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remember years ago that for mot test they revved diesel cars at high revs for a period of time. this is to check emmissions at high load. (at lot of engine failure occured due to cambelt breaking) is that practice still current. i was at a mot test centre and a tester just revved a diesel slightly checking emissions at lower rev range
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I've never seen my tester do anything at high load, only high revs. My last test was Aug.2005, and it hadn't changed then. The advised way to pass the test was to drive several miles first, doing the high revs bit yourself to blow out the soot. A shot of conditioner may help too.
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Test isnt done at high load.its free revving this should show any problems anyway,ie any problems with injection system would show on standing test
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Steve
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They floor the accelerator for 5 seconds, think they repeat that process 3 times if I remember rightly. Cambelt would have to be absolutely knackered if it snapped during that.
Cars that fail the smoke test are usually taxis or other cars stuck in town. The tester I know drives the cars on a 10 mile round trip at 70mph in 3rd gear, after which they always pass!
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My tester just blipped the throttle three times and it passed with flying colours.
Believe they do it until the machine indicates it has a valid reading.
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A little suspect the tester should not be driving the car on the road or do you mean the man that takes your car for the test
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My tester never revs our diesels up to the limit. He says its a recipe for snapping a customers cambelt and how would he explain it to a customer who came back to find his engine destroyed. He also goes on to say this practise does not relate to real life useage of a car and isnt something he would do with his own car.
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In a previous test (about 18 months ago) they had to take mine to high revs to get a reading. I had 15% biodiesel in it at the time and they couldn't get a reading at low revs, I think the machine thought the test probe wasn't inserted correctly.
But at my last MoT they didn't have to rev it, it passed at idle and the emissions test printout says "Fast Pass".
At my local place they now have signs up and leaflets, warning of the risk of a snapped cambelt, and saying you should tell the tester if you think yours may be suspect.
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"But at my last MoT they didn't have to rev it, it passed at idle and the emissions test printout says "Fast Pass"."
It would have been revved - the test still requires a run to full governed engine speed - if a diesel smokes at idle there's something very wrong with it! The 'fast pass' allows it pass on the first acceleration if the measured reading is below 1.5. If it doesn't pass this, then 2 more are supposed to be carried out when the average must be <2.5 (non turbo) or <3 (turbo) for a pass. If still no pass, then up to 3 more are allowed to get the average under the pass. If not, then it fails.
It's a very brutal sounding test, and best not to watch / listen if it's your car....
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RichardW
Is it illogical? It must be Citroen....
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Sorry I stand corrected, I must have not been paying attention at the last test!
It's a very brutal sounding test, and best not to watch / listen if it's your car....
Yes the previous time, I was stood behind my car when they tested it, it sounded pretty terrible and made me wince!
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my advice is always to find a diesel mot tester that knows what he is doing.
Dont let him practise shrapnel attacks with your pride/joy
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Took my C5 2.2hdi for the MOT 2 weeks ago.
This engine has a particle filter which captures most of the soot.
I think the tester thought his smoke meter was defective as he reved the engine to it limiter at least 6 times & held it there for a few seconds each time - certainly made me wince.
The printout showed 0.1 & as others have stated the limit is 3.0.
You dare not say anything in case the tester finds something else subjective to fail it on.
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The MOT test procedure of running a diesel to the governor off load is brutal and potentially damaging. When my own diesel goes in for an MOT, I reduce the max governor speed setting from 4540 to 3500 rpm. As an ex diesel engineer I have done this for the last few years and it has not been detected.
Drives like a pig to and from the test station though.
659.
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That Honest John website had something about this recently.....
www.honestjohn.co.uk/forum/post/index.htm?t=6459&v...f
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Hi dieselnut,Barty here would it be possible for you to read my posting 26.06.06 it starts with S.O.S new to site. It relates to a similar problem you had about your passat losing power. I'd be very grateful if you could reply to it. Thanks
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Sorry dieselnut the message header now starts with "Camper van with 1.9tdi"
Barty
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What is so fragile in a diesel engine that you lot think it will collapse into a heap of metal shards if it runs at the rev limiter for a minute or so? Why would a cambelt go on a diesel engine at say 4000 rpm, but a petrol will run happily at 7 to 8k rpm or my bike at 13k rpm.
Martin
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I'll second that, diesels are often touted for their robust qualities yet so many people seem afraid to rev them. Maybe its just me but there is nothing I enjoy more than running my engine to the limiter in the first couple gears.
