No start on hot days

[Cabriolet]

My friend has a 1990 325i. And on hot days (90F+) he has no crank.
changed all relays.
to get the car to start he jumps a line from the battery to pin 11 on the diagnostic port on the engine.
then the car starts. or we can push start it, to get it to run.

do you guys have any ideas of whats going on there?
we are thinking ignition or the starter interrupt. thoughts on testing this?

[bmwstephen]

it sounds like the starter but that could only be if you hear nothing engaging when turning the key all the way. when i first bought my car it would not crank up immediately after the first drive. I would have to wait an hour or so for the engine bay to cool off before it starts again

[MaksV]

I sometimes have this issue too. Changed batteries - issue dissappeared.

[matt0300]

What about coolant temp sensor?

[rares]

check ignition coil and check valve in the fuel pump. my bet is it is one of those.

[jlevie]

If the engine won't crank (i.e. turn over) by the key yet will by the diag connector, the ignition switch or wiring from the switch to the starter are the suspects.

[Cabriolet]

thx guys for all the info. we'll be tearing into his car soon.

[matt0300]

Sorry the coolant temp sensor wouldn't not make it start but everyone else's suggestions are good.