I've been doing some reading and speaking to a mechanic trying to diagnose a hesitation and rough idling issue. I think I've ruled out a vacuum leak by spraying flammable gas around the seals and not noticing any change in behaviour. I'm seeing low manifold pressure with the scan tool (around 27 - 37 kPa vs expected range of 64 - 69 kPa) and the mechanic thinks that as long as there isn't a vacuum leak, this is probably related to valves. Obviously a vacuum leak is the faster and cheaper thing to rule out, but since it's been mentioned I'm also just curious - is it even possible for a vacuum leak to be the cause of a drop in MAP? I would have thought with more air coming in through the leak, pressure would increase.

The motor is a 2.3l Duratec HE petrol I4, non turbo, in a 2010 Ford Transit.

  • Are you sure that the wiring and sensors giving you to low reading are working correctly? Apr 15, 2019 at 13:48
  • I haven't tested them... does 27kPa seem too low to be a correct reading? I am also seeing around +18% LTFT (long term fuel trim). My reasoning so far: if this is compensating for air ratio (not bad injectors for example) and since the engine runs, I reckon there must be more air than normal (lean). Then I can only think of 2 things that could cause the MAP to be low - the engine pulling too much air out of the manifold, or the throttle not letting enough into it. Again assuming the cylinders are lean, this seems to point to the first option. I could be missing something though. Apr 15, 2019 at 15:53
  • On a side note, why wouldn't the ECU throw a code for such low MAP? Apr 15, 2019 at 17:21
  • Also if there is too much air, and fuel is being added to compensate, why doesn't this raise RPM? Is it somehow burned in a way that wastes the extra energy? Apr 15, 2019 at 17:23


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