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Diesel - Common Fault Diagnosis & Rectifying

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Overview Guide
Ford Model: All Diesels
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Checking For Air ingress

  • Obtain some 8mm id clear pipe (approx 3 ft should be plenty) and suitable hose clips (6)
  • Replace the black nylon pipe going from the filter housing to the fuel pump
  • Run engine for 5 to 10 min then observe the pipe, a couple of small air bubbles every minute is acceptable but anymore indicates a problem so read on
  • Now disconnect the fuel heater from the steel supply pipe and the filter housing with clear pipe and repeat step 3. If air bubbles disappear or are reduced this indicates a problem with the fuel heater and requires replacing
  • Now connect a piece of clear pipe from the steel feed pipe that should go to the fuel heater direct to the fuel pump and repeat step 3. If you still have lots of air bubbles chances are there is a problem with the flexible pipes at the fuel tank, but this not very common. If the air bubbles have gone you also have a problem with the filter housing and requires replacing

Checking glow plugs

  • Connect a test light/multi-meter (set on DC volts) to the glow plug bus bar and switch on ignition
  • Check voltage should be battery voltage
  • Observe the dash warning light for glow plugs when the light goes out the plugs should stay powered for several more seconds
  • If there is no power, check the fuse - an 80A fuse located in the fuse box next to the battery. Check this carefully!!! As the fuse is known to fracture and this can be very hard to see through the little window, it is best to prise off the window and carefully check the integrity of the fuse. Either that or use a multimeter forn continuity checking. If the fuse is ok, suspect the controller
  • Remove all 4 glow plugs and connect them to a battery using a suitably-sized cable - see if they glow (be careful NOT to hold the plugs whilst testing for at least 10-15 mins after testing as they get very hot. If any plugs fail to glow replace the entire set with good quality plugs
  • I would advise against purchasing cheap or unknown brand glow plugs, as I have known them to fail after only 1 day - the main culprit seems to ba a brand called Wellman. The best available are Beru, Delphi, and NGK
  • Whist the plugs are removed check the integrity of the bus bar if faulty replace

Fault diagnosis

Problem 1

  • Car lacks power
  • Sometimes revs up without actuation of accelerator pedal
  • Increase in fuel consumption

Possible cause

  • Split boost diaphragm


  • Remove tapered pipe from boost box on the side of the fuel pump (front as you look at it facing the radiator) and check for presence of diesel fuel


  • Replace boost diaphragm - some people will tell you that it is possible to change in situ but this is not recommended as once disturbed, the fuelling will be out of calibration and will need resetting on a calibrated test bench (or you could do it yourself by measuring the amount of visible threads and putting it back how you found it)

Problem 2

  • Uneven idling
  • Presence of blue smoke from the exhaust
  • Poor driveability

Possible cause

  • Faulty advance solenoid
  • Needle lift sensed injector
  • Throttle pot


  • Have an assistant switch on the ignition whilst you listen closely to the fuel pump. You are listening for a series of evenly spaced clicks. If not present or uneven solenoid replacement will be required
  • With the engine and ignition switched off disconnect the plug from the injector and check the resistance across the terminals. The reading should be between 95 and 110 ohms. If not, replace injector
  • With the engine switched off mark the position of the throttle potentiometer that is located on the top of the fuel pump above the throttle lever then carefully loosen the 2 torx screws. Start the engine and move the potentiometer until the engine runs smoother. If no improvement is possible or any of the cables are in poor condition where they exit the potentiometer, replace with a new component.

Problem 3

Excess black smoke from the exhaust

Possible cause

  • Blocked air filter
  • Faulty EGR valve
  • Faulty Mass Air Flow sensor


  • Check and replace if necessary the air filter element
  • Locate the EGR valve (on a Mk1, under the intercooler follow the black vacuum pipe from the control valve on the nearside inner wing, just near the windscreen. On a Mk2, it's in front of the intercooler on the left hand side as stood looking at the front of the vehicle). Remove the vacuum pipe and place a tight fitting ball bearing or bolt into the pipe (if bearing fitted reconnect pipe, if bolt fitted secure out of the way) and take vehicle for a good hard drive. If the fault has been cured, leave EGR valve disconnected
  • Check and replace if necessary the Mass Air Flow sensor