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Hi i have been having intermittent starting problem on my car for a while. It sometimes starts and stalls then takes ages to start or starts first time now problem. I've found when i've let the engine warm its usually fine. The car is a 07 1.8tdci and it's only started happening after i had my MAF/MAP sensor replaced due to a pipe was loose on it. I purchased a used one would this cause the engine to start and stall/not start?


regards Neil


The MAF (mass air flow) and MAP (manifold absolute pressure) are 2 differen t things -


MAF sensor -



Product with useful info in description -




The MAF sensor goes between the airbox and turbo, and measures the airflow, it often uses a hot wire that cools more as the airflow increases, so the IAT (intake air temprature) is important - changing the IAT is a basic way to tune an engine with a simple tuning box



MAP sensor -



The MAP sensor goes somewhere in- between the turbo and inlet manifold (usually after the intercooler) it measures boost pressure, when you turn the ignition (before starting) it measures the barometric air pressure (this changes due to weather conditions/ height above sea lovel etc, so is nessesary to get a correct reading) once the engine is started, it measures the pressure in the pipe its situated in  



The sensors are analog, and the resistance changes the output voltage when an input voltage is applied, this analog output voltage has to be converted to digital  (for the ECU/ digital microprossesor to read) so the voltage is converted to digital with an A-D (analog - digital)convertor  



There are other sensors as well as the IAT, MAF and MAP, there is a knock sensor and the head or temprature sensor, crank or cam position sensor and possibly other sensors (depending on the design) - the ECU interprets all of these signals 



Often, a fault code is produced - MaF sensor out of range etc


So the owner/ technician/ mechanic assumes that the sensor itself is faulty, but it may be that the sensor itself is working perfectly, and there is a fault in the boost control/ turbo (eg - too much peak boost) if, for example the ECU reads a signal that represents too much boost from the MAF sensor, the ECU only reads the signals that are fed to it and cannot differentiate from eg, too much boost fed to the MAF (and the MAF, its connector, wiring etc working perfectly )  or the MAF is fine and its connector or wiring is faulty


So there is a chance your sensor is fine, and the replacement one is fine too (whether is actually the MAF on MAP )  and the fault lies elsewhere (it could simply be a loose connection onto the plug)


You have not specified whether it is the MAP or MAF you have replaced, or if you have read the fault codes/ what fault codes came up, or if you took the car to a mechanic/ garage - and other potentially important information - because it is difficult enough to diagnose faults remotely (without seeing or hearing the car/ engine)  

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