Fuel rail pressure leak

A6 All/ S6 All
#1

Hi, I have an issue with fuel rail loosing pressure after turning engine off in my S6. While driving actual fuel pressure is pretty much spot on with specified fuel pressure. After stopping the engine it loses 20 bar of fuel rail pressure in around 5 minutes. Lpfp after engine stop does not go above 8 bar.

Would that indicate leaky injector, or regulator valve fail?

Car takes around 3 seconds of cranking on cold start, but if I leave the car for a couple of days then I would say it cranks 1-2 seconds.

Long term fuel trims are:
Bank 1 1,8%
Bank 2 2,0%
Bank 3 2,3%
Bank 4 2,8%

#2

Did some testing today.

Cold start, 1,5 km drive, engine about 70 degrees celcius. I turned the engine off and checked group 140 in Engine II. Fuel rail pressure raised about 15 bar. Good engine start, but 5 misfires noted in ECU cylinder 3. No codes

Then i did drive around 20 km, came back home and same test, turned engine off. Fuel pressure did not drop, and started raising. I started the car with no issue, no misfires in ECU.

After one minute turned engine off. This time fuel pressure started dropping and dropped to around 19 bar in a few minutes.

Started the car again two times, both of them fuel pressure did drop to 15 bar, and the second time 10 bar.

Would this indicate injector leak, fuel pressure regulator valve leak, or what?

Sometimes when I start the car on warm engine i feel some hesitation, and like i said before car cranks up to 3 seconds on cold start if I was driving the day before.

Im confused where should i look exactly.

Any thoughts? Let me know.

Ps. No fuel smell in engine compartment

#3

I am here browsing today because I do have a fuel smell, and it looks like I have a leak somewhere on the fuel rail. I did notice my car cranks longer than usual to start.

As for your issue, do you have one of those cheap borescopes from Amazon? I had some leaking injectors, and I was able to confirm by pulling spark plug and looking inside with a borescope. I want to say just turning the key on I could see uncommanded spray into the cylinder, but it’s been quite a while. I know it was really wet where it should not have been.

#4

I don’t have a boroscope, but I will buy one. Either way I decided to pull the injectors and get them serviced.

#5

Probably a good call depending on your miles. I thought the source of my leak was an injector leaking at the fuel rail. I was in a rush so I just pulled the manifold and resealed the 1 injector that was wet.

Turns out it wasn’t the injector at all. The fuel rail cracked at the back end where the manifold ends and the rigid line begins. It’s a result of poor maintenance by a previous shop. The bolt all the way at the back that supports the bracket where the manifold and the line to the HPFP meet was not installed, someone just decided it wasn’t needed and was too lazy to put it back in. Over time the vibration caused a tiny crack.

#6

So a little update. Injectors were tested, no leak issues. They all have even flow. #2 and #7 were clogged a little, but after cleaning they are ok. Any other ideas where could I have pressure leak? No visible leaks near fuel rail or fuel pump

#7

Any fuel smell under the hood after shutting down? Have you been able to use a mirror or a borescope to observe the fuel rails after shutting down? My fuel rail crack started out very small, and with the warm engine the fuel evaporates so fast there was no puddles, but watching closely after shut down I could see a find mist near the crack.

#8

No fuel smell under the hood. I haven’t checked the valley with boroscope or mirror yet, but I might just do that. However despite fuel injectors getting good results on test bench I still think that they are the culprit. Test bench uses 12 or 14 bar for testing IIRC, when injectors have 30 bar of pressure during idle.

#9

Update. Replaced 10 injectors yesterday with brand new OEM ones. No more fuel rail pressure leak, so it seems old ones were the culprit.

Also after old injectors were cleaned my fuel trims went south with one bank being over -11%

#10

Yeah cleaning the injectors generally isn’t going to be testing them for holding pressure or fuel weep. Especially if you’re not getting them tested by someone that leak tests them.

Great job following up and letting us know. This is so helpful for people in the future that have the same issues you had.

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