Petrol v Diesel Emissions Test

[helidan]

All things being equal which fuel type usually passes emission testing easier?

Something I've always wondered.

[BFG TERRANO]

For mot? In my experience diesels. It's just a smoke test rather than a probe gas tester.

[helidan]

BFG TERRANO said:

For mot? In my experience diesels. It's just a smoke test rather than a probe gas tester.

Yes, MOT emissions test.

I always thought diesels were more difficult to get through emissions, guess I was wrong.

[Turkey]

A Diesel has to be spewing smoke to fail, mine is fairly smoky now as the injectors are worn, but still achieved a "fast pass" on the test, i.e. much lower than the limit so it goes straight through.

Even newer Diesel with DPFs have the same limits to stick to, hence why even after removing the DPF they can pass - a visual check for the presence of the DPF is done though.

with petrol cars the fuel:air mixture is critical to getting the balance of emissions to be good, so if a sensor or cat is playing up then they can fail. With a Diesel it can be running quite rough but still pass.

[MG CHRIS]

We are getting an ever increasing number of new cars diesel which wont record anything during the smoke test. The car has to be belching out loads of black smoke to fail a diesel.

For this reason yes easier for a diesel to get through the test.

[podwin]

So diesel isn#t tested for stuff like CO or NOx etc?

[MG CHRIS]

podwin said:

So diesel isn#t tested for stuff like CO or NOx etc?

No just smoke test.

[podwin]

Well that's rubbish.

[M4cruiser]

Worth noting though that petrol emission is tested at idle (and fast idle) but diesel emission is tested at maximum revs.

[TheAllSeeingPie]

If you have a friendly MOT tester and sports cars they usually let you go shed thrash the car before the emissions test. This makes it easier to pass as they work better when you're trying to melt them.

[aw51 121565]

An "Italian Tune Up" before the MoT blows any accumulated particulates* out of the exhaust system before the emissions test, rather than them being disturbed and blown out of the exhaust system during the emissions test (which naturally gives a falsely high reading) . It's nothing to do with the operating temperature of the engine in terms of particulate production - the emissions test is only linked to engine temperature in that the oil temperature must be at or above a certain level (60C from memory).

*(Admittedly a DPF will catch almost all (if not all) of them, so it's a bit of a rubbish & generic reply - but anything without a DPF will certainly behave as described and any particles which get past the DPF will also be ejected.)

NOx and CO etc are tested at type-approval; the cost to so-equip MoT stations with these test beds would be prohibitive so it ain't going to happen.

[anonymous-user]

Assuming your car engine isn't actually knackered, gasoline engines are more likely to fail an MOT test, as they have a much narrower AFR window in which they must run. And, there are multiple things that can cause the engine to operate outside this window (exhaust leak, air leak, dodgy MAF, Lambda sensor etc)
Compare that to a modern CR diesel, that has no lambda target to meet on the MOT test, and because it will only put in enough fuel to make enough torque to keep the engine spinning, no matter how much you accelerate, it will always run lean during the test. Hence, unless the injectors are completely knackered, or your HP fuel pump has fallen off, it will pretty much pass regardless.


So, i'd suggest that most older cars fail the MOT emissions test due to minor issues with sensors or plumbing (cracked vacuum pipes etc) rather than fundamental issues (worn engine / failed CAT etc)

[Huff]

Max_Torque said:

And, there are multiple things that can cause the engine to operate outside this window (exhaust leak, air leak, dodgy MAF, Lambda sensor etc)

+1 There are even a few small gotchas that can cause 'fails' when the engine and its operating parameters are quite legal!

Example, my BEC (Fisher Fury with an R1 bike negine) - only 998cc to fill 2" dia exhaust /silencer, which is all of 750mm long past the cat. With a long 4-1 header before the cat that means enough reversion pulsing at idle that I had a heckuva time getting it through the MOT until I twigged - test looked hopelessly lean everywhere, due to entrained outside air regardless of actual AFR; and I now use a blob of mineral wool to block 2/3rds the tailpipe cross section while the probe is up there. That done, no problem at all.

[not even given the ecu runs alpha-n, and no lambda probe - this map is juuuust right where it matters ]

Edited by Huff on Saturday 21st February 23:00