Examples of Common Rail Injector seat damage. This damage will cause fuel to pass by and make a common rail injector system on an engine hard to crank.

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Diesel Truck Parts Damage

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Fuel and Water contamination





Diesel Engine Trouble Shooting Chart


All diesel engines are not made the same, but the information listed below is a generic cause and effect list of items to check for when diagnosing diesel engine trouble.  Newer engines have High Pressure Common Rail pumps and injectors, and some of this information is geared to that fuel system.


Probable Cause Engine not Starting Hard to start engine Runs rough at lower RPM Lack of Power Diesel knock / pinging noise Black Smoke White Smoke Blue Smoke
Low compression X X X
Low fuel pressure X X X X X X
Low cranking speed- weak battery X X
Bad glow plugs or faulty relay X X X
Insufficient fuel supply X X X X X
Fuel quality and or contamination X X X X X X
Air –  vacuum in fuel supply X X X X
Blocked fuel supply – filters, hoses X X X X X
Faulty diesel injector(s) X X X X X X X
Faulty high pressure pump X X X X X
Faulty pressure regulator sensor X X X X
Faulty low pressure supply pump X X X X
Air intake restriction X X X
Turbo problems – wastegate X X X X
EGR problems X X X
Injector blow-by seat leaking back X X X X
Cam / Crank sensor X X X X X X
Injector wiring harness X X X X
Internal engine problems X X X X X




Low compression

Low engine compression will result in insufficient heat being generated to ignite the fuel and cause hard starting.  This is more of a problem with older or high mileage vehicles.  To find out the compression, perform a cold engine compression test.  Compression should be between 20 to 35 bars or 300 to 500 PSI.   Anything below this will cause starting problems.


Low fuel pressure

It is in this area that most fuel supply problems occur.  With high pressure common rail fuel systems, the problem could either be poor fuel supply to the rail / injectors or the rail / injectors are not holding the fuel within the system.   The best way to diagnose this is to look at the fuel supply in 3 areas.


  1. Low pressure supply from the tank to the High Pressure Pump (HPP) – some vehicles rely on the HPP to suck the fuel from the tank while others have an electric pump in the tank or fuel line to supply fuel to the to the HPP. The supply from the tank to the HPP should be about 2 to 5 bars.
  2. Fuel is delivered from the HPP to the rail / injectors at about 200 bars during cranking, 300 bars at idle and anything from 1200 to 1800 bars running.
  3. Once the fuel is delivered into the rail / injectors at the relevant pressure it must be maintained within the injectors or rail. Any loss of rail pressure will cause problems.


Low cranking speed

If the engine turns over too slowly, the pump cannot generate enough fuel pressure to activate start of injection causing hard starting problems.  This is usually seen more in the colder months especially if the battery is run down.  Check your battery voltage and starter to make sure they are working correctly.


Glow plugs or relay faulty

The engine depends on the glow plugs to generate heat to help with the combustion cycle.  Some engines only use the glow plugs during cold weather, but others will allow the glow plugs to work when the ECU (vehicle’s computer) needs them to be on to help with combustion.  Problems in this area will cause diesel engine starting problems, uneven running and white smoke when the engine is cold.


Insufficient fuel supply

This speaks for itself-  not enough fuel in the tank or a problem with supply pipes being cracked, bent, collapsed, restricted or some kind of restriction in the tank.  We have seen a fuel tank with a shop rag in it which stopped up the fuel line out of the tank.  If you are not sure about the fuel supply, put clear hose from a fresh tank of diesel and run it to the feed pump or injection pump.  This will bypass all hoses and filter bases and will let you know if there is a problem in that area.   The fuel tank breather can sometimes be blocked causing a vacuum in the tank which in turn draws the fuel back to the tank.


Fuel quality – contamination

The topic of fuel quality and contamination solutions is controversial.  Working with diesel injectors we see the result of poor fuel quality- damage to the internal components. I know that if a good quality diesel and additive solution is used regularly, it will prolong the life of diesel injection equipment.  Poor fuel quality and general wear and tear are not the only cause of injector failure. The newer types of common rail injectors sometimes fail due to design problems.   We would estimate that about 85% of injectors fail due to fuel related issues and the balance due to design problems.


Air – Vacuum in fuel supply and Blocked fuel supply

This is similar to insufficient fuel supply, but dirty fuel filters or a faulty filter head assembly may also cause fuel supply issues and hard starting.  Air getting into the filter base through cracks in the filter base, loose hoses, or fuel filters not seating correctly will cause problems also.


Faulty injector(s)

Probably the biggest result of injector failure for Common Rail injectors is due to the injectors having excessive return flow or back leakage.  This is due to worn parts which allow excessive fuel to go through the diesel injector and to return back to the tank or fuel system. This causes a drop in rail pressure (see “low fuel pressure“) which results in hard starting or not starting at all issue.  Another problem resulting from worn parts is a delay in the start of injection which in turn results in rough running at low RPM or your diesel engine not starting.


Faulty high pressure pump

If the high pressure pump is faulty, there will be a low fuel pressure problem on the injector rail.  This problem arises if the pump “breaks up” internally causing fine pieces of metal filings to get into the fuel system.   Normally this causes damage to the diesel injectors.  If this is not corrected, the problem will arise again.  This is an expensive failure and no short cuts can be taken.    For Common Rail systems that have metal damage, it is recommend that the fuel rail, fuel lines, and injectors be replaced.


