Diesel particulates filter clogs if low mileage?

400ixl

Mr.Generous said:

GDB2222 said:

What happens when the DPF is full of ash? Can it be emptied out, or do you need a new dpf?

Need a new one, but sometimes they are cut out of the exhaust, stripped out completely and its welded back up - but an MOT failure if spotted. Also might fail the smoke test at MOT time. Ive also seen a video where they cut it out of the pipe, pressure washed it (unbelievable amount of crud came out) and refitted it. None of these options are cheap.

You also have the issue with EGR* valves on low mileage diesels - I owned an Astra once and had to strip and clean that twice.

*Exhaust gas recirculation - any unburnt fuel in exhaust and it goes through the engine again resulting in sooty clog up. 

That's not quite correct.

Step 1 is to do a forced regen. Essentially it puts the car into a cycle of running the engine at revs whilst stationary which heats the exhaust enough to the do the regen cycle. Most cars have this capability, just requires the right device to plug in and activate it.

Step 2 is to do a chemical clean in place alongside a forced regen

Step 3 is to remove the DPF and do a direct chemical flush

If all of that fails then you would need to replace the DPF. Although many garages can't be bothered or understand 1-3 and just rip off their customers with straight to replacement.

I would agree though, a diesel is not the right choice for this situation. A petrol or electric would make far more sense.

macman

jimjames said:

GDB2222 said:

So, really, taking ulez into account , the only options are to go for a very recent diesel or a petrol car. Even then, the diesel is likely to have problems.

Petrol car could be pretty much any since around 2001 so that gives a very wide choice. Even my 1998 petrol vehicle is exempt from ULEZ

To avoid the ULEZ charge, a petrol car must be Euro 4, so any vehicle reg'd after 1/2/05, not 2001. Although some vehicles reg'd earlier will still be Euro 4 and so compliant.

Flight3287462

My diesel Superb would hardly be warmed up after 5 miles and in cold weather that would be a pain.  Fuel economy will also be poor in cold weather.

Goudy

EGR's can be a problem.
They can be fitted to both petrol and diesel cars but they tend to cause more issues on diesels.

It's a valve that opens to allow exhaust gas back around the engine into the inlet side.

Engines tend to run lean under certain conditions, like on a light throttle cruise or on the over run where it'll inject almost no fuel at all and just pump air.

When they are lean like this they produce more nitrogen oxide, so the valve's job is to feed exhaust gas which lacks oxygen, back into the inlet to cool combustion and reduce the nitrogen oxide.

Trouble with diesels is their exhaust is very sooty and like most engines, they vent their crankcase pressure into the air intake to get burnt.
Diesels run much higher compression ratios, so crankcase pressure oil side tends to be much higher as well, so they tend to throw out more oily vapour from the crankcase vent.

So now you have sooty exhaust gas meeting oily crankcase vapour in a valve just before the intake manifold.
This is a recipe for a sh*t pie inside the EGR valve which the hot exhaust gas tries to bake to perfection!

All the oily sooty crud jams up the valve's operation, this screws with the fuel air mix into the engine causing running and emission problems.
Now with a lack of air in the mixture which is now taken up with unwanted exhaust gas, the engine effectively runs rich, producing more soot which causes more trouble for the valve and the DPF.

There are oil separators that try and condense off some of the oil from the crankcase vapours and return it to the sump, most are pretty poor but some big diesel engines in trucks and buses etc tended to use better oil separators built into the crankcase venting system which helps keep the oil vapour away from the soot, but these separators tend to be bulky and obviously adds a cost to manufacturing the cars.

Again, short tripping and more cold starts tends to cause more soot, which doesn't help the problem.

Contessa

Goudy said:

A DPF is designed to trap soot in the exhaust, then when it detects the filter is filling with soot it will need to actively regenerate.

This means the engine injects extra fuel on a post combustion cycle so it makes it way into the DPF unburnt which it ignites and burns the larger particles of soot to smaller particle of ash, which it can hold more of.
Even if it all works perfectly it will eventually fill with ash and require attention.

To do this it needs the right conditions, to burn the soot to ash temps have to be really high (some French cars use another fliud, EOLYS to reduce the temps needed) and if you don't quite get them you can start having trouble.

Obviously if the regen cycle doesn't start or complete because you have switched the engine off or it can't meet the right conditions, you fill the filter with soot far quicker than it would fill with ash and the car will detect excess back pressure from the filter and complain via the on board diagnostics.

Another issue if the cycle doesn't complete is the fuel that is injected post combustion.
This can seep down past the pistons and dilute the engine oil.
Not only does this alter the oil properties, but if it happens enough the oil level will rise and if the oil rises enough the engine can "run away" on it (run uncontrollably on it's own engine oil).

If you get the the point of having lots of incomplete regens, it becomes a vicious circle.
The car will try more and more to regen, injecting more and more post combustion fuel which then might not complete, exasperating the problem.

The issue with short tripping a diesel is then tend to make a bit more soot when started cold, the chances of reaching the ideal temp conditions for a regen are greatly reduced and the chances the engine might not be running long enough is also high.

Some cope with this better than others, as already mentioned there are systems that can help the regen process on some cars plus everyone's driving habits are slightly different, but short tripping one isn't ideal.

With your trips being so short, you probably aren't going to see the cost difference in fuel back any time soon either.
Diesel is far more expensive than petrol and to make that back you will need to do a lot more miles.

Some petrol cars have particulate filters but they work very differently.
For one they don't produce as much soot, plus it's a passive system.
Heat from the exhaust gas on the over run (when you take your foot off the pedal) is really really hot and it uses this to burn any soot to ash in the filter.
As there's less soot to start with and the regen is passive and happens many many times when you are driving, these tend to be far more reliable.
In fact, short tripping usually means you're on and off the pedal far more often which helps the process.

