Car engine ruined

Ayr_Rage

@Goudy I think you've hit the nail on the head, especially as the car is being used for driving instruction and therefore unlikely to get decent long runs at higher speeds to burn off any accumulated crud.

born_again

AlisonLow said:

The garage stripped the engine and said there was nothing wrong with the timing belt but when they tried to drain the oil it was "like treacle". They said that they could also see that the engine had been seriously damaged as a result and was ruined. The car did crank but would not start

If they had stripped the engine, then they would already have drained the oil...

Which engine is this?

Goudy

Ayr_Rage said:

@Goudy I think you've hit the nail on the head, especially as the car is being used for driving instruction and therefore unlikely to get decent long runs at higher speeds to burn off any accumulated crud.

That might not have helped matters. but I don't think it's the full story.

Partial and light throttle tends to cause the EGR system to operate more as that's when the engine is running lean and producing more NOx. So you're recirculating more exhaust gases which have been cooled slightly back into combustion, which are full of soot.

If it's left idling a lot, that won't help as the fuel/air ratio tends to be richer at idle.

And of course DPF regeneration tends to dilute the engine oil over time, so the black treacle found in the engine won't be just engine oil mixed with excess soot, but a dilution of diesel and engine oil full of excess soot.

As it cranks and doesn't start, if I'm anywhere near right the engine has probably suffered heat damage and the pistons have nipped up badly in the bores rather than the bearings seizing, but the bearing will probably be heat damaged as well.

Engine oil carries a lot of the heat away from the engine components and on modern diesels tend to go through an heat exchanger somewhere down by the filter that is cooled by the engine coolant system.

Peak combustion temps of a diesel can be around 2500c where a normal petrol might be around 1000c or so.
It's the heat of compression that ignites the diesel unlike the spark of a petrol engine.

tedted

Gaudy this an 2021 208 hdi which uses ad blue that is injected before the dpf and doesn't contaminate the oil

Goudy

Ad Blue has nothing to do with the Diesel Particulate Filters (DPFs).
Ad Blue is the additive for the Selective Catalytic Reduction system (SCR) to reduce Nitrogen Oxides.

A DPF traps soot (which are larger particles) from combustion and periodically burns it to ash (which are much much small particles) and stores it in there.
Soot is just fuel that hasn't fully burned.

On a diesel engine they are active.
To "regen" it has to burn the soot to ash and this requires a high temp, 500c or more.
To get this to works mean fuel is injected into the cylinders post compression which finds it's way to the DPF to burn. (though some of this post injected fuel finds it's way past the piston rings into the engine oil to dilute it).

Modern Petrol cars have passive filters.
Petrol's don't produce as much soot as diesels and the exhaust gases are usually hot enough on their own to burn the soot to ash when on the over run (taking your foot of the accelerator while still moving) so "regen" more often without the need to add any fuel or other intervention.


On modern diesels, the SCR is positioned after the DPF to help reduce Nitrogen Oxides.
These are harmful pollutants that generally proliferate in a diesel engine when the fuel to air ratio is lean and combustion temps are very hot.
Take your foot off the accelerator in a diesel and it will inject no fuel whatsoever, it just pumps air and as a result it produces NOx's.

EGR systems help reduce these NOx's as when the EGR valve opens it allows spent exhaust gas back around into combustion when the engine is lean and combustion hot. This  replaces some of the air being compressed, so lowers the temp in the combustion chambers thus lowering the amount of NOx's being produced.

But they are not enough for modern emission standards so SCR's were introduced.
  
Ad Blue is a urea solution (around 32.5%)  that is injected into the Selective Cat where is reacts with elements in the Cat (titanium oxide, vanadium, molybdenum and tungsten) and reduces the NOx to nitrogen and water and a tiny amount of carbon dioxide.

tedted

ad blue is injected into the dpf  to clean it and reduce emissions ,oil has progressed massively in the last forty years and i haven't come across any diesel engines with  thick oil for years including a diesel 308 2.0 hdi that did over 25000 miles before an oil change

WellKnownSid

tedted said:

ad blue is injected into the dpf  to clean it and reduce emissions

No, SCR is about reducing NOx released to the atmosphere, it happens after the DPF which is there to filter the particulates.

Goudy

Citroen/Peugeot used an EOLYS system in their DPF's a few years ago, you might be confusing it with that.

The EOLYS fluid was added to the DPF as it reduced the temperature needed to turn the soot to ash.
I seem to remember the system could get a reliable regen at temps around the 400c mark rather than 500+c.

This is completely different to SCR and Ad Blue.
DPF (FAP if your motor is French) and dealing with soot.
SCR and Ad Blue is dealing with NOx.