Misfire 4.2 BFM
 

Hello.

I was using my A8 to explore the length of Australia's east coast last week and I experienced what felt like extreme misfiring. The symptoms were:

Car made popping noises
Extreme loss of power
Had to rev the nuts off it to maintain speed.

These are most acute at highway speeds on light throttle when climbing a gradient.

It seemed that it was bad fuel at first because its never done it before and the car was driven many highway miles the previous day without issue. I parked the car and went to the beach, left it an hour or so and it drove fine.
The next day it was fine again until about the 3rd hour of highway.

It was 37 degrees C that day also.

The car actually misfired so badly the engine stopped twice.

The codes I got when I arrived home were:

17538 - fuel trim bank 2 (multi)
P1130 system too lean

17536 - fuel trim bank 1 (multi)
P1128 system too lean

No codes for coil packs or plugs and all have been tested and renewed in the last service.

Fuel pumps are ruled out as the car is fine after a restart (yes, I am aware that the secondary pump is activated after 3000 revs or a restart idle) but the car drives fine only until a few hours of highway.

I cleaned the MAF yesterday and I will take the car out on the highway again soon to test it.

Any other ideas?

Looking at codes and symptoms I would be wanting to fit fuel pressure gauges and watch what fuel pressure is doing while its faulting .

thanks Lee,

I have cleaned the MAF with special cleaner solvent and it seems to run much better (Like I told a very fat passenger to get out)

No more fault codes after either.

The thing that ruled out the fuel pumps for me was the fact it only happened after a long drive on the highway where the MAF would really take effect (light throttle openings or adjustments and reliance on torque). The car didn't have issues after restarting either.

I have to go on the highway soon so I shall monitor it.

Thanks for the reply

Mike

Submerged roller cell type fuel pumps can work ok at first then go off after hours of driving .........seen this loads of times , yes air mass can do similar faults too , if it re occurs you have to rule out fuel pressure with guages .

You can also rule out air mass by diss connecting the electrical connector ...this defaults the ecm to a 'middle of the road ' air mass map .

Ok, So after a few weeks of normal operation, I had to take the car on a long drive to collect a new alternator for another car in the fleet.

Since this drive was 3.5 hour round trip, it was long enough to cause the engine to misfire again.

I have discovered the following:

As long as the car is working for less than about 2 hours, it will behave no problem.
The codes it throws after that are always "system too lean banks 1 & 2"

The MAF has been cleaned and is in spec.

There is a slight hiss (not the suspension) that tapers when the car is turned off.

I believe a vacuum leak in conjunction with the EGR is allowing too much air into the intake and leaning out the system. The hiss would be a vacuum leak.

Has anyone had a similar one? I know the storage tanks are around the suspension compressor and could be involved in the hiss.

Mike

For an air leak to come on after more than 2 hours of driving and be that bad it spits and coughs and some times cuts out is a fair old air leak that would show its face much sooner I would of thought .
My money's on fuel pressure dropping off .

I don't think so. Fuel pressure would be dropping off regardless of how long the car is used. If I drive the car all day around the city it is totally fine. It fails specifically on highway work.

That and the hiss means I should hunt a vacuum leak I would have thought. There isn't much info on these having such leaks though.

I have had loads of cars where fuel pressure has been good for hours of driving only to show its face when the submerged roller cell type pumps have over heated and failed .

Or ....there is a low voltage or resistance issue in the wiring to the roller cell pump , relays ..wiring at sender unit on the tank and so on

I hear what your saying about the hiss but... you say it become almost undriveable and the engine cuts out and so on and you had to "rev the nuts of it to maintain speed " ...for this to happen the air leak would have to be massive .It just doesnt make sence that such an air leak could come and go , but it could be posssible i suppose but i dont know what it could be .

But with regards to diagnosis , i would fit fuel gauges and run the car till it faulted and see if fuel pressure dropped .....if it didnt ....yes move on to other things ..... structured diagnosis .....rule out ...move on to the next testable items .

well, the hiss is the aircon system. So not a vacuum leak. It all seems quite good under there. Hoses are all soft and sealing well. Everything is working underbonnet.

The fuel pumps overheating seem to be the next thing as now that I think about it, both times its been a 37 Deg C day. At night it didn't cause any issue.

Bugger.

I think you are correct but I will have to organise some gauges for the overall faffing I may just replace the pump.

Did these have a twin fuel pump set up .... where one of the pumps shuts off after so long .....remember posts on this a bit back .

It maybe worth having a look at the sender unit plugs , look for burned pins then find which fuses supply the fuel pump relays ...again check for burning on fuses and find the fuel pump relays ...flick the plastic Lids of them and see if you can see the points of the relay burned .

It's always worth going that bit extra with the diagnostic process as if you replace items that ' it could be ' and it doesn't fix it you have wasted fitting time and the price of the items you have replaced .