06 WRX slight hesitation/bucking, no CEL

[ninefourteener]

06 WRX, VF39, Cobb ots map.

Same setup for 6 years.... and this is the first issue Ive ever had. Plus.. there is no CEL to look up a code.

Car pulls great at high rpms... hits full target boost (18).... no backfiring..... etc.

During acceleration..... car hesitates and bucks a little between 2800 and 3800 rpms..... and the "grabs" and pulls strong all the way through redline.

Spt intake... clean filter.... clean maf. 3" exhaust... no cat to clog.

Any ideas?

[blkbugeye]

I was having the same problem with my vf39'd 02 wrx, but I never figured out what was wrong before it spun a rod bearing. Are you running an ebcs?

[EJ257]

Datalogs?

[06wrx4me]

Slipping clutch? OEM clutch?

My stage 2 protuned wrx would do the same until I changed the clutch and pressure plate with a Exedy Stage 1 .

The stock pressure plate looses clamping force overtime as the fingers work harden. There is a pic of mine in the clutch thread in my sig.

Otherwise, clogged injectors? Clogged fuel filter?

[efmd3]

Have you checked for boost leaks?
Does the issue depend on weather at all?
When did you last replace spark plugs and/or O2 sensors?
Bad gas?
Exhaust leak?
Check all vacuum fittings and your fuel filter.

[ninefourteener]

Stg1 exedy clutch with lightweight flywheel, only 20k miles.

Fuel pump and filter were replaced roughly the same time.

I guess a clogged/faulty/leaky injector could be the culprit. I kinda figured if I had a lean condition in one of the cylinders it would throw a cel.

Still old-school... using accessport v1 since 2006. Never saw the need to upgrade. Cant datalog though. Think I might hunt for a faulty injector issue this weekend.

Any other ideas?

[efmd3]

I figured it was running rich, not lean.

[ninefourteener]

Well..... after a little more research...... ok.... a lot more research:

No leaks..... trust me.... I looked.

I did however discover...... that I may be a bit low on fuel pressure.... which may explain why it is only an occasional hesitation between 2800 and 3800 rpms.... which as per Cobb Tuning, is when the engine is at it's heaviest load.

My fuel pressure is sitting at 24psi at idle, and 50psi at full boost (18psi)..... that is precisely about 10psi less than what it should be at... across the entire powerband. It "may" be enough to meet parameters though... which is wht it isn't throwing a CEL. Of course... it could just be a bad gauge.

So.... I'm going to swap out the fuel pressure regulator with a high-quality aftermarket one with a built-in gauge.

Then... I am going to re-check the sock filter, and the fuel filter I'll likely change just "because".... which requires cracking open the tank..... again.

Then... I'm going to check the fuel pump control unit in the trunk.... it's an unlikely culprit.... but it "could" be the problem.

Lastly.... I think I'm going to replace both fuel check valves in both the supply and return fuel lines.... just to be safe.

More to follow. I'll report back with my findings. There are about 15 different threads out there on numerous Subaru forums with similar problems.... and all of them end with "So was this issue ever resolved? I'm having the same problem. What was it?"............. With no answer.

[efmd3]

That's much lower than 10 psi at idle isn't it? I thought the correction free fuel pressure is supposed to be 43.5psi +/- manifold pressure

[ninefourteener]

Well, generally I am at 12lbs vacuum at idle.... so I figure 34psi of fuel pressure at idle would be just about right.

Im sitting at about 24psi of fuel pressure at idle.

Anyone have a good explanation of what could be causing this?

[ninefourteener]

Just did the fuel pressure regulator test. Pulled the vacuum line off... and fuel pressure jumped up 10psi to 34 psi....

Which is exactly what it is supposed to do.

Still 10psi of fuel pressure lower than I should be across the rpm range.

The walbro and the filter have less than 20k miles on them.... I cant imagine that could be the problem......

Any suggestions? Anyone?

[efmd3]

have you opened up the tank to make sure its clean of debris?

[ninefourteener]

Not recently.... not since I installed the new walbro and new filter.... but that was only about 20k miles ago.

I guess I am going to have to. I still have the oem pump.... if it supplies the correct pressure.... at least I know what the problem is.

If the same thing happens..... not sure what to do.

[ninefourteener]

When I turn the car on (not start)..... I hear the pump cycling for about 2 seconds..... my gauge hits about 40 psi..... then drops off to zero.

When I start the car.... it idles at 24psi of fuel.

Wtf?

[efmd3]

so it builds pressure properly with the injectors closed, but as soon as they open the pressure drops, sounds like blockage of some sort to me.