Daimler Double Six (X300) Running Lean on Left Side

My father and I have been gifted a lovely Daimler Double Six 1995 in dark blue. It is in really great condition and had done 80,0000km.

It is under powered and we diagnosed that the left hand side of the V12 is running lean, the right hand side appears to be running well. The left hand side exhaust was cold, while the right hand side was warm.

During our diagnosis the car stopped being able to turn on so we opened up the ECU and found a blown capacitor and replaced this. This resulted in the car being able to be turned on but still running lean on left hand side.

The diagnosis continued:

  • So being a V12, it has duplicated sensors on each side so we proceed to remove, clean and swap the side (MAP, O2) but still the right hand side runs well, left hand side does not
  • Replaced MAP sensor inlet pipe
  • Replaced all 12 spark plugs
  • Tested voltage of MAP sensor at sensor and output same at idle and at 3.5k RPM
  • Tested voltage of MAP sensor at ECU wiring and output same at idle and at 3.5K RPM
  • Tested voltage (average) across fuel injectors and we noticed a significant difference between left and right hand side. Left hand side not increasing as RPM of engine rises, where as the right hand side is increasing
  • hooked car up to view live data, and saw that the ECU seemed to believe the left MAP sensor was reading lower than the right hand side, so the theory is it’s not fueling the left hand side. So we thought it’s another ECU fault(remember we had a blown capacitor). I sourced an ECU (from Sweden from a running car (had been crashed) with close match on part number, LNA1410NA/004 is the original ECU, and LNA1410NA/005 is the purchased ECU. We swapped the ECU in the car turns over but will not start.

Our current assumption is potentially the car is now immobilized but are not sure and thought it’s time to get a post up on the forum - we do not want to give up!

Not familiar with the later V12 engine management system but my two cents:

That’s not running lean, it’s not running at all.
Remove the left side spark plugs or even better the injector sockets and see if it makes any difference.

But do you have spark in all 12 plugs?

Can you hear the left side injectors click?

If the MAP sensor values are the same but the ECU thinks they are not, the ECU or the wiring are suspects.

Something seems to be wrong with the new ECU.

Thanks for your response - I think you are right that prior to fixing ECU capacitor the left bank wasn’t running at all. Now it seems to only be running partially. The voltage entering the ECU from map sensors is the same at the map sensor so made assumption wiring is OK. Had mechanics test voltage drop, but they seem to believe not dropping.

One other fact that is odd is there was two times where the car was running well, very powerful and smooth.

I do think ECU fault but confused why new ECU that has same part number won’t allow car to start. Do the 1995 have an immobiliser that I may need to override?

I don’t think that any immobiliser scheme would be incorporated in the ECU.
The inertia switch maybe?

The ECU can very well have an intermittent fault, a cold solder joint for example, so it’s not that odd that sometimes the engine worked fine.

as suggested by Aristides, cold exhaust would be engine bank not running at all

you should be able to check if the sparks and injectors are ok :
if you have access to injector noids, insert one in each injector command and check it flashes in the same way on each cylinder
also a cheap stetoscope is an easy way to check for the injectors licking or not

is there any error code showing ?
before ODB, the codes were shown on the trip computer when powering the car , before engine start

if the engine still behaves after swapping most sensors around, a O2 sensor may be wrong, and making the ECU lower fuel on this side ?

for some markets, cat temp sensors were mandatory, and would cut fuel to the side overheating to prenvent catalyzer fire : that could be another suspect ifyour car is fitted with such device