Jet engine sound

[awesM]

Whenever I start the car up cold (i.e. mornings, parked for extended periods, etc.) it sounds like there's a jet engine spooling up. After the car warms up it stops.

Any ideas of what it could be?

Thanks in advance!

[Fern3]

My guess is it's your secondary air pump. Pop your hood before you cold start your engine and listen to the top left part of your engine bay where the pump is. If it's coming from there then you have a vacuum leak somewhere that's why it stays on for longer than usual. Check connecting hoses and such

[InfiniteDice]

Either a belt or bearing... I'd look at alternator or water pump.

Bearings usually make that sound, belts tend to whine/screech

Press on the belt, if it drops more than 1/2 and inch then it needs to be adjusted. ( 2 pulley vs. 3 pulley belt have different deflection ranges, check a book) Look at the belt for glazing, the rubber will be shiney on the part that rides on the pulley, look for cracks in the rubber, it may need replacing.

Just noticed 36's comment, I'm not sure what an air pump is... but if your car has an air pump for some reason check that. LOL

[Fern3]

Lol any obd2 e36 has a secondary air pump that pushes more oxygen into exhaust gases to comply with emission limits.

[PedroBMW]

its the air pump for sure.

[Mad Cow]

Originally Posted by e36.M52B25

Lol any obd2 e36 has a secondary air pump that pushes more oxygen into exhaust gases to comply with emission limits.

Seriously?T hat is one of the greatest ideas I've ever heard, but how did BMW even get away with this? That would just dilute the mixture, not lowering the emissions in any way.

[Fern3]

Lol this idea was patented in 1966 and it wasn't by BMW. Obd2 emission standards were stricter than obd1 so most cars got an air pump installed after 1996, all it does is creates a more oxygen enriched enviroment and therefore combustion is fuller = cleaner emissions.

[propr'one]

^^why would the combustion be fuller if the secondary air pump blows air into the headers?

[Fern3]

I'm not really sure. I think it's cause the air that leaves the headers is still so hot that there is still some form or chemical reaction or combustion going on.. But that's just my guess

[BimmerDriver]

Those are the turbos spooling up for sure man...

[awesM]

Originally Posted by BimmerDriver

Those are the turbos spooling up for sure man...

Thank you for you contribution.

[PedroBMW]

Originally Posted by e36.M52B25

Lol this idea was patented in 1966 and it wasn't by BMW. Obd2 emission standards were stricter than obd1 so most cars got an air pump installed after 1996, all it does is creates a more oxygen enriched enviroment and therefore combustion is fuller = cleaner emissions.

The secondary air pump reduces emmisions by decreasing catalyst light off time (the time it takes the cat to get to its operating temperature after a cold start.) The engine is purposely run over-enriched after the cold start so that the excess fuel (that wasn't burned in the cylinder) combusts with the air being pumped into the exhaust (specifically the catalyst brick), thus causing an exothermic reaction, ie. heat. This is pretty much standad on any car these days that is trying to attain the ultro low emmisons standards like PZEV, and will only become more and more used.

[awesM]

Update: out of town for a couple days, and now she won't start. Can the noise be the cause?
Engine is turning over but it won't catch.
Going to look at the plugs...

[mclovin]

I get that noise when car sits outside in -C
My noise is coming from the driver side of the engine and it goes away within a minute of a cold start.

[Eurotakeover]

my 325i sounded like it had a turbo, then my exhaust cracked, now it sounds like it has a super charger apparently