Barometric pressure correlation issue
09-06-2020, 04:58 PM
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w203 220 CDI
Barometric pressure correlation issue
2005/6 C220 CDI w203, 5 speed auto
Worked through a long list of problems with this one but stuck at the last hurdle. Cars had a new turbo, intercooler, MAP sensor (genuine bosch), intake manifold cleaned out (already had flap delete manifold fitted), fuel system checked, injector seats recut and honda washers installed.
Starts and idles forever nice and smooth no problem.
Almost constant limp mode, will occasionally pull out of it and drives like it should for all of about 5 seconds then goes back into limp or stalls.
Only fault code that comes up is barometric pressure correlation. Can clear it but as soon as ignition is on, this pops up immediately. If you drive around a bit, turbo boost circuit low comes up (to be expected given the following).
Ignition on, engine off...
Barometric is given as 101pa (pascals I assume).
MAP is given as 139pa
MAF gives a reading of 1.8g/s
Ignition on, engine on..
Barometric is still 101
MAP nevers goes below 139pa, but does increase as actual boost increases
MAF gives a reading that follows engine load nicely, not worried about MAF.
Ignition on, engine off. Now I start pulling sensors out.
Barometric, when I disconnect the sensor, the ECU still reads 101pa. Surely this should fall to zero as there is no sensor (or maybe its just showing me the default value of 101, which is sea level).
MAP sensor, when disconnected the value drops to 49pa, not zero and not 101. Would expect this to fall to zero, or to fall to 101pa like the barometric sensor, but 49pa is weird
MAF sensor, should have zero reading when engine is off, but gives a solid 1.8g/s, until its disconnected and then ECU sees 0g/s. Might just be mild wear on the sensor?
With everything plugged in the car drives in limp, but is jerky as if the ECU is trying to push/pull extra boost to try and figure it out.
With MAP sensor unplugged, car drives in limp but nice and smooth, it doesn't try to adjust anything
Checked the +5v on both sensors, sits at a nice steady 4.99v all day long with no drops, tried poking and moving as much of the wiring harness as possible and these voltages stay the same.
Haven't checked GND yet
Haven't really been able to check the signal wire on both, means have to pull the ECU from its main connector right and then check for voltage drop or leakage?
Questions....
Anyone know how the ecu sees unplugged sensors on these cars. Should the values fall to zero or to a default value?
Any ideas on checking grounds around the PCM/ECM and is this a common thing? Imagine 2 of my 3 weird values could be attributed to a bad ground or leaking ground from another source.
My new bosch sensor is somehow faulty, but why does the value go to 49pa when its unplugged, surely it should be zero or 101pa?
Is there a long learned value for MAP that is adaptive (like fuel trims on petrol cars), as this car had a hole in the top of the old MAP sensor from the engine cover resting on it, so before my ownership the ecu would have been seeing less and less boost via the MAP sensor so maybe demanding more, or maybe adapting its value from the MAP sensor?
Worked through a long list of problems with this one but stuck at the last hurdle. Cars had a new turbo, intercooler, MAP sensor (genuine bosch), intake manifold cleaned out (already had flap delete manifold fitted), fuel system checked, injector seats recut and honda washers installed.
Starts and idles forever nice and smooth no problem.
Almost constant limp mode, will occasionally pull out of it and drives like it should for all of about 5 seconds then goes back into limp or stalls.
Only fault code that comes up is barometric pressure correlation. Can clear it but as soon as ignition is on, this pops up immediately. If you drive around a bit, turbo boost circuit low comes up (to be expected given the following).
Ignition on, engine off...
Barometric is given as 101pa (pascals I assume).
MAP is given as 139pa
MAF gives a reading of 1.8g/s
Ignition on, engine on..
Barometric is still 101
MAP nevers goes below 139pa, but does increase as actual boost increases
MAF gives a reading that follows engine load nicely, not worried about MAF.
Ignition on, engine off. Now I start pulling sensors out.
Barometric, when I disconnect the sensor, the ECU still reads 101pa. Surely this should fall to zero as there is no sensor (or maybe its just showing me the default value of 101, which is sea level).
MAP sensor, when disconnected the value drops to 49pa, not zero and not 101. Would expect this to fall to zero, or to fall to 101pa like the barometric sensor, but 49pa is weird
MAF sensor, should have zero reading when engine is off, but gives a solid 1.8g/s, until its disconnected and then ECU sees 0g/s. Might just be mild wear on the sensor?
With everything plugged in the car drives in limp, but is jerky as if the ECU is trying to push/pull extra boost to try and figure it out.
With MAP sensor unplugged, car drives in limp but nice and smooth, it doesn't try to adjust anything
Checked the +5v on both sensors, sits at a nice steady 4.99v all day long with no drops, tried poking and moving as much of the wiring harness as possible and these voltages stay the same.
Haven't checked GND yet
Haven't really been able to check the signal wire on both, means have to pull the ECU from its main connector right and then check for voltage drop or leakage?
Questions....
Anyone know how the ecu sees unplugged sensors on these cars. Should the values fall to zero or to a default value?
Any ideas on checking grounds around the PCM/ECM and is this a common thing? Imagine 2 of my 3 weird values could be attributed to a bad ground or leaking ground from another source.
My new bosch sensor is somehow faulty, but why does the value go to 49pa when its unplugged, surely it should be zero or 101pa?
Is there a long learned value for MAP that is adaptive (like fuel trims on petrol cars), as this car had a hole in the top of the old MAP sensor from the engine cover resting on it, so before my ownership the ecu would have been seeing less and less boost via the MAP sensor so maybe demanding more, or maybe adapting its value from the MAP sensor?
Last edited by faster4tec; 09-06-2020 at 05:02 PM.
04-13-2024, 08:58 AM
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w203 220 CDI
Yes!
Gave up for a while and pushed it into a corner.
Took all the sam out and cleaned every connection with a very small very fine emery file.
There was water entering car when parked on a certain angle into fuse box near steering wheel. Coming through bonnet release grommet.
New grommet, all new fuses, cleaned and applied contact cleaner to everything.
Fixed. Rogue voltage gone.nightmare as it wasn't even obviously corroded
Gave up for a while and pushed it into a corner.
Took all the sam out and cleaned every connection with a very small very fine emery file.
There was water entering car when parked on a certain angle into fuse box near steering wheel. Coming through bonnet release grommet.
New grommet, all new fuses, cleaned and applied contact cleaner to everything.
Fixed. Rogue voltage gone.nightmare as it wasn't even obviously corroded
