Random misfire code P0300, P0301-2-3-4

i hope this writing has enough detail. i am writing about my scion xb with 2azfe 2.4L, i have added tri y headers and sheet metal intake manifold. But after installing Kelford cams 239-b i am now getting random misfire code when the motor is idling or running low rpms. i can tell its not a true misfire it seems to be the small off beat lope of the cams at idle being picked up by the computer. i have use hi perf. cams in other motors like motorcycles and v8s. any help would be great. the check engine light when on turns off the vsc and i think abs. thanks Kelvin

Hi Kevin,

sounds like the OBD monitoring is particularly sensitive to changes in airflow.

the overlap from the cams is probably causing issues with the map signal, you could try moving the sensor pick up to a different location, bringing all cylinders together in a manifold or adding an accumulator in the vac line to damp the pulses out - small dirtbike fuel filters work well for this. But it’s a difficult one to say for sure so a bit of trial and error is needed and what you can do will depend on the control algorithm somewhat.

thanks for the help, I think we have the same idea. but this engine doesn't use a map sensor it uses a maf. is it possible to tune down the sensitivity in the ecu.? thanks Kelvin

Ah ok, then this is not that simple.

There are a few options, stand alone ECU to get rid of MAF and run alpha N or speed density would be the top option but costly. There are some piggyback modules out there that could allow you to scale the MAF yourself or replace it with a higher flow sensor. And I think in some applications there are some options to flash and rescale the MAF in the OEM ecu. There no easy option I’m afraid but I’m sure you can find a solution with some more targeted research.

thanks Scotty I do have re-flash software to re scale the Maf sensor but I wasn't sure that was what I needed to do. that will be what I do next. I will let you know how it goes. thanks again Kelvin

hello Scotty I re scaled the Maf sensor and I have about 1.5% short term trim 0 long term trim. this had no effect on the misfire count my scan tool is showing at idle. if this was a motor with a carburetor I would be trying to richen up the idle mixture and up the timing and idle a little to get it to idle smoother. still not sure what to do for EFI. thanks Kelvin

Sounds like its pretty marginal all round in terms of OBD monitoring.

Have you tried to increase the idle speed and move the ignition timing around at idle. I’d be reluctant to blindly add fuel as I’ve seen too much bore wash due to poorly calibrated idle and transient strategies. Is there anyway with the OEM diagnostics to monitor the missfire percentage? Working with OEM management is always difficult when going too far outside the boundaries and ends up in a less optimal compromise.

hello Scotty are you saying the mods I have made the stock ecu just doesn't like, and my scan tool does monitor misfire margin and load and rpm. I have to say I don't understand what this is telling me. the margin at idle is moving up and down from -45 percent to about 10 percent. I increased the idle and the total misfire came down but the idle would hang at about 1500 after pushing in the clutch. timing is moving up and down between 11 degrees and 14 degrees. thanks Kelvin

hi

I think that's the case, it sounds like its marginal as to what the OEM can deal with now. from a bit of research i can see that the misfire detection becomes active around the 1.5-2% mark so if you can get it down to this level then the ECU should be happy. how is the idle strategy managed? IACV, DBW or ignition timing? if you can calibrate a flat timing map in that area it might help. Are you able to try another MAF sensor that capable to measure higher flow? maybe its worth doing some digging around to see if anyone has done this, the pulsation you stock sensor is picking up will be at a lower level on a MAF that designed to measure higher flow. failing that we've kinda hit all the low hanging fruit now so it'll be down to looking at piggy back modules and stand alone ECU's.

hello Scotty that leaves me with a lot of question. first i still don't understand about the misfire margins or percents? i think the IACV is idle air control valve that I don't think this has, it is drive by wire. and I don't know what DBW is? and I can't say I know how the idle system works, just have two control maps in the reflash software. Can you recommend a new MAF sensor? and how would a piggyback module help me? OK i get it DBW is drive by wire. LOL. as always thanks for your help Kelvin