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Discussion Starter · #1 ·
I've read through all the old forum posts I can find and tried quite a few of the solutions. However, given the limited conditions under which the misfire happens I figure its worth its own post.

I've been in the final stages of finishing my 2ZZ swap getting it ready for inspection and I'm getting a P0300 Random Misfire code only during the initial warmup after cold starts.

The sequence is as follows:
  • Car starts without issue
  • Car warms up to the point the idle is down around 900-1000RPM
  • Only during the period between the car being warm enough to idle down and fully warmed up I get the misfires. The car is actually misfiring as it can be heard and felt if you pay attention. The misfire however is not constant, perhaps one or two misses every 30 seconds or so.
  • Once the car is warm, the misfires stop.
  • The code normally doesn't get past the pending code phase. If it does, the code can be cleared and will not return until the car sits long enough to be at a cold start again no matter how much or what kind of driving is performed. I haven't observed or popped any pending codes after the warm-up is complete.

I have done the following so far:
1. Checked for vacuum leaks
as part of diagnosing a previous P0171 Lean code which was solved by pressurizing the intake to find the leak. That turned out to be air leaking by the dipstick. Added some o-rings to the top of the dipstick, retested and no air could be heard escaping under pressure. That code has not returned.

2a. Replaced the B1S1 O2 sensor with a known good sensor to eliminate that as a possibility.

2b. Replaced the B1S2 O2 sensor due to a failure of the heating element in that sensor which was causing a P0141. That code has not returned.

2c. Sensor in the other position on the header is known BAD. My understanding is that the 2ZZ ECU doesn't use it, so I'm using it only for appearances. Unplugging it has no effect as far as I can tell. Just want to make sure there isn't something I don't know here.

3. Cleaned MAF sensor. Didn't note any issues with it but cleaned it anyway to be sure. The ratio of MAF to RPM is generally linear in the scan data I've recorded so far, so I don't currently suspect that.

4. Swapped CYL3 and CYL4 coil
as part of diagnosis. Earlier in the process before I got rid of the lean code, I did see a P0303 code a couple of times. I , but I haven't seen that or any other cylinder specific codes since.

5. Recorded data.
  • Fuel trims: During my last recording session, LTFT was consistently around +2.5% at idle. STFT varied but was is generally between +5-10%
  • O2 sensor: Appears normal. Bounces up and down in the usual fashion
  • Timing Advance: Consistently 16-17deg at idle
  • MAF: Consistently 3-3.4g/s at idle. Linear response to RPM as stated before.

- I'm using the MWR swap header with the B1S1 O2 sensor only reading CYL1. I've been wondering about the effectiveness of this configuration with a 2ZZ ecu. I don't know that it would cause my misfires, but it seems like the ECU is working from incomplete data regarding its fueling strategy. Should I consider having a bung welded in after the collector?

- The only warm driveability issue I've noticed is an occasional hesitation around 3500-4000RPM but only at partial throttle. Higher throttle input results in passes through that range without hesitation. I don't know if its related, but I do have what I feel is excessive play in the throttle cable. Even at its maximum adjustment at the bracket on the engine, I have about 1/2in of very noticeable takeup in the gas pedal before the cable goes tight any any throttle response occurs. I would love to hear your thoughts on this as well.

Any advice on what I should try next?

Thanks in advance,


4 Posts
Discussion Starter · #3 ·

Thanks for your reply!

I tried to be as clear and concise as possible because I know how I feel trying to diagnose problems from a vague set of information!

In reference to the O2 sensor placement, I’m glad to hear that I’m not the only one who thinks that doesn’t make any sense! I’ll get a bung welded in further down the line.

I’ll get techstream going, see what I can figure out, and come back with the results.

Thanks again!
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