Wont rev past 2500...

[Marky3005]

Hi all, complete newbie so please excuse the stupid!

The b8 3.0 quattro has developed a fault where it won't rev past 2500 rpm under high throttle positions. And after a second or two, (thought it might clear after a second) the coil light flashed and went into limp mode. Plugged into my friend's diagnostics and it showed low fuel pressure. Went for a drive with live data on and could see that injector 4 was dosing well above the other injectors.
Ok, injectors gone, replaced it today, but not had it coded yet. Tried it up the road and same fault. Only at 3000 instead.
Am I worrying here and it will go away after its been coded?

 

[pelhamsdetailing]

What engine code is yours? CAPA, CCWA? That'll give other members a bit more to go on. If the fault code has not been cleared, there is potential for it to not rev past 3,000RPM. Does your coil light flash or come on when you try to push it past 3,000RPM for more than say a few seconds?

 

[Marky3005]

Sorry, its a CCWA. The code has been deleted and no light has come back on when I tried it, I only went a mile up the road as I didn't want to push my luck and damage anything. It won't rev past 3000 now, stutters and almost feels like it's hit a limiter.

 

[pelhamsdetailing]

Thanks. Your high pressure fuel pump could potentially be unable to push any more due to ongoing failure. The next step would be to rule out if you are getting adequate fuel pressure or not, which can be accurately logged with VCDS while driving. This can be done cheaper using an EML327 adaptor and a car scanner app that is able to read live data on your vehicle, which I have personally found adequate in the past on my old A5 3.0 TDI CAPA. I had the same issue as you at 3,500RPM, although no stuttering, and it turned out to be a blocked DPF that would not regen no matter what. That could also be a potential cause for your 3,000RPM limit.

 

[Marky3005]

Thanks. Your high pressure fuel pump could potentially be unable to push any more due to ongoing failure. The next step would be to rule out if you are getting adequate fuel pressure or not, which can be accurately logged with VCDS while driving. This can be done cheaper using an EML327 adaptor and a car scanner app that is able to read live data on your vehicle, which I have personally found adequate in the past on my old A5 3.0 TDI CAPA. I had the same issue as you at 3,500RPM, although no stuttering, and it turned out to be a blocked DPF that would not regen no matter what. That could also be a potential cause for your 3,000RPM limit.

I have had that read whilst driving, it was showing a healthy pressure in the fuel rail, we also monitored the 6 injectors variance to what was required, all were more or less where they needed to be except no4 injector which was almost permenantly +1.6mg per stroke and stayed there almost constantly. Hence changing the injector. Would a blocked dpf not show up on a code read?

 

[pelhamsdetailing]

It depends on the version of your ECU software and whether it's been tampered with. Mine was in both those cases and never threw a DPF fault code, only a "turbo underboost" due to it not regenerating, becoming blocked and heavily restricting air. i.e. huge back pressure. If yours is blocked (oil ash volume, soot mass measured, soot mass calculated - worth looking at live data for these on your vehicle), then that would explain why you can't rev above a certain RPM.

Your engine needs fuel and air. If it's getting adequate fuel, then you need to look at what could be stopping that air from escaping out of the exhaust or getting into the engine in the first place, e.g. top and lower air intake. These engines can get very dirty air intakes due to the EGR. If it's blocked up badly, then that would explain why it's not able to rev that high. They are easy to clean on these, but with the CCWA engine you must use VCDS to prime the high pressure fuel pump before starting the engine after cleaning the air intake and putting it back together, otherwise you'll damage the HPFP.