Diesel car, worth considering?

Andrea15

https://forums.moneysavingexpert.com/discussion/comment/81914930#Comment_81914930

From what I read, you can easily buy it for like £15 for 10L, and put it in your car yourself, so what is the big deal?

daveyjp

Modern diesels have far more to potentially go wrong than old school. DPFs, adblue systems and all the associated sensors.

if they do develop a problem you really need to have contact with someone who can diagnose thoroughly before deciding it needs lots of new and expensive parts.

A few weeks ago a friends 17 reg Peugeot 3008 decided to have problems and his mecahnic was changing part of the adblue system in a car park.

Mildly_Miffed

https://forums.moneysavingexpert.com/discussion/comment/81914785#Comment_81914785

Modern diesels certainly prefer use to lack of it.

DPFs (Euro5, late 00s on) clog up if the exhaust isn't got properly hot, allowing them to regenerate and burn off the collected soot.

AdBlue (Euro6, mid 10s on) crystallises in the tank, nozzle and pump if it's not consumed fairly quickly.

People regularly remove and map out both of these… Minor issue, it's illegal…

Herzlos

I think it's a combination of fluid age and air exposure. Leaving the fluid low seems to result in more crystalization issues.

When my system complained last time I just put a couple of bottles of decrystalizing additive in and filled up the tank, and the warning went away pretty quickly. Which saved me a fortune in diagnostics.

Frozen_up_north

By way of a comparison, I moved from a diesel when I retired as most of my journeys are relatively short these days. My current car is a Seat Leon 1.5 turbo petrol. Courtesy of the mobile phone app, I can share the driving data for the whole of 2025:

612 journeys, total distance 9,231 miles, average journey 35 mins, average fuel consumption 47.7 mpg.

Many of my journeys are just 2 miles from cold, not something you would want to do with a diesel engine. Longest runs are mostly 60 miles with the very occasional 100 + journey.

One problem we recently encountered with our other petrol car was a failure in the EVAP system… this is a carbon filter and valve assembly that filters petrol fumes from the fuel tank, it proved expensive to diagnose and replace. The likely cause of the problem is filling the fuel tank until the filling nozzle "clicks", at which point petrol may overflow into the charcoal filter, needing a replacement filter at a cost of around £150 + fitting and vat! A totally bonkers system.

Arunmor

https://forums.moneysavingexpert.com/discussion/comment/81914550#Comment_81914550

Why are you so pass-remarkable?

We have 3 cars in the family 1.0 2020 plate petrol Kamiq, 1.2 2016 plate petrol Fabia and 2.0TDi 2016 plate Superb. Guess which one is the most frugal?

I know you will find it hard to swallow but of course it is the 2.0TDi. For me and by extension the family will not have an EV.

daveyjp

https://forums.moneysavingexpert.com/discussion/6658997/mazda-2016-1-6-diesel-issues

A real life example. Ten years old and suffering DPF issues.

goldieandblackie

I purchased a 2016 Mercedes E220d Bluetec Euro 6 last year for 5k, it had 100000 miles on the clock but its behaved faultlessly in the past 9 months.

My other car was written off and hence why I picked up this one, as it was a one owner car with full Mercedes service history and ideal for my daily 50 mile commute.

Goudy

https://forums.moneysavingexpert.com/discussion/comment/81914801#Comment_81914801

This is pretty much the only answer you really need to take notice of. Research your cars before hand.

There are pros and cons on all the different engines and power trains available and not all will suit everyone equally and you might even find yourself between one, two or three differing types of power train.

The major problem with combustion engines is emissions. We know Co2 is an issue and a diesel generally produces less than a straight petrol but they also produce more NOx and much more particulates than a petrol which has lead to some complicated and sometimes unreliable emission controls.

There are three main elements to these emission controls, one or two are shared (but to a much lesser extent) with most modern petrol engines and that is EGR or Exhaust Gas Recirculation and Particulate filters. SCR is diesel only.

EGR. Some exhaust gas is fed back into the air intake when the engine is likely to produce lots of NOx (lean running) as it cools combustion and therefore reduces NOx due to the lack of air in it. Diesels produce a lot of soot which is just fuel that hasn't burnt fully so tends to clog the intake side of the engine after a period of use due to the valve sends this gas/soot back around into the engine.

DPF or Diesel Particulate Filters. These trap the soot and periodically burn it to ash. Ash has smaller particles than soot so the filter can hold more. It burns or "regens" by injecting fuel through the cylinders to the filter but not on a compression stoke (so not on a bang stroke). Any unfavourable engine conditions that produce more soot means more regens and they only take place when conditions are correct (lots of heat in the exhaust).

SCR or Selective Catalytic Reduction. This is another NOx control. A fluid (DEF/Adblue) is injected into a catalyst within the exhaust system that turns up to 90% of the NOx in the exhaust gas into nitrogen and water.

This had lead them to be used in a way more favourable to those controls. It's not necessarily about mileage alone, it's the length of trips. They are better for longer trips. Short trips have a couple of problems. There's not usually enough heat in the exhaust. Diesels generally produce more soot when running cold. You are more than likely to turn off the engine before a regen cycle has completed.

It's worth noting at this point, some are better at all this than others.

Petrol engines had a slightly easier time when it comes to same emission controls as above but they are still not without fault. They have EGR and now Particulate Filters but don't naturally produce the same levels of soot and NOx as diesel (though NOx has climbed with more modern engines), so they often work in a different way, particularly the filter which is more than likely passive (no fuel injected to create a burn, it burns the soot on exhaust heat alone).

Catalytic Converters have been around for years and are pretty reliable, which only really leaves Co2 as an issue and there have been various ways of tackling that including hybrid and mild hybrid systems, some of which make the petrol engine as/ or nearly as efficient as a diesel when it comes to mpg.

There is of course electric vehicles and they have there pros and cons as well, but those have been discussed on the forum many times.

Andrea15

Thanks @Goudy. The problem is that I lack the knowledge to know what is relevant/trustworthy and what is not. Would I be any the wiser if I scoured the internet for a week? Alas, I doubt it.