We were presented with this Skoda to diagnose an intermittent fault on from another garage.
The vehicle arrived on a trailer after breaking down on the way to us and wouldn’t start.
Initial description of the fault was holding back poor running and cutting out but would restart along with a fast idle speed.
The vehicle had apparently been showing a fault for the throttle pedal and had been fitted with a secondhand unit. Initial investigation showed that the injectors were not being triggered
On the PD diesel engine the cranking speed needs to be above 270-280 rpm to trigger the injectors this vehicle was very slow at 230 rpm.
After some initial testing the stater wouldn’t turn the engine over at all and just clicked.
The stater was replaced and the cranking speed rechecked it was now a healthy 300 rpm and the injectors were being operated but it still wouldn’t start.
We next moved onto the fuel system and a pressure gauge was installed on the main fuel line from the tank.
After extended cranking the vehicle started but it was observed that the gauge was showing a negative pressure.
This was obviously a fault as the vehicle is fitted with an electric pump in the tank and a normal reading is 0.5 to 1.0 bar pressure.
The negative pressure was being produced by the engine driven tandem pump sucking fuel through the fuel filter all the way back to the tank.
The rear seat was lifted and the pump feed and ground were tested and load tested all tests were good.
The in tank pump was removed and bench tested and confirmed as a failure.
A new in tank pump unit was installed and the vehicle fuel pressure was re check and showed a healthy 1.0 bar positive pressure this cured the intermittent cutting out fault.
All that was left was to investigate the high idle speed which was sitting at a constant 1200 rpm.
There were no faults stored in the engine management system but live data was showing a pedal sensor reading at 8%.
The old throttle pedal was in the vehicle this was reinstalled and the reading dropped but was still high at 5%.
We quickly found that if the pedal was pulled up the reading would drop to zero and the idle speed was correct.
Conclusion unusually there were three separate faults with this vehicle and the use of second hand parts made the situation worse.
A methodical approach with a good system knowledge led us to a correct speedy diagnoses.
Faulty starter motor..
Vacuum in the fuel line from the tank..
Faulty in tank pump..
Correct positive pressure from the in tank pump..
