Dr. A8

Hello all, I have a '99 A8 that I have been working on for the last several months. The list of things I did include:
R&R transmission
Timing belt, WP and all associated tensioners/idlers
All oil seals (engine)
Coolant pipe replacement (oil cooler)

I removed the engine and trans together and did most of the work out of the car. I also did remove the intake manifold because one of the rear coolant pipe o-rings had torn on install.

Now the car starts but will only run for about 30-60 seconds. It runs smooth and then just slowly sputters until it dies. I did have some trouble getting it to start before but that was because the starter signal wire was not connected to the correct terminal. As soon as I got that connected, I started it up and it produced multiple check engine lights for O2 sensors and injectors. I had forgotten to tighten the engine ground after working on the starter. After tightening it I have this current situation but no CEL's. Any ideas what might be causing it to die?

jfrahm

An easy test is to give it some starting fluid when it is dying, you can pull off a vacuum line to feed it. Sputtering sounds like fuel so maybe the fuel pump is dying or the filter is half-clogged.

Dr. A8

Thank you, I'm thinking fuel as well. I am going to go take a look and let you know.

Dr. A8

Ok so I sprayed carb cleaner in and the engine idled up. It was still pretty rough but definitely jumped up.

The car sat for several months after I bought it and probably for several more before. It has 1/3rd tank of gas. I did drive it home in transmission limp mode but it ran smooth. The problem started after I had it apart. This is why I don't think it's the fuel filter or bad fuel.

Do these cars have 2 fuel pumps like the D3 A8? If so, it's possible that the main pump isn't working. I can definitely hear a pump priming when I turn the key. Time to head back out there I guess.

jfrahm

It has one fuel pump and a byzantine system of fuel-powered venturi pumps that scavenge fuel out of the tank "feet" The pump is mounted somewhat high in the tank on the RH (passenger) side, if it's at 1/3 of a tank that's right when it runs out of gas if some muppet has installed the wrong fuel pump (lacking the jet pump outlet) or has bungled the installation of the correct pump.

If it's the scavenge system, you can put in 1/4 gallon of gas some will be directed to the fuel pump's collection cup and it''ll run great for a minute or two and then start dying out again.
-Joel.

Dr. A8

Joel, you were spot on, I added 3 gallons and the car runs perfectly now. Now I just need to finish filling the transmission and take it for a drive. This is the FUN code trans you sold me a while ago.

THANK YOU!

Dr. A8

Okay so now that we have that figured out, I've been driving it and there are a couple of issues.
1. Hard starting
2. While driving, it feels like the car suddenly loses power and quickly gains it back. You might mistake it for a sudden gust of wind or a bump but it is happening pretty often.

I wonder if both of these things might also be fuel related. Looks like I should dig into the fuel tank and see how bad the situation with the fuel pump is.

Any thoughts on this?

audinutt

I am thinking your fuel pump is dying

Dr. A8

That's definitely a possibility. I'm thinking that I have a leaky injector that is causing the hard starts. (It won't start at all now). It also ran very well even when revved high so fuel pump doesn't seem to fit quite right. I'm going to do some injector cleaning today and re-install. If that doesn't work, I'll pull the pump (needs to happen no matter what)

jfrahm

I had some long cranks when my A8 fuel pump was dying, but otherwise it ran fine once the pump woke up. It then actually died on the side of the road but later unstuck so I could drive it home. My S8 started sometimes taking a while to light also so I replaced the fuel pump at 107K or so. Since I replaced the pump that has never happened again. It ran great under full load, high RPM all that time also.
It was a CA car so it might have sat in traffic a lot of hours, running the pump. I too think you might have a dying fuel pump, if your fuel pump access panel and the plastic screw-down retaining ring show no signs of the pump having been replaced that'd be a clue that the pump is on it's last legs.