1996 728i Starts fine then struggles to start after its been running

[GrandClam 1]

Hello there guys.

This isn't a desperate problem yet but I figured I'd better get out in front of it now before it becomes one.

My 728i has recently been struggling to start but mostly only after its already been running (say i drive it somewhere then leave it to sit for half an hour or so then try to start it again)

What will usually happen is the first time of the day I start it it will start no problem and then every time after that it will struggle. Either crank for longer than it should before starting or once or twice its failed to crank at all.

I have never had it completely unable to start yet thank god usually it starts are a bit of struggling maybe 2-3 seconds. But a couple of times it has failed to crank at all and then after a few tries it starts up.

Once its running absolutely nothing is off or weird about the car so its definitely just a starting issue.

Any ideas what this could be? Its got a brand new alternator in it so I doubt its that maybe the battery or the starter motor? also had the crank shaft sensor replaced very recently as well so don't think its that either.

This might have nothing to do with it but I did accidentally hit a curb just before this issue started. No damage to the car that I could see and it wasn't a major hit just mis judged where it was and drove up onto it at maybe 15km or something.

your thoughts are greatly appreciated thanks in advance

Edited January 3, 2022 by GrandClam

[polley 916]

Check wires of the crank sensor at the plug underneath the rubber boot.

[GrandClam 1]

5 hours ago, polley said:

Check wires of the crank sensor at the plug underneath the rubber boot.

ok will do tomorrow maybe

[jon dee 500]

9 hours ago, GrandClam said:

Either crank for longer than it should before starting or once or twice its failed to crank at all.

If by "failed to crank" you mean that the starter did not attempt to turn the engine over, then it is unlikely to be a crank sensor issue. I'd suggest starting off by loosening each battery terminal clamp, wriggling it slightly to break any corrosion buildup, and then tightening them again. If the battery is grounded to the chassis (I'm not familiar with the 728i) do the same thing with the ground connection. Likewise there is probably a starter motor to chassis ground strap that you can check.

Cheers...

[polley 916]

13 hours ago, jon dee said:

If by "failed to crank" you mean that the starter did not attempt to turn the engine over, then it is unlikely to be a crank sensor issue. I'd suggest starting off by loosening each battery terminal clamp, wriggling it slightly to break any corrosion buildup, and then tightening them again. If the battery is grounded to the chassis (I'm not familiar with the 728i) do the same thing with the ground connection. Likewise there is probably a starter motor to chassis ground strap that you can check.

Cheers...

Woops I didnt read the original post properly hehe. 

 

[GrandClam 1]

14 hours ago, jon dee said:

If by "failed to crank" you mean that the starter did not attempt to turn the engine over, then it is unlikely to be a crank sensor issue. I'd suggest starting off by loosening each battery terminal clamp, wriggling it slightly to break any corrosion buildup, and then tightening them again. If the battery is grounded to the chassis (I'm not familiar with the 728i) do the same thing with the ground connection. Likewise there is probably a starter motor to chassis ground strap that you can check.

Cheers...

Ah ok so it sounds to you like a battery problem. that could make sense as when I replaced the alternator the battery went fully dead as it was sitting for 2 months. I know that is surposed to be quite bad for them but i used a battery charger to get it back up to full charge and havnt had any issues with it since and then was a couple months ago. will check the connection today. perhaps some wd40 or some kind of connection fluid would help?

[jon dee 500]

38 minutes ago, GrandClam said:

perhaps some wd40 or some kind of connection fluid would help?

What I am suspecting is that you have a high resistance connection that is causing the starting circuit voltage to drop. The usual culprit is a loose or corroded terminal or ground connection. WD40 won't help... as a minimum you need to loosen the connection just enough that you can twist them by hand before tightening again... that will be enough to break the corrosion layer. Longer term it is better to break the connections and remove any corrosion by wire brushing or sanding so that you can obtain a solid bare metal connection to ground.

If you are handy with a multimeter, you can test the resistance of your connections providing one end of the cable is isolated.

Cheers...

PS: If your battery is dodgy, then that can cause much the same issues... one good start and then difficulty with subsequent starts. If the battery is borked it will take a surface charge (takes maybe 15-20 minutes to show full), but if the battery is good it should take 8 hours or more to fully charge from flat. Got another known good battery you can try ?

Edited January 4, 2022 by jon dee
PS

[BM WORLD 1283]

bad crank or cam sensor will cause this ,