The Simps

2004 UK spec 4.2 V8 Sport also running a high quality LPG set up.

I had a problem earlier in the year of when the car was warming up and switching over to the LPG system it was sometimes causing a misfire causing the ecu light to flash. If I was to switch off the LPG at that point and go back to petrol the misfire almost instantly clears and all is well. Except ecu light remains on!

The lpg was due a service anyway so I had that done and got it all looked at. The cars ecu came back with errors of:

17819 - Secondary Air Injection System Bank 2 - P1411 - 002 - Insufficient flow
17831 - Secondary Air Injection System Bank 1 - P1423 - 002 - Insufficient flow
17536 - Fuel Trim; Bank 1 (Mult) - P1128 - 001 - System too Lean - Intermittent
16684 - Random/Multiple Cylinder Misfire Detected - P0300 - 001 - Intermittent
16689 - Cylinder 5 - P0305 - 001 - Misfire Detected - Intermittent
16690 - Cylinder 6 - P0306 - 001 - Misfire Detected - Intermittent

The LPG ecu was recording a fault regarding the Coolant Temp Sensor.

The engineer who is a general mechanic and lpg specialist (Audi don't deal with LPG), advised that the cars CTS was the issue and in the meantime raised the kick in point of the LPG system by 5deg C and advised to have the CTS investigated.

This raising of the kick in point and the service stopped the misfire on warm up. I did however take the car to my Audi specialist to look at the fault codes further. He checked the Secondary Air Pump and confirmed it was all working as normal. He tested the air lines and they flowed as normal. He physically checked the air lines for leaks with the exception of the section going under the manifold as it require pulling all the inlet off which was added £££s for potentially nothing. He wasn't concerned by the CTS fault as the ecu wasn't picking anything up, the car was warming and running temp absolutely fine so no reason to suspect any issues. So as the car ran fine we left all be. Except for ecu light coming and staying on

So several months of happy motoring getting used to seeing an ecu light I notice a light coolant leak coming from the left hand side of the engine bay. It was only minor and I was just topping up the level as needed until I had time to investigate. Coincidently, and it may just be coincidence, but the misfire returned when on LPG. This time however it wasn't from cold it was only when starting the car when warm, but not running temp, it would occasionally happen.The idle speed generally now under petrol doesn't seem too smooth to me either but I may just be overthinking it!

On investigating the leak it is coming from the LPG vaporiser of which now I understand the LPG system works by using the cars water system to warm the LPG liquid into gas form. On this vaporiser is also a sender of some regard that I can only assume is coolant temp sensor.

The car is booked in a few weeks (I'm away from next week for 12 days) with the LPG specialist to fix the leak and investigate the misfire under LPG further. My thoughts are that the CTS for the LPG on the vaporiser is infact faulty causing the misfire but would it also cause problems with the secondary air pump issue??

I would like to get your thoughts and experience on the Secondary Air Pump issue as to what you think it might be and where to go from here....

Thanks in advance from an otherwise happy A8 owner

MN-A8

I don't know much out LPG systems but I'll try to help out the with SAI codes.

Did your mechanic verify the vacuum solenoid and combination (Combi/Kombi) valves are operating normally? The SAI will only be able to pump air into the side of the heads when the combi valve opens. If the solenoid is not working, the valve will not open. Based on previous VW/Audi engine experience, the pump is less likely to fail than the solenoid or valve.

Sometimes the valve can get "stuck". See the below pictures for a dirty combi valve.

The Simps

Thanks for taking the time to reply.

I will have to check with him when it goes in for service next month. Are they parts I can easily test? What about replacement parts cost? OEM only option?

The Simps

Just to bump this thread...

How do you test the secondary air pump solenoids? Is it something only a Audi dealer can do?

MP4.2+6.0

every time I have personally seen those codes (on my C5 4.2) or most times I have read posts about them, it usually ends up being the vacuum lines related to the air injection system, rather than anything about the valves themselves/motors/solenoids, etc. Thus, I would review all the vacuum lines. There is an underhood vacuum schematic label/sticker to guide you, though I have no idea if your LPG system might have affected this layout.

From my C5 4.2 experience (and if I remember right, the D3 W12 too), you can test the relevant stuff from VAG COM. Don't remember the specifics, but look under the ECU and start poking around the self test routines.

The Simps

Thanks for your reply.

The garage checked the pump and lines when the fault first appeared. They stopped checking getting close to the inlet telling me that to inspect a section of the pipework will require 4-5hrs labour stripping parts etc. but they had checked either end and flowed ok. So i kinda wanted to check the solenoids to rule them out. I'll see whats on vagcom.

MN-A8

You could also try the emissions readiness test. This might go through the SAI system and check its functions.