Jerking on low rpm when pressing acc pedal (WRX 2004)
 

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, which 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: 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?

Try unplugging the MAF and see if there is any change. The other sensors should throw a code if faulty. It's worth getting a code reader or find some one with a Tactrix scanner or similar to let it go any faults. Swap the stock bov back on and check for any air leaks.

Thanks for a reply. As I mentioned, I have checked the ECU for fault codes and there are none. I do have laptop with romraider.
I do not have stock DV, the GFB BOV was there, when I bought the car.

Has the fault been present since you've had the car?

Yup, on a pre-buy inspection it has been diagnosed by a subaru service as 'nothing major, probably worn out spark plugs'

Get a stock BOV from a breakers so you can at least rule that out first. Always suspect any non standard parts initially until you can prove otherwise.

Normally they are quite good at throwing out a code if it's something electrical. I'm with above, i think swaping out the re-circulation valve for a oem unit would be a good start point.
When you swapped the plugs what did they look like? Any smoke or fumes?

that sounds to me like a boost leak check all your pipework

Also if you have romraider then take some datalogs,should help diagnose

How much petrol is in it? Are you hard cornering and accelerating when it happens?

If yes to the above then it's fuel surge, the petrol sloshes around in the tank and the pick up sock can lose contact with the fuel causing very harsh stumbling until the fuel pump gets pressure again.
I can get it in my car with even half a tank.

OK, first thing I am going to do is swapping recirc valve to oem.
About plugs: I am not sure, because it has been done by a shop. I asked them and they told me that spark plugs looked quite OK.

I do have romraider. Do you need anything specific to be logged?

It's for sure not that. It has nothing to do with acceleration. It's the moment I push the pedal, so it starts jerking instead of starting to accelerate. Also If I put pedal to the metal (when consumption for fuel is greatest) there is no jerking. And the effect is biggest of the 5th gear (so about 2000rpm there is hardly any acceleration there).
It is happening during normal cruising. If I am cruising with one speed everything is ok. But when I want to accelerate a little (lane change, etc.) the engine jerks 2 or 3 times before it starts accelerating.

Datalog,AFR,timing,injector duty,fuel pump duty,fuel correction,boost and WG duty also have a good look at your LTV's they will tell you how the car has been driving and how much the ecu is having to compensate.

OK, I will log that, just to clarify:
Datalog - is there a param called that?
LTV - you mean learning view tables?

Cleaning the MAF doesn't always work, been there done that :) swap out the MAF with a known good one then go from there.

Had a faulty one on a bug a few years ago, didn't show up any errors specific to the MAF sensor, just a few missfire errors now and again. The car has run error free for the last 6 years

MAF, the MISFIRES AND FART sensor :D

I have same problem, heads exhaust valves gave up maybe thats why
Found that after leaking down test
No codes and engine light is flashing
Or air leak somewhere but cant find it

OK, I have logged some data. First log (with temperature below 60 Celcius degrees) is when everything is running OK, so I press gas pedal and car accelerates normally.
The other log is when temeperature is higher and car is jerking when pressing gas pedal.

Steve001 did you have the same symptom as I do? I mean 'jerkiness' from low rpm?

Yes, it was bearly noticeable at first and sometimes I would get a flashes from the CEL on boost, bank 1 missfire if I remember correctly. Changed plugs then coil packs didn’t do anything. Cleaned the MAF still no changed over a couple of months it got gradually worse, stuttering on low revs mainly 2 gear in traffic. Changed the MAF and no issues since :)

I'm not saying you have the same thing, but well worth investigating IMO Do you know anyone with a bug, blob or hawkeye WRX or STi as the MAF is the same on them all, just to borrow one to see if that cures your problem. Dead easy and simple just 2 philp screws and a plug connection :)

I will look for someone who would like to lend me a MAF, thanks for input.
Can anyone read anything useful from the logs?

In Romraider, what voltage is your maf showing with the ignition on, but engine not running?

My DD software won't read your log files,I've had a look using data zap and I can't see anything strange,maybe the fuel pump duty is high 100% at light throttle but this might be normal? Can you take a screen shot of learning table views,might be some answer there?

There is a really helpful mapper @bludgod ,sometimes can be found in the engine management section,he might spot something unusual with your data.

in romraider logger - hit the "show LTV" button on the left hand side. Save as image and upload here we'll see what's up.


Also - cleaning maf sensor rarely works if its at the stage where it's causing problems just replace it.

Are you using an aftermarket air intake or a standard airbox?

I will check it.

What is DD software? These files are pure csv generated by Romraider.

I have it somewhere I will publish it later.
I have OEM intake and airbox. Car has about 160k miles of mileage.

DD is delta dash which I use for my ESL,it does read csv but for some reason won't read your romraider files,not sure why tbh?

its because files aren't CSV - they are delimited by semicolons not commas, quick find and replace and DD will happily chew them up for you. Do a replace first for , to . then ; to , and you'll be able to view.


For a standard airbox the A/F corrections look a little high but would need to see the LTV info to confirm it - if so then it's a leak or sensor fault. Leak testing with spray won't show you much unless it's really bad, you need to get it properly smoke/pressure tested to confirm.

Nice one,I've just tried this,edit and replaced all , to . then ; to , but i still can't view. :wonder:

I attach screen from LTV. One note though, it's quite fresh, I reset ECU lastly when I cleaned MAF. I think that before reset the value in 10-50 range was smaller, about 6% (and there where maybe 3 spots on FLKC table with values about 3.

I have the same logs, my bet exhaust valves

@fawor, I assume that no symptoms during warm up and under full throttle exludes a general problem like with exhaust valves.
How does your car behave during warmup? Or if you press full gas pedal immediately? In my case under these conditions car is working with no problems.

When is cold it is jerking till 3k, looks like it is loosing preasure below, engine stops when on neutral

Warm similar but no engine stops
Cold or warm feels ok after 3k revs
Engine menagment light flashing constantly but no codes in ecu
I do not think You will get other symptoms when Your exhaust valves gave up

So your symptoms are different. I have no symptoms until engine reaches 60 celcius degrees.

Bludgod, do you know how the ecu alghoritm differs when engine is during warmup and with normal temperature? Which sensors are not being used during warmup?