Contrôle Technique - Pollution

Normally I take my Clio to Renault for a pre contrôle technique service, have them mot it, ready for me to pick up.

This time I was unable to do so and took my car to be mot’d myself. It had a major failure - L’opacite depasse la valeur de reception ou les mesures sont instables’. I went to Renault for advice and I was told to pour a 300mls can of ‘Renault Diesel Injection Cleaner’ into the tank and go for a good run - heat up the engine for 30 minutes or so - to clean out my clogged up diesel engine.

I asked if that was sufficient to satisfy the CT requirement under pollution regulations, and Renault said YES!

However there is no mode d’emploi on this can of additive. Do I pour it into a full tank, a half tank and go for a fast drive…or what?

Grateful for any advice!

Get Renault to do it for you :grin:

I have an old bottle of Redex cleaner. It says 20ml to 10 litres of diesel.
I suspect that the contents of the bottle is much the same, regardless of brand name.
I would recommend an hours drive before going for the re-test. A wise man once advised me that diesels take a long time to reach optimum operating temperature. No point in risking a failure!

Have a look in the supermarket for a bottle of wynn,s diesel cleaner, not to expensive, just add the whole bottle to a tank of fuel, worked a treat for my car,

Right, I have just seen that online. One bottle to a minimum of 20 litres.
Possibly Renault’s is the same stuff, which might explain the absence of instructions.

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What I have found works well is to add the entire contents of the injection cleaner to a half full tank, then immediately top up the fuel tank to full, and then go for an hours drive as Mike suggested. Try to use lower gears when possible because you need to heat up the exhaust system as well as the engine. Heating up the exhaust system will help to get rid of any condensate build up within it (often caused by repeated short journeys) that can be the cause of the ‘opacity’.
On the day of the re-test, choose a route of at least 20kms to reach the testing station, and aim to arrive just 5 minutes before the appointed time. Again, use lower gears to keep the revs up in order to make sure that both the engine and exhaust system are up to full operating temperature.
If the above doesn’t work, then you probably need a full diagnostic with attention paid to such things as piston rings, valve stem seals, fuel injection system etc. Excess exhaust emissions cost money in wasted fuel as well as being bad for the environment.

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Thanks so much Robert and others. I shall do what you suggest Robert - sounds eminently sensisible - and is something I was contemplating doing. The Renault engineer said something about pouring the additive into the fuel tank when about quarter full and then topping up. But he seemed impatient with me at the time so didn’t pursue the matter further.

I am also contemplating an engine overhaul as it’s done more than 170,000 kilometers - which could be looked into if necessary when I have my next service due in 10 days time.

A compression test on each individual cylinder is probably a good idea at that level of kilometrage.

Well, did what was advised about pouring a can of diesel injector cleaning fluid into the fuel tank, raising engine revs to heat up the engine, to flush or burn out diesel gunk. Worked! Passed by Contrôle Technique.

Thank you all.


Worth pointing out that any vehicle with injectors will benefit from the use of injector cleaners. That’s petrol as well as diesel. They help to improve fuel consumption and general performance, as well as reducing pollution and getting you through the CT.

You are welcome. You should find that you will get a few more miles to the gallon now due to increased engine efficiency.
It’s worth doing annually if you only do short journeys. I keep a spare can of the cleaner in the boot and then use it when I top up the tank prior to a long journey.

My car was booked in for a Renault service which had to include brake inspection - CT minor failure - resulting in new pads and disks. I drove the car away when finished and was surprised at the new life in the engine, because of the injector cleaning fluid, and the increased efficiency of the brakes.

It’s so easy to get used to a sluggish engine and tired brakes. I have learned something here. Feels like a new car now!

I’ve just received a quitus fiscal for a vehicle I’ve owned for many years but hardly driven, so absolutely delighted. One of the next steps is to go and get a controle technique, but just wondering if it will pass, as when I start it there is a plume of smoke emerging from the exhaust. This does dissipate the longer I have the car running. I’ve read a few posts, and this one seemed to be the most relevant, so will certainly try the injector cleaner. It’s a 1998 petrol car, in immaculate condition, but it has been standing for some time with a full tank of fuel. I understood that the injector cleaner needs to be ‘mixed’ in the fuel, but if I add it to the full tank of fuel, I wonder if it will indeed help, as I really don’t want to drain the tank.

If it’s been standing idle it might be an idea to have it serviced before the CT… ??
(just a thought)

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The fuel itself might be no good if its been in the tank for a year, and injectors can get a bit gummed up. J would still put a fuel system cleaner in, and take it for a long run. As @Stella says, a service might be in order before a CT

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I did service it myself, as far as changing oil, all filters, plugs and various other bits and pieces. Before servicing it wasn’t running, but that was soon fixed as an air intake sensor had simply failed - thanks the lord for youtube!

I really wanted to avoid having to empty the tank :scream: wondered if there is another way of ‘refreshing’ the fuel??? e.g. something I could add???

AFAIK there’s no way around this. In the UK there are people who specifically offer a fuel tank emptying service (usually for diesel owners who put petrol in their tanks) so you might find something similar.

That’s what I’m afraid of - will keep the thinking cap on a little longer, as it’s a Summer car so the priority is to get it back on the road for the good weather in Spring!

You could drain the fuel by disconnecting the fuel delivery line under the bonnet, connecting a suitable length of flexible hose to the fuel pipe, and then switch the ignition on to use the vehicle fuel pump to empty the tank for you into suitable containers. Best to do this outside in the open air and remember to ensure that there are no sparks or other ignition sources nearby.

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It’s a mid engine car, so need to take a look at the fuel line routing, but thanks for the suggestion