Any Diesel 4 x 4 - DPF's. Still a problem?

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Tue 26 Apr 2011 22:33
Any Diesel 4 x 4 - DPF's. Still a problem? - Dieseldoubter

There are vast numbers of diesel cars on the roads and many makes/models are now only available with diesel engines. Not all of these vehicles can be being driven under 'ideal conditions'. If there were continuing problems then surely owners would be 'up-in-arms' and there would be plenty of media cover. My own straw poll has not found anyonewho has experienced a problem (many do not know what a DPF is and that their vehicle has one!). So has there been a marked improvement in DPF technology that has overcome previous problems? Finally is there any sort of league table of manufacturers problems (or lack of) with DPF's? Do any cover DPF's in their warranties and is there any correlation between engine size and DPF problems?

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Tue 26 Apr 2011 23:55
Any Diesel 4 x 4 - DPF's. Still a problem? - unthrottled

4 X 4 work their engines much harder than equivalent sized cars. as far as DPF is concerned, this is good. Higher load leads to higher exhaust gas temperatures which makes regenerating the filter much more straight forward.

DPF problems tend to manifest themselves with engines that are underworked for most of their duty cycle.

Big engines allied to small cars, short journeys and gentle driving present the biggest challenges to particulate traps.

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Wed 27 Apr 2011 05:12
Any Diesel 4 x 4 - DPF's. Still a problem? - oldroverboy

Excellent reply!

If anyone is getting a modern diesel give it the chance to regenerate, mine does a 40 mile run at least once a week. (motorway speeds)

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Wed 27 Apr 2011 12:52
Any Diesel 4 x 4 - DPF's. Still a problem? - Vrev

Hi,

I came on here to start a new thread but here seems a relevant place to ask the question. I am looking to shortly buy a secondhand Mercedes ML320 diesel. I'm looking at cars around 2.5 - 3 years old.

There is one currently available at around the 10k mileage mark. Otehrwise I am looking at 30-45k. Am I better off going for the higher mileage choice? Is the 10k option potentially more risky given the lack of use and likely short journeys?

I'll be doing around 12-15k a year myself.

Thanks in advance

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Wed 27 Apr 2011 13:03
Any Diesel 4 x 4 - DPF's. Still a problem? - unthrottled

Almost definitely. There's an old adage that it's not miles that kill a car it's age.

10K over 3 years works out at ~3K a year-a ludicrously low mileage wholely unsuited to diesels. Taxis frequently clock up huge mileages-often from cars of very average quality. The main reason is that they fire them up on Monday morning and shut them down on Friday night. The engine virtually never cools down.

Low mileage cars are often owned by people who refuse to spend money maintaining them because they assume that because they don't drive much, the mechanicals are prestine.

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Wed 27 Apr 2011 17:42
Any Diesel 4 x 4 - DPF's. Still a problem? - jamie745

Very good point. Taxi firm near me still uses 10 year old Peugeot 406 HDi's, i was in one a few weeks ago which had 257,000 on the clock and was still absolutely fine.

Edited by jamie745 on 27/04/2011 at 17:45

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Wed 27 Apr 2011 19:02
Any Diesel 4 x 4 - DPF's. Still a problem? - Vrev

Thanks - I figured as much.

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Wed 27 Apr 2011 22:51
Any Diesel 4 x 4 - DPF's. Still a problem? - OldSkoOL

First reply was spot on

I have a 3.0 twin turbo diesel, loads more power than i need but the autobox is excellent at managing this. It will cruise at as low as 1k, typically around 1.3k. It will hold onto a gear this low down the RPM range when going uphill, almost feeling like it is labouring. Massive load, it will also change up just as it hits max torque where loading typically drops in most conditions. Checking the temp's they are around 90-93C.

If i flick it into sport mode the revs range between 1.8k and 2.2k cruise. I do this when i go for a bit of a spirited drive. I struggle to keep the temp's over 80. Often running in the high 70's. There is hardly any load on the car at 2k RPM.

