Idling/battery/running trouble

1. user_64763 Established Member Joined: Jun 2013 York Posts: 2134 Citroen Saxo VTR (2001)

   #1 5th August 2014, 00:04

   So I recently posted about my car bogging down at around 2k revs when accelerating...well tonight I parked up with radio and headlights on and after 15mins the radio went off then I realised the battery was dead brilliant!
   After getting it bump started, it was idling at 5k revs which with a straight through, isn't the best of things!
   
   I put it in gear, brought the clutch up and used the brake to slowly bring the revs down and it then idles at 2k revs but was revvin up and down by itself! Then instead of bogging down when accelerating it fully cut the throttle off and on making it so juddery unless I floored it then it was fine but idling was still high and lumpy!
   
   After a 25mile drive home it's fine again except the same slight bogging down and revs sometime sticking at 2k but when I just kiss the accelerator it goes down and idles fine but why did the battery die so quickly? just a dead battery?
   And what is with this bogging down/high idle stuff?!
2. user_39461 Super Moderator Joined: Nov 2009 Cambridge(ish), UK. Posts: 10581 BMW 5x M50d.

   #2 5th August 2014, 08:03

   The ECU controls the engine idle / fuel etc.
   
   With shit battery voltage it has no idea what the sensors are reading because it's calibrated to read the sensor outputs from a car with 13v or so battery (so a sensor might put out 3.4v or whatever). With the cars battery flat, the same sensor might be putting out 2.2v under the same conditions because it's input voltage is down - so the ecu tries to adjust not knowing any better.
   
   Batteries aren't just there for running the headlights and heater fan
   
   All aftermarket ECU's (and presumably the standard ECU's) have a separate voltage adjustment table to compensate for fluctuating battery voltages, but you can still go outside of these boundaries if you have a really shit battery charge level.
3. user_64763 Established Member Joined: Jun 2013 York Posts: 2134 Citroen Saxo VTR (2001)

   #3 5th August 2014, 08:55

   Quote:

   Originally Posted by Ross The ECU controls the engine idle / fuel etc. With shit battery voltage it has no idea what the sensors are reading because it's calibrated to read the sensor outputs from a car with 13v or so battery (so a sensor might put out 3.4v or whatever). With the cars battery flat, the same sensor might be putting out 2.2v under the same conditions because it's input voltage is down - so the ecu tries to adjust not knowing any better. Batteries aren't just there for running the headlights and heater fan All aftermarket ECU's (and presumably the standard ECU's) have a separate voltage adjustment table to compensate for fluctuating battery voltages, but you can still go outside of these boundaries if you have a really shit battery charge level.

   
   I was thinking along those lines but that's awesome thanks! Great bit of info really helpful!