The faults that fill owner forums and workshop diaries on modern diesels are mostly the exhaust and fuel systems, not a mysterious difference in the bottom end. Short, cool journeys leave soot in the EGR path and in the particulate filter. A rough idle that shakes the seat is more often an injector or a fuelling fault than a burnt valve. Coolant loss and overheating use the same tests as a petrol engine, with one extra place to look: the EGR cooler. This page is the index. Each linked page is the diagnosis.
Nothing here is a measured failure rate. The AA records that its patrols are often called to a blocked diesel particulate filter, and workshop write-ups keep returning to the same clusters: a filter warning after town driving, an EGR valve full of soot, a turbo that is short of boost, and an injector whose return flow has risen. Those are patterns with a mechanism, not a league table. An engine code can have its own extra fault, such as a timing chain or a swirl flap, that this page does not pretend is universal.
These pages describe how the faults behave and how a workshop separates them. They do not describe how to remove or defeat emissions equipment. The emissions page covers why the hardware is there, and the MOT inspection of it.
Where to start
| What the driver reports | Look here first | Easy to confuse with |
|---|---|---|
| Filter warning, limp mode, fans running after a run, fuel use suddenly worse | Particulate filter | EGR or injectors making extra soot; a thermostat that never lets the exhaust get hot |
| Rough idle, hesitation at low speed, black smoke, soot in the intake | EGR | A sticking variable-geometry turbo, a dirty air-mass meter, low boost |
| Hard start, shakes at idle, one cylinder out, diesel knock | Injectors and fuel pumps | Low compression, air in the low-pressure fuel line, a failing lift pump |
| Power loss under load, whistle, black or blue smoke | Turbocharger and charge-air leaks, below | A loaded particulate filter, EGR flow that is wrong, a split intercooler hose |
| Coolant loss, white exhaust that does not clear, temperature rising | Overheating and coolant and head gasket | An EGR cooler leaking coolant into the intake |
| Hesitation that is really a lack of fuel under load, with no misfire shake | Fuelling | Boost control, a blocked filter, a sensor the ECU believes |
Read the fault memory and the freeze frame before replacing the part the forum thread named. HELLA’s EGR guidance makes the same point for one system: an air-mass reading that is wrong will make the recirculated quantity wrong, and the valve is then blamed for a calculation it did not make.
Soot: EGR and the filter
A diesel meters load with fuel, not with a throttle plate, and the spray leaves soot. The emissions page explains that chemistry. On the road it turns into two complaints.
The filter stores the soot and has to burn it off. The AA’s account is the one that matches the forum pattern: passive regeneration needs a hot exhaust, the sort of temperature a motorway or fast A-road produces, and many cars never see it. The manufacturer then commands an active regeneration by injecting extra fuel. If the journey ends first, the soot stays. The AA lists the other blockers as well: the wrong engine oil, a fuelling or EGR fault that makes excess soot, a stored fault code, an overdue service, a low fuel-additive tank on engines that use one, and a low fuel level. The detail, including ash, oil dilution and the petrol filter, is on the particulate filter page.
EGR puts exhaust back into the intake to cut NOx, and on a diesel that gas still carries soot and oil mist. HELLA describes the valve opening closing up until the recirculated quantity falls away. A valve stuck open instead fills the intake at idle and the engine runs lumpily or stalls. Passages and coolers block even when the actuator still moves. That diagnosis is on the EGR page. Carbon elsewhere in the intake is covered on carbon deposits and cleaning.
Fuel supply
Diesel fuelling faults split into the low-pressure side and the high-pressure side. Air leaks on the suction side, a blocked filter, water in the filter and a tired lift pump all starve the high-pressure pump. The engine then cranks for a long time, dies under load, or loses rail pressure just when the demand rises. Those supply faults are on the fuelling and fuel pump pages.
Common-rail injectors are allowed a return flow, because that flow is how the injector operates and how it is lubricated. Pico’s description of the back-leakage test is the workshop version of the forum complaint “it starts badly and shakes”: one injector returning far more than the others pulls the rail down. The comparison has a procedure and a limit for that injector family. It is not one number for every diesel. Injector coding, cylinder contribution and a bench test are separate checks. See injector faults.
