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Re: BMW 320i year2002 e46
I don't think your trouble is on the ignition side of it but I am sure that you must have a look at the idle speed control(stepper motor).
If with temperatuer your "misfire"changes to better, it means that it's probably running too lean at cold, normally the stepper motor or ISC does this job.
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Replace spark plugs and look at coils. Sounds like a misfire at idle. Car is trying to keep idle high enough with 1 or 2 cylinders misfiring. Car compensates then revs up as the cylinders catch and run again. These cars often have bad coils. Scan car and look for codes 301 through 306. Replace all plugs and if misfire comes back then replace the coils on the cylinders that occasionally misfire. Look at boots and spring wires on coils and replace coils that have bad boots.
code p300 refers to random/multiple misfires--misfire detected==causes--spark plugs--HT leads --injectors--ignition coils--low compression--wiring
If you are only now experiencing the problem after changing the ECU, I would suspect that wiring and connections to the new ECU may not have been fitted properly. There may be problems in the ECU CPU (chip) and it may be necessary to use a scanner to reset the CPU program to recognize the sensors of the vehicle correctly
wiring faults to coil/s --coil/s --HT leads ---Spark plugs --low fuel pressure --injector wiring--injectors -- faulty injector tips are some of the causes for misfiring. Run the fault codes to check where the fault may lie.
Please check the periodicity of the pulse to the LT circuit. If there is any irregularities here the spark can get absent. Check when the engine gets warmer as the circuit might also have problems coming up at this stage.
Finally check on the HT cables from the coils to the spark plugs.
Sounds like it may be running a bit lean - do basic checks for air leaks in the induction system (such as leaking manifold gasket, loose hose fittings to air flow sensor etc).
Condition of HT leads also worth a check.
This is a pretty common problem. I would change your valve cover gaskets. They run around ~ 50 bucks. If they are bad it will leak oil into spark plug well and cause a misfire. Very known on the 740.. Also if tune was not anytime not too long ago id change spark plugs while you at it. Then clear the computer and if you still get misfire on 8 try changing the ignition coil which is right about the cylinder with cylinder ignition coil 5. clear the computer. if you get random misfire and misfire cyclinder 5 after then its your ignition coil. parts store runs around 100 bucks