Might indeed be that even though it was higher, it was actually right where the ECU wanted it to be. Yeah, I totally forgot to look at what the desired idle value was when the MAP was unplugged. Only worth changing if fuel economy or rough idle or poor running is the result. So the test is not meaningless, it is useful info, but inconclusive on its own.Īn accurate test on the manifold pressure (1 to 2%, or +/- 1 to 2 kPa) would be ideal, failing that, just replace the TMAP if it is not too expensive (I have not checked, but they are usually about £20 to £30).īut if the only symptom is the fuel trims, then I would just continue to monitor the situation. I have used similar double level control loops in several applications.) RPM is then used as a second, slower control loop deriving a MAP setpoint. Faster response in a control loop gives tighter control. (I suspect that normally, the MAP is used as a direct control over the idle valve setting point, as it will respond faster to valve movement. Thus the pcm would have to open the IAC wider. The 900 rpm idle could be the pcm recognising the failed TMAP, and setting a more conservative idle, as it had poorer control over the idle speed loop.
The fuel trims being normal could be a result of the higher idle speed, or the TMAP out of calibration giving the wrong computed air flow. All the engine controls are so inter-dependant. Only issue I could see was the idle speed getting about 150 rpms higher (so from 750 to 900), but, weirdly enough, the IAC was also opened around 6 % more Idle fuel trims returned to normal (overall around 3-4 percent positive trim). I know a bit about pressure transducers (I design & manufacture them!), but not so much about the pressures in Sigma engine inlet manifolds!. The 12kPa & 25kPa readings don't sound too far out to me, if anything a little high (not enough vacuum), but that is a bit of a guess. A good quality absolute device that will have <1kPa error (1%) near zero may not be cheap. So they will not read true at pressures near zero. Some low cost vacuum gauges are just gauge pressure devices that read negative.
Have you correlated the top readings with Met office data (corrected for your altitude by -12Pa per metre of elevation)?Ītmospheric pressure can go from 970mB to 1030mB, that is +/- 3kPa.
So span and zero errors will both alter the top readings. It is here that the transducer diaphragm is being bent the most, as an absolute transducer must have vacuum on the other side of the diaphragm. Most calibration errors will affect the top of the measurement range (atmospheric pressure). I average around 25kPa on OBD2 MAP reading at idle