Thank you-that's a really clear explanation. One question though-how do you know when regeneration is happening and when it's complete? I drive an old diesel and have never been aware of regeneration taking place.

400ixl

With many cars it is very subtle, slight raise in revs when stationary, drop in reading on real-time mpg. Some do actually have a regen light on the dashboard, but thats much less common these days.

Bigwheels1111

Mine smells hot and metallic, burnt brakes.
Then a metal cooling sound, like ticking.

Johnmcl7

I'd agree with the comments about diesel not being worth it for the mileage, when I bought my first DPF equipped diesel I took up cycling to avoid short trips in the car but even then I had a problem with the EGR at one point, a DPF sensor failed the night before a long trip which meant a hire car and latterly the car seemed to be having more problems regenning possibly meaning it was needing the DPF cleaned out or replaced.

By that point I was only doing around 3000-4000 miles a year so changed to a petrol, the higher VED and lower mpg did sting a bit initially but in ten years of owning the car it's never had an issue with the engine.

35har1old

GDB2222 said:

We were thinking of getting a diesel car, perhaps 8 or 9 years old. We live in London and mainly do short journeys- typically 5 miles. Are we likely to have problems with the filter?

Yes a waning light will appear and if ignored engine may go into limp home mode. Before this happens  you will have to go on a long journey keeping the rev up in fourth gear

Goudy

Contessa said:

Goudy said:

A DPF is designed to trap soot in the exhaust, then when it detects the filter is filling with soot it will need to actively regenerate.

This means the engine injects extra fuel on a post combustion cycle so it makes it way into the DPF unburnt which it ignites and burns the larger particles of soot to smaller particle of ash, which it can hold more of.
Even if it all works perfectly it will eventually fill with ash and require attention.

To do this it needs the right conditions, to burn the soot to ash temps have to be really high (some French cars use another fliud, EOLYS to reduce the temps needed) and if you don't quite get them you can start having trouble.

Obviously if the regen cycle doesn't start or complete because you have switched the engine off or it can't meet the right conditions, you fill the filter with soot far quicker than it would fill with ash and the car will detect excess back pressure from the filter and complain via the on board diagnostics.

Another issue if the cycle doesn't complete is the fuel that is injected post combustion.
This can seep down past the pistons and dilute the engine oil.
Not only does this alter the oil properties, but if it happens enough the oil level will rise and if the oil rises enough the engine can "run away" on it (run uncontrollably on it's own engine oil).

If you get the the point of having lots of incomplete regens, it becomes a vicious circle.
The car will try more and more to regen, injecting more and more post combustion fuel which then might not complete, exasperating the problem.

The issue with short tripping a diesel is then tend to make a bit more soot when started cold, the chances of reaching the ideal temp conditions for a regen are greatly reduced and the chances the engine might not be running long enough is also high.

Some cope with this better than others, as already mentioned there are systems that can help the regen process on some cars plus everyone's driving habits are slightly different, but short tripping one isn't ideal.

With your trips being so short, you probably aren't going to see the cost difference in fuel back any time soon either.
Diesel is far more expensive than petrol and to make that back you will need to do a lot more miles.

Some petrol cars have particulate filters but they work very differently.
For one they don't produce as much soot, plus it's a passive system.
Heat from the exhaust gas on the over run (when you take your foot off the pedal) is really really hot and it uses this to burn any soot to ash in the filter.
As there's less soot to start with and the regen is passive and happens many many times when you are driving, these tend to be far more reliable.
In fact, short tripping usually means you're on and off the pedal far more often which helps the process.

Thank you-that's a really clear explanation. One question though-how do you know when regeneration is happening and when it's complete? I drive an old diesel and have never been aware of regeneration taking place.

That's the issue.
If you do enough miles in a modern diesel fitted with a filter you don't really need to know when it happens or when it finishes, it just happens when the conditions are right, mainly when it's DPF is hot and it needs too. It works out the percentage of soot in the filter via sensors that detects back pressure in there.

Some cars have a warning light, others don't but you can tell as described, increased rpm, starts smelling a bit hot with a slight decrease in mpg.

With some diagnostic software you could probably monitor the percentage it's full and anticipate a regen, but it's a lot of bother.

Some cars will monitor the oil as well.
If it increases in volume it's a sign that the post injected fuel in making it's way into the sump diluting the engine oil.
As always, it's a warning something isn't right, it shouldn't replace a periodic manual check of the oil (level, consistency and smell)

As I mentioned it earlier, some French cars like Citroen use a special fluid called EOLYS.
This is fed into a catalyst before the DPF and helps reduce the temperature needed to burn off the soot.
It usually needs 600+c to burn off the soot. EOLYS reduces that to around 450c which is more easily reached.

A full EOLYS system is meant to last around 70,000 miles, but I seem to recall reading the ECU correlates the fuel tank size, miles ran and how often the fuel flat opens.
So if you continually top up rather than fill up when near empty, it will think the system has less in it than there is and set a low level DTC.

My dad had a car fitted with this system and I took it on after, well you know.
Either of us ever noticed it regening, it must have as some point as it was close to 150k when I moved it on.
Then again, it shortest trip I did in it was 300 or so miles.

I currently drive a new petrol Clio with a FAP (French name for particulate filter).
As described, it's a passive system and works every time I take my foot off the gas, as that when the exhaust temps scream mad hot and burns off any soot trapped in there.
Bonus is it's nice shiny tailpipe never gets sooty, though the exhaust does appear to be steamy at times.

I have heard of some petrol systems causing trouble though, like a diesel DPF it all depends on where it's fitted.
If it's close to the mainifold, the chances of it getting hot enough, regularly is higher.
Some manufacturers fit them further along the exhaust system due to packaging problems and they tend to suffer.