Rough throttle

Q

After some success recently by correctly diagnosing and rectifying a turbo fault on my 2007 Mercedes E320 CDI, I’m still troubled by another problem. If, when the engine is at normal temperature, I stop say for five minutes and then continue the journey, I will for about a half mile get rough throttle at lowish throttle. Then all is well again.

The car still returns the same mpg as always and there are no fault codes. Gareth Owen

A

The problem does appear to be a temperature related issue, and it is the case that whilst standing the engine temperature will alter as the heat percolates through the system.

The problem may be with the reading of one of the temperature sensors either air or coolant, and reading off the live data may help to determine more closely what may be occurring.

As your Mercedes has two MAF sensors it also has two ambient air temperature sensors which are incorporated into the MAF sensors. Observe live data when the engine is warm to both check the accuracy of the sensors and to ensure that they are both giving the same reading, would be the first point to check.

If the readings do appear accurate the coolant temperature sensor reading should also be checked.

From the data I have on your Mercedes engine, a fuel temperature sensor is also incorporated into the high-pressure pump and again, this should be checked.

The engine running rough without leaving a code does point more to either a MAF failure or an incorrect temperature reading being received by the ECU, and as things do stabilise again after the journey is resumed, this would certainly be the area that should be checked first.