Start/stop problem

[Gollings]

I'm just after a bit of advice.

My Golf is having a random start/stop problem. Most of the time it doesn't kick in, and then if/when it does the engine will restart itself after about 10s.

I've scanned the car with VCDS and there are no faults (apart from an ongoing problem I've got with the cruise control) and the battery level is 11.9V with the engine off and about 13.8V with it running at idle.

Do you think it could be battery related? I think it's on the original and I'm coming up to 5.5yrs/94k.

Thanks

Rich

[Alex5555]

I would check battery first. What if you run on motoway for at least an hour, how does it change? 10s increased if you try straight after? Voltage of the battery doesn't say much, more important internal impedance. If battery is bad impedance increased. Voltage can be high at idle but under the load drops quickly.

[Gollings]

Thanks. Long motorway journeys don't seem to help. It has a 180 mile round trip most weekends and maybe one or two short journeys in the week.

Most of the time, the start/stop won't kick in on the first journey of the day even if it's immediately after getting off he motorway, but stopping the engine for a period of time and restart and it'll work as I described above. If the engine restarts itself after the SS kicks in, it won't function for the rest of the journey.

I should add that it's got progressively worse over time - it used to work perfectly. When it started to not work properly, I put it down to the fact that I'd halved how much I use the car, but now I'm not so sure this is the case.

Will a battery test confirm if the impedance increases under load?

[zollaf]

if the battery voltage is below a certain level the s/s wont work. i think you need a new battery. years ago a battery would last until it would just get the engine started. now they last 3-5 years, until the float volts are above about 12.5 volts.

[Gollings]

I'll start looking for a battery.

Does it need to be coded to the car or is it a straightforward swap? Google isn't very forthcoming.

[Alex5555]

Yes, bettery test will tell that. I think Halfords do bettery test for free. They also check charging in the car

[zollaf]

it wont though, a battery can test ok but still have too low float volts to allow s/s to operate. all depends on what test is used, but all you need to do is put a multi meter across the terminals and see if the volts is above the threshold or not.

[stevie5tapes]

I can only talk from the experience of my Land Rover. My stop start didn't work very often, changed the battery and everything is fine now. If you change your battery make sure the battery spec is the correct one for the car. When I wa looking online for a new battery it was surprising how many companies quoted for the wrong battery capacity when using my car reg details.

[Alex5555]

it wont though, a battery can test ok but still have too low float volts to allow s/s to operate. all depends on what test is used, but all you need to do is put a multi meter across the terminals and see if the volts is above the threshold or not.

Checking voltage on the battery by multimeter is not the test. Simple proper test is when you have charged battery and connect calibrated load. Voltage drop will tell you
how capable battery is. This is exactly what is going on when you start the engine. Battery may show high voltage but drops very quickly when high load connected

[niall campbell]

in some cars, using the rear demister stops the stop/start working