I have an bit of an issue that no mechanic can seem to figure out. The car is a 2009 Ford fiesta that the battery light keeps popping up when slowing down at crossroads, the battery light will mainly just pop up and then go away after but I can feel the car starting to die (sometimes cuts off but not always). This has been an ongoing issue for two years but recently the car has been cutting off a lot more, so the car completely goes off and won’t turn back on for a minute or two so when I turn the key it’ll kind of shudder and the battery light will pop up and then disappear a few times and then it’ll drive 100% fine after. It’ll only come up when the car is slowing at a junction. 5 mechanic men have looked at it- one said he emptied the petrol filter (which another mechanic says my car does not have) another mechanic said the car could just be idling too low and to just deal with the issue, one man changed the crank shaft sensors and another man thinks it’s maybe injectors but he doesn’t know I’m thinking it’s an electrical issue or maybe the alternator intermittently cutting off? We changed the battery in the car and the car drove 100% for a day no battery lights popping up and then we were right back to square one the next day where the car was cutting off and taking a few minutes to turn back on. The battery and (I think) alternator was tested when the car was just turned on but the battery light will only ever come up when slowing down to a junction, thinking it could be an electrical issue or something along them lines? The car also has apple car play (that was already installed when I got the car) so I wonder would it be draining the battery when in use or possibly still using the battery when the car is turned off? I’m just lost at this point now so any advice would be greatly appreciated!

Edited May 19May 19 by Ford2009

Hi,

If 5 men have failed to find the issue then I think you need to try a woman. 🙂

If this is a petrol engine then the cutting out is often caused by a split PCV hose under the plastic manifold. This is a very common fault though so I would expect at least one of the mechanics to know about it.

Really we need to see some live data from the car when it cuts out to see whether the voltage drops because the engine idle drips too low, or whether the alternator is faulty and not charging enough at idle speed. That can't be done in one stationary test though, it needs to be done over at least a few minutes with a laptop or wireless dongle plugged in.

Yes, don't confuse the battery warning light coming on as low voltage.

The light is coming on because the car is about to stall so the alternator has dropped below charging speed.

That's why the battery light is on on ignition, because the alternator isn't turning.

You need to investigate why the idle speed is dropping off, could be electrical, could be mechanical.

First thing to check is what @TomsFocus has suggested.

Also, make sure the engine is fully serviced, new, correct oil & filter, new air filter, new plugs and leads.

Then check all the electrical connections to the engine are clean and fully secure.

Also check that the engine/gearbox is earthing to the body correctly.

Check & clean (fully) the throttle body as it could be sticking

Run it on better quality fuel as well, see if the issue goes away after running on E5 instead of E10 (Tesco is the cheapest)

But it's probably just the PCV hose like Tom has suggested