I knew I had a minor vacuum leak, but it got worse all of sudden while at a stoplight.

In trying to get through to the next block, the engine started to knock. (My guess is preignition thanks to the air/fuel ratio being off)

Now when I turn it on in neutral, the idle is intermittent and sometimes turns off but there is no sign of knocking.

First gear, however, is a no go.

The car did have a vacum leak (which the mechanic fixed), but it came back again.

  • Was wondering why did this leak come back again?

  • Is there another reason for it occurring?

  • Why is it sometimes better when the engine is started cold? (Tighter rubber?)

  • Furthermore, is there any other inspection I should do because of the knock?

There are no check engine lights.

1 Answer 1


Preignition occurs when a hot spot inside the combustion chamber ignites the fuel before the spark does. The exhaust gas recirculation system (EGR) dilutes the air/fuel mixture slightly to lower combustion temperatures when the engine is under load. This reduces the formation of oxides of nitrogen and also helps prevent detonation. So, I would check the operation of the EGR valve and system.

  • On a 1991 BMW E30? EGR valve? Nov 23, 2015 at 12:08

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