2001 Hyundai Tiburon A/T - Random Misfire codes and I believe we have tried nearly everything. Car throws P0300 all the time between 40-60 mph, Throws P0301, 0304 nearly as many times at the same speeds, and once in a while will throw p0302. The vehicle has 75K miles on it and runs well - no noticeable misfire problem, however, some have said it feels under-powered. Here is what's been done, looked at and verified its correct or working. (Not necessarily in this order). My question is what else could it be and how can we test it?

Crank trigger replaced (2 times - 1 of the replacements was bad)
Compression checked out within spec
No intake vacuum leaks - all vacuum specs within factory tolerance
Brand new coil
New spark plug wires
New Iridium spark plugs
Mass air flow sensor verified to be functioning within spec
Timing belt (double verified when replaced to be properly installed on cam and crank marks).
Both o2 sensors within spec and functioning - one was replaced because it was a bit off.

Basically, we have gone through the typical and Atypical issues that can cause these codes. Neither my nor my mechanics equipment is sensitive enough to detect why this code continues to be thrown?

Our last thought is it's the ECU "brain Box" and it's malfunctioning? We have not tried a different ECU.

Open to ideas?

 

Are you able to get fuel trim numbers when the problem presents itself?
You didn't mention the fuel filter , replaced??
The fuel trim #s could provide a clue if the misfires were due to a lean condition.

 

Does your mechanic has access to a scanner??
If he does then you should be able to see fuel trim numbers.

P0300 is misfire.
P0301 is misfire on number 1 cylinder
And P0304 is misfire on cylinder number 4
Check to see the fuel pressure
It is possible it is the fuel pump or injectors
Or even a bad EGR system

Good luck