A couple of gremlins

I have heard vehicles run with inlet manifolds off and the engine is a lot louder. If you look on realoem.com, the intake is actually called the intake 'muffler' it smoothes the airflow going into the cylinders and also covers the actual part the air is sucked in past the valves. This process is quite loud and the intake manifold does muffle the sound. If there was a significant leak, the engine would be louder and I'd be losing a lot of vacuum causing all sorts of fuelling issues.

This is why my thoughts are simliar to jetset's, I'm thinking either a vacuum pipe is leaking (though I did renew some of these when replacing the PCV) or a pipe is possibly trapped. One thought that just ocurred to me now.... there are two pipes coming off the PCV valve. One one the drivers side that goes onto a fuel pressure valve on the rail and on the passenger side it goes off down under the nearside somewhere (passenger footwell area) The pipe on the passenger side was totallyperished and would not have survived being re-attached so I took a normal length of vacuum pipe and used that. When I took the old pipe off, it looked like there was a tiny little piece of plastic tubing half way down the pipe with an internal diameter of about 2-3mm. The mechanic said it looked like a joiner which must have been used previously and had broken off leaving a piece in the pipe. My thought now is, could this have been some type of restrictor that was meant to be there? if the vacuum has to suck through a smaller gap, it makes it more powerful does it not? My theory is, could this be causing the vacuum to be sucking too much and causing an imbalance?

It's only an aside theory but wondered if anyone else noticed this in their pipe?

Didn't manage to look on Saturday. I have been using a work van so not had the car either. I'll do the brake cleaner test tomorrow.

Mike

Hmmm - well I hope he's talking about the exhaust manifold then because the inlet manifold is the one with the vacuum!! and is the one that will hiss when it has a leak. It's the exhaust manifold that will blow and be quite loud if it's leaking.

Also, the problem with our engines is that they are so torquey and monstrous anyway that even if there is a problem and not running on full power it's actually quite hard to tell! The only time I've ever noticed a true difference in performane was when I renewed the spark plugs and it seemed like a different engine and felt so responsive and so much more potent at high revs.

Back to topic though...! (Sorry) I'm sure that your seal is good, just throwing out possibilites! It could be something completely different, but just from everything you've described I think the first thing you should try is to test for leaks around that rear left area of the air intake manifold and all connected lines. Once that's done and you can be sure it isn't that, then it'll be easier to let it go and think of other things :)

I'm still a bit concerned that you've been told by a mechanic that it's very unlikely, because most people will tell you that a hissing sound from the engine bay of a V8 BMW is MOST likely to be the inlet manifold gasket because they are prone to going. And considering that's just been tampered with it's the first thing I'd have thought he would think of!! Anyway, I hope you find something tomorrow - good luck!

I'm convinced it's a vacuum leak near the rear left side of the manifold area - but the vacuum in the manifold definitely decreases as you open the throttle, as it allows more air in :) so the noise should get quieter.

It could well be the PCV gasket, the crack in the manifold (hope not though) or any of the little vacuum pipes round there. As Jetset says, take the cover off and just have a damned good listen and spray brake cleaner or something around the area. I'm sure you'll find something.

EDIT: Oh, sorry Jetset - just seen your edit!

Hi, I noticed you had posted while I was writing my last post! I've just been looking for diagrams but can't find any at the moment, at least not of our engine bays. I also seem to remember my brake booster being on the driver's side but I could well be wrong!

That's strange what you say about your dealer - perhaps we're a bit behind times here in Suffolk but they are always happy to see me going in to see them and bug them with questions (considering I give them very little money other than for parts, for which they normally give me a 10% discount anyway!) and the technician I usually talk to is a mature chap who's worked with BMW for over 20 years now, so he says. The way he talks about my car without even looking at it though I believe him!

The specialist I mentioned earlier has just one technician and he started at BMW in the late '80s and so is also very clued up. Then my local mechanic who I entrust all the jobs that I can't do to worked at both Volvo and BMW throughout the '90s and so has always been perfect for me! Perhaps I'm in a lucky position but I wouldn't use anyone else.

I have, in the past taken my car to other garages when my mechanic's busy to ask for advice and frankly they didn't have a clue. Yes, they are mechanics, but my brother-in-law is a lecturer and trains mechanics and I've seen what he teaches and is in the syllabus.....! I'll say no more, but following a manual when you have a good set of tools is not a difficult task and that's what I try to do now! Having someone that really knows a specific car not only saves time (as you did suggest) but also will often result in correct diagnosis and, more often than not, will save you money. I love my brother-in-law like my own syblings, but I wouldn't let him touch my car :)

Still, your chap's started it so you might as well let him finish! I will have my glovebox out tomorrow as I'm looking for a ZV module to plug a keyless entry/alarm system into and so will poke around for anything that looks like a vacuum pipe! Best of luck for tomorrow!

I had a look at mine today and it's easy enough to see the crank sensor and to get to it - there's a large, toothed wheel behind the main pulley on the crank on the front of the engine and the sensor is at 10 O'Clock if you're looking in from the front of the car.

If I were you and could find the time I would trace all the vacuum pipes going to and from the manifold and as Jetset said just spray brake cleaner around with the engine switched on to find any leaks.

Really, to save yourself all this worry you could just get it looked at by a proper BMW technician. Then again I understand it's expensive and throwing cash at it might not be an option just yet!

It could just be a disturbed vacuum line or the manifold gasket (