P0301 only during idle - 986 Forum - The Community for Porsche Boxster & Cayman Owners

I just finished a rebuild on my 2000 boxster and I'm getting a sporadic P0301 only at idle. If the keep the engine above 1k rpm, it never gets the misfire. I done the following: * Swapped coils and plugs with a couple other cylinders - still only 301 * Replaced cam follows and inspected the cradle * During rebuilt I replaced variocam solenoids * Inspected all electrical and vacuum connections to make sure they are connected. * swapped O2 sensors from bank 1 with bank 2 Any thoughts on where to look next ?

Go to a parts store that provides free code readings and also has a scan tool that can read freeze frame data (snap short of many parameters when the CEL was set). From the freeze frame data, look at the short-term, and long-term fuel trim values for both banks (these values have a +/- range), and post them here. This way, we can see how the fuel adaptation is working at idle (it seems OK above idle since the CEL goes out then). Also, swapping the fuel injector may show the misfire moves to the other cylinder. Since the FIs aren't easy to get to, try to get that freeze frame data. Regards, paul...

Here is the freeze frame data: Calulated load = 3.53% Coolant temp = 95F Short-term Bank 1 = 25% Long-term Bank 1 = -0.78% Short-term Bank 2 = 25% Long-term Bank 2 = -0.78% engine RPM = 840 Let me know your thougths on the data. I was thinking on swapping the #1 injector tonight. Paul

Thanks PaulV for your info ..... I think you have me looking on the right path for my issue. Here is an update to the things I did and learned and I would like to share with the forum: * I swapped the fuel injector of #1 with #3 - results no change the only CEL error is 301. * I then did not quite believe the freeze frame data of the short term being 25% that I previous listed. I did some more real time data collection. I found that BOTH banks on the short term fuel trim would be ramping up during idle. When they hit close to 25% the it would cut back to 0 . That is when the engine cuts out and the CEL would flash and either give a pending error of 301 of eventually over many flashes, a steady CEL error of 301. I'm thinking that it is just throwing a P0301 is because #1 is the first that is in line when the software decides that it hit the 25% limit (being #1 always has its benefits:-) * attached is a chart of the short term over time where the DME is ramping the mixture until it hits @25% ..... then it drops off to 0. This is cyclical with about every minute or so. * at RPM above 1000 the mixture is right on the long term of .078 and no CEL codes are thrown. The engine is purring and I have all the power I want :-) When I drive it it's fine until I have to stop and sit at idle. my conclusions: it's not just a #1 cylinder issue. it's how the DME controls the mixture during idle Might be : a vacuum leak which is causing the DME to over compensate on the mixture? a bad senor which is confusing the DME ? Now I need to fine the thing causing the problem. Any thoughts are appreciated

Here are some reponses I published on another forum ...figured it made sense to keep up to date on my findings: Thanks for all the comments. So I'll try to clarify some of the questions. One major thing that made a significant improvement is that I found I had not properly connected the oil filler tubes (I need to fire the mechanic ). I believe I was getting a huge air leak at that point. With connecting it, things have really improved. The vehicle “feels” much smoother at all speeds. However I still get the CEL pending but not as often. And after some driving, and letting it idle it does not trigger. To clarify, on the initial freeze frame data I published. I see the same when FF when the engine is at 190F. The STFTs plots I published were when the car was at temperature (+190). On whether the tool measures absolute, I'm not sure. I’m using the OBDwiz tool with ELM adapter, so I did not spend a lot for the tool. BTW the data I published was before I fixed the oil tube connection. So I had not driven the vehicle too much, since the rebuild. I decided on the weekend to give it a good run after I fixed the oil tube connection. I put a good 150+ miles on it. When sitting after the drive, hooked up the ODB tool and there were a few pending CELs but no stored (I had cleared memory before the drive). I got the following: 301 1128 1130 1117 The 1117 ( engine temp disconnection) is a first time occurrence and was interesting as it could relate to both banks and cause the DME to keep raising the mixture??? So the instances of the CELs have changed since my last reply. After the oil tube repair and the past couple days of driving, I still get 1 or 2 CELs, but it’s not as repetitive. Since my temporary tags expire today, I actually needed to take the vehicle in for it smog test. It did pass the smog test and the CEL never came on (whew). It has come on since the smog test with the above codes. So, I spent some time seeing how the vehicle did just idling. Letting it sit on idle, and after getting it to 190, it quit ramping to +25% on the STFT and the STFT stays near 0 (both banks). And I never got a 301 after that STFT was stable and not ramping 25 % meaning if the fuel trim stayed stable and within range, the misfire never occurred.. When looking for my air leaks, I have also noted that there is significant oil in the tube from the AOS to the air manifold. I did do the disconnect test and block the openings, but did not see a huge difference and I still encountered a 301. I also have seen a couple mornings where I get a bunch of blue smoke on start up …not ever morning??? I am considering replacing the AOS. I will consider doing the compression and leak down, but I feel the computer data was telling me different and that the vehicle runs so sweet after getting beyond 1-2 miles and over 190F, that I’m struggling with doing the tests since the data is only good if the engine is at temperature which to do these test at temperature are difficult. Oh …. I replaced the flywheel during the rebuild with new. I was careful on the install of the flywheel (not as careful as I was on the simple oil tube connection). Could the wheel sensor be acting up? I’m still need to find root cause. Now that it passed the smog test and I can get real plates, the panic is not as bad; however I want to resolve the issue. I do not want to do permanent damage to the vehicle .So any feed back is appreciated. Thanks Paul

I figured I should put some closure to this thread, as I finally resolved the issue. So I drove the car for a few years with this issue. I could avoid the CEL if I kept the idle speed variable by just revving periodically. This winter decided to do some maintenance and this included replacing the AOS as I was getting some small smoking on startups. After replacing the AOS, the CEL light and the P0301 no longer showed up. The issue was a vacuum leak, as suspected. hope this might help someone.