Hi Everyone,

I'm hoping someone can point me in the right direction- there are not any Audi experts close by, and I don't want an outrageous towing bill on top of the repairs! My dad is very good with mechanical things, knows his way around American cars, but not so much foreign ones. If someone could point us in the right direction, he could try some things and possibly fix it- he doesn't know where to start with diagnosing it.

ANYWAY, it's a 2005 Audi S4, 4.2 V8. I started it yesterday, and hit the gas, and it proceeded to do nothing for a few seconds, then lunged forward, the RPMs dropped again, and then it lunged forward again. It continued to do this across a parking lot, and I stopped. I had it towed a short ways to a local mechanic. The check engine light did not come on. There are 2 cylinders not firing, right next to each other. They swapped the coils around, those are fine, and the spark plugs are fine as well. I did not get the cylinder numbers, but I can call tomorrow and find out.

What the heck could it be? I've found some similar issues mentioned online. One thing that my soon to be ex husband mentioned was that there is apparently some kind of chip in the MAF, and he noticed something was broken, and he thinks that chip may have fallen out, and could be causing my problems.

Similar cases I've seen online turned out to the MAF, O2 sensors, fuel filter, fuel pump, and ICM. Ignition related, a few times recently in the very cold weather, I've turned the ignition to start it, and it would take a good 5-10 seconds for it to actually start. The lights would all turn on, but the engine wouldn't respond right way. Not sure if this is related.

However, this mechanic (not super experienced with Audi) believes that the cylinder issues would not be happening if it were something listed above. He suggested a head gasket.... which about made me faint.


Any help would be appreciated- either possible culprits or to rule out the head gasket thing. I don't know what it costs to change a head gasket in this thing, but I assume it's not pretty. I've only had the car about 10 months, so this is my first real issue with it. Thank you to anyone who is willing to help me out!

 

You can disconnect the MAF sensor and see if it runs better. The ECU can run the engine with the O2 and other sensors just fine. The MAF gives the ECU more accurate info on the air intake flow but if it's sending crap information, the ECU can do wild things to get the engine to run including misfires and bucking... It could also have glazed a spark plug or two in the process.

 

Thank you for the feedback- I will definitely give this a try. I saw the MAF mentioned a lot, but the mechanic thought it was a ridiculous idea. I know more about cars than the average woman, but I'm by no means an expert, so I didn't know if he was right and it was a total dead end. It's good to know it's still a possibility, so thanks again!

I'll be sure to update as I go.

 

Believe it or not, it really does work. I determined a bad MAF on my A8 like that. I thought it was crazy but once you think about it, it makes sense. The programming in more recent ECU's is pretty complex and allows for failure of some sensors to not cause a complete no-run condition. Crank and cam sensors that may fail will cause a no-run condition since those two are critical for timing and ignition. The MAF just determines how much air is flowing into the intake due to opening or closing the throttle. Should the MAF stop sending information, the ECU can use the O2 sensors to determine how much fuel to add to the combustion chamber. It's not as efficient as there is a slight delay in the sensor reading since the reading can only take place after the combustion process but it's not any less effective than, a carburetor. You just lose the more efficient fuel delivery which means fuel economy and better emissions. It will trigger the obligatory Check Engine Light and a couple of other warning lights (I forget exactly which ones) on the instrument panel but it'll run perfectly fine. MAF sensors are about $200 but they're easy to swap out because they are either in the air filter box or located in the plastic intake trunking just after the air filter box.

 

So! Long time for an update. There is basically no one in the area that knows how to work on my car. The car has been moved to someone that has some idea what they're doing, finally.

MAF sensor is working. Cylinders 3 & 4 are showing as misfiring. Again, moving coils didn't change which cylinders were messing up. On a compression test, these cylinders have lower compression than the rest.

My dad is worried about timing belt. I have no idea if it was ever changed, I got the car less than a year ago with 130k on it.

Any new ideas or feedback is always aporeciated. Thank you guys for taking the time to reply to me.

 

Low compression on those two cylinders will cause a misfire. It's possible there could be broken piston rings or slightly bent valves on those two cylinders. BUT. It also could be something as simple as come carbon build up on the valves causing it not to seat fully and cause a slight leak. This can result in a burnt valve of not resolved. Cleaning that carbon is not going to be easy with it misfiring like that. So running a can of Sea Foam through the intake while it's running may not be an option. You can at least try to determine whether the valves are leaking by pumping up each of those cylinders with compressed air. This will push the piston to BTC and the valves on that cylinder should be fully closed at that point. If you hear air leaking back in the intake, the intake valves are not fully seating (bent or just carbon deposits). If you hear air leaking out the exhaust, it's the exhaust valves. You will hear some air leakage regardless because air will slip by the rings because they are only to provide a momentary seal on the compression stroke. Plus there are small gaps (ring gap) that will allow a little air to pass by. A broken ring will cause a motor to smoke more than a misfire.

Anyway, I hope you get it sorted out. If you do and it turns out to be something minor, DO get that timing belt replaced and have the water pump replaced at the same time. An extra $50 is a small price to pay because you're already in there with the timing belt job. If the water pump were to fail at some point after the timing belt is replaced, you have to do all that work ($$$$) again..

Keep us posted on how it works out. Good, bad, or ugly..

 

It took so long to figure out what was wrong with the car, I forget about this forum at first. I figured I'd at least give a conclusion to what we FINALLY found wrong!

The MAF sensor was pretty beat up, it may have contributed to the problem but it was not the primary issue (I got a new one on anyway).

Apparently- and this is just what I was told, so don't mind me if I'm off on this- my car has two gas tanks, and the gas equals itself out in order to keep the weight equal. The tube (or whatever it is) that connects the two tanks somehow got disconnected/screwed up, and it blew one of the fuel pumps. New fuel pump, and we're finally good to go.

That's a very condensed version of all the troubleshooting that went into it. I think it took a month to get the thing fixed, and $1500 (for both the MAF sensor and fuel pump, and a lot of diagnosis).

The only set back is that the car won't let me fill it past 3/4 of a tank. When it gets up to that point, I can't pump any more in. I'm hoping I can just deal with this minor annoyance, and it isn't anything that's going to cause big problems. Oh, the low fuel light also won't shut off (I assume there's just some switch that needs to be re-set, ah well!).

Thanks for all the help some of you provided. Hopefully this post will help someone else out in the future as well! Now to fix all the other 100 little things going wrong (window switch, wipers, stereo, wonky starting sometimes). But I guess I should have expected this when I bought it, eh?

 

Is the gas level indicator working? The sending unit for the gas has a mechanical contact at the "reserve" point and does not lie. Unless it's dirty or corroded. If you had issues with the in-tank fuel pump, somebody has bound to have disturbed the gas/fuel level indicator.