Advice On Buying A Used Diesel Car

The appearance of the Diesel Particulate Filter on modern Euro 4 and Euro 5 diesel cars has made buying a second hand used diesel car, even more of a minefield than previously. Because the Diesel Particulate filter is considered to be a consumable and therefore outside of the usual warranty cover (Unless you live in the U.S) it can mean that this expensive potential for failure is not even covered if you buy from a dealer or a used car showroom.

Prior to the DPF being fitted to all new diesel cars since 2008 (Some manufacturers fitted them from as early as 2005), finding a low mileage older diesel car was considered something of a bargain, however these days, a lower than average mileage DPF equipped vehicle should best be avoided – let me explain why.

Modern Diesel cars with DPF’s are now only recommended for drivers who cover at least 15k miles per year, ideally on motorway (highway) based roads. This is because the DPF system needs to regenerate itself on a regular basis (often at least once every 300 – 500 miles) and to do this, requires a set of criteria to be met, a criteria which can only be achieved if the car is being driven above 50mph (90 kph) for a prolonged period on a fairly regular basis. Generally in the UK, the only place you can maintain these types of speed for 20 – 40 minutes is on a Motorway.

Finding a car with a below average mileage for its age would suggest that the previous owner(s), rarely used it on long journeys and it probably was only used for driving slowly around a town or in stop-start city traffic, directly in conflict with the driving style recommended by the majority of diesel car manufacturers, and the use which is likely to make the DPF block and fail due to excessive soot and ash loading, prematurely. For this reason, avoid “low mileage bargains”, because what may be a low mileage engine, could easily cost you well into four figures for a new DPF as it will probably already be severely blocked, and you could be inheriting a problem child with expensive demands.

Make sure that the service history is accurate and the car has been serviced within the recommended intervals, also check that the car has been given the correct (low ash) oil. Avoid cars which have been serviced at typical drive in “quick service” outlets as there is a huge chance that incorrect oil has been used at some point, since due to its expense low ash oil is not generally stocked or routinely used. If in doubt make sure you supply your own oil!.

Check the vehicle exhaust outlet for signs of soot, and run your finger around the inside of the outlet. A car with a correctly working DPF should produce no soot and so the exhaust outlet should be soot free. A dirty finger indicates the presence of soot, and this means that either the DPF is faulty and will need replacing (Big Money) or worse still that the DPF has been bypassed or the internals knocked out, which in the UK and Europe is illegal and now an MOT (inspection) failure.

Many early DPF equipped diesels had their DPF filters removed (because of the frequent & expensive problems that they caused) this was normally done by removing the DPF, cutting a hole in the case and removing the internals before re-welding and refitting. In March 2014, making these modifications became illegal and an instant MOT failure, it may also void the insurance cover and make the vehicle unroadworthy under “construction and use” laws all of which can leave the owner open to prosecution as well as effectively being uninsured. It is therefore vital that you avoid any vehicle where you suspect that the DPF has been modified or removed, and because such modifications may not be visible it may be wise to pay for a third party inspection before buying the vehicle, just to cover yourself.

Take the vehicle for a long test drive before buying, make sure that there are no warning lights on the dash and that the vehicle pulls correctly and doesn’t feel sluggish. Run the vehicle up to normal operating temperature and check that the coolant temperature sits correctly in the middle of its normal operating range. Luxury Cars such as BMW’s will display the condition of its DPF on the I-Drive system, so this should also be checked.

A DPF is also a consumable, and just like a timing belt, it will eventually reach the end of its service life and need to be routinely changed, when the level of ash reaches a certain point. Since a replacement DPF can cost between £1000 and £3000 it is important that you check the current mileage of the vehicle, and the recommended DPF replacement intervals in the vehicle manual. Most DPF’s need to be changed every 100k to 120k intervals, but some can be as low as 70k to 90k. Do you really want to be buying a High Mileage car which may need a £2000 ‘service’ soon to replace its end of life DPF system?, well when the DPF reaches a certain mileage / condition, that is exactly what you may be facing.

It is also worth remembering that DPF systems can be problematic and can and do fail prematurely before these normal replacement milestones, depending on how they have been driven and used by previous owner(s). Either way, if you are considering buying a diesel car which is close to approaching its DPF replacement interval then you should factor this into the price if the current owner / dealer cannot provide proof that it has already been replaced, and when it was done.

