Runs rough at low RPM

Low Fuel Pressure

Have a look at “Engine, not starting.”

Low/ Uneven Engine Compression

Low or uneven compression will result in poor combustion causing the engine to be unbalanced. To find out the compression perform a cold engine compression test or a cylinder leakage test. Compression should be between 20 to 35 bars or 300 to 500 PSI. Anything below this will cause starting/running problems. We have come across bent conrods, valves not seating and damaged cylinder head gaskets that will cause uneven running.

Insufficient Fuel Supply

Not enough fuel in the tank or a problem with supply pipes being cracked or bent. The fuel tank breather can sometimes be blocked causing a vacuum in the tank which in turn draws the fuel back to the tank.

Air – Vacuum in Fuel Supply

Similar to “Insufficient fuel supply” (above) but dirty fuel filters or a faulty filter head assembly may also cause fuel supply issues and running issues.

Blocked Fuel Supply or Dirty Fuel Filters

The first thing to do when fault-finding is to replace the fuel filter with and O.E. one. Also, make sure that there are no blockages in the fuel supply lines.

Faulty Diesel Injector(s)

Probably the two most significant results of injector failure are due to the injectors having excessive return flow or back leakage and delayed start of injection. Worn parts which allow excessive fuel to go through the diesel injector and to return to the tank or fuel system, causing a drop in rail pressure which results in hard starting or not starting at all.

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 the engine not starting.

We are also seeing problems with the later Bosch Piezo injectors that are causing white smoke at idle. The vehicle builds up excessive white smoke at idle but then tends to clear on acceleration. We have found that the nozzles dribble resulting in unburnt fuel.

For peace of mind, send your injectors in for testing, ask us for a quote.

Faulty Pressure Regulator or Sensor

Most vehicles have a pressure regulator fitted on the high-pressure pump and a sensor installed on the rail. If either of these is defective, there will be running issues like hard starting, uneven tick over and the vehicle cutting out when increasing the RPM or load.

Cam and Crank Sensors

Check that the sensors are not loose, cracked or damaged. They are not very expensive to replace if in dought.

EGR problems

Now including water-cooled EGR valves leaking water into the engine.

The idea of an EGR valve is that while the engine is at tick over, a valve opens and allows some of the exhaust gases to pass back into the nice clean air 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 black smoke. This black smoke is then drawn back into the air intake via the EGR valve. A vicious cycle then starts, thus causing air intake problems resulting in uneven running.

A new problem with modern EGR valves is that some of them are water-cooled, great idea but as they age, they become porous and start leaking coolant into the engine through the EGR valve. To check remove the EGR valve, block of the coolant outlet connection, connect compressed air into the coolant inlet, drop into a bucket of water and see if you have any air leaks.

Injector Blow-By or Injector Seat Leaking

Injector “blow-by” can be the cause of some of the following symptoms.

Hard or difficult starting,

Erratic or uneven tick over,

Lumpy running,

Smoke on 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.

On some engine applications if this continues severe engine damage could result because the ECU will over compensate the fuelling, causing piston wash or 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 only way to remedy the fault is to reface the injector seat in the head using a seat cutting tool.

Injector Blow-By or Injector Seat Leaking (link to Instruction)

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