97 318i knackered air boot now won't start

I been trying to help a friend with his 318i and it had a knacker inlet duct leaking air after the air meter. This was fixed for a few weeks with tape and the engine problems have returned, but now it won't start
even with a new boot.
Before getting worse it was reving up and down, and then wouldn't run. I did try and start it yesterday and it ran for a bit (badly) and I suspected the tape was not sealing it. Now it won't start atall.
I havn't seen the car since he's replaced the duct and he says the battery is still ok, so maybe he hasn't been trying hard enough. He also says the rev counter is jumping around when he is trying to start it.
Should I keep trying or is it likely there is another fault? Flywheel sensor?
-- Tony
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There's a leak somewhere.

The revving up and down was because the system was leaking air, and the control loop wasn't closing properly.

Could be. You don't know anything until you look at the engine codes. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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Scott Dorsey wrote:

Got it - there was another fault, the alternator +ve terminal at the battery was loose, a bit like a bad earth, causing the voltage to go all over the place.
Car is running well again.
-- Tony
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