A new one on me this (having only owned a diesel from new for 2 years and it's not strictly relevant yet). The Vehicle Inspectorate's card at my local MoT workshop states:
"The test has been developed so that it will not damage your engine. After checking the oil temperature to make sure your engine is warm, the tester will accelerate the engine 3 - 6 times from idle speed until the governor cuts in. Your engine will not be run like this for any length of time, and if it has been reasonably well maintained the test will not damage it."
Although my car has been maintained scrupulously, the above has (to me) worrying undertones. Am I being neurotic? :(
Oz (as was)
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Even though you wouldn't rev the engine like this yourself, you've no option!
Just make sure your OK cambelt wise, not nearing time for renewal,oil level is correct, a new air filter is a good idea, as well.
A brisk drive before test may be advisable, if you've had mainly shortish journeys, recently.
Then it's fingers in your ears time!
Mark
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Is this really supposed to be a realistic check on emissions.
Maximum revs under zero load really does equate to normal driving conditions, doesn't it!
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And a quick dose of injector cleaner does no harm either, just before the test. Your car is obviously fairly new but I would advise leaving the area before the test if your car were an old faithful. Hearing it being abused to test it when you have cared for it and driven carefully for several years would bring tears to your eyes!
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This test for Diesel smoke has been a standard test for decades. The engine is not running under no load because it is accelerating itself and will be injecting maximum fuel for some of the time.
The governed speed is about 10% above max power engine speed and is within the engines normal operation range. Don't worry about it unless engine is badly maintained.
I would certainly recommend running the engine at full throttle and toward max revs before the test. Best done uphill to prolong time you can run flat out.
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Thanks for your collective views. I do also take note of HJ's recommendations about regular stints at high revs in order to clear out the system. Just prior to the MoT test would therefore be a good time for this.
Oz (as was)
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adjust the throttle stop on the injector pump so that it cant rev over 4000revs, much kinder on the ear and the engine
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Well yes, except that in the course of adjusting the throttle stop to this setting, and then testing it, you're going to rev the engine very hard 3 or 4 times anyway!
A lot of modern diesels are drive-by-wire, so wouldn't have a throttle stop anyway.
Most people don't seem to remember that the induction system on a diesel works back-to-front compared to a petrol engine, ie the air intake is running at maximum all the time, and the accelerator pedal varies the amount of fuel being injected. A diesel engine can't be "over-revved", because at full revs you can't physically cram any more air/fuel mix in.
So as long as your timing belt is in good condition (and diesel engine timing belts have to work at lower speeds, which should add to their longevity) then there shouldn't be anything to worry about in the MoT. I would be more worried about pulling out onto a dual carriageway and having to accelerate up to 60 or 70mph quickly than I would about the MoT emissions test.
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It's good to have a proper scientific answer to this point, DtD.
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What disturbs me most about this whole question is that the onus is placed on the owner to be sure that relevant engine components (e.g. cambelt) are AOK.
I am able to guarantee that I have followed the manufacturer's servicing schedule to the letter. However, I'm not able to guarantee that individual components since the last service haven't suddenly deteriorated for some reason. (SHMBO's Pug 205 just failed MoT only a few months after a major and costly service by the local Pug dealer - reason: split CV gaiter).
In the event that something goes 'pop' during the MoT test, I will complain bitterly, to which they will answer - 'tough'. :(
Oz (as was)
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My friendly garage did manage to break a cambelt on one of these occasions, but they did put it all right again at their own expense. It was a fairly aged Escort, too, so it was probably about to happen anyway, but the mechanic was a bit red-faced about it...
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stand by my theory on vehicles with throttle cable, had a hgv that bombed at annual test , lengthend cable - ie less revs - passed no probs, sometimes it pays not to think too deep
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OZ I guess everything was OK? I have just booked my 5 Series 2.5tds in for an MOT and was asked if I was confident that everything was ok for the garage to rev the car to the limit 3 -5 times. I started to sweat, even though these cars have cam chains and not belts. Think I'll dump a load of Redex in before hand.
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Dave the Taxi Driver wrote;
".. ie the air intake is running at maximum all the time, and the accelerator pedal varies the amount of fuel being injected. A diesel engine can't be "over-revved", because at full revs you can't physically cram any more air/fuel mix in."
I don't really agree with that Dave. I'm sure someone with more authority can comment but whilst they do run with an unthrottled induction system diesels can rev to self destruction.
