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HELP!!! 98 Boxster Bucking/Misfires at various throttle ranges

  • 1998
  • Boxster
96 000
Fuel System

Hello, I have been scouring multiple forums to try and find a fix for this issue but am not having any luck.

98 Porsche Boxster with 96k miles

Surging and bucking under various throttle loads
Some smoke from tailpipe
Increased fuel consumption

Error codes:
P0410 air system, bank 1
P1411 air system, bank 2
P1319 misfire, emission relevant
P1313 misfire, cylinder 1, emission relevant
P1314 misfire, cylinder 2, emission relevant
P1316 misfire, cylinder 4, emission relevant
P1317 misfire, cylinder 5, emission relevant
P1140 throttle position sensor (NEW, showed up after vacuum test)

Replaced AOS and Upper Vent Line, Spark Plugs, and MAF. Cleaned the throttle body, Idle Control Valve, and Intake Plenum. New oil and added fuel system cleaner.

I read that it could possibly be the AOS failing and I did notice a bit of oil residue in the intake plenum, manifold inlets, and the AOS vent pipe. (I cleaned them very recently, so this buildup was only after a few hundred miles of driving). I removed the vent pipe that connects the AOS to the Intake plenum and capped off each end thinking too much oil may be getting into the cylinders and causing the misfires. I then drove the car for 30 minutes or so and it ran much better. No surging and no misfire codes. After replacing the AOS and the vent line today, the misfire codes are back. Is there something in the air/vacuum system that may be having issues which was bypassed when I removed that vent hose?

I tried doing a crankcase-side vacuum leak test by removing the oil cap, and the car nearly died, which makes me believe that isn't the source. I also built a diy smoke machine and tried using that but I didn't see any visible leaks (maybe I used it incorrectly).

I also performed a vacuum test via a digital manometer connected to the oil cap. The readings were approx 5.26-5.58 on startup with the SAI system running, then dropped off to 4.90-5.14 when the system shut off. The numbers were all over the place when I pressed the throttle. Here is a video I took of the readings when engaging the throttle. I also took a video of the RPMs when giving some throttle after I finished the test and replaced the gas cap. You can see that when I just barely press the pedal the RPMS briefly drop real low and the car almost dies. After that RPM test video the RPMs were were irregular and the car eventually died. (I tried to run over to the engine bay to get a video of that but it died before I could get out of the car)

Vacuum test -
RPM capture -

Can the P0410 and P1411 secondary air system codes be related? I have not yet begun to search that system. There is a disconnected sensor plug under the passenger intake manifold. Similar to what connects to the fuel injectors and ICV. I think it may connect to the SAI change-over valve (PN: 7PP906270 ) but I haven't been able to verify that.


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