Low power and 6500 rpm limit

4.2 V8 32v Naturally Aspirated - 414 bhp
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RS4RAB
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Low power and 6500 rpm limit

Post by RS4RAB » Fri Mar 12, 2021 10:00 pm

Evening All,

Car has started to run rough, cold start is extremely rough and the revs move up and down a little during the process, once cold start is finished it settles a little but still stumbles and is not smooth

Car lacks any power at all

Whilst trying to log a 3rd gear pull and capture some data the car hit limited itself at 6500rpm

Any ideas?

No fault codes or dash lights at all

Plugs are new and injectors have been out and cleaned recently, coil packs haven't been changed but i would of though i would of had a flashing EPC light if it was a coilpack?

Getsumadis
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Re: Low power and 6500 rpm limit

Post by Getsumadis » Fri Mar 12, 2021 10:17 pm

Sounds slightly, similar to what was happening when my high pressure fuel pumps were failing. Though I've not gotten the new ones yet so not sure if it'll be fixed....

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RS4RAB
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Re: Low power and 6500 rpm limit

Post by RS4RAB » Fri Mar 12, 2021 10:28 pm

Getsumadis wrote:
Fri Mar 12, 2021 10:17 pm
Sounds slightly, similar to what was happening when my high pressure fuel pumps were failing. Though I've not gotten the new ones yet so not sure if it'll be fixed....

Sent from my SM-G991U using Tapatalk
I logged some data with VCDS the other night with fuel pressure etc both low and high pressure, my low pressure seems to be around 5 bar at idle and my high pressure seemed to actually be higher than specified in quite a few areas in the log, sometimes peaking at 9 bar higher than specified so was wondering if i have a problem with one of the HPFP's

My long term fuel trims are also around -6/7 on both banks so ECU is subtracting fuel

RichT128
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Re: Low power and 6500 rpm limit

Post by RichT128 » Sat Mar 13, 2021 12:54 am

Check the lambda sensors. Sounds a little like the edge of it tripping into limp mode (not sure from the post how great the loss of acceleration is)

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RS4RAB
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Re: Low power and 6500 rpm limit

Post by RS4RAB » Sat Mar 13, 2021 7:26 am

RichT128 wrote:
Sat Mar 13, 2021 12:54 am
Check the lambda sensors. Sounds a little like the edge of it tripping into limp mode (not sure from the post how great the loss of acceleration is)

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Power loss is pretty substantial, only other thing to note was just before I noticed any real power loss and really rough running I could feel a vibration through the accelerator pedal

RS4RAB
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Re: Low power and 6500 rpm limit

Post by RS4RAB » Sat Mar 13, 2021 7:27 am

RichT128 wrote:
Sat Mar 13, 2021 12:54 am
Check the lambda sensors. Sounds a little like the edge of it tripping into limp mode (not sure from the post how great the loss of acceleration is)

Sent from my SM-G973F using Tapatalk
What is the best way to check lambda sensors? I have VCDS

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12th
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Re: Low power and 6500 rpm limit

Post by 12th » Mon Mar 15, 2021 12:43 pm

The RS4 has two fuel pumps, and that sounds similar to what happened when one of mine failed. The dips in power seemed to happen at the same point in the rev range, and changing one did the trick - the other is still going strong. There was no warning light.
Sprint Blue B7 RS4 Saloon with MRC tweakery.

RS4RAB
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Re: Low power and 6500 rpm limit

Post by RS4RAB » Tue Mar 16, 2021 8:41 am

12th wrote:
Mon Mar 15, 2021 12:43 pm
The RS4 has two fuel pumps, and that sounds similar to what happened when one of mine failed. The dips in power seemed to happen at the same point in the rev range, and changing one did the trick - the other is still going strong. There was no warning light.
The only thing that confuses me is the fuel trims indicate I am running rich as they are negative and my exhaust tips look black again indicating rich running so if I had a fuel delivery issue I would expect to be lean

RS4RAB
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Re: Low power and 6500 rpm limit

Post by RS4RAB » Mon Mar 22, 2021 11:10 pm

Fuel trims seem to have returned to just slightly negative on both banks now but car still runs rough and lacks any power, have noticed a fair petrol smell when removing oil filler cap so maybe a hpfp is leaking into the valve cover!

If the throttle is blipped in neutral the revs seem to hang quite badly also and the exhaust note is quiet compared to normal when the car is running well

Getsumadis
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Re: Low power and 6500 rpm limit

Post by Getsumadis » Mon Mar 22, 2021 11:40 pm

Have you checked fuel rail pressures? Line 140 on obdeleven not sure if the same on VCDS. Shows requested vs actual pressures

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Getsumadis
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Re: Low power and 6500 rpm limit

Post by Getsumadis » Mon Mar 22, 2021 11:43 pm

Heres a link to my thread about my hpfps

https://r.tapatalk.com/shareLink/topic? ... source=app

B7 Rs4 nasty lag/missfire at WOT between 5500/6500rpm

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RS4RAB
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Re: Low power and 6500 rpm limit

Post by RS4RAB » Tue Mar 23, 2021 12:40 pm

Yes high pressure and low pressure seem ok although the hp reading does seem to move up and down more than I remember from when the car was running good but the problem is there from the minute the car is started so even present at idle when the hp actual reading is near bang on what the specified reading is

Getsumadis
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Re: Low power and 6500 rpm limit

Post by Getsumadis » Tue Mar 30, 2021 5:31 pm

I'm at a loss then. I just got my new hpfps in and installed and that seems to have fixed my problem. Any luck diagnosing yours?

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