Hi all, might be useful to someone else.....
My E320 coupe failed the MOT on a broken front spring and on emissions .
The car passed at idle.
At 2,500rpm it passed the calculated lambda and HC tests but failed on CO - about .5% where the limit is .3%.
Came as a shock as the car drives very well and idles (almost) perfectly....
"You need a new cat mate" - hmmmmm.....seen too many new cats that "fix" this kind of problem, only to fail in a year or so because the underlying cause is still there.
Back to basics:
High CO means incomplete combustion at the tailpipe.
Cat needs sufficient oxygen to combust HC and CO (the reason why O2 sensors are used to rapidly swing the mixture and so supply the needed O2).
Working cat will be hotter at the tail end than at the front end because of the heat generated by combusting the HC and CO.
Tests:
Quick infra-red check of the cat - its NOT much hotter at the back than the front (so could be dead/dying) OR could be short on O2 to combust the pollutants.
So, its an M104/HFM car - what influences mixture?
O2 sensor measured - cycling correctly and cycles speed up at higher revs (not broken!).
MAF cleaned (it was clean already) and then measured - correct voltages and quick voltage rise with a throttle blip (not broken!).
Intake air temp sensor - correct resistance at 20C (not broken)
Coolant temp sensor - correct resistance (pins 1 and 4) at 20C and 80C (not broken)
Car "knows" when its in Park and when the A/C clutch kicks in so those parameters should be OK.
Careful listen to the idle at the tailpipe - its hunting a bit - weak mixture?
Could be injector(s) or could be an air leak after the MAF (unmetered air).
(As a side-note, I wish I had access to an HHT to be able to see live date from this engine.....).
Taped a drinking straw to my soldering torch and start (carefully) feeding the inlet manifold with propanefrom the MAF forwards...
Found a leak! Where the inlet cross-over pipe sits on the cylinder had cover there are 2 crankase vent connections sealed by rubber bungs - no2 admits the propane and causes a change in idle. Whipped off, cleaned up and replaced - leak gone.
Found a second leak! Under the inlet manifold are 2 rubber vent pipes - one for the main crankcase vent and one that links upper and lower manifold. The PCV one has hardened and cracked.
Used some self-amalgamating tape as a botch/temp repair while new one on order 3 or 4 days....(why is nothing in stock any more??)
Idle now smooth so looks like that might have done it....CAT now 100C hotter at the back than before!! (not a scientific test at the conditions were not the same as the first test).
Re-test. CO now .11% - total cost less than £5......and no unnecessary CAT (a fresh one would probably have been able to deal with the CO but I would rather fix the issue).
Sometimes this is fun......
My E320 coupe failed the MOT on a broken front spring and on emissions .
The car passed at idle.
At 2,500rpm it passed the calculated lambda and HC tests but failed on CO - about .5% where the limit is .3%.
Came as a shock as the car drives very well and idles (almost) perfectly....
"You need a new cat mate" - hmmmmm.....seen too many new cats that "fix" this kind of problem, only to fail in a year or so because the underlying cause is still there.
Back to basics:
High CO means incomplete combustion at the tailpipe.
Cat needs sufficient oxygen to combust HC and CO (the reason why O2 sensors are used to rapidly swing the mixture and so supply the needed O2).
Working cat will be hotter at the tail end than at the front end because of the heat generated by combusting the HC and CO.
Tests:
Quick infra-red check of the cat - its NOT much hotter at the back than the front (so could be dead/dying) OR could be short on O2 to combust the pollutants.
So, its an M104/HFM car - what influences mixture?
O2 sensor measured - cycling correctly and cycles speed up at higher revs (not broken!).
MAF cleaned (it was clean already) and then measured - correct voltages and quick voltage rise with a throttle blip (not broken!).
Intake air temp sensor - correct resistance at 20C (not broken)
Coolant temp sensor - correct resistance (pins 1 and 4) at 20C and 80C (not broken)
Car "knows" when its in Park and when the A/C clutch kicks in so those parameters should be OK.
Careful listen to the idle at the tailpipe - its hunting a bit - weak mixture?
Could be injector(s) or could be an air leak after the MAF (unmetered air).
(As a side-note, I wish I had access to an HHT to be able to see live date from this engine.....).
Taped a drinking straw to my soldering torch and start (carefully) feeding the inlet manifold with propanefrom the MAF forwards...
Found a leak! Where the inlet cross-over pipe sits on the cylinder had cover there are 2 crankase vent connections sealed by rubber bungs - no2 admits the propane and causes a change in idle. Whipped off, cleaned up and replaced - leak gone.
Found a second leak! Under the inlet manifold are 2 rubber vent pipes - one for the main crankcase vent and one that links upper and lower manifold. The PCV one has hardened and cracked.
Used some self-amalgamating tape as a botch/temp repair while new one on order 3 or 4 days....(why is nothing in stock any more??)
Idle now smooth so looks like that might have done it....CAT now 100C hotter at the back than before!! (not a scientific test at the conditions were not the same as the first test).
Re-test. CO now .11% - total cost less than £5......and no unnecessary CAT (a fresh one would probably have been able to deal with the CO but I would rather fix the issue).
Sometimes this is fun......