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Possible Causes Of TD4 Power Loss

Repair Guide Possible Causes Of TD4 Power Loss

There are quite a few requests for help on this subject so here are a few things to check if your TD4 looses power. If the MIL is illuminated in the instrument cluster have the stored fault codes read from the ECU to help diagnosis.

As with any diesel an increase in fuel generally gives more power up to the point where the production of black smoke from the exhaust makes any further increase undesirable. Therefore the reverse is true; any decrease or limitation in the fuel supplied will cause a reduction in power.

A common cause of poor throttle response/acceleration is a faulty MAF sensor. Its output is used along with other sensor outputs, by the ECM to calculate the correct quantity of fuel to be injected. Simply remove the electrical connector and road test the vehicle with it disconnected. If performance is restored then 99% the MAF is faulty. I say 99% because it can improve drivability when other sensors are faulty.

Poor or intermittent LP fuel pressure is first evident during conditions of high demand, i.e. at motorway speeds or when climbing hills.

The fuel filter is probably the first thing to consider. It is often overlooked in routine maintenance and should be changed every 60k miles so if you don’t know for certain if/when it has been changed consider eliminating this first.

Early TD4 before 05/2002 m/y have a combined submersible low pressure fuel pump and fuel gauge sender unit in the fuel tank. Check under the right rear seat if you can hear this electric pump running with the ignition turned on, it should run for about a minute and then cut out when the fuel pressure has built up. These models also have a secondary low pressure pump in the near side of the engine bay next to the fuel filter which should be running simultaneously but not making any excessive noise or vibration as this would be an indication it is faulty. Sometimes the intank pump fails and for a time the engine can run on the underbonnet pump only.
There is a pressure sensor located in the filter head which monitors the LP fuel pressure and should it fall below 0.8bar the ECM reduces fuelling to the engine and a reduction in power will be felt eventually stopping the engine to prevent damage to the HP fuel pump. If the engine fails to start at all, temporarily disconnect this sensor and see if the engine fires up, if so it confirms the problem is low LP fuel pressure. (Stop the engine asap though)

Later TD4s after 05/2002 have only one low pressure fuel pump located along with the fuel filter at the front of the right rear wheel arch behind the plastic arch liner. Again this pump should be heard running with the ignition on, low output will give the same symptoms of reduced power and complete failure will prevent the vehicle from running. The LP fuel pressure sensor for these later models is fitted in the fuel supply line in the near side engine bay chassis area.

There is a HP fuel pressure sensor located in the end of the Common Rail. The connector for this sensor can become dirty or corroded resulting in false readings being transmitted to the ECM so remove this connector and check the contacts are clean and wiring is sound. Wiggling a faulty/dirty connector or wires may cause the engine to misfire. If necessary there is a harness repair kit available.

If the problem is not a fuel supply issue, it may be Turbo/ Boost pressure related.
The Variable Vane Turbo has a speed control mechanism rather than a wastegate. The VVT system enables the turbo to spool up quicker to give useful boost at low engine rpm and also reduce turbo speed at high rpm to prevent over boosting. An initial check for the operation of this is to view the actuator and control rod fitted to the turbo from where the steering rack tie rod passes through the right front inner wing. You should see the actuator mechanism move when you snap the throttle wide open and as the rpm increases it should move back again.
If it doesn’t move, first check all the braided vacuum pipes in the engine compartment for leaks/chaffing. Check the whole system from the vacuum reservoir on the near side across the engine under the acoustic cover and down the back of the engine. If no leaks are found it’s likely the boost control solenoid is faulty or its vent filter is blocked. Remove the vent filter and recheck for movement of the speed control actuator and linkage.

To test the turbo actuator and linkage with the engine idling take the vac hose that comes from the vacuum reservoir off the solenoid and put it to the actuator (bypassing the solenoid) and you should see the actuator linkage move and hear a slight change in the sound of the exhaust. If the linkage moves all is well with the turbo and this again indicates the likelihood of boost control solenoid failure.
Lastly, occasionally the MAP or Boost Pressure sensor fitted in the inlet manifold can fail resulting in no boost control operation which also restricts exhaust gas flow through the turbo causing power loss.

Hopefully there will be some benefit in having this information in one post. :)
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Brilliant to have all of this information in one post! Thank you for this!!
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