Starting problem

[Grantus]

I have an E34 520I with the M50 non vanos engine. When I start it it runs ok then starts to surge the revs a little with an irregular pop (backfire?) on the inlet side, definitely not the exhaust, all this at idle.

Is this likely to be a vacuum pipe leak somewhere under the inlet manifold? I checked the brake booster pipe and it seems ok. There doesnt seem to be a fault code system on this car ( I tried the stomp test) no warnings on the dashboard either apart from service light.

Fuel pump and filter checked out ok, I'm about to check some sensor voltages.

Any help will be much appreciated.