BinsuJiro

I recently had the flex joints damaged on my A6 by road debris left on the highway. The Avant was running fine before that, but now all of the expected trouble codes are being generated by the separated cats.

I'm sure the cats are still good, and I am exploring the possibility of having V-flanges welded on the pipes and also to a set of stainless steel flex joints.

In the mean time does anyone know if the differential in the readings between the front cats and the rear cats would be the cause of warm/hot starting problems. The issue does seem to be temperature related; it will state after the engine cools to a certain point and seems to run fine once started.

Also, does anyone have any advise for adjusting the computer settings to accommodate this condition until the work can be done ... without increasing emissions or damaging the components.

Thanks in advance to anyone with helpful information

Kris Hansen

Hot starting problems usually have more to do with the coolant temp sensor.. but you now have a large exhaust leak by the sounds, and it's possible that is throwing the primary O2 sensors off enough that they are screwing up the A/F mix... I'd suggest getting the flexes replaced, and see how it goes.

BinsuJiro

Kris, thank you for the information. I was suspecting that the differential in the O2 sensor readings might be part of the problem, and I definitely will check the COOLANT TEMPERATURE SENSOR, but I recently took a look at the vacuum hoses and found that they were not in very good condition.

So, I may have enough of an accumulative vacuum leak to cause stalling when trying to start a hot engine and to stall the engine at low RPMs when driving in very slow traffic. That combined with the possibility of a bad COOLANT TEMPERATURE SENSOR might be the cause of no starting after the engine stalls.

Then there are those pesky FLEX JOINTS; if they were only connected by stainless steel V-FLANGES, and thus made easy to replace, many hours and dollars would be saved on this very weak exhaust system design deficiency.

If you read this please let me know if you concur with my analysis, and thanks again for your initial response.

Vincent

Prospeeder

No the cats and all that work has nothing to do with it not starting. Crankshaft position sensor will fail hot but usually set a code. Fuel pumps can fail hot, as will fuel pump relay. Temp sensor, meh, i have yet to see one cause a no start, but long starts and rough starts yes. Nothing to do with oxygen sensors is going to cause any kind of startup issue. Oxygen sensor input is ignored untill the sensor has had time to heat up and give a reliable signal.

BinsuJiro

Prospeeder, thanks for the update on possible causes. I have gotten no code for the CRANKSHAFT POSITION SENSOR, but am wondering what function that depends on temperature and RPM could cause this problem.

Could the vacuum leak combined with a hot engine condition, an engine that was turned off after a prolonged period of use, prevent the car from starting until the engine cooled off?

And, could the vacuum leak cause a stall at low RPMs with a hot engine in very slow traffic?

I have documented the surging idle at around 900 to 1200 RPM which was associated with a rough idle and some near stalls which I would describe as a sort of buck or bucking.

I'm thinking that simply replacing the old vacuum lines might solve the problem. What do you think?

Thank you,
Vincent

Airbag

I think you have a air:fuel ratio issue a CPS will not differentiate between hot and cold satrt , unless of cours you had a minor crack that may change with expanding aluminum , but its a long shot.

I have been teacjing troublshooting for ovr 25 years and i always give the same advice in every course i give.
In large complex systems where everthing is interrelated, always fix what you see , before you fix what you cant see.
What this basically means is that if you see obvious things wrong fix these first , dont look past these for soemthing more complex.
Fix your vacuum hoses on the top end . Fix your flex hoses on the botom end , then lets see where you are.

I cant remember if there is a measuring block for fuel pressure in VAG COM for our cars. But i would look at that too

Prospeeder

Just so you know, almost every crankshaft sensor i have replaced the car will start cold, and die hot or not start after a hot soak after driving. The temperature does affect the CPS, I think its just resistance and heat go hand in hand, so more heat more = resistance until it makes too weak of a signal to start. Thats my theory anyway lol. But they always set fault codes on Audi and VW cars so this probably isnt the issue.

WHen it wont start you need to check for fuel and spark. Your flying blind if your not for sure it has both of those during the no start situation.

Airbag

that is interesting empiricle data on the sensor. op can check with a voltmeter on ac and look for a pulse. when mine died it was dead as a doornail.

clancy

I will first confess my ignorance. I am assuming the CPS is the same as the speed sensor. Had my speed sensor replaced a few months ago. It was a hot no start issue. I Ran a VCDS scan and got 16706 Engine speed sensor G28 P0322 no signal - intermittent. But not every time it failed... Was told by my tech to check for the engine RPM readout on VCDS (block 4 I think) when cranking. If the speed sensor was not bad the RPMs would show (200 to 400?) as the starter cranked and that meant I had another problem instead. If speed sensor was bad then there would not be a reading. Pretty diagnostic.

BinsuJiro

Lots of great advice from a number of contributors has been given on this thread. Thank you for sharing your thoughts and suggestions. I was planning on replacing the vacuum hoses first, because there are clear indications of a leak, and then move on to the other suggested problems.