I have problem with my 2004 WRX: during a drive, while rpms are below 3000, when I press acceleration pedal, the engine starts jerking like crazy. The car is totally stock, apart from PPP (only ecu and y-pipe) and GoFastBits regulaed recirc valve.
Everyone told me, it's for sure something with ignition (the typical symptom of misfire), probably the spark plugs. So one of the first things after buying the car was changing spark plugs (to original iridium ngk, but one degree colder) - no effect at all.
Then someone told me to check if the gap is 0.7mm, so I have them checked and gap set to 0.65-0.7mm - no effect at all.
If sparks are ok, it must be coilpacks, they said. So my mechanic swapped a whole set of coilpacks to another set from properly working scooby - no effect at all.
After that, someone suggested checking the head valves clerances (but I have checked service history of a car and previous owner has done that one year before).
Apart from that I also did:
- changed PCV valve;
- added missing gasket under recirculation valve;
- do a leak test on idle with autostart spray - no leaks found;
There are no fault codes in ECU, no check engine. The car passed emission test with no problem.
Last time, I noticed some situations in which problem does not occur (what might be crucial in finding a solution):
- when I press the pedal fully and very fast (what in my opinion eliminates ignition as source of the problem);
- when the engine is cold, during a warm up .
So from my understanding, the engine runs well in open loop (when cold, when pressing pedal to the metal, and when pressing gas pedal above 3-3,5 krpms) and jerks when pressing acc pedal in a closed loop.
Firts thing I thinked of was MAF, I watched this video:
https://www.youtube.com/watch?v=QwGICKqWgM4 and I thought: it must be it! I am probably missing the first spike on maf reading and this is exactly when the jerking comes on. Unfortunately, after cleaning MAF with carb cleaner and a soft brush, there is literally no effect.
Please give me some advice, what should I investigate next. Can MAF be faulty despite cleaning? How can I check if it is OK?
After reading the forums, other suspected sensors are: first lambda and Coolant Temperature Sensor. How can I check if they are faulty?
