Fuel pump problem?
09-24-2013, 04:16 PM
#31
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Join Date: Oct 2007
Location: Denver, Colorado
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Just a few reasons,
Full size spare tire, maximum trunk & backseat leg room squishes tank into horizontal and vertical layout separated by tire.
Tank is weird 2 sided shape that has to get fuel from one side to the other all the time to maintain center of gravity.
When low on fuel tank needs to be able to get fuel from lower part of tank to main parts of tank where fuel pumps are to get last drop.
Sporting quick changing motions of the car need to be able to maintain fuel supply in strangely shaped tank.
Large variance in fuel consumption due to efficiency of A8 at normal driving and high fuel demand needed in massive acceleration with secondary air chamber in intake manifold opened. Probably difficult to maintain constant pressure with changing velocity in fuel supply.
And regarding changing the pumps, it is not that hard, but it is awful with the fumes. I recommend a respirator with organic cartridges. And there is the fire hazard.
But one reason to do it preventatively and one reason only: when the pump goes out, you will be stranded. And it will go out. If you are OK with that don't sweat it.
Full size spare tire, maximum trunk & backseat leg room squishes tank into horizontal and vertical layout separated by tire.
Tank is weird 2 sided shape that has to get fuel from one side to the other all the time to maintain center of gravity.
When low on fuel tank needs to be able to get fuel from lower part of tank to main parts of tank where fuel pumps are to get last drop.
Sporting quick changing motions of the car need to be able to maintain fuel supply in strangely shaped tank.
Large variance in fuel consumption due to efficiency of A8 at normal driving and high fuel demand needed in massive acceleration with secondary air chamber in intake manifold opened. Probably difficult to maintain constant pressure with changing velocity in fuel supply.
And regarding changing the pumps, it is not that hard, but it is awful with the fumes. I recommend a respirator with organic cartridges. And there is the fire hazard.
But one reason to do it preventatively and one reason only: when the pump goes out, you will be stranded. And it will go out. If you are OK with that don't sweat it.
11-08-2013, 09:09 PM
#35
Hello guys. Based on this thread and others I believe at the very least I need to change fuel filter and possibly left pump. My symptoms are that I have about a 5 second crank before car starts if it's been sitting for longer than an hour. Would it be a dirty filter that is slowing down the pressure needed to build at initial crank. Most importantly, where is the fuel filter so that I can try changing that first. Your help is appreciated. Thank you!
11-08-2013, 11:04 PM
#36
AudiWorld Super User
Fuel filter is just in front of the right rear wheel. It can't be the reason for longer cranking. Left pump may be but it should make some problems on high load (rpm). Check the spark plugs too.
11-09-2013, 04:32 AM
#37
Thanks Mishar. From the post I've read, longer crank is also a symptom bc it takes slightly longer to build enough fuel pressure due to partially clogged filter. Just to make sure I understand your reply. Bad fuel filter will cause issues at high load or bad left pump? I believe I've noticed a slight hesitation at high rpm recently. That's only because I don't really get on it much.
11-09-2013, 10:24 AM
#38
AudiWorld Super User
Replacing fuel filter is always good, but if it cause slower pressure buildup it would choke it totally on high load (rpm).
Left pump kicks in for 30 seconds on stat and above certain(?) rpm. You can check if it is working by disconnecting the right one. It should start but work just 30 seconds and die.
Left pump kicks in for 30 seconds on stat and above certain(?) rpm. You can check if it is working by disconnecting the right one. It should start but work just 30 seconds and die.
11-09-2013, 12:28 PM
#39
AudiWorld Member
Ayrula,
I agree with Mishar that a dirty filter will not cause the extra long crank. Dirty filter just causes pumps to work harder and wear out faster. The pumps have a pre-filter at the bottom of each one that sits at the bottom of the tank. The pre-filters block any large particles in the tank from entering the system and the main filter stops the really small particles. Thus a dirty fuel filter still allows flow, that flow is just more restrictive than it needs to be and makes the pumps work harder. The pumps, if good, will still supply the required pressure to the engine.
It is still a good idea to replace the filter always, but not likely the cause of your extended cranking problem.
Failing pumps could be the cause. When mine were failing, there was an extended crank condition. However in my case I did not notice an extended crank until after I first noticed the car stalling because the main (right) fuel pump had failed out during driving. Upon restart there was an extended crank because the system had dried up under normal driving with only the main (right) pump being demanded. Upon restart, in that condition, the secondary transfer (left) pump is repressurizing the system for the first 30 seconds of startup. With intermittent failure of the main (right) pump I believe this kickstarts the system to get it running again. But if that is the case it is only a short matter of time until the main (right) pump fails completely and your car will not run at all.
This may not be your problem, as Mishar suggest, could be spark or an electrical side related issue. If you seriously suspect fuel is your problem, I recommend removing the interior of the trunk finishing to expose your pump covers. Remove the pump covers, then listen carefully upon startup with your head in the trunk to check the run condition of your pumps. Your main (right) pump should run 100% of the time under all conditions of engine on. Your secondary transfer (left) pump should run for 30 seconds at startup and when fuel is demanded above 3000 rpms.
I agree with Mishar that a dirty filter will not cause the extra long crank. Dirty filter just causes pumps to work harder and wear out faster. The pumps have a pre-filter at the bottom of each one that sits at the bottom of the tank. The pre-filters block any large particles in the tank from entering the system and the main filter stops the really small particles. Thus a dirty fuel filter still allows flow, that flow is just more restrictive than it needs to be and makes the pumps work harder. The pumps, if good, will still supply the required pressure to the engine.
It is still a good idea to replace the filter always, but not likely the cause of your extended cranking problem.
Failing pumps could be the cause. When mine were failing, there was an extended crank condition. However in my case I did not notice an extended crank until after I first noticed the car stalling because the main (right) fuel pump had failed out during driving. Upon restart there was an extended crank because the system had dried up under normal driving with only the main (right) pump being demanded. Upon restart, in that condition, the secondary transfer (left) pump is repressurizing the system for the first 30 seconds of startup. With intermittent failure of the main (right) pump I believe this kickstarts the system to get it running again. But if that is the case it is only a short matter of time until the main (right) pump fails completely and your car will not run at all.
This may not be your problem, as Mishar suggest, could be spark or an electrical side related issue. If you seriously suspect fuel is your problem, I recommend removing the interior of the trunk finishing to expose your pump covers. Remove the pump covers, then listen carefully upon startup with your head in the trunk to check the run condition of your pumps. Your main (right) pump should run 100% of the time under all conditions of engine on. Your secondary transfer (left) pump should run for 30 seconds at startup and when fuel is demanded above 3000 rpms.