2018 Audi RS3 high fuel pressure dropping out

Wealer · Post by **Wealer** » Mon Oct 09, 2023 2:52 pm

Hi There,

I`m hoping someone here can help me with a wierd issue.

This Audi RS3 with a a 2.5 5 cilinder engine (DAZA) loses its high fuel pressure under full load acceleration. It sets a P0087 "high fuel pressure to low" code. In live date I can see the high pressure completely dropping out.
There are some TSB bulletins about simmilar issues regarding software, low pressure fuelpump module, intermediate shaft sensor. These have all been checked, didn`t solve the issue. Also the high pressure fuelpump and high pressure sensor where replaced by another workshop.

At first I suspected a low pressure fuel delivery problem because it only occurs under full load past 5000 rpm (when it needs a lot of fuel), at lower loads it runs fine past 5000 rpm and the high fuel pressure is maintained. It has a low and high pressure injection system so I checked the live data from the low pressure sensor G410 when the problem occurs, it doesn`t drop. I would`ve liked to verify the low pressure supply when the problem occurs by placing a WPS500 inline, sadly I don`t have one.

: rail

Because this engine uses an intermediate shaft to drive the high pressure fuelpump it uses intermediate shaft position sensor G265 to time the actuation of the pressure regulation solenoid N290. I`ve scoped the signal of G265 when the problem occurs, it looks fine.

: Timing chain

Next I scoped the actuation of pressure regulation solenoid N290, when the problem occurs I get this wierd double actuation of the valve. The first 3 actuations are normal, when the problem occurs (last 3) I get an extra actuation between the normal ones. To me this makes no sense, by actuating the valve between the pressure strokes of the pump it shuts the valve during the intake stroke of the pump. This causes the loss of fuel pressure. Be why would the ECU do this?!?

: Actuation N290

Any help would be greatly appreciated.

vasek · Post by **vasek** » Tue Oct 10, 2023 9:14 am

Hi.
.
It is very likely, that this is a reaction of the control unit to a problem related to fuel high pressure.
As a rule, the transition to emergency mode is accompanied by a 50/50 - MPI/FSI injection ,and a decrease in pressure in the high-pressure circuit.
That is, initially, it is not a change angle in the pulse on the quantity valve that leads to a decrease in pressure, but the nominal pressure disappears with the transition to the emergency injection control mode.
It is necessary record the VCDS log (.scv) in motion under load, where the values will be: low fuel pressure actual, high fuel pressure specified, high fuel pressure actual, angle of closing of the quantity valve.
Then, it will be clear what happens earlier, a drop in high fuel pressure with an emergency mode, or a change in the status of the valve and the injection status with a change in the quantity valve angle.

wiki38andy · Post by **wiki38andy** » Tue Oct 10, 2023 9:05 pm

Scope the HP rail sensor and monitor the pressure drop for each injector as one or more may get sticky open and bleed the rail. Amp clamp on HP pump regulator and coil 1 sync signal. Post the all 3signals psdata file.

Wealer · Post by **Wealer** » Thu Oct 12, 2023 6:23 pm

vasek wrote: ↑
Tue Oct 10, 2023 9:14 am
Hi.
.
It is very likely, that this is a reaction of the control unit to a problem related to fuel high pressure.
As a rule, the transition to emergency mode is accompanied by a 50/50 - MPI/FSI injection ,and a decrease in pressure in the high-pressure circuit.
That is, initially, it is not a change angle in the pulse on the quantity valve that leads to a decrease in pressure, but the nominal pressure disappears with the transition to the emergency injection control mode.
It is necessary record the VCDS log (.scv) in motion under load, where the values will be: low fuel pressure actual, high fuel pressure specified, high fuel pressure actual, angle of closing of the quantity valve.
Then, it will be clear what happens earlier, a drop in high fuel pressure with an emergency mode, or a change in the status of the valve and the injection status with a change in the quantity valve angle.

Thanks, I hadn`t though about reading data pit of the closing angle of the quantity valve. Also there might be a data pit of injection status / emergency mode.

