MOT Diesel Smoke Test....  
MOT Diesel Smoke Test.... - Railroad.
I've worked in the motor trade for all 26 years of my working life, starting in a garage as an apprentice in 1980. I worked there for 15 years until 1995 when I left to work for a motoring organisation as a patrol. I was an MOT tester from 1985 - 1995, and conducted countless tests on cars and light vans.

Can someone please explain to me why it is necessary to rev a diesel engine from idle right up to the governor, hold it for a second and let it fall again, and do this 5 or 6 times to measure exhaust smoke? What exactly is it trying to achieve? Nobody, and I mean nobody ever drives their car under these conditions, and revving an engine like this makes me cringe especially when there's no load on it. The test alone must take many miles off the life expectancy of the engine, and to me it's completely unnecessary. If someone wants to rev their engine up in this way then good luck to them, but I'd rather they didn't do it on mine. I like to look after my car, not let some MOT tester {wording changed, as the tester is only following VOSA guidelines - DD} rruin it in this way.

Surely a better test would be to gently increase engine speed to about 2500rpm and hold it for about 10 seconds, and measure exhaust smoke this way.

Over to you.......

Tags: technical issues MOT maintenance and servicing

MOT Diesel Smoke Test.... - 659FBE
There is a problem of practicality here. Having been in the diesel fuel system design business for many years, I can only agree with your sentiments but to perform a proper smoke test, the engine has to be under load. It was deemed to be far too expensive and impractical to equip every test house with a rolling road dynamometer (and engine cooling support) to perform this test.

The off load test is at least comparative and the reason for repeating the test several times is that most engines clear themselves to a large measure given this treatment. Many engines will not reach the acceptable smoke limit on the first pass if they have been considerately driven.

My smaller diesel is goverened to 5250 plus or minus 50 rpm. I reset it to 3500 rpm for every MOT test and I haven't been rumbled yet - the smoke at this engine speed is brilliant.

Your last suggestion of holding the engine at 2500 rpm really apes my MOT method - nearly all engines would pass this test, because at that speed off load, the fuelling would be very low.

It's a bad job - I hate it, but we are lumbered with it now.

659.
MOT Diesel Smoke Test.... - Number_Cruncher
659,

How do you reset the engine speed to 5250, plus or minus 50 without submitting the engine to the type of torture you are originally trying to prevent?

Number_Cruncher
MOT Diesel Smoke Test.... - 659FBE
On the Bosch EP/VE pump, there is provision for an additional max speed stop screw. All you have to do is to fit an additional stop screw and locknut in the tapped hole provided and set this to the required (lower) speed. The engine will then govern at the lower setting.

After the test, my screw and locknut comes out and is put in a box for next year.

Don't try this unless you know what you are doing.

659.
MOT Diesel Smoke Test.... - Number_Cruncher
>>After the test, my screw and locknut comes out and is put in a box for next year.

Ah, that's a nice easy solution - duly noted, thanks!

Number_Cruncher
MOT Diesel Smoke Test.... - ib33
Got my Citroen AX 1.5D test soon, doesn't seem to kick out alot of soot, unlike some modern diesels I have followed. Are there different levels for older diesels?
Regards, Sparky
MOT Diesel Smoke Test.... - 659FBE
The smoke limit for a naturally aspirated engine such as the TUD5 unit in your Citroen is appreciably lower than that for a turbocharged diesel. With any turbocharged diesel, you have to put in the extra fuel to generate the boost which eventually builds up to provide an air charge which is commensurate with the fuel you put in first. That's why they smoke more.

659.
MOT Diesel Smoke Test.... - Robin the Technician
I agree....as a vehicle technician of many years on both car and commercial, its painful to hear the guts being thrashed out of your car. I have an '02 pug 406 estate with 45k on the clock and when i got it secondhand it was as quiet as a mouse - after it had its mot its now noticabley noisier. The question is - 'what if' it lets go. I know they ask you if the timing belt is ok etc and we all say yes. But is there any claim against the garage (or ministry) if an extra vent hole with conrod appears in the crank case during these tests? I would be extremely miffed if the tester said 'tough' and you're suddenly facing A: a bill of £2000+ for a new engine or B: scrapping the car.

Any views on this?


--
These are the views of Robin the Technician with 35 years in the trade. I fix, therefore I am...
MOT Diesel Smoke Test.... - Number_Cruncher
>>But is there any claim against the garage (or ministry) if an extra vent hole with conrod appears in the crank case during these tests?

No, it's your problem.

