Fuel rail pressure

[Dm85]

Hi,

I’ve been adamant somethings not right as on cold start I can feel more engine vibrations. I would not say misfire but it’s possible they aren’t aggressive. I have OBD11 and I’ve been monitoring fuel rail pressure. When start stop cuts in at traffic lights I can see the reading drop from 32000kpa roughly, to 200kpa in a matter of about 10-15 seconds.

With a warm engine this should go higher not lower, so there’s a leak somewhere. Can the fuel rail pressure regulator be faulty? Could be an injector issue but not getting any exhaust smoke I can notice?

Does the tandem pump have a none return valve on it?

 

[n10n]

When start stop cuts in at traffic lights I can see the reading drop from 32000kpa roughly, to 200kpa in a matter of about 10-15 seconds.

Here's one stop/start event from my CXEB diesel engine for comparison. As you say the pressure starts to drop from stop event and drops in about 15 seconds to neighbourhood of 200 bar until picking up again. Interestingly, there is only a small pressure bump on engine start, but overall rail pressure keeps dropping well into the acceleration following the stop/start event.




I usually run stop/start disabled so don't have much stop/start events recorded. However, here's another one but it's so short the pressure does not really change at all. This is also a warm engine although only some tens of secs into the drive.

With a warm engine this should go higher not lower, so there’s a leak somewhere. Can the fuel rail pressure regulator be faulty?

I don't think this alone says anything about the regulator valve. If the regulator valve would be faulty I would expect the pressure to be unstable or not able to reach highest >2kbar in peaks.

 

[Dm85]

Here's one stop/start event from my CXEB diesel engine for comparison. As you say the pressure starts to drop from stop event and drops in about 15 seconds to neighbourhood of 200 bar until picking up again. Interestingly, there is only a small pressure bump on engine start, but overall rail pressure keeps dropping well into the acceleration following the stop/start event.

View attachment 235141


I usually run stop/start disabled so don't have much stop/start events recorded. However, here's another one but it's so short the pressure does not really change at all. This is also a warm engine although only some tens of secs into the drive.

View attachment 235142



I don't think this alone says anything about the regulator valve. If the regulator valve would be faulty I would expect the pressure to be unstable or not able to reach highest >2kbar in peaks.

That’s great info thanks! I was thinking rail pressure should rise but that’s the petrol fuel rails.

How do you check the injectors are within specs other than a leak off test? VCDS only shows time deviation but not actual injector fuel values. Mine just says 0.00mg/stroke. Only thing I can see is overall quantity of around 6mg/stroke. I believe this is because it’s UDS and not CAN in the Engine module.

 

[Dm85]

That’s great info thanks! I was thinking rail pressure should rise but that’s the petrol fuel rails.

How do you check the injectors are within specs other than a leak off test? VCDS only shows time deviation but not actual injector fuel values. Mine just says 0.00mg/stroke. Only thing I can see is overall quantity of around 6mg/stroke. I believe this is because it’s UDS and not CAN in the Engine module. Unless you need to login in access and submit a code

 

[n10n]

How do you check the injectors are within specs other than a leak off test? VCDS only shows time deviation but not actual injector fuel values. Mine just says 0.00mg/stroke. Only thing I can see is overall quantity of around 6mg/stroke. I believe this is because it’s UDS and not CAN in the Engine module.

Not sure how to actually check injectors but post #67 and #68 have some injection graphs from mine - although not measured with VCDS they are named after VCDS advanced measuring values and use the same data source on ECU so may help in comparing. I don't think you can see individual injector quantities from anywhere; just the duration and timing.

EDIT: "I don't think you can see individual injector quantities from anywhere; just the duration" - I mean duration is effectively same as quantity, just in different units. A single injection by a single injector does not really drop rail pressure all that much so the quantity is pretty much a linear function of duration.