If buying from a dealer who is offering a warranty on the vehicle, then don’t just assume that the DPF system is covered, in most cases it isn’t, in fact some manufacturers don’t even include them in the standard warranty. Ask for a list of exactly what is covered, and if the DPF isn’t included, then the warranty isn’t going to be of much use as the DPF is perhaps the most expensive part of the car after the engine itself, and the component which is also more likely to fail than anything else, so either negotiate DPF cover, a discount for future DPF problems or better still walk away and consider an older non DPF car, or a  petrol.

When selling a used diesel car with a high mileage which is close to the end of the DPF’s expected life cycle in the manual then you should also factor in that the new owner will be buying the car on the basis that they could very well be facing a £1000+ bill in the future for replacing the DPF, and you should expect an offer which reflects this.

So, the DPF will negatively affect the residual value of your car, whether you buy new or used, the higher the mileage the more likely the DPF will need to be routinely replaced as a service consumable.

If you are still intent on buying a used DPF equipped car ensure that the Glowplugs are all working, and the car quickly reaches its operating temperature, the car should also idle smoothly from cold and not appear to be lumpy. Failed Glowplugs and Thermostats are the biggest cause of failed regeneration attempts which in turn can cause the DPF’s to fail and need replacement. A lumpy engine from cold, also could indicate a blocked and failed EGR valve, again a cause of premature DPF failures.

However probably the best advice I can offer, is to avoid any new diesel car which has a DPF fitted, and instead choose an older pre-DPF diesel, as they were far more reliable before they started bolting emissions based crap onto the engine block!. Always a problem when you start letting tree huggers design cars, rather than engineers!.

However, if you do decide to buy a used diesel car with a DPF, it is important that you service it and look after it to prevent expensive problems biting you in the future. Once bought, make sure that you get it serviced using low ash oil, and also pay for the EGR Valve to be cleaned at the earliest opportunity – which is often the underlying cause of DPF Blockages.

Avoid using the car for short journeys and school runs (the type of journey where the engine doesn’t get to its normal operating temperature), ensure that you routinely take the car for at least one blast at above 50 mph (90 kph) for at least 20 minutes every few hundred miles of normal use, in order for the DPF to regenerate.

Using a DPF Cleaning additive which is added in small amounts to every tankful of diesel fuel is also a good idea, and isn’t expensive.

6 Responses to Advice On Buying A Used Diesel Car

  1. Gary. Sticka says:

    Hi can you tell me if the 2007 mitsubishi grandis 2.0 did engine has a dpf thanks

    • admin says:

      The Mitsubishi 2.0 DI-D engine was sourced from VW, and is actually the same engine as was used in their 140BHP 2.0 TDI Models. This was known as a PD engine, and at the time VW declared it as being unsuitable for DPF fitment, hence their 2.0 TDI cars didn’t get a DPF until around 2009 (as late as 2010 in some Skodas) when the engine changed from PD to CR design. However, a quick of the aftermarket parts catalogues, does show a replacement DPF available for a 2007 – 2010 2.0 D-ID Grandis, which complicates matters.

      Since Mitsubishi were one of the manufacturers who never responded to my enquiry email regarding their DPF fitment when I created this blog, I am unable to say for certain whether the 2007 does have a DPF or not. I would advise, when you view the car to do the “Soot Test” – this simple but effective test, carried out by running a finger around the inside of the exhaust tailpipe, can give you a good idea as to whether a DPF is in place, if there is a large amount of soot / blackened exhaust tail pipe exit, then there will be no DPF.

  2. Terry says:

    Hi
    I have a Zafira 2012 with dpf
    Unfortunately
    I do all the things you say not to example short school runs
    From time to time my dpf waning light coils flash
    I take the car on s long run and the waning light goes off
    Maybe a couple of weeks later the light is on again
    This is often a lot less than 300 miles
    Please can you tell me if the car is only used for short journeys
    Does this mean the car will need to be regenerated well under the usual 300 mile s
    I think I’m doing about a hundred miles of short school runs wen the warning light come on

  3. admin says:

    Yes, short journeys will result in more frequent DPF regenerations, due to the fact that the DPF isn’t cleaning itself passively (The DPF will burn off the soot load that is collected through normal driving when used on long journeys, and its this passive regeneration which extends the mileage between the forced (Active) regeneration attempts. If you exclusively only use a car for short runs ( under 15 miles) where the car will barely get up to its normal running temperature, means that the car won’t burn off its soot loading, naturally through the exhaust gas heat from normal driving and if it can’t keep the soot level below the threshold passively, it will then rely solely on those forced active re-generations.

