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Tech Article

Audi C5 A6 Quattro 4.2 V8 Fuel Pressure Regulator Replacement

Last Updated: 09/24/2017

Parts Group: L-Jetronic fuel pressure regulator retaining clip

This article discusses the fuel pressure regulator for the Audi C5 A6 with the 4.2 V8 as made from 1999 to 2004, with the main focus being on how to replace it. It's really simple but there are a few points of confusion that tripped me up. I hope my write-up saves you some time and frustration.

On our project car, a silver 2000 Audi A6 Quattro 4.2 V8, we were chasing a problem: the fuel line had no fuel pressure after the car had been sitting for an hour or so. This made for a slow start requiring much cranking to get the fuel line and fuel rail full of fuel and repressurized. I figured that two likely suspects were the fuel pressure regulator perhaps having a slow leak (less likely), or the check valve in the fuel pump being faulty (more likely).

I figured that a fuel pressure regulator could hold fuel pressure at 4.0 bar (its official rating) yet have a slow leak too, causing the fuel to drain back into the tank. So, replacing that seemed at least somewhat logical -- and easier and cheaper than replacing the fuel pump, which is located in the fuel tank. I figured I'd eliminate the easiest variable, first.

I have removed and replaced a fuel rail before, on this type of engine, and I wasn't looking forward to doing it again. It seemed necessary because the body of the fuel pressure regulator is an integral part of the fuel rail. However, on closer inspection, I saw that the body is just a hollow metal cylinder that is hardly ever likely to be the problem. It's what's inside that needs replacing. The fuel rail need not be removed.

I bought a new Bosch fuel pressure regulator for less than $40 from FCP Euro. The part requires two O-rings; a small one and a large one. I bought that too, at just under $10 per set, not realizing that the part came with its own O-rings pre-fitted.

If all goes well (which it didn't) then the process is simple:
- Use a screwdriver to pry up the black upside-down-U-shaped vertical clip. Remove it.
- Pull out the old inside of the fuel pressure regulator. Disconnect the vacuum line.
- Push in the new inside of the fuel pressure regulator. Reconnect the vacuum line.
- Push in the black upside-down-U-shaped vertical clip.

That's how it's supposed to work. Here's how the events actually played out:

Too late, I learned that it's prudent to FIRST connect the vacuum line to the new part, so when I drop it then it doesn't free-fall behind the engine.

I didn't lose the clip, but if you do, I typically have a few extra for sale.

I couldn't push the unit in far enough to get the clip into position. I'd asked my tech-savvy friend if I should lubricate the part before installing, and she recommended I do, using either fuel or a small amount of assembly oil. The fuel didn't do it for me. So, off I went to Autozone to buy assembly oil. They didn't have oil, but they had a $5 bottle of assembly lube. Good enough. However, even that didn't do it. My friend suggested that I tap the unit into place with a rubber mallet. That sounded terrifying but I did so and it worked. I was able to push the the black upside-down-U-shaped vertical clip in.

I started the car. The problem I had been hoping to solve was still there, but at least I hadn't made it worse. I checked for fuel leaks; none. I have been checking every few days, and driving with a fire extinguisher, but ... so far so good.