Re: Multiple missfire & lean code- ideas? (Hugh Gordon)
I talked to the service manager yesterday, and he pulled the "run lower octane fuel" bit on me. He tried to say that with day to day driving, if you don't drive the car hard, the slower burning fuel (91 octane) leaves carbon deposits in the combustion chamber that cause misfires
Of course, after sending him the following email, I just realized how much more assinine that theory is since if anything, higher octane would leave more deposits at high rpm since it doesn't have as much time to burn..... Anyway, here's the email that I sent him, we'll see what his response is:
After thinking about the codes our car had when we had a friend scan it (misfires on all four cylinders and a lean condition), I don't believe that fuel octane is the issue. I don't know for certain that the most recent CEL is for the same issue, but the same issue has popped up a few times in the past, so I'm assuming that it's the same one right now. My reasons are as follows:
1) The factory designed the engine to take advantage of fuel with an octane rating of at least 91. This is shown by the rather high (9.3:1) compression ratio when used with some form of forced induction. I sure this was done to improve off boost drivability.
2) I don't think it is even possible to build an engine that is so sensitive to octane that you need low octane for day to day driving in traffic to keep the combustion chambers clean, and can't benefit from higher octane fuel.
3) Misfires won't cause a lean condition, but a lean condition can contribute to misfires. If there are carbon deposits creating hot spots in the combustion chambers, thereby causing pre-ignition, it can't cause the O2 sensors to read lean. They will either read read rich (due to improperly burned or unburned fuel), or have a reading consistant with the mixture that the ECU asked for. In order for the O2 sensors to read lean, there has to be more air or less fuel in the cylinder than the ECU requested. Carbon deposits can't introduce more air into the cylinder and can't reject fuel (I suppose deposits on the injector side of the valve could stop fuel from making it into the CC, but I would certainly be disappointed in your attempt at cleaning the valves if that was the case since the CEL came on so soon afterward).
4) This leads me to believe that the root of the problem is either in the air measuring sensors (MAF of MAP sensors), in the fuel system (fuel pressure regulator or something impeding the flow), or in the ECU tuning (since the problem has been more frequent since the ECU was re-flashed at or around the 60,000 mile service).
5) It is also possible, I believe, that the parameters for the CEL may simply be too stringent. These are misfires unnoticable to the driver, and there's no doubt that many cars in the past have had these issues, we just weren't aware of them.
I believe that VW did a god job designing this engine, but may have come up short on the electronics side of things, as evidenced by faulty MAF's and coil packs. The engine itself has been proven numerous times to be capable of making in excess of 500 hp without changing anything inside the longblock, so I would hope it's able to run properly for more than 62,000 miles in stock form.
Thanks for your attention and consideration on this matter.
Kelly Farrington
BTW, my friend's website is
http://www.EFI101.com, and I believe he is holding a class in Renton in October.
