Bit of a weird one today.

Driving a long at 70 on a dual carriageway today and all of a sudden the CEL started flashing. It did this for about 10 seconds then went away. At no point did the car drive differently or change. I have Vag Com so will hook it up tomorrow and get more details to post here.

The car has had a stage 1 and is running about a Bar of boost.

Any ideas?

 

Do a smoke test and check for leaks.

 

So after running VAG com it came back with cooling fan obstructed. I think this may be a nuisance fault as the fan is clear.
both 02 sensors are open circuit which I already knew.

Then the one that I think was the cause of this is multiple cylinder missfire. Then an individual fault for each cylinder 1-6.

I've done another 50 miles since with no issues or dash warnings so not sure what the cause was?

I'm suspicious of maybe a bad value in the remap?

Any suggestions welcome. I think it's too early to be point the finger at the vacuum hose.

 

I was going to suggest the vacuum leak until I reached the very last two words of your post.

With Both O2 sensors open circuit, you're fueling on a (the) backup map? Or using purely speed density?

 

Backup map? I don't follow you? The O2 sensors are recorded in Vag com as intermittent. The post cat ones are still running fine which should keep fueling from running away.

 

The front sensors are responsible for fueling feedback to the ECUs. Post sensors cannot do that since the exhaust vapors have been scrubbed by the cats. Post sensors' first is to monitor the cats' performance.

If your front sensors are intermittent, then correct/accurate fueling is intermittent. If the ECUs cannot get an accurate FB (rich/lean) signal from the front sensors, they usually revert to a backup fueling map and ignore the primary fueling map. Backup fuel maps are always notoriously rich. Eventually, at cruising speeds, the plugs are going to foul and can account for P0300s. If you got a series of P0301 - P0306 those are all cylinders on the motor right side (Driver's if RHD) which is an indication that O2 sensor has stopped, or long enough for the number of misfire cycles to set a CEL.

 

Ah I see.

Am I right in thinking those sensors are a pig to change? I'm almost tempted to see if this develops before dropping the engine to change a pair of O2 sensors

 

The front O2 sensors are vitally important. I know the dilemma with changing them, but not much joy without them.

Are you getting any P-codes that show the front sensors are failing or have failed? Any (or many) P-00xx codes, starting with P0030 all the way through P0175 (P0171/P0174 lean; P0172/P0175 rich)? Or P0173? If you know the front sensors are open circuit are you seeing any P-codes? Anywhere from a P-0030 to P-0038? Or more? If so, not much you are left with but changing them. It would be an awfully odd chance both ECUs failed, although II has to be synced with I.

You could DL the front sensors just to verify.

 

I don't recall seeing any of those codes. Just the faults I mentioned I didn't note down the specific P codes as they didn't relate specifically to anything I didn't expect to see. I'll drive the car over the next week and do another scan.

The ECUs are synced as ECU1 has the message to check ECU 2. Plus with a dud ECU or ECUs I'd expect to see more codes. Scuttle panel is also debris free and draining properly

Sorry but DL the front sensors?

 

DL - Datalog the front sensors. A good running DL will show the voltage span of each sensor as they transition rich/lean. A better Data logger would also show the sine wave which, to me, is more valuable than the raw voltages. As front senors age, one sign of a failing one is a "lazy" switch from lean to rich. Pretty easy to spot if you DL both sensors on the same screen. Datalogging the sensors is like a health check on them.

Yes, drive it more, post anything you can. First, clear the misfires, see it you can clear the CEL on the dash, then note what the conditions are when the light sets again.

 

Ah okay VAG Com can data log I planned to log the injector duty cycles anyway .

Sorry if I wasn't clear the CEL cleared itself the CEL flashed for 8-10 seconds. Then went out. If I hadn't have looked at the dash during that time period is of never have known.

 

Very good. DL is best to give you a snapshot of your engine's health in a running state. Some tips about the DL

Only run a DL on a completely warmed up (up to op temps) engine.
You might be DLing other things, but for fueling purposes, the primary fueling input is the front sensors in conjunction with the MAF.
They need to be in closed loop (obviously) to DL any useful info.
On a turbocharged engine like the W12, the CL window could be small before the EMS switches to power enrichment.
In PE, the loop is opened and O2 sensors will flatline on a graph, so make sure you stay in CL.
CL should be safe around the rpm range of 2200 to 3000, maybe as high as 3500, but safer to stay around 2200 to 2500
Find a nice stretch of road where you can drive and hold 2200 to 2500 rpms for a few to five minutes.
If you need, hold the trans in a lower gear if the speed is too high for road conditions.
If your DL allows a continuous recording 5 mins long, capture that and save it.
If not that long, stream a continuous steady rpm run as long as you can.

Can't think of anything else, until the moment I log off, of course.

 

So I've driven and re scanned the car and I have the following faults;

P1453 - exhaust gas temp sensor 1 (G235)
P1950 - radiator fan jammed - intermittent
P1672 - coolant fan control open or short - intermittent
P1457 - exhaust gas temp sensor 2 (G236)

I​​​​​I must have read it wrong the O2 sensors are fine then. So not sure why it would have misfired at all.

The radiator fan is definitely not jammed.

 

So, a couple of things:
The P1453 and P1457 are exhaust temp sensor failures on both banks. Both of these codes report a failure in the sensor (open or short), and not a performance code such as P1455/P1459. These sensors are very important on turbo charged cars. They monitor the CAT temps so they do not reach a temp so high they might melt. The EMS will command a rich (overfuel) AFR in the case(s) of P1453 and/or P1459. The overly rich AFR will eventually cause fouled plugs and carbon deposits if driven with these conditions very long. The car is not going to run very well when seriously overfueled.

Don't know what to make of the coolant fan issue or if it's coincidental or causal. I'm leaning causal primarily BC of the "intermittent" possibility.

If it were my car, I'd focus first on the fan control module. Can't answer for the Bentley, but on other cars I've experienced, the fan control module is usually a seriously high amperage transistorized (fancy) relay. On these, because the current can be so high, contact points in the control, as well as connectors in the harness can get hot, burn and melt.

Hope this helps a little...

 

Thank you very much for your help! Searching around on the forum I can see post about the EGT's being linked to the cooling system and causing faults on that system. SO hopefully replacing the sensors will resolve this.

Can these exhaust sensors be changed DIY? or are they a pig to access?