9 times out of 10, a DPF regeneration will occur after a spirited run, it can be as frequent as every 500 miles in sport mode. However in normal cruise mode i can go many tanks, potentially 2-3k miles before i notice a DPF regen. Whether there are passive regens going on i dont know but it is very clear that the levels of soot are greatly increased driving under light loading.

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Wed 27 Apr 2011 23:03
Any Diesel 4 x 4 - DPF's. Still a problem? - RT

If your temp is dropping into the '70s, it's out of it's efficient range - sounds like a faulty thermostat.

I've not know temperature variations of more tha 5 degrees once \an engine is warmed up ASSUMING the engine has no faults.

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Wed 27 Apr 2011 23:30
Any Diesel 4 x 4 - DPF's. Still a problem? - unthrottled

I've not know temperature variations of more tha 5 degrees once \an engine is warmed up ASSUMING the engine has no faults.

If you use a scan guage to record actual measured coolant temperatures, you'll find they fluctuate over a considerable margin. Dashboard temperature gauges are heavily damped and will sit bang in the middle from 75~95C. Manufacturers do this intentionally to prevent needless concern from drivers worrying about wandering temperature gauges.

Think about it. A diesel goes into overrun and carries on pumping cold air through the engine, yet adds no fuel. Of course the engine will cool down. The actual coolant temperature as measured by the temperature sensor is, in itself, unimportant. But it is an indirect indication of the temperatures of the exhaust port, cylinder liner and piston crown. And these temperatures do matter.

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Thu 28 Apr 2011 00:31
Any Diesel 4 x 4 - DPF's. Still a problem? - OldSkoOL

Thanks for your concern

The temperature readings i obtained are from the hidden service menus available on my vehicle. They are the actual sensor readings so not the averages you get on the dashboard display and i should point out the last time i took the car for a "hoon" was when the ambient temps were about 6 or 7C lower. That perhaps contributed to the difference too.

Edited by OldSkoOL on 28/04/2011 at 00:31

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Thu 28 Apr 2011 11:34
Any Diesel 4 x 4 - DPF's. Still a problem? - RT
Think about it. A diesel goes into overrun and carries on pumping cold air through the engine, yet adds no fuel. Of course the engine will cool down. The actual coolant temperature as measured by the temperature sensor is, in itself, unimportant. But it is an indirect indication of the temperatures of the exhaust port, cylinder liner and piston crown. And these temperatures do matter.

I still don't understand why the coolant temperature drops so much. Once the thermostat closes, the coolant will only be circulating around the block and oil which still has plenty of thermal capacity to give to the coolant plus the rate of cooling without the radiator in circuit will be low - so the coolant will rarely drop below the thermostat opening temperature.

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Thu 28 Apr 2011 14:59
Any Diesel 4 x 4 - DPF's. Still a problem? - unthrottled

Actually getting the engine up to operating temperature can be the real challenge. I forgot to add that the engine itself is subject to significant direct air cooling via air moving through the engine bay. This is particularly true when the coolant in the radiator circuit is cold.

The other point to note is that the temperature gauge is usually mounted in the cylinder head-which is one of the hottest parts of the engine-and the first to warm up. The actual engine block typically lags behind the head in terms of engine warm up.

I've got a couple of SAE papers buried somewhere where they tested a 2.0 petrol engine held at constant low throttle setting from a 20C cold start. The thermostat opened after about 8 minutes, but the oil temperature took 20 minutes to stabalise at 'correct' operating temperature. A diesel engine would probably never reach operating temperature under these conditions.

You're right though, once up to operating temperature, the termal mass is huge and the temperature will be fairly stable.

This is all of rather academic interest since there is not much a driver can do about it-apart from worry! I do partially block off the front grill and air vents in winter though. Ugly as sin but it does help with the warm up.

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