S6 timing

[HMS-S6]

That's my thought process too Dave and I have to say that although it varies slightly it sits about 13.5-13.8 at WOT around 3,500-4,000 RPM. maybe the 0.2 bar lack of fuel pressure is enough???
A friend is breaking a 96/7 S6 avant and I'm thinking of putting the complete tank pipes and everything into mine.

[UrS4boy]

Is 0.2 bar low on fuel pressure enough to cause a problem?

The ECU assumes that there is 4 Bar of pressure available across the injector(s) when it selects what it thinks is the appropriate injector duty cycle (i.e. the period of time that the injectors are cycled open) for the given situation, e.g. RPM and load. As I understand it, when the car is NOT at WOT, there is feed back from the O2 lambda sensor that the ECU will use to adjust (increase in this case) the duty cycle to correct the lack of fuel that less than 4 Bar across the injector would cause. However, there are limitations on that adjustment capability. At WOT, the O2 sensor signal is ignored by the ECU so adjustments are NOT possible.

prj will correct me where I am wrong.

[HMS-S6]

Is 0.2 bar low on fuel pressure enough to cause a problem?

[HMS-S6]

The 1.5 was obviously on my mapped one but since it's poorly yet again I switched to stock so any findings weren't in doubt of a bad map.

[HMS-S6]

No I'm back on the stock ecu the car left the factory with Dave. Plus these readings tally with my mechanical gauge.

[UrS4boy]

Sorry but I can't remember if your ECU has a 250 kPa MAP sensor or 300 kPa MAP sensor. The OE digital boost gauge was calibrated to the 250 kPa MAP sensor and if you changed the MAP sensor from a 250 kPa one to a 300 kPa one, *ALL* the readings on the OE digital boost gauge are wrong, across the board. So these readings might be another "red herring" in the search for the real problem.

Or not.

[HMS-S6]

At idle, the engine is making vacuum so the 4 Bar differential across the injector = less than 4 Bar pressure in the fuel rail.

That's what I'm saying. The math doesn't add up. It's showing 0.5 on the dash which agrees with the mechanical gauge and the fuel pressure is at 3.7 bar (should be 3.5 surely?) and at 1.0 bar of boost its showing 4.8 bar fuel pressure (should be bang on 5?)
At 1.0 on the dash it does sit at 4 bar.

[UrS4boy]

Think that's playing on my mind is that also, at idle it sat at 3.6 bar occasionally swaying to 3.7/8

At idle, the engine is making vacuum so the 4 Bar differential across the injector = less than 4 Bar pressure in the fuel rail.

[HMS-S6]

Think that's playing on my mind is that also, at idle it sat at 3.6 bar occasionally swaying to 3.7/8

[UrS4boy]

Power lost again today on a still full tank so I sneaked off to my workshop and fitted the 'Stack' electronic fuel pressure gauge and went for a blast. Think I've found my issue. At 3,500 and 1.5 bar I have 5.2 bar fuel pressure, at 5,500 at the same boost I have 4.9 bar fuel pressure!

Done the relay on the pump which is new. Just also realised that on a long steep hill at WOT, the dash boost is reading higher than my mechanical gauge.

Stock ECU at 1.0 bar (mechanical gauge) fuel pressure gauge is showing 4.8 bar. Is this enough to cause the fault?

Just to review the "math": A 4 Bar fuel pressure regulator should always provide 4 bar across the fuel injector, i.e. at zero boost/vacuum, the fuel pressure should be 4 Bar, 58 psig. So at 1.5 Bar boost, you need to have 5.5 Bar at the fuel rail, i.e. you're already too low at 5.2 Bar. Obviously 4.9 is way too low = running lean = higher air/fuel ratio = potential pinging = knock sensors "tell" the ECU to dial back the timing = down on power.

So fuel delivery is the cause of your problem.

In my opinion you might want to (in no particular order):

1. Check the voltage at the fuel pump under various load conditions (I have done this by connecting long leads from "Scotch" taps I add to the 12V and ground wires at the top of the fuel pump and then running them up to my digital multimeter on the dash and then driving around, testing.

2. Consider replacing the fuel pump. I know you say it is new but "new" today could mean made in China stuff that fails in very short order (or never worked properly from the get go).

3. Clearing out the fuel lines at the fuel filter by running the pump via apply power to the No. 17 fuse (down stream leg), as described previously. THEN replace the fuel filter with new.

4. Check the fuel lines from the fuel pump to the fuel filter for kinks/bends/crushes. Something is restricting flow/burning pressure.

[HMS-S6]

Stock ECU at 1.0 bar (mechanical gauge) fuel pressure gauge is showing 4.8 bar. Is this enough to cause the fault?

[HMS-S6]

Yes. Done the relay on the pump which is new. Just also realised that on a long steep hill at WOT, the dash boost is reading higher than my mechanical gauge. This car is a f*****g mess!

[AndyP]

Do you have the pump relayed? Just wondering if a weak voltage is causing the flow to drop under high load?

[diesel des]

Bingo

Sent from my SM-G900F using Tapatalk

[HMS-S6]

Power lost again today on a still full tank so I sneaked off to my workshop and fitted the 'Stack' electronic fuel pressure gauge and went for a blast. Think I've found my issue. At 3,500 and 1.5 bar I have 5.2 bar fuel pressure, at 5,500 at the same boost I have 4.9 bar fuel pressure!