I have checked the high and low actual pressure when the problem occurs, the high pressure (orange) completly collapses until I let of the throttle. While the low pressure (blue) climbs (because the quantity valve is closed during the intake stroke of the pump?)

Wealer · Post by **Wealer** » Thu Oct 12, 2023 6:28 pm

wiki38andy wrote: ↑
Tue Oct 10, 2023 9:05 pm
Scope the HP rail sensor and monitor the pressure drop for each injector as one or more may get sticky open and bleed the rail. Amp clamp on HP pump regulator and coil 1 sync signal. Post the all 3signals psdata file.

I`ve scoped the HP rail sensor, the signal is digital so hard to decipher the pressure from the signal. I checked the oxygen sensor value when the problem occurs, it goes lean instead of rich so I dont think a leaking injector is bleeding the rail.

Wealer · Post by **Wealer** » Thu Oct 12, 2023 6:29 pm

Thanks for both replies, will have to get the car back in for more testing.

wiki38andy · Post by **wiki38andy** » Fri Oct 13, 2023 10:47 pm

When the air/fuel mixture enrichment goes beyond flammability point, combustion no longer takes place in cylinder which becomes an air pump. The whole amount of O2 which should have been used in fuel oxidation will pass by upstream A/F ratio sensor(together with fuel vapor) whose signal will show a LEAN mixture. In catalyst at high temp a good amount of this fuel will burn overheating/melting the cat. Yes, the HP rail sensor signal is in SENT protocol which has to be decoded. Anyway, the question is where the fuel from the rail suddenly goes. Another scenario could be: HP pump does not deliver enough fuel flow under heavy load to rise the pressure at the expected level, ECM gets the pump in emergency mode and so rail fuel pressure suddenly drops. This could happen due to a HP pump timing issue. Which one switches first, rail pressure or HP pump command signal?

wiki38andy · Post by **wiki38andy** » Sat Oct 14, 2023 4:26 pm

Nissan QQ 1.3 petrol, HR13DDT engine. Fuel rail pressure/HP pump regulator current during acceleration/deceleration. Double pulses regulator control at low speed/load.

Wealer · Post by **Wealer** » Sun Oct 15, 2023 7:24 am

wiki38andy wrote: ↑
Fri Oct 13, 2023 10:47 pm
When the air/fuel mixture enrichment goes beyond flammability point, combustion no longer takes place in cylinder which becomes an air pump. The whole amount of O2 which should have been used in fuel oxidation will pass by upstream A/F ratio sensor(together with fuel vapor) whose signal will show a LEAN mixture. In catalyst at high temp a good amount of this fuel will burn overheating/melting the cat. Yes, the HP rail sensor signal is in SENT protocol which has to be decoded. Anyway, the question is where the fuel from the rail suddenly goes. Another scenario could be: HP pump does not deliver enough fuel flow under heavy load to rise the pressure at the expected level, ECM gets the pump in emergency mode and so rail fuel pressure suddenly drops. This could happen due to a HP pump timing issue. Which one switches first, rail pressure or HP pump command signal?

Thanks again for your usefull reply. It makes sense that the oxygen sensor would read lean when the mixture goes beyond the flammability point, hadn`t considerd that.
My first aproach was to find out if the double actuation of the quantity valve was the cause or a reply from the ECM to the issue, but I couldn`t decipher the SENT signal from the HP sensor. Is there a way to decode the SENT signal in the Pico software?
The HP pump was allready replaced and the cam was checked. The pump is timed directly from the intermediate shaft with its own position sensor G265. So I doubt its a pump timing issue.

Wealer · Post by **Wealer** » Sun Oct 15, 2023 7:26 am

wiki38andy wrote: ↑
Sat Oct 14, 2023 4:26 pm
Nissan QQ 1.3 petrol, HR13DDT engine. Fuel rail pressure/HP pump regulator current during acceleration/deceleration. Double pulses regulator control at low speed/load.

At low pressure / flow it would make sense to close the valve during the intake stroke of the HP pump.

PicoScopes
& kits

For vehicle manufacturers

EV kits and
accessories

Noise, Vibration and Balancing

Milliohm tester

Pressure sensors

Current clamps

Breakout
leads/boxes

Accessories

Automotive Forum