Maybe such use will cause the engine to grenade before the year is out, but if you can't enjoy it (or are afraid of it breaking), get rid is what I say.
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Maybe such use will cause the engine to grenade before the year is out,
There's a difference between revving out through the gears, and free-standing revving. The latter is under a no-load situation.
Even fore-going that, have you tried revving your engine to the limiter with the car at rest in neutral ? It takes a bit of nerve, and doesn't feel at all natural... mechanical sympathy = nil.
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>>What is so fragile in a diesel engine that you lot think it will collapse into a heap of metal shards if it runs at the rev limiter for a minute or so? Why would a cambelt go on a diesel engine at say 4000 rpm, but a petrol will run happily at 7 to 8k rpm or my bike at 13k rpm.
Its referring to engines that have not had cambelt change *as per warning in MOT station*when they should have done,ie risk of cambelt breakage is high and are disclaiming any problems relating to it if it fails during test.
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Steve
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The cambelt doesn't know whether it's no-load or full-load.
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The cambelt doesn't know whether it's no-load or full-load.
My thoughts exactly and considering it's running at abound half the speed of a petrol engine..
Running an engine at full revs when it's stationary is no noisier than seeing a car or bike on a rolling road dyno - it's just that when driving, with enthusiasm, you're in an insulated car with wind and road noise masking the screaming engine.
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Rate of acceleration could be greater under no load though. Generally most mechanicl failure occurs when you close the throttle from WOT.
Although agree no issue with warmed up engine going to the rev limiter.
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Nobody here ever heard of piston ring float which occurs when a engine is reved hard under no load conditions
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Nope - so a more elaborate post would be handy. ;o)
Every day's a school day!
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If you have ever seen a truck diesel on the test bed,running at full load,max revs-the manifold will be near white-hot,the whole turbo and first 6 ft. of the exhaust will be well red-then a short smoke test is nothing.
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when an engine is under load forces act on the piston ring caused by combustion on top of ring even in the sides in the piston, this tends to keep the ring in the same position.
Engine under no load hardly any fuel being burnt minimal pressure on ring & high revs ring can chatter in groove and break. more likely to happen on a worn engine
Also a similar effect on big ends too, again more likely on a worn engine
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Thanks for that - makes perfect sense now.
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Rate of acceleration could be greater under no load though.
Could be...?! I think not much doubt the tacho needle will go around a bit faster with no load...!
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I used to work with a Rover trained mechanic back in the 70's. His factory supplied info was that the OHC (P5 or 6?) 2000 engine should never be revved unloaded above about 3000 rpm as the camshaft was likley to shatter.
When loaded the engine was safe to rev to it's limits.
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Engines should never be revved off load due to the danger of torsional oscillation in the crankshaft and timing drive. The load on an engine acts as a damper to these oscillations, which explains why an engine can be taken to its rated speed when loaded with relative safety.
The free end of the crankshaft (usually the pulley end) always carries a torsional vibration damper on a decent engine - usually the pulley is made in two parts, separated by a rubber section. Larger engines have a significant decoupled mass at this point, usually fluid filled.
659.
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>>The load on an engine acts as a damper to these oscillations,...
How effective are the springs in the clutch driven plate in decoupling the torsional oscillations of the engine from the rest of the drivetrain?
Going a little further, as the inertia of the vehicle, as seen from the flywheel diminishes rapidly in the lower gears, does this mean that revving in low gears is more of a danger than revving in higher gears?
Number_Cruncher
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A clutch driven plate is very carefully matched to the drivetrain in order to avoid torsional resonances. If you look at a typical driven plate, there will be either springs of differing rates or windows of differing working lengths (you need to look carefully to see where additional springs become effective as torque is applied) or both. Almost without exception, the torsional stiffness of a driven plate assembly increases non linearly with applied torque in order to control drivetrain oscillation.
There is indeed more danger of torsional oscillation damage through revving in the lowest gear, both from the point of view of increased engine acceleration and the reduced damping effect of the load. However, the engine acceleration rate even in a low gear is usually about an order of magnitude less than the unloaded acceleration rate. I spent many happy hours designing big diesel governors which would catch an engine off load and yet give stable control on the governor run-out. For some large scale applications the engine has to be protected even if the load is suddenly removed as in the case of a shaft breakage. The diesel engines we can afford will not necessarily enjoy this level of protection.
659.
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