Car has 100,000 miles on it, almost stock, K&N filter, like any other A4 I'm on the 2.73 gears. changed spark plugs, battery, and the catalytic converter recently. It runs just fine, but the gas mileage is just pitiful. Even with coasting to redlights with engine shut off and going only 55mph on the freeway, it still barely manages 14mpg combined city and highway.

Also kind of a rough shake at idle and in 1700-2000rpm in higher gears. When I wanna over take someone at those rpms the car starts to shake very bad and when I'm past those rpms it goes smooth again.

I thought i just have to live with it because its a V8 but people are getting over 27mpg with their LT1s. Now this car is meant to get 22mpg as suggested by EPA, but that's just impossible.

Some suggestions I got: PCM flash (AFR edit to run leaner), PCV valve swap, Amsoil, and gas pills. I'd appreciate any explanation on any of these, and whether you'd recommend them or not. Or anything else that you'd recommend besides these. Thanks.

 

why would you shut the engine off to coast to red lights ? starting causes the most ware to an engine

 

are u getting any codes? ive got kind of the same issue, changing the 02 sensors helped for a short time and now i get a bad prom code sometimes, from a new prom. so got another ecu/ecm coming. ill let u know if it works for me. i would say look at the tps, iac, 02 sensors, map/maf whichever u have, and injectors and if they are getting the right pulse from the ecu/ecm. the last one is whats wrong with mine. gets a semi steady volt on two injectors. the rest pulse with rhythm

 

Check the easy things first: Replace plugs, wires and fuel filter. After that see if the problem persists...

 

Just changed plugs. I'll have to look into the other two. Would you recommend "low resistance" wires? Are they any good?

 

"Low Resistance" sounds like a bunch of crap to me. You can go with an 8mm wire (stock is 7mm) if you really want, but it's not really necessary in my opinion unless you're running the MSD opti which has a tendency to fire through the stock wires.

If you replaced the plugs, did all the tips look ok? No miscoloring (black or red?)...

 

The new ones had a silver-ish color, with a sharp tip. The old ones didn't have a tip at all. Looking and the condition of the old ones (that were in there since 1994) I thought the problem would be gone. But the problem remains.

The wires I'm looking at are these:

http://cgi.ebay.com/ebaymotors/MSD-3...Q5fAccessories

Also I got this from someone:

"You need to put oxygen sensors extenders before the catalytic converters in your system. Otherwise, the computer will work against you and add more fuel than it would have before. You have to fool the computer to think that it is experiencing a good burn. Also consider changing MAP sensor feedback."

Is this bogus or should I look into it?

 

Where do you find these people?!

You have 2 O2 sensors before your cats that your computer uses along with the MAF sensor to determine the correct mixture. You only have 2 O2 sensors on a 94, 1996 and later have 4, 2 before and 2 after the cats, but you don't need to worry about that. Also, changing anything with the MAP sensor isn't going to do anything since the car uses the MAF after it enters closed loop mode. The MAP is only used before the car warms up and the O2 sensors are hot enough to moniter the mixture.

Take a picture of the spark plugs you removed if you still have them...

 

This is one of the old spark plugs:



And this would be the cat I put on it. There's only one.

 

Can you get us a picture of the plug from the top down (so we can see the coloring from the tip). From that angle it looks like it may be running a little rich or that plug wasn't firing for some reason.