. Comments?

I have cleared the codes 2 or 3 times now and gone on test drives and the codes keep reappearing after about 20-30 miles of driving.

Any other suggestions short of replacing more parts like the MAF?

One other thing I have noticed is that when I take it out for a test drive with the engine cold the engine will stall. This has happened once and almost happened a second time though I say it coming and blipped the throttle. I live on a big hill so when I leave it is down hill for a half a mile with a stop sign at the bottom. When I stop the car at the bottom of the hill the car is pointed down slightly. When I put the clutch in when I stop at the stop sign is when it has stalled. I also notice that after the stop when I accelerate from the stop sign I get a slight hesitation in the acceleration. After that initial stall/hesitation I do not notice the symptoms again. Low fuel pressure maybe?

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Same with me. I have a 98 with at the time 98K. Same symptoms stumbles when cold when warm runs fine. Disconnect the MAF and run it for a while chances are the only code you'll get is for no MAF signal. You may want to try to clean it but in my case it didn't work.

What tool are you using to get your data?

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