Hi, I wondered if anyone can help. My Octavia mark2 1.9tdi 2005 failed MOT on excessive emissions (5.88 m-1  max should be 0.8 m-1). He suspected a sensor was at fault or maybe a clogged cat. I was getting (sometimes) hesitation then surging on hill climbs, occasional rough idle and black smoke (unsurprisingly!). I've cleaned the grounds in the engine bay and checked leads voltage supply and ground to both coolant temp sensors with a multimeter (both 5v and 019 continuity) , MAF (12v and 019 con. + 5v ref wire), MAP (5v and 019 con.) and fuel temp sensor (again 5v and 019 con.) which all seem fine. However, when I do a back probe on MAF black posn.5 signal wire (engine running) I get start reading of only 1.6v, increasing to 2.8v on acceleration. Does anyone know what the starting voltage should be? Same problem with MAP sensor. Apparently, voltage (from back probe on purple wire posn.4)  should decrease the more vacuum is increased which it does from 1.91v to 0.73v but what is the normal range for my car?  Same problem with the other test on the yellow IAT signal wire - with other makes of car on youtube, voltage should be a constant 4.8v - 5.2v - mine:- 2.7v?!? Sorry a bit longwinded but I'm no good at car electrics! Any help much appreciated or even where I should look to find these figures.Thanks, Rich.

A diesel engine cannot run rich, if there was excess fuel there would be a corresponding increase in RPM.

 

When did you last replace the air filter?

 

Check the air intake at the Anti Shudder Valve/EGR for severe soot build up.

Edited 10 October, 20214 yr by MicMac

Hi I changed air filter about 14k ago. Will check as you have suggested soot build up  in EGR. I doubt its EGR itself as I renewed this about 45k ago...Car has been smokey for a while as I struggled to get it through the last MOT and it only just passed even though I had all the high mileage injectors rebuilt!

Many thanks for your reply...

Rich.

2 hours ago, MicMac said:

A diesel engine cannot run rich

So what causes black smoke then, if not excess unburnt fuel?

Any unburnt fuel is excess.

3 minutes ago, KenONeill said:

So what causes black smoke then, if not excess unburnt fuel?

An excess of diesel to air without corresponding increase in engine RPM is technically an overfuel condition.

 

In a diesel engine the air is not throttled like in a petrol engine, a diesel engine will exhibit symptoms like the OP's if there is a restriction in the designed intake system as insufficient air is present to fully combust the diesel, it will run but you are wasting diesel.

 

If the air intake was restricted I imagine modern systems would increase the diesel injection quantity to maintain the ECU commanded idle RPM so as not to stall, this would take into account engine variables such as temperature, fuel grade and air intake so it's possible for these symptoms to creep up over time without notice until it's obvious.

 

To be considered are the owner's driving habits which may be as much a factor as something like a clogged air filter.

 

Let's see how this one unfolds.

 

34 minutes ago, MicMac said:

An excess of diesel to air without corresponding increase in engine RPM is technically an overfuel condition.

AKA "running rich". The same thing happens (at least with mechanical pump turbo diesels) if you use full throttle at very low revs, and even with electronic pump engines if you pop a boot hose, have leaky injectors, the engine starts dieseling because of a leaky turbo oil seal...

@KenONeill

 

How about assisting the OP with his issue instead of kicking the can for something to do.

Many thanks for your thoughts - have looked inside flap motor + ECU as suggested by MicMac and its pretty coked in both and the end of the air pipe that plugs into flap motor - will de-coke these before I refit them. I may be barking the wrong tree with the sensors but I took the car for a drive with MAF plugged in and then out and couldn't notice any difference - it still hesitates going uphill/under heavy acceleration with occasional rough idle. Should I notice any difference? Maybe you can't tell that quickly given its been smoking for a while and therefore so coked. On paper I suppose it shouldn't be the MAF valve given I replaced the original with a recon Bosch one 50k ago but I imagine stranger things have happened! The point about my driving habits is probably valid as I do a lot of local stop/start travel.

Regards,

Rich.  (sorry for the late response - as my neighbours understandably think I have no life working on the car till 1am, I'm going to bed!)

Hi Another thought occured to me along the lines of what the MOT tester suggested in his potential list of sensors/CAT......I think on a couple of occasions I may have overfilled the engine oil - could this clog up/wreck the cat? Just a thought....

Rich.

7 hours ago, richwen said:

I may have overfilled the engine oil - could this clog up/wreck the cat?

Possibly; certainly, depending on how overfilled the sump was, you could have had air whipping of the oil, clogging the PCV system, which may have an effect on the fuelling.

Many thanks Ken - will see if I can check this too...

Regards, Rich.