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Misfire, no acceleration, rough idle

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Misfire, no acceleration, rough idle

Old 07-21-2018, 10:25 AM
  #21  
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Good point on the fuel filter. If however the pump has not been replaced and you are over 110K or so it's a good idea to do it. If it's over 140K it's risky not to replace it IMO. And the fuel filter at some point around 100K.
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Old 07-21-2018, 10:48 AM
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Completely agree. Anymore (I'm trying to get this S8 to 300K-- just because...) I think the list of stuff to change every 100K is:
Timing belt
Tensioners
Water Pump (no plastic impellers!)
Fuel pump and filter
O2 upstream sensors

Because if any one of these fail out on the open road somewhere, well your day's going to get interesting.
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Old 07-21-2018, 11:51 AM
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I am also trying to deal with everything that might leave my S8 stranded. In 100K my A8 only stranded me once and I was 5 blocks from home and parked legally (fuel pump).
The serpentine belt tensioner will never make it to 300K (often not even 200K), you can fit a Hyundai 2.7 wheel to the Audi tensioner assembly though. The idler will also need to be replaced or repacked. Fan support bearing, repack or replace.
Also get to 300K I'd also expect to do the cam chain tensioners and for an S8 the valve springs. The tensioners should warn you but the valvesprings not so much.
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Old 07-21-2018, 12:46 PM
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The car has 160K on the clock.

As far as the fuel filter goes I don't believe its ever been done. So I plan on doing that as well as the pump.

Is the fuel filter located under the car near the tank? Do I need any special tools to do it or just some flare wrenches?
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Old 07-21-2018, 12:56 PM
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under rear passenger,get new oem washers from a dealer because old will leak(i was asked for a vin number to get washers from the dealer),jiffy lube charged me 75$ to replace it,i brought the parts.
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Old 07-21-2018, 01:23 PM
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Yep, passenger side-- in front of rear wheel well. Banjo bolts and crush washers-- nothing exotic. There's a video on Y/T but the guy's a future organ donor, doing it with his rear wheel off and without jack stands! Removing the rear wheel isn't necessary. The filter just lives behind a cover panel down there.

for stuff like this I just like elevating the car on sturdy service ramps. With a couple of jackstands-- just in case.

If you are into masochism, and still wondering about your fuel pump-- start to do the fuel filter, and after you crack the input fitting but BEFORE detaching the fitting entirely-- have someone turn the key to ON, while you watch for a spurt of fuel out of that input fitting. The pump should generate 4-5+ Bar of pressure and you should get some good fuel ejection. Seeing NO observable squirt is a pretty good indicator of your fuel pump being near death.
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Old 07-21-2018, 02:47 PM
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The crush washers are common sizes, 12mm and 14mm ID. Usually OK to re-use, you could re-anneal them first if you want. Napa should have them, motorcycle shop, etc. Amazon has them too.
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Old 07-21-2018, 04:43 PM
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Video on replacing fuel filter.


Good luck.
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Old 07-21-2018, 05:12 PM
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PSA: And please use a jackstand!
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Old 07-22-2018, 05:17 AM
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I replaced the S8 fuel pump last night, it was working fine at 110K but I am staying ahead of stuff like this.
A few quick notes:
It'd never been done before, clearly all undisturbed under there yet there were already cracks in the top of the assembly, I guess from the pressure cycles. I finished with some spray sealant and a generous dollop of RTV over the long bolt head and the cracked area.

In the past I've tried to start the car with the electrical plug pulled to de-pressurize the fuel system (run the injectors w/o a pump) and it did not seem to work, I got a good spray of gas when I cracked the banjo bolt (please loosen this and the long bolt with the big outer ring still tight). This time I left the car sit for a couple days and the pressure was down.

Getting gas on the body coating in this area, sound deadener I think, makes for a sticky brown goop and runny brown liquid that gets all over everything. This will be important when you lift out the assembly and maybe even the level sensor.

You'll have two oeticker clamps to replace, one at the pump and one on the return line. On the pump it's good to use a fuel injection clamp or oetiker if you have it (I had none in that size on hand). I used a worm clamp on the return.

The cup/strainer that holds the fuel pump has to key into the outer housing and drop in all the way or you will not be able to get the assembly back together, it does not really feel like it is keying in (like the pump-to-cup does), you cannot easily rotate and feel it drop but it has to be keyed right or it'll be too proud and you will not be able to snap the big tabs back into place. That's never easy but if it's impossible the inner cup is not in all the way. WD-40 on the inner cup to outer cup helps. Or petroleum jelly might be smarter seal-wise (some say WD is bad for O rings but unlike some types of grease or carb cleaner I have not observed this to be the case) Anyway there is no O ring there.

I put the assembly in w/o the big floppy seal and get it to the point where it'll pop up and down but not turn, where the little dingus that feeds the scavenge system is all lined up and able to insert. Then I lift the assembly up and work the big seal in around it. Some wd-40 and patience and it eventually can fit with all the looseness tucked in. Feels like no way that'll happen but eventually it's all in there and when you are close some pressing down with a small prybar braced on the car body opening will pop it in satisfyingly.

If the assembly is not flat wrt the tank top and car body, it's not right. If the feed line banjo bolt does not thread easily, the assembly is not flat, it's too proud, it's not right.

Eventually you can start the car with the level sensor still out and then get a light and an eye over the opening and see the scavenge system roiling the fuel. Or if it was dry it'll be bubbling out air for a while but that should stop. If the tank is pretty full you might not see the fuel movement, I was at 1/4 tank and it was obvious.
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