Number_Cruncher
MOT Diesel Smoke Test.... - DP
Do any other countries force people to torture diesel engines like this on an annual basis, or is it just us?

659's post was very educational. So not only is this test brutal, but it's a "second choice" option which was basically a cost saver.

I know I shouldn't expect any more from HM Government, but it really does make my blood boil.

I can't watch when the Mondeo is tested each year.I never treat it like that, and it annoys me that the law forces someone else to.

Cheers
DP
MOT Diesel Smoke Test.... - jc2
Yes;this is very similar,in fact slightly easier,than the test specified in ECE 24(the EEC test is similar but I can't remember the number).They were originally published back in the 70's.All new engines go thro' this test and you will normally see the results of this test shown on a yellow sticker in the engine compartment.Any engine in reasonable condition with conform easily unless people have been mucking about with the settings of the pump.I regularly drive my diesel upto these engine speeds on the road,let alone at no-load.
MOT Diesel Smoke Test.... - DP
Don't get me wrong, I'll take it up towards its redline occasionally under hard acceleration to give it a clear out, but it's up over 4,000 RPM for maybe a second until the next gear is engaged. I still consider that very different to holding it against the governor in neutral with no load though. That is not a situation that ever arises on the road under any driving style.

If you saw someone revving a petrol engine to its limiter in neutral and holding it there, the word "abuse" would come to mind, surely. I don't see its any different for diesels.

I'm not concerned about lack of servicing and things like timing belt failure, but more the actual mechanical effects of this, particularly on an older higher mileage engine like mine. It does it each year and passes the test comfortably (<10% of the prescribed limit last year IIRC), but it does make the kind of noise that you wouldn't be surprised to see punctuated by a bang and a lot of smoke.

Hence, why I can't watch it! :o(

Cheers
DP

MOT Diesel Smoke Test.... - mss1tw
Would turning the air con and everything electrical on before hand help load up the engine?
MOT Diesel Smoke Test.... - mph_turbo

Re: Do any other countries force people to torture diesel engines like this on an annual basis, or is it just us?

Here, In Ireland, we have a National Car Test (NCT) performed every two years on vehicles 4 years old or older. These tests are all carried out by teh same company with locations in every county.

Technically, the testers are supposed to rev the engine in a manner similar to that described earlier in this thread..............but it really depends on the tester. Older guys seem to have sympathy for the engines and only moderately rev to around 3000 rpm...............other guys thrash the engines!!!

Commercial vehicles are tested yearly and my own experience of this is that the testers use common sense and only rev the engines moderately.
MOT Diesel Smoke Test.... - Railroad.
I like to look after my car, not let
some MOT tester {wording changed, as the tester is only following
VOSA guidelines - DD} ruin it in this way.


My apologies Mr Mod. I think I may have made the wrong choice of words there, I wasn't suggesting that all MOT testers have some kind of brain deficiency, after all I was one myself for ten years and probably still would be if I worked in a garage now. There are some very good and honest ones around.

However I do like to think that I've learned something over the course of my working life, and I take a pride in my job. Sadly I don't think all vehicle technicians think the same way, and like it or not some more unscrupulous MOT testers may take pleasure from seeing a diesel engine fly apart during the smoke test, and then look to the owner to blame.

I just think that this part of the MOT test is fundamentally flawed.....
MOT Diesel Smoke Test.... - mss1tw
How much damage are we talking about here?
MOT Diesel Smoke Test.... - 659FBE
A customer I used to deal with (engine manufacturer) had a typically British way of describing the sort of damage possible.

"This engine has lost its structural integrity". (=Con rod sticking out of the block).

659.
MOT Diesel Smoke Test.... - PhilW
OK, so I don't know anything about this, except as a diesel owner who listened to a 170k BX going through the smoke test (yes, I cringed because I don't think I ever took it to the governor limit) but isn't the limiter set at a rev limit that the engine can cope with? And if it is not under load isn't that better than thrashing it up a hill at full revs with a caravan on the back?
Must admit that I don't do that with my cars but with a full van/minibus I have often taken them up to the limiter (in low gears usually) and it doesn't seem to do them much harm.
--
Phil
MOT Diesel Smoke Test.... - bell boy
Phil


the engine hasnt got a load on the end of the wheel/diff/gearbox/clutch/crank and therefore the crank can decide too much too young as the specials said
MOT Diesel Smoke Test.... - 659FBE
The problem with taking an engine up to the governor run out (max goverened speed) off load is one of torsional oscillation. The load on an engine damps the torsionals in the crankshaft to a large measure and the engine is, of course, designed to provide mechanical energy into a load.

