Heres the deal, about a week ago my 03 A4 1.8t had a problem with it, and i did a piston isolation procedure only to find it was running on 2 cylinders and the timing was all off. I had the dealer come pick it up and fix the coil pack and such. I got the car back and there was no problems, but the next day a check engine light came on. The car is running great, but that light worries me. When i hit the little diagnostic button on the dashboard it says everythings ok also. Im not sure what to make of this problem, from what i can see, nothing is wrong. also, unfortunatly theres more:-(, my miles/gallon reading is doing crazy stuff like going from 19.2 mpg to 180 mpg back to 22.5 mpg and just hitting random numbers, then it will clear itself with this "--- mpg" and then it will start doing the random number thing again...if anyone can help either of these situations it would be much appreciated

 

if u put in neutral or hold the clutch or let the car iddle at a high speed the miles per gallon will increase of course.. if it goes past 200 mpg is shows "---mpg" it is normal...ur car is automatic right???

 

yeah, its an automatic...anything about the check engine light?

 

You're going to have to get the car scanned for codes. If the CEL is on you will have some codes stored in the ECU. Get the codes and report back. Use VAG-COM or get a local autoparts store to scan the car for you with a generic code reader. AUtozone and Advance Autoparts will scan your car for free.

 

So you did a cyl isolation test and found two dead cyls-OK no mystery there

But how exactly did you determine that the timing was all off????

And the MPG has two part one is AVG and one is instant (well kinda instant)

 

i got a code that said catalyst efficiency is below threshold...im pretty sure that its from the previous problem and is tripped on, is there a way to turn the light off manually?

 

sounds like your looking at the "current mpg" not the average. The current will change constantly based on your current RPMs etc. Toggle through the options until you see the average MPG.

bah, should have read the whole thread

 

well it is quite possible you may have damaged the catalytic converter, I am very well trained in driveability issues such as what you had experinced, the problem is when cylinders are cut like that it will result in a catalytic damaging misfire condition, which is where unburnt fuel is dumped out the exhaust and into the cataytic converter, now normally the computer should see this is happening and cause a FLASHING cel to indicate catalyst damage is happening and indicate which cylinder is misfiring at that moment (flashing CEL always means catalyst damage is occuring, contrary to popular belief where people say it means your engine is about to blow up or something it does not neccesarily mean that, just means cataytic damage is occuring) now it is possible that this problem is what is referred to as a SOFT code, at which point the computer is going to recheck time and time to see what is going on until it determines it is an ongoing issue then it will turn into what is referred to as a HARD code and illuminate the CEL light on your dash. now as to the problem your having now, your car has 2 o2 sensors per bank on the engine , pre cat and post cat, the purpose of the pre cataytic sensors are to control the emmsions of the vehicle, basically what happens is the fuel mixture is switched between running rich and lean about 5 times per second, this provides a balance for carbon monoxide, hydrocarbons and ground level ozone (when car runs rich it will produce hydrocarbons and carbon monoxide but very low ground level ozone which is referred to as NOX, but when it runs lean it produces virtually no hydrocarbons or carbon monoxide but lots of ground level ozone so that is how your motor strikes balance before it passes exhaust into catalytic converter) now the point of the post cat o2 sensors is to check to make sure the catalytic converter is cleaning up the exhaust properly if it notices that the exhaust is not cleaning up and is out of the threshold of reference voltage it is set to in the powertrain control module, it will throw that code you have right there which is catalys efficency below threshold, now i know this post is very technical but i feel this stuff helps people out to know how things work in there vehicle, what i would do from here is clear out the code, and then drive the vehicle about 50-75 miles to set your readiness codes again if it pops back up your catalytic converter is shot and needs to be replaced.