Faulty pressure regulator – sensor

Most vehicles have a pressure regulator fitted on the high pressure pump and a sensor fitted on the rail.  If either of these are faulty there will be running issues like hard starting, uneven engine running and the vehicle cutting out when the RPM is increased.


Faulty low pressure pump

Not all vehicles have a low pressure supply pump but if they do, it can be found either in the tank or on the fuel line near the tank.  If your low pressure pump is faulty, you may experience symptoms similar to those of a faulty high pressure pump.  With low fuel pressure to the injection pump, the engine will ‘starve’ for fuel.


Air intake restriction

This would be due to a dirty air cleaner, blocked pipes or a stuck butterfly valve found on some vehicles.   In addition, a faulty air flow sensor on the air intake will cause problems.  There will also be excessive smoke.


Turbo problems

We are seeing more turbochargers failing with newer vehicles.   I put it down to a combination of things-  high revving engines demanding more power, incorrect driver actions (not allowing the engine to idle a while when started and before switching off), poor maintenance and not replacing old oil with a good quality oil.   A turbo spins at about 42,000 revolutions per minute, the average washing machine at 1,000 rpm.

As vehicles get older, the turbo waste gate sticks causing the vehicle to either shut down, go into limp home mode or smoke excessively.  If a vehicle has a variable vane turbo, problems can arise if the vans carbon up.  The symptoms of stuck vanes are lack of power, black smoke and hesitation on acceleration.  Also make sure that all the vacuum pipes and sensors that operate the turbo are operating correctly.  Another problem is if the air pipes to and from the inlet, inter cooler and turbo leak due to damage or loose clamps.  Air leaks after the turbo will cause lower engine power, excessive smoke, and in some cases, and noise like a whistle when there is a pin hole air leak.


EGR problems

EGR (exhaust gas recycling) valves cause more trouble than they are worth.  The idea is that while the engine is running, a valve opens and allows some of the exhaust gases to pass back into the good clean air into the intake manifold.  After a while the gases containing dirty, sooty carbons start to cover and coat the intake area and valves causing the air to fuel ratio to become unbalanced thus resulting in more black smoke being emitted from the exhaust.   This black smoke is then drawn back into the air intake via the EGR valve.   A vicious cycle then starts with the engine producing more smoke and sootier carbons being drawn into the intake, a major problem.    EGR coolers, especially on Ford 6.0 engines can leak water and this will cause problems until the EGR cooler is replaced.


Injector blow–by, seat leaking

Injector “blow-by” can be the cause of some of the following symptoms.  Hard or difficult starting, erratic or uneven running or idle speed, rough running,  smoke while running  or acceleration, black tar around the injectors and a chuffing sound from the engine when running.   Injector “blow-by” occurs when the injector does not seal against the injector seat in the cylinder head.  Often a chuffing sound is heard or black “tar” can be seen around the injectors.   On some engine applications if this continues, serious engine damage could result due to the fact that the ECU will over compensate the fuelling on the cylinder or cylinders with the seating problem and will cause a piston washing or cylinder over fuelling.  Even if the injector is removed, cleaned, a new copper washer fitted and then replaced it will not always rectify the problem.  The reason for this is that the seat in the cylinder head has been eroded by the escaping combustion gases resulting in damage to the seat.   The only way to reface the seat is to use a seat cutting tool and gently reface the seat in the head.


Injector wiring harness

Problems occur with vehicles that have the injectors under the valve cover and allow oil to come in contact with the electrical connections.  Even though some diagnostic machines will condemn the injector(s), many times the fault lies with the wiring harness.  Check that the electrical connections on the injectors are good and are making contact.


Internal engine problems

This is generally mechanical failure such as problems with bearings, pistons, rings, oil pressure, overheating, valves and more.  The list is endless, and it is best to get an engine specialist to diagnose the fault.


The Answer is in the Smoke

We can generally understand what is wrong with a diesel engine by the color of smoke emitted from the exhaust.  There are three basic colors – black, white and blue.


Black Smoke

This is due to a air to fuel ratio imbalance, either the fuel system is delivering too much fuel into the engine or there is not enough clean air (oxygen).  Here are  a few things to look for :

  • Faulty injectors (injectors need attention at about 100,000 to 120,000 miles)
  • Faulty injector pump
  • Dirty air cleaner
  • Turbocharger or intercooler faulty
  • Problems within cylinder head, valves clogged up due to faulty EGR (exhaust gas recycling unit)


White Smoke

Normally means that the fuel injected into the cylinder is not burning correctly.  The smoke will burn your eyes.

  • Engine and or injection pump timing is not correct
  • Fuel starvation to the pump causing the pumps timing not to operate correctly
  • Low engine compression
  • Water and or gasoline in the fuel


Blue Smoke

The engine is burning engine oil

  • Worn cylinders or piston rings
  • Faulty valves or valve stem seals
  • Engine over full with engine oil
  • Faulty injector pump and/or lift pump allowing engine oil to be mixed with the diesel

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