Vacuum oddities - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Over the past week or so, I have noticed a slightly unstable idle when previously the idle was completely solid around 760-780 rpm. The 'unstable' idle will creep upwards very slightly but it doesn't creep over 1000rpm. I also notice on my OBD2 reading of vacuum pressure that it is a bit higher than normal; ~25in/HG compared to a previous ~22in/HG. I would normally say this is a vacuum leak, ICV needs a cleaning, MAF cleaning, TB clean etc. The HIGHER than normal vacuum confuses me, but I had wanted to clean the intake assembly anyways. So I did all of that very thoroughly, in addition to inspecting all vacuum hoses for leaks and did not find any. So far its storybook vacuum gremlin (except for higher vacuum???). I quickly checked the oil cap for signs of AOS failing and it seems the same as always, easy to take off, but engine will stumble slightly after a few seconds of having it off. I then got a pending P1126 (one of the cylinder banks at maximum richness at idle speed), which would explain the slightly unstable idle, and would also make me think small vacuum leak even more than I previously did. I drove ~200 miles this weekend and the pending code never activated the CEL. After cleaning all of the above, I cleared the code and drove ~80 miles and nothing. The slightly unstable idle remains however (so I assume the code will return). After cleaning everything(ICV was a bit stuck), car runs better and my OBD2 display of 'engine load' (not sure how this is calculated; it is somewhat REAL however because with the AC on at idle, load goes to 1.6%) has dropped from a consistent 1.2% (warmed up, california-hot day) at idle to a 0.8% at idle under same circumstances. Higher vacuum tells me AOS is starting to fail (J tube was dry inside), but does a beginning-to-fail AOS cause a P1126/all of the other symptoms? I want to invest my cookies into a more thorough vac leak search but not sure this makes sense either.

Your train of thought seems to be correct. Those were my thoughts. Other thoughts could be maf needing cleaning(cheap and easy)...I'd also wonder how your o2 sensors are doing. Looking at resistance levels could rule that out. Sounds also like a good smoke test is in order.

I cleaned the MAF 3-4 times, probably used about half the MAF cleaner can, let it dry etc. The 02s are something I also thought about. P1126 is only for one side of the engine so a bad drivers side pre-cat 02 would explain that. Could it also explain a slightly odd fluctuating idle/higher vac? This is a stretch, but the DME enriching the mixture to account for a bad drivers side 02 (without the mixture needing to be enriched) could raise the vacuum/idle slightly...maybe?

Quotes source post: 465201

A bad o2 made my car run rough at idle especially at startup. Same sensor. My mechanic saw the sensor had a high resistance and when he replaced it things went back to normal.

Now I am more confused. Drove home 1.5 hr trip and new pending codes: P1126 P1124 P2424 P2411 P2600 I don't think my cat on bank 2 went bad over night so oxygen sensor is a good idea. Do the other codes make sense? I couldn't find a good description of the p2xx codes.

Now only pending codes P1126 and P1124 remain. Another odd thing: the vacuum reading via OBD2 does not drop when removing the oil fill cap??? I would have expected it to. Also, is something supposed to happen when I remove the gas cap with engine running? Doesn't seem to be any pressure or vacuum at the cap with engine idling.

My tablet in the dashboard paid off: the torque app will alert you instantly if any pending codes are found long before the CEL ever goes off. Audio and visual alerts.

Someone like jfp needs to weigh in. New oil cap, oil fill tube o2 sensors. A smoke test and someone with a Durametric to look at o2 voltage would knock all out in one. At least you'd know where you stand.

Did the best testing I could tonight with the time I had. I checked most vacuum junctions under the 'hood' with an unlit propane torch. Slight idle rise was not changed by anything I was doing in the engine compartment, whether it be wiggling connections, oil filler tube or using propane. Interestingly: the slight idle rise/pending code throw happens only after ~30 seconds of starting with the car warm. Additionally, the slight idle rise doesn't seem to happen until the car is warmed up. I think tomorrow night I will check the fuel EVAP area. The CEL has yet to illuminate, so far only pending codes come up. While idling for long periods, 1124,1126,EVAP leak detect pump LOW signal (2401?) and AC distribution valve OPEN (2611?). I don't know if those last two are the correct numbers or description but I struck out in the engine bay so will check the rest of the EVAP system. I have never replaced the AOS (maybe PO did) but with ~105k I'm thinking it could be contributing here. The idle increase happening only after car is warm/after ~30 seconds if the car is already warm puzzles me.

The mystery is solved, and I didn't even have to check the EVAP system! No wonder the propane wasn't having any effect...

Quotes source post: 465528

What was it? Oil filler tube?

In the picture is the rubber tube connecting the crankcase and AOS. It had a large tear in it.