2000 1.8t a4 b5 sudden loss of compression
 

yesterday, and a couple of weeks after a regular service job at the dealer, i had a sudden loss of compression while driving down the road doing about 40mph. there was no event, no noise, just suddenly bogged down and i limped it 2 blocks home at 10 mph with 4Krmp. I got home popped the hood and the Cat was flaming red hot under the heat shield. I checked all the things the shop would have touched and there was nothing disturbed, loose or missing.

After cooling down for an hour or so (me and the car), i started it, and there was either a high pressure sucking or blowing coming from under the hood near the firewall passenger side, I couldn't get out and identify it in time. this stopped after 10-15 seconds. it did the same thing this morning before I said screw it and rode my MC to work. any clue, i am buying a scanner tonight, but would like some insight so i am not throwing money at every code that pops up if all i need is a little Mystery Oil in my cylinders.

Loss of compression must be a regional expression. Anyway, sounds like a power loss, and with the cat red hot you are likely running rich, maybe even dumping raw gas into it. So I'd look for codes that indicate an ignition problem, like a misfire. Not sure what would have caused the noise, but the exhaust is there, and things got really hot, so if you are lucky maybe something popped open, then settled back into place. If not lucky maybe something cracked. I would fix the ignition problem, then wait and see if the noise came back.

thanks. I am sure code checking will direct me to the right spot, i was just hoping i had run into a common issue that. I am using VAG-Com so my scan should be fairly accurate and to the point

misfire codes
 

okay, i got the code PO302. swapped coilpacks out and cleared codes, then 301 came up, went and bought new CP's, plugs and i had a wire connector repair kit already. i swapped out the connectors and installed new parts. It ran fine for about 5 minutes in the driveway and then around the block 1 trip. I turned it off, cleared the codes, drove around the block and before i made my 4th corner the CEL started flashing. I got back in my drive with smoke coming from the base of the CAT where it meets the downpipe. CAT red hot inside the shield again. checked codes- again only says PO301. thats all. misfire cylinder 1. I pulled the coil to look at my connection and the side of the coil boot was cracked and oozing what looks like lava. new 45.00 coil pack!

is it possible my ICU is shot? My cat is less than a year old- is it possible that bad fuel mixture from running so rich could pack the cat so soon?. i am lost. the engine sounds good and strong, but no get up and go when in gear. turbo engages. everything seems fine except bogging down in gear.

and white hot CAT burning my engine compartment

^^This^^ I'm guessing your coil pack problem means a cylinder isn't firing and is dumping raw gas into the downpipe/CAT. Solve the misfire first, but you might have already sacrificed your CAT.

What kind of plugs did you buy? These cars are pretty picky about plugs. Too much resistance and you can kill a coil. Stay away from aftermarket, just guy what the book talks about.

Could be an ECM, but I'd get a Bentley and go through their tests for the grounding and wiring before I threw a new ECM at it.