HP Fuel rail pressure

4.2 V8 32v Naturally Aspirated - 414 bhp
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glscholes
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HP Fuel rail pressure

Post by glscholes » Fri Oct 16, 2020 12:53 pm

Hi Gurus! Looking for some assistance with my 2007 B7 RS4.I am a pretty logical thinker, a mechanic, and I like to work through things in a sequence.
My RS4 takes a few seconds of swinging to fire when cold, and when she does fire, misfires, or stutters for a second or two before settling down to a smooth idle. I have replaced a few coils based on misfire data from VCDS, and she runs a lot better, but after start up there is an incredibly strong raw petrol smell from the left tail pipe, and she does run rich.
Now, with all that info in mind I need to ask what the High Pressure fuel rail pressure should be. Before starting up, the pressure on VCDS is reported as 5.8 Bar, after starting she idles at 33.6 Bar. I have a strong suspicion that one, or probably a lot, of the injectors are leaking, causing the poor start up and misfire immediately after starting. What I would like to know is should the High Pressure fuel rail remain static after shutdown? By this I mean, if I see a reading of 33.6 Bar before turning the key off, should this pressure remain (or at least stay close) until the next start? Bearing in mind that turning the key off disrupts power to VCDS, after stopping the engine and restarting VCDS, the High Pressure rail pressure is only 5.8 bar, which I assume is the tank pump output pressure. Is there perhaps a return valve that could be leaking? Although that would not explain the fuel smell from the tail pipe that a leaking injector would cause. She is heavy on fuel, I have a KMH version and usually get about 12.6 Litres/100 km if I drive sensibly, but who has an RS4 and has to drive sensibly?

Any and all assistance would be greatly appreciated, I love my RS4 and want to give her the best life I can.

Thanks in advance, Graham

AA_954
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HP Fuel rail pressure

Post by AA_954 » Fri Oct 16, 2020 4:44 pm

glscholes wrote:Hi Gurus! Looking for some assistance with my 2007 B7 RS4.I am a pretty logical thinker, a mechanic, and I like to work through things in a sequence.
My RS4 takes a few seconds of swinging to fire when cold, and when she does fire, misfires, or stutters for a second or two before settling down to a smooth idle. I have replaced a few coils based on misfire data from VCDS, and she runs a lot better, but after start up there is an incredibly strong raw petrol smell from the left tail pipe, and she does run rich.
Now, with all that info in mind I need to ask what the High Pressure fuel rail pressure should be. Before starting up, the pressure on VCDS is reported as 5.8 Bar, after starting she idles at 33.6 Bar. I have a strong suspicion that one, or probably a lot, of the injectors are leaking, causing the poor start up and misfire immediately after starting. What I would like to know is should the High Pressure fuel rail remain static after shutdown? By this I mean, if I see a reading of 33.6 Bar before turning the key off, should this pressure remain (or at least stay close) until the next start? Bearing in mind that turning the key off disrupts power to VCDS, after stopping the engine and restarting VCDS, the High Pressure rail pressure is only 5.8 bar, which I assume is the tank pump output pressure. Is there perhaps a return valve that could be leaking? Although that would not explain the fuel smell from the tail pipe that a leaking injector would cause. She is heavy on fuel, I have a KMH version and usually get about 12.6 Litres/100 km if I drive sensibly, but who has an RS4 and has to drive sensibly?

Any and all assistance would be greatly appreciated, I love my RS4 and want to give her the best life I can.

Thanks in advance, Graham

I don’t have any logs at hand but to recall at WOT I was peaking at ~120bar at the HP and LP would go down to around 3.8-4bar, additionally the following are a couple of items that has been discussed in the RS4 owners whatsapp chat:

When you turn the car off and keep the key at the ACC position the pressure on the HP rail should temporailly keep building up, if it doesn’t that could be a sign of leaking injectors.

The other item is that the reading you are getting I believe is from only one of the fuel rails as there is only one sensor, I can’t recall who clarified this to me and which bank it is but don’t take the readings as both HPFP.

To help guide you a bit better whats the mileage on your car?

When was the last CC performed?

Have you ever changed the injectors as part of your preventive maintenance?

Have you cleaned the TB?

When was the last change of Spark Plugs and all coils?

For reference my car starts on the click but I’ve done all this in the last 5k miles including the 2 fuel pressure regulators


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glscholes
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Re: HP Fuel rail pressure

Post by glscholes » Fri Oct 16, 2020 7:55 pm

Thanx for the input. The High pressure does fluctuate, and does go quite high, must admit to not knowing how high, glancing at my laptop screen while thrashing the engine at WOT is not something I recommend doing!

