Low power diesel

My partner recently bought a 2007 Renault Laguna 1.9 diesel estate. It drives normally and with full power for the first five minutes. You can feel the turbo kicking in and the car drives smoothly. But when you put your foot on the accelerator, there is no power surge, as if the turbo is not working, and the speed slowly climbs to about 50-60mph. Your foot can be flat on the floor in fourth gear. It is worse when going uphill, as you put extra load on the engine. If you then stop the car, turning the engine off and back on again, it works as it should, then the problem reoccurs after five to seven minutes of driving. No fault codes were found when the car was hooked to a diagnostic device. David Kay

As the problem can be reset by turning the ignition off and on, I would rule out any mechanical problem and concentrate on the electrical side of the control systems.

First on my list to check would be the MAF (Mass Air Flow) sensor. This can cause running problems and will often not throw up a code. It can sometimes be diagnosed by disconnecting the connecting plug and driving the vehicle for a short distance. If the vehicle performs without the loss of power, this will confirm that the unit is not operating correctly. It may be worth trying to clean the MAF sensor by spraying an electrical cleaner on the element. If you do this, ensure that you do not touch the element or start the engine while the element is still wet.

The other component I would check is the EGR valve, which could be sticking once the engine has warmed up. Try removing the EGR valve to see if it is overly coked up. The EGR valve can be cleaned using a toothbrush and a carb cleaner spray. If you have access to an advanced code reader, you should also be able to check the operation of the EGR valve using the actuator operations programme.