Torsional oscillation is potentially very destructive and will cause damage if a resonance is found. Consequently, it is inadvisable to run engines off load at speed. The governor is there to protect the engine in the event of an overspeed at light load - maybe once in a lifetime off load if a shaft or return spring breaks.......until the MOT test came along.

659.
MOT Diesel Smoke Test.... - FP
I'm reading this thread with great interest. How do I (temporarily) adjust the maximum revs on my Peugeot 306 HDi?
MOT Diesel Smoke Test.... - 659FBE
I'm not going to even begin to tell you how to go there. You could borrow several sets of thick floormats though.

659.
MOT Diesel Smoke Test.... - FP
:( Didn't realise I was getting into a sensitive area!

I suppose increasing the slack on the accelerator cable would have the same effect.

Somehow I was hoping for a more elegant solution... A deft tweak with a screwdriver... Re-setting a control somewhere...

After all, there would be nothing to stop the tester removing the floormats, would there? Or maybe they aren't allowed to do that.
MOT Diesel Smoke Test.... - Dynamic Dave
Be aware that adding extra floormats *could* also affect how far your brake pedal travels.

That aside, I don't why so many people are afraid of having their engines revved. If engines were really that delicate then the manufacturers would set the rev limiter at 2000 rpm, not at 5000 rpm (or 6000 odd rpm for a petrol engine)
MOT Diesel Smoke Test.... - bell boy
depends on the vehicle ChrisPeugeot ,but i have certainly turned down old diesels before an mot,
Thankfully my mot man these days has what is called mechanical sympathy and i dont need mirrors/ smoke etc to get a pass on what would be a good engine but maybe past its prime
MOT Diesel Smoke Test.... - PhilW
Thanks for that oldman and 659 - interesting and educational - as I said I don't know anything about it but now I know much more!
Cheers
--
Phil
MOT Diesel Smoke Test.... - cheddar
My Mondeo passed its second MOT yesterday in May at 4 years and 105k miles, no problems. The Opacity test measures the transmission of light through the exhaust emissions, I understand that the car can go through up to six test cycles to be within the max 3.00, mine came out at 1.46 first time so one application of full throttle for a second or so was all it required.
MOT Diesel Smoke Test.... - Number_Cruncher
>>The load on an engine damps the torsionals in the crankshaft to a large measure...

Is this really the case? If there are springs in the hub of the driven plate, or even a dual mass flywheel, do these not serve to decouple the torsional oscillations of the crank from driveline oscilations?

Number_Cruncher
MOT Diesel Smoke Test.... - bell boy
yes thats why you dont have engine mountings anymore,not needed see ;-)
MOT Diesel Smoke Test.... - 659FBE
Driveline oscillations are a separate issue. Torsional oscillation in the crank is of a higher frequency and is brought about by the perturbations arising from firing pulses. If the crank is a "free" mass, ie substantially unattached to anything at either end, at certain speeds the equivalent of standing torsional "waves" are set up within it. The firing order of an engine is chosen to minimise this effect, but there will always be a few speeds at which oscillation occurs, and it is something carefully watched for in engine development. The driveline "load" plays a significant role in damping these oscillations.

Remember some very high output engines had centre power take-off? Now you know why.

659.
MOT Diesel Smoke Test.... - jc2
Those high-output engines had negligible flywheels to lower the inertia mass unlike a diesel!!
MOT Diesel Smoke Test.... - bell boy
easier way to describe it is look at the seats on a bus, at certain engine speeds they all vibrate ,why? because this is their natural oscillation frequency
is that helpful?
MOT Diesel Smoke Test.... - Number_Cruncher
Indeed, the placement of take offs at nodes of the crankshaft torsional modes is good practice - although it sometimes leads to difficult engine layouts. I'm thinking of those engines with timing chains or gears mounted near the nodal point which is typically just ahead of the flywheel.

I suppose the bit I'm having trouble with is how significant the external load is. I can't believe it actually moves the crankshaft resonant frequencies about too much, and owing to the impedance mismatch, I can't see a great deal of vibrational energy passing accross the clutch, and out of the crank system.

I have only passing experience in this, and have never taken any measurements - so I don't "know" the truth of the matter. I worked on an isolation system for a six cylinder diesel, connected to a very large alternator, and we had great difficulty in making sure that we avoided trouble with third order torsional vibrations at the operating speed of the system. We did employ a sprung isolator in the prop shaft, but that was really to protect the alternator rather than the engine.

