ct_esq

causes bad cats?

Witt_4rings

Well I know too much oil would damage the cats. Maybe a bad air/fuel ratio. Reason I ask I because on start up there Is a slight rattle, found out it was one of the cats making the noise.

tenspeed

Try a new air filter. Air up the tires a few psi over the recommended pressure.

According to Fuelly.com, you should be seeing MPG's in the low 20's.

http://www.fuelly.com/car/audi/a4/gas%20v6

ct_esq

I'm not convinced that a rattle on startup is the cats. for a cat to rattle you need to have some of the honeycomb material dislodge within the cat, and that rarely occurs as that is a complete failure of the cat. you would see it glowing as the dislodged material would block the exhaust gas.

after having drive an 03 a4 3.0 6mt more than 200k miles, there are a number of simultaneous things that cause problems on the 3.0 that lead to bad mileage, failing cats, and other issues.

they are, in part, a combo of design and intermittent failures. let me review: 1 - crackcase breathing system must be carefully maintained. proper and timely oil changes with proper oil keep the sludge out and the system working. most gasket seal leaks are the result of breather problems. 2 - coil packs. any a4 3.0 in the b6 chassis started with crappy coil packs, had crappy ones replaced under warranty, and crappy ones installed after 100 or 120k miles. failing coil packs result in unburnt fuel into the exhuast. this equals dead cats. 3 - the cats for the 3.0 just sucked to begin with. they were manufactured with many failing well before 80k miles, the federal warranty. so separate and apart from fuel in the mix, this could be the culprit too. 4 - the 02 sensors are easily fouled and last 100k max.

all of these could be working together or separately in your case. But again, I doubt the rattle is the cat. If anything, it is a loose heat shield or ticking cam chain tensioner in the 3.0. I'm guessing the rattle/ticking goes away after the engine is warm.

anyway, I would have gotten another 100k out of my a4 except i rear ended a car at an on-ramp. repairs = the book. so now I drive an allroad. I miss the manual, for sure.

Prospeeder

Id like to see this magical cam chain tensioner.....since you know, it doesnt HAVE a timing chain. Theres a oil pump drive chain, and it has a spring loaded tensioner for chains no the 3.0.

Speedy72t

Just for comparison I just wrecked our C5 Avant 3.0 tip. As much as we love the car we did not replace it with another C5 because the gas mileage was atrocious. 14.7 was the highest we saw in 3.5 years of ownership. The car was fine, new coilpacks, low mileage, clean motor. Now that's all city and a fair amount of idling but it's still horrible.

I understand different car, heavier, all that stuff. But in my experience the 3.0 is a pig. When I asked about it I got a lot of people with similar stories, although none quite as horrible as mine. 6MT would have made a big difference I'm sure. The tip was always hunting for the right gear on the little hills around here.

lyleswk

Probably cam adjustor which I assume the 3.0 has. Everyone seems to keep calling it a "tensioner" for some reason.

shwndh

I would start with tracing all the vac lines and the PVC pancake thing on the top of the engine. See if the engine is breathing right. Also, when you start the engine, see if the intake drive on the front of the engine moves to a compressed position.

Here's a pic below. Those arms circled in red should get sucked in by vacuum. If they don't, you got a vac leak in the front.

Witt_4rings

Looks like I have a few things to consider and try to tackle this weekend. Thanks for all the input guys!

Witt_4rings

I don't know if this is okay but the arm on the right is broke....but it still gets sucked inm