    There is a potential complication to this. Largely the fact that you are more likely to run the risk of a regeneration attempting during a school run, and you switching off the engine before it completes, resulting in unburned diesel leaking into the engine oil, which is not only detrimental to engine wear (as the oil lubrication qualities have been diluted), but that there is also a small chance of significant damage being caused to the engine if the oil level goes far enough above the maximum mark on the oil dipstick to cause it to “run away” – fueled by its own engine oil, so its important to check your oil level frequently – ideally at least once a week, and also perhaps change the oil every 6000 – 7000 miles or 12 months.

    From how you describe in relation to the warning light coming on every 100 miles, it seems that the car isn’t even getting up to the temperature required to carry out a forced active DPF Regeneration itself, hence the light coming on, which indicates that the DPF has reached the upper point where the amount of soot that it has collected is affecting normal engine operation. You are doing the right thing, by taking the car on a long journey, as soon as possible when this light comes on, otherwise if ignored, it will reach the point where it is so blocked, that you will need to pay the dealer to do a forced regeneration using their diagnostic kit, or worse still, require a new DPF

    The DPF also contains a compartment at the bottom of its housing, which holds the ash, which is created every time the DPF regenerates and the soot burned off, when this compartment becomes full of ash the DPF essentially reaches the end of its life and will require either a professional cleaning or a replacement, neither of which is especially cheap. Of course the more re-generations the car does, the quicker the ash compartment will fill, and so regenerating every 100 miles instead off 300+ isn’t an ideal situation, as you may hasten the demise of the DPF at a much lower mileage, instead of the usual 100k – 120k mile designed life expectancy.

    All of this is basically indicating that you really should be considering a petrol engine for this type of use (or an older non DPF diesel), as DPF equipped diesels really shouldn’t be considered by those who do less than 12k miles a year or a lot of short journeys. The difference in economy between diesel and petrol for such short journeys really doesn’t even factor into the equation, and the £1000+ which will eventually be required to replace the DPF buys you a lot of petrol, even at today’s prices.

    In the meantime, if you continue to use the Diesel car for this type of use, I recommend that you use only premium diesel (Shell Nitro / BP Ultimate etc) which produce less soot, and that you take the car on a few long journeys before the DPF light comes on, so the car can regenerate its DPF passively and so extend the interval between active regeneration attempts. The DPF warning light is exactly that, a warning that the car isn’t being used in relation to how it should be used, it really shouldn’t be illuminating on a regular basis as part of your normal use, as you are just putting additional strain and potentially decreasing the longevity of an extremely expensive part of your car!.

  4. Stephen Owen says:

    Very informative read My 406 Estate is getting a little tired 187,000 miles and have been looking for a replacement. The Skoda Octavia is top of the list at mo and as I only have £2500 max to spend I’m guessing I’d be getting a non DPF model (around 2008 i’ve been looking at) Any other issues with these that you know about ? I know can belts can be expensive to replace and have to be done at 60,000 Thanks

  5. admin says:

    Hi Stephen, the 1.9 Tdi PD Engine is the one to get in order to avoid DPF problems as it never had one fitted, since the engine and fuel system design made it impossible to use one. Both the 1.9 Tdi and 2.0 Tdi have their share of expensive problems though, which are far too extensive to go into in any detail here. I would seriously advise googling “1.9Tdi engine Problems” and “2.0 Tdi engine Problems” before making a purchase though. It seems that the 1.9 Tdi engine became less reliable from about 2005, and one of the problems that they are known for appeared after the changeover to the ‘PD’ engine which use Siemens injectors, as opposed to the Bosch injectors used on earlier versions of the 1.9tdi, and the Siemens piezo injectors are known to fail and are extremely expensive to replace.

    Probably the best engine that VAG ever produced was the 1.9 Tdi 130 Bhp engine, which was used in the earlier Skoda range, these were pretty much bulletproof if looked after and are still the choice of some taxi drivers today, for good reason!. Sadly production ended in 2005, and cars of this age have huge mileages and are showing their age now, but it would be the version I would favour over the newer 1.9 and 2.0 versions, I personally wouldn’t purchase a car with either the 1.9 or 2.0 between 2005 and 2015, but out of the two and to avoid the DPF, the 1.9 CR Tdi is the better option.

Leave a Reply

Your email address will not be published. Required fields are marked *

*