Number_Cruncher


MOT Diesel Smoke Test.... - PaulFromMelton
I've got a 405 due for a MOT at the end of the month. It's good to know I'd probably get away with lowering the maximum revs to about 4000rpm. I'm also thinking to totally lower the fuel boost compensation and slightly lower the fuel pressure to pass.....

Any thoughts?
MOT Diesel Smoke Test.... - bell boy
just lower the revs and stick some diesel cleaner in the tank and get it nice and warm before the test.
the idea is not to get an unsatisfactory engine through the test but just to make sure your engine if in good condition passes first rev

ok? :-)
MOT Diesel Smoke Test.... - cheddar
>>I suppose the bit I'm having trouble with is how significant
the external load is. >>



Of course the engine mounting system is designed to counter vibration and resonance at idle or at higher revs under load from the transmission therefore I suppose it is possible that the engine will be allowed to resonate dangerously at high revs under no load.
MOT Diesel Smoke Test.... - David Horn
For crying out loud, it's only for a second or two. I have an MOT test on my 1.9TD XUD with 85k on the clock on Friday and it's going to get a good thrashing on the way there, followed by an oil flush and change prior to the MOT. Every year it barely registers on the smoke test, mostly because the contents of the exhaust have been dumped over whoever chose to tailgate me on the day of the MOT. (I once got an open-topped convertible with cream upholstery - those were the days.)

The diesel engine was invented solely to deal with tailgaters and fast cyclists who are difficult to overtake.
MOT Diesel Smoke Test.... - Martin1981
My 1994 306 1.9TD always gets a dose of Redex diesel additive and a good thrashing on a motorway/ dual carriageway- up to 75-80mph in fourth gear for a short while before driving it to the MOT test centre and this always does the trick for me. I always make sure that the air and fuel filters have been changed shortly before its MOT. The engine is on 140k and runs like a dream (touch wood!), although it does tend to chuck out a fair bit of smoke when starting from cold and when accelerating heavily after driving steadily for a long time, but that's normal for these engines. The car does get plenty of long motorway runs at a steady 70-80mph which partly helps keep it sweet.

Martin
MOT Diesel Smoke Test.... - Greg R
I must be thinking wrong. But if an engine is in neutral, reving it to 7000 rpm or whatever shouldn't do any damage I thought...it is not moving the vehicle so the load on the engine is minimal. However, with these revs with the engine pulling the car would really cause the wear to engine if done daily!

Am I wrong?
MOT Diesel Smoke Test.... - Greg R
What do others think about my theory of the revs?
MOT Diesel Smoke Test.... - 659FBE
The problem is not wear, but damage. Very few if any automotive diesels will rev to 7000 rpm and all diesels are fitted with a governor, one function of which is to closely control the maximum engine speed.

Very simplistically, there is a square law relationship between engine speed and stress in the various components where engine work load is not taken into account (as in the case of the MOT test procedure). This arises from the simple relationship that the force generated by a rotating off-axis engine component is proportional to M x r x omega^2. In this instance, M is the mass or effective mass of the component concerned, r is its radius of rotation and omega is the angular velocity, which is proportional to "speed". It is the fact that this term in the equation is raised to the power 2 (squared) that makes an engine overspeed so risk prone.

In a properly governed engine, none of this will be a problem, but off-load resonaces certainly can be - see my earlier threads.

659.
MOT Diesel Smoke Test.... - 420 Turbo Diesel
Hi,just add mypeneth,I have for many years put a spacer on the throttle cable which stops the diesel pump opening up so far,I have got away with it for a long while.I thought the rules had been changed as regards the "reving",if they pass on the first rev they don't have to rev them anymore,if they fail the first rev they can do another one,upto a maximum of 9 I think? This is how my MOT tester does it.
--
I do like my Diesels to be Turbocharged for that extra OOMPH!!!!
MOT Diesel Smoke Test.... - David Horn
Just had a read of the MOT smoke test instructions (mine sailed through today with a reading of 0.61 on the fast test).

In summary:

1. Check the governor has not been tampered with.
2. Run the engine at 2500RPM for 30 seconds to purge the system.
3. Raise the engine speed slowly to the maximum to check the governor.
4. Following prompts on the machine, push the accelerator rapidly to the maximum and hold until told to release.
5. Repeat up to 6 times if test fails.

Interestingly, even if the smoke test fails, the garage can make a subjective opinion as to whether it should be allowed to pass. Explains why the buses that pour out black smoke are still on the roads.
MOT Diesel Smoke Test.... - mondeo1306

my chevrolet epica 59 plate 2.0vcdi passed with a reading of 0.17 today

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