Renault Kangoo van 1.5 dci 2007 model

Hi Please Help I am new to this i have a problem with my van it starts fine but when i go to pull away it pulls back feels like i not got a lot of power so i bought a air flow sensor cleaner and replaced the air filter at the same time a little improvement but now the fault is coming back.

i also noticed when i filled with a tank of diesel that my mpg has dropped from average of 58mpg to 42 mpg still the van is having a lack of power i pluged the van to check for fault codes even though no lights have come on the dash and a fault code p0670 came up which when googled means its to do with the glow plug module control circuit.

I took the leads off the glow plugs and tested to see if power was coming through which it is i then checked the omms and i had no reading on 1 2 & 4 but no 3 was showing a reading of 1.6 which reading the haynes manuel is too high as the reading should be between 0 and 0.6 so i am going to replace the glow plugs and see if that makes a difference if anyone knows what else could be causing the lack of power issue i would be greatful to know as i thought the glow plugs only worked to start the van not the running of the van.

regards



david