If you get self dieselling, e.g. by excess oil splashing up into the bores then the engine will just rev and rev, and of course the governer will do nothing. For example its a problem on badly marinised diesel engines which can do this if cranked up when the boat is over on its ear.
The point is that the air intake is not necessarily the limiting factor.
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My Ciroen synergie with a 1.9 turbo diesel engine used to smoke badly at the MoT emissions test (because it has an easy life and I don't thrash it) causing me to sweat a bit at MoT time. After reading HJ's advice about Millers Dieselpower Plus, I dose the tank during the summer and the car is towing (therefore flat out most of the time). Come MoT time in September I remove the air filter (thank you John S, for that one), take it for a 3rd gear Italian tune-up on the way to the test centre, and the last twice, it has sailed through the emissions test.
The smoke test is and always has been a deeply flawed and totally unrepresentative test of emissions.
(runs off to find rabbit's foot)
H.
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Sounds like good tips, although contary to HJ's advice, I have found Millers makes no noticable difference to the running or fuel consumption. It is not the failing of the emissions test that worries me, but the 'thrashing of the engine'. Tip, filling up biodiesel also reduces particulate matter. Fact.
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If your engine runs OK and there is no smoke, then subjectively you will see little or no difference. I CAN assure you from experience that it will help a smaokey engine and improve MPG ETC. by cleaning it up. Because it cleans and lubricates the fuel system, I will continue to use it in my current 'clean' diesel to stop it deteriorating.
I must say however that Redex Diesel additive had no effect whatsoever on my last 'smokey'diesel.
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Our test centres are dotted all over and we have to get the cert before we go register the vehicle. The procedure sounds the same as yours except my local test center is part of a battery shop in a whole parade of motor accessory fitting shops so I bet they love the noise and smoke. My Harley has to go too, but bikes are only tested on CO not hydrocarbons (that's because of the vast number of two-strokes hauling passenger sidecars which wouldn't pass otherwise.
So last week I took the Hog along for its smog test. It has V & H drag pipes for street use, no baffles or mufflers. So our man asks me if he can wind it up I say sure go ahead. He does so and not only does the racket set off all the car alarms within about 100 feet but the 12" jets of flame really get him going. I got the pass certificate.....
Life's little pleasures.
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Don't know if it's relevant but the HGV smoke test is pretty much the same as the car test. An engine with no problems will pass on the first 'rev'
Today I took a two year old 7.5t lorry for MOT. This engine had been modified by the manufacturer to prevent the engine from revving at more than 1200rpm unless the speedo was registering more than 5mph. This means that at full revs (stationary) for a smoke test the engine was revving at less than half maximum revs. The tester didn't know what to do as this was his first encounter with this type of modification. Eventually he decided that as 1200rpm was the Max available revs, that would have to do.
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Nice story, Carrick. I suppose if the tester's really brave he could jack up a driven wheel and test it to the limit in gear. Something to be watched from a safe distance, I'd say.
H.
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Hawkeye...."Come MOT time...I remove the air filter".
This is a new one to me. Please would you explain the reason / advantage of such action. What am I missing?!
Thanks.
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PS...Do you leave it out for the Test?
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Yes I leave it out for the test.
Why? I tried to type a plausible explanation but it came out complete rubbish so I deleted it and typed this instead. Dunno, but I seem to remember the learned John S posting an explanation on the old forum - the one with the edit button, allegedly.
Help me out here, someone ...
H.
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One question. There are suggestions of lengthening accelerator cable, removing air filters and other techniques of getting past the MOT test. From my understating this would be an illegal action (please do correct me if I am wrong). If someone is prepared to do that what?s the difference with that and paying a garage £20 for a dodgy MOT slip?
And I do agree, for what it counts, that the missions test is completely unrealistic. My AX TDG NEVER smokes if you rev it in neutral, but frequently if you are pulling away from lights with two or more people in the car you can see a puff of smoke gracefully exiting the exhaust.
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Because a diesel engine runs unthrottled on the air side it is calibrated when built to inject the right amount of diesel that will burn with out making excessive black smoke. If you inject more diesel (worn injectors/pump) or strangle the air (blocked filter) you get black smoke. Removing the filter allows more air in and tends to reduce the smoke, by compensating for either a dirty filter or worn injectors / pump.
I do not know if this legal or not!
RichardW
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Well where I come from (Northern Ireland) most people get round the diesel emissions by adding one part petrol to ten parts diesel: 20 litres diesel and two litres petrol. Strangely I haven't tried it in my new (204BHP) BMW 330d ...
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