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Ok, I have so far been tracing a misfire that originated in the LPG installation through equipment fault, the installer is currently working diligently on trying to resolve the problem, but has hit a bric wall with a phantom problem.

So far, this is what has been undertaken to trace theropt of the rough running, Which incidentally appeared at first to have cleared then materialized after about 15 miles, all on petrol! Put it on vag-com and no error codes at all... Very bizarre, especially as I know it works because it diagnosed two problems which kicked this whole episode off...

LPG system completely disconnected
All Audi wiring checked
All petrol injector inputs and outputs checked with oscilloscope and
ohmmeter waveforms all good
All coil inputs and outputs waveforms checked
Conducted cat test
Removed all spark plus and refitted after checking ok
Performed compression test all cylinders even
Performed cylinder leak down test
Checked again no error codes in Audi ECU
Replaced air flow meter
Checked for air leaks
Live data record shows closed loop fuelling and fuel trims all good

This car should run perfectly but it doesn't!! Can anyone please offer any suggestions?

Thanks in advance!

Model? Year?

What engine rpm range?

Well spotted! Audi s4 B6 4.2 V8

This is on idle even...

Ok, turns out I have blocked cats... So what should I do? Car has passed it's MOT not long ago, and I plan on getting a full system made by AAS next year, including sports cats, but for now I need to get the car running again.

So, shall I rip the contents out of the cats to unblock them? I know that will mean an MOT failure in theory, it will react just like a decat I guess, then when I get the full system next summer I can go up in exhaust width, get sport cats and but a non resonated system.

Any thoughts?

Blocked with what? Sounds unusual. Gutting the cats can be quite a big job - you have to get all of the catalyst out or anything left will rattle and make it sound rougher than ever. If you really want to do this then start with the precats which shouldn't fail the MOT.

Have you checked the fuel trims (block 32 IIRC)? Lambda sensors can fail without throwing a code but you should be able to pick this up from the O2 trims going awry. Quickest way to test this is to clear engine DTCs and see if she runs more smoothly.

hey, thanks for the feedback! we actually got to the bottom of the problem by replacing all the coil packs on one bank, voila! car runs really strongly now.

Because we had everything in bits, we gutted the pre-cats anyway and the difference is amazing. its a really easy job to do if you can put the car on a ramp and go from there. reassembling the downpipes was a pain though as the sleeves started to blow, took two more attempts to solve that but she's perfect now.

I cant wait to get the cats back replaced with a non-resonated setup and get her dynoed! she was making 366bhp before the pre-cats gutting.

I am finally happy with the car and the LPG

lee_whitehead wrote:I cant wait to get the cats back replaced with a non-resonated setup and get her dynoed! she was making 366bhp before the pre-cats gutting.

Don't be surprised if the power output is lower... the reduction in back pressure from a freer flowing exhaust can often result in a loss of power, until you have a custom remap done.