Piston Slap: The B7’s Bemoaning Fuel Pump?

2006 Audi S4, Image: Cargurus

Paul writes:

Hi Sajeev, I have an opinion/advice question for your column:

I have an ’05 Audi S4 (B7 generation), and this is not a question about this model’s notorious timing chain that so frequently scandalizes car website comment sections.* I have a longstanding issue with fuel pump vibration. It is extremely noisy when it primes, making a rapid clicking or clunking noise that is audible both inside and outside the car. It only lasts for the priming process, and afterwards, the car starts and runs as normal. There is no hesitation in starting, misfiring, or power loss. It is only a noise.

I assumed it was the fuel pump, so I replaced it, but the problem did not go away. I took it to a German car specialist that I’ve used for many years, someone trusted but also somewhat expensive. He confirmed that the pump is not at fault, but suspects that the housing for the pump — which is built into the fuel tank — is broken. There is some kind of vibration dampener assembly that not doing its job and the noise is from the pump rattling the housing around during priming.

Repairing this, according to him, would involve replacement of the entire fuel tank.

This is obviously not a cheap part, and the tank straddles the driveshaft and rear differential, so replacing it would involve dropping both components and that’s beyond my ability to DIY with the tools I have at my disposal. So it would be a very expensive job at a mechanic. Accordingly, I’ve left the problem unfixed for quite some time now.

Should I suck it up and get this fixed? Aside from being unsightly and annoying, it doesn’t seem to be affecting the running of the car. On the other hand, there’s always the possibility that I may be damaging the pump or the fuel tank and setting myself up for an even bigger expense down the road.

Have you ever heard of anything similar to this? I couldn’t find a whole lot online about other B6/B7 owners with this problem. It seems to be a completely aberrant failure and I was thought maybe you had some insight.

* The timing chain problem on these models is completely overblown and can easily be avoided by making sure you change your oil on schedule and maintain the check valves and oil spray nozzle in the heads. The chain slap comes from oil draining out of the heads while the car is parked, slackening the hydraulic chain tensioners until the engine restarts and builds the pressure back up. Audi was smart enough to foresee this issue and build in a pair of check valves designed to maintain oil pressure in the heads while the car was off, but they can fail and stick open over time. Replacing them proactively will keep your chain properly tensioned and avoid the slap issue. I did mine as soon as I started to hear startup noise 25k miles ago and it went away immediately and never returned. Spread the word!

Sajeev answers:

Consider the word spread on the timing chain problem, now get people changing oil at recommended intervals, LOLZ!

Regarding your fuel pump vibration problem, I have an old Fox-body Ford that had a terribly loud aftermarket fuel pump after a terrible conversion from wussy Central-EFI to manly 4-bbl carburetion. The solution was to either to add an isolating rubber bushing/gasket between the pump and the frame (and pray that was enough), or run an old-school mechanical fuel pump off the front of the engine.

But I digress, your Audi has an easy access port under the back seat to eyeball the vibration problem yourself.

I also doubt your vibration damages the fuel tank, and there’s likely a DIY solution.  Sharpen your knives, Best and Brightest, I smell a wrong answer forthcoming. 

Remove the back seat/fuel pump cover, get a mechanic’s stethoscope, and have someone key-on the car to trigger the fuel pump. After reading this thread, I reckon you’ll isolate the noise quickly without removing anything, and hopefully silence the noise with a GENTLE push or pull on the fuel pump top or the retaining trim ring. If so, you know the source of the vibration may lie elsewhere but can be silenced at the point of contact. A4 B7 Fuel Pump, Image: Audi Forums

See the rubber ring with the little nubs at each end? I bet that’s the place that needs extra silencing initiatives.

And yes, any tech that values their reputation will replace the whole tank, as I only see proper replacements for that big orange gasket (above). Armchair quarterbacking this problem is a bad move, so my notions involving a thin bead of silicone are going out the window. It really depends on the vibration’s location, how the fuel pump (top part) sits inside the recess, how the locking ring operates, etc.

But I will have faith that this is a cheap fix after some NVH diagnostics on your part.

[Images: Cargurus, Audi Forums]