Older distributor-pump and unit-injector engines fail differently. Timing and quantity are tied to the pump or the cam, and a return-quantity test written for a solenoid common-rail injector does not apply. EDC15 injection systems records that split. EDC15 diagnostic data is the worked example of reading quantity, air and cylinder balance before condemning a part.
Air, boost and smoke
Most light-vehicle diesels are turbocharged. Loss of power under load, a whistle, and black smoke when the fuel arrives without the air are the usual boost complaints. Split charge-air hoses, a sticking variable-geometry mechanism, a seized wastegate, shaft play and oil fed through a failed seal are different faults with the same “it won’t pull” story. Blue-grey smoke with oil use points at oil entering the intake or the exhaust, which may be the turbo seals or the engine itself. The construction of the chargers is on forced induction. Replacing the turbo without finding why it lost its oil supply, or why the filter downstream is blocked, repeats the failure.
Black smoke alone does not name the part. A loaded filter, EGR that will not close, a boost leak, an air-mass meter that under-reads, and an injector that dribbles all make smoke. Live boost against the commanded value, air mass, and injector corrections separate them. The emissions page explains why a diesel smoke test is not a NOx test: a car can be dirty on NOx and still pass an opacity check.
Coolant, the head gasket and the EGR cooler
Overheating, a heater that stays cold, and coolant that disappears are shared with petrol engines. The tests are the same: level, leaks, the pressure cap, whether the thermostat opens, whether the pump actually moves coolant, and whether combustion gas is in the coolant. That material is on engine overheating and coolant faults and head gasket failure.
The diesel-specific trap is the EGR cooler. It is a gas passage jacketed in coolant. A cracked core can drop the coolant level with no puddle on the floor, put white vapour in the exhaust, and pressurise the cooling system, which is the same set of signs as a head gasket. A combustion-gas test proves gas is reaching the coolant. It does not say whether the path is the gasket, a cracked head, or the cooler. The EGR page and the gasket page both say how to separate them.
Burnt exhaust valves happen on diesels, but they are not the everyday complaint the forums are describing when an engine idles badly. A leak-down test that hisses at the tailpipe is the finding that moves the job from injectors to the cylinder head. Confirm the timing first. A jumped chain or belt leaves air at the intake or the exhaust as well.
Faults that belong to one engine, not to diesel
A cold-start rattle from the timing cover, a manifold flap that sheds its spindle, a dual-mass flywheel that rattles at idle and goes away when the clutch pedal is pressed, and an AdBlue countdown are all real. They are not diesel-wide. The flywheel is a transmission part: if the noise changes with the clutch or with load in gear and the engine is smooth in neutral, chasing injectors will not explain it. AdBlue and selective catalytic reduction are a separate system from the particulate filter; the emissions page describes them. What the car does when the fluid is empty or the quality sensor disagrees is in that vehicle’s handbook and fault memory, not in a single mileage that suits every manufacturer.
Glow plugs matter for a cold start on an engine that uses them. A long crank and white smoke on a cold morning can be the plugs, the fuel supply, or compression. The plug circuit has its own faults in the memory. A lamp on the instrument panel is only a glow-plug fault if the handbook says that lamp is the glow plugs.
Shared with petrol
The same pages are linked from common petrol engine problems where the failure is shared:
- Engine overheating and coolant faults
- Head gasket failure
- Fuelling faults
- Fuel pump faults
- Injector faults
- Burnt valves
- EGR faults, which covers petrol EGR as well as diesel
- Particulate filter faults, including the petrol filter on direct-injection engines
Sources
- The AA, diesel particulate filters. Patrol call-outs for blocked filters, passive and active regeneration, and the conditions that stop a regeneration. Page updated 7 August 2017; the driving pattern is the part used here.
- HELLA Tech World, exhaust gas recirculation. Soot and oil mist closing the valve, vacuum leaks, and an air-mass error changing the calculated EGR quantity.
- Pico Technology, back-leakage testing. Why a common-rail injector has a return flow, and what an excessive return means.
- Petrol and diesel emissions on this wiki, for soot, EGR, filters and selective catalytic reduction.
- RAC, loss of power. A blocked filter, injectors and an underperforming turbocharger among the causes of lost power.