Will try looking at the pressures after turning the key to ACC, but to be honest, only the electric pump in the tank may then be running, and the pressure would then not rise above the specified low pressure system maximum of 6 Bar.

Mileage is 172000 KMs, Last Carbon Clean was 22000 Kms ago, the injectors have not been changed since I have had the car, for just over a year now, I have not cleaned the throttle body, is this something I should be doing? Checked all plugs, they are in good condition, apart from some soot build up which, if my theory about the injectors is correct, is as a result of the leaking injectors, and I have only had to replace 1 coil due to a persistent misfire on number 1 cylinder, I swopped the coil to number 3 and the misfire shifted, so knew it was definitely the coil.

Honestly, I am looking for confirmation that the rail pressure should be maintained at at least the idle pressure of 33 Bar after a shutdown, otherwise a leak of some kind is obviously bleeding the rail pressure off. Next step would then be to ascertain if it is an injector, or possibly a non return valve, if there any on the outlets of the high pressure pumps.

Any more input will be gratefully accepted.

p.s. I am currently living in South Africa until I can get back home to the UK and parts for the RS4 are stupidly expensive here, so just changing the injectors, if you can find them, is not wife pleasing!! There is a cleaning service I can try, and they will give a report on the injectors, but I would like to get some more definitive data before stripping them out.

glscholes
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Re: HP Fuel rail pressure

Post by glscholes » Fri Oct 16, 2020 8:05 pm

Oh, and from what I see on ERWIN and the literature I have, both High Pressure fuel pumps feed into a single pipe that feeds both rails, which is why there is only one pressure sensor on bank 2 left hand bank, cyls 5 - 8 (see pages 30 and 31 of the attached PDF)
Attachments
A065S00232V-Nr_377__Audi_4_2-litre_V8_FSI_engine.pdf
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AA_954
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Joined: Thu Mar 29, 2018 9:23 am

HP Fuel rail pressure

Post by AA_954 » Fri Oct 16, 2020 8:07 pm

glscholes wrote:Thanx for the input. The High pressure does fluctuate, and does go quite high, must admit to not knowing how high, glancing at my laptop screen while thrashing the engine at WOT is not something I recommend doing!

Will try looking at the pressures after turning the key to ACC, but to be honest, only the electric pump in the tank may then be running, and the pressure would then not rise above the specified low pressure system maximum of 6 Bar.

Mileage is 172000 KMs, Last Carbon Clean was 22000 Kms ago, the injectors have not been changed since I have had the car, for just over a year now, I have not cleaned the throttle body, is this something I should be doing? Checked all plugs, they are in good condition, apart from some soot build up which, if my theory about the injectors is correct, is as a result of the leaking injectors, and I have only had to replace 1 coil due to a persistent misfire on number 1 cylinder, I swopped the coil to number 3 and the misfire shifted, so knew it was definitely the coil.

Honestly, I am looking for confirmation that the rail pressure should be maintained at at least the idle pressure of 33 Bar after a shutdown, otherwise a leak of some kind is obviously bleeding the rail pressure off. Next step would then be to ascertain if it is an injector, or possibly a non return valve, if there any on the outlets of the high pressure pumps.

Any more input will be gratefully accepted.

p.s. I am currently living in South Africa until I can get back home to the UK and parts for the RS4 are stupidly expensive here, so just changing the injectors, if you can find them, is not wife pleasing!! There is a cleaning service I can try, and they will give a report on the injectors, but I would like to get some more definitive data before stripping them out.
Unfortunately my car is in storage so can’t help you with a read but I have asked in our chat if someone could run a quick log and share I’ll dm you the result.

Given that the car has about 105k miles I thinks its best you assume that the injectors were never changed and I would recommend you do that as soon as you can as preventitive maintenance, their are confirmed cases of engines taking a crap due to faulty injectors...

If you can get Rockauto or FCP to ship to SA without significant costs to you in duties you are looking at an investment of around 550 usd plus the clips, I know some folks in the UK have them cleaned and have not mentioned any problems




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AA_954
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Re: HP Fuel rail pressure

Post by AA_954 » Fri Oct 16, 2020 9:00 pm

I found an old obd11 log I had done back in December and at 760 rpm (idle) the specified value for fuel rail appeared as 20 and my values oscilated between high 19’s and low 20’s with a peak of 20.5 bar

Hope this helps, I only started seeing specified and actual values of 30 bar at aproximately 3200 rpm


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