my 11yr old diesel smokes a bit on hard acceleration now and again. Its got its MOT next month and just trying to make sure it’ll pass on emissions.

Do the diesel additives from STP / Redex etc work and if so any reccommendations? Is it the injector cleaner i should use or just the general diesel treatment bottle?

Or do i just take it for a drive with some hard accelerations just before the test?

thanks

Ime option 2 is best.

Use some top end diesel like BP Ultimate or Shell Vpower for a couple of tankfuls, and regular long runs with some hard acceleration as you say (Italian tune up)

Just boot it’s few times. I know many a MOT tester who would give a car a few minutes of high revs to clean out the crap

Italian tune up it is then 😀

When did you last change your oil and air filter?

Italian tune and a bottle of STP injector cleaner and expensive fuel for a couple of tanks should clear soot build up….and obviously a service and filters before MOT will help. But……if it’s blue smoke its oil burning, if it’s white smoke it sunburnt diesel (or head gasket gone but you woul have other symptoms ie coolant dropping), if it’s black smoke that’s soot or can be also a low pressure turbo pipe split or popped off.
Never failed me on any of mine or the families diesels and I’ve had some old high mileage ones!

at its last service i guess, probably about 9000 miles ago, its on variable servicing but says it doesnt need another yet. Might get them done at the MOT though as it’ll save me taking it in again later in the year.

When I first MOT’d my car it only just passed the smoke test at 2.97 (fail is 3.0)despite giving it a good Italian tuneup before going in. I was recommended to use BG244 fuel treatment. This I did each year as part of my servicing schedule. Same machine and tester it went down to 0.46 within the next 3 MOT’s. It is even less now which isn’t bad for a 10 year old Mondeo.
No doubt there will be others saying snake oil but the figures speak for themselves. I now use Archoil AR6400-D same benefits but cheaper as buy it bulk.

Just give it a good thrashing. IIRC there isn’t actually a hard limit on the smoke test and the tester can use their discretion if they want (this is why you see buses still belching smoke).

Italian tune up it is then

get ready to familiarise yourself with the engine management light and limp-home mode 🙂

For the relatively low cost, a tin of ‘Fuel System cleaner’ isn’t a bad idea.
As others have said – tankful of posh diesel the week before the MOT + Italian tune-up on the way to the test centre.

Worked OK for my various smokey old Landrover Diesels over the years..

I used to remove the fuel filter and tip a big slug of injector cleaner straight into the can before screwing it back on – not sure if it was a good idea though…

I’d say unless you fail and need a retest why worry? 14 year old van with 330k miles – never seen a drop of premium fuel or a bottle of homeopathic fuel treatment and emission tests have never been a concern

IIRC there isn’t actually a hard limit on the smoke test and the tester can use their discretion if they want (this is why you see buses still belching smoke).

You recall wrongly, a bus test is a very different test to a passenger car, hence a passenger car is a class 4 test and a bus is class 6 if operated for hire or reward.

The test criteria used is based on age, unless it’s very where it is a visual, otherwise it is 3.0 or newer vehicles 1.5, and this again depends on whether there is a turbo fitted.

A good vigorous drive with the vehicle up to temperature usally clears problems, unless there is a fault with the vehicle. Tend to see the most issues with smoke, with diesels that are pootled round town only doing the school run.

I was having power problems with my D5 and was told to give it a good drive out just ticking the redline in the lower gears, out of roundabouts, junctions etc
Lo and behold it made a world of difference.
Every drive, if you can when it’s warmed up, get used to that redline in the lower gears, it’ll do it the world of good 😀
My car is 12 yo and has 135,000 miles 😀

A mate with a 200k+ diesel Mondeo had such low emissions on the MOT the testers comment was “you really drive this don’t you”. And yes he does. A regular ‘smoking’ will help prevent smoking.

BG244 comes well recommended, I threw a can in mine when I got it and it made no difference whatsoever but that’s ok, it might have been fine already. The bonus of the italian tuneup is that even if it doesn’t work you’ve still had a a laugh, if a fuel treatment doesn’t work you’re just poorer.

Maybe getting off the topic but if a bit of mad blasting doesn’t resolve it, have you considered maybe there’s actually something wrong with it? Like, mine with its flakey maf sensor smoked at full power (and lumped occasionally at idle), it could probably have got through an MOT but fixing the fault didn’t cost much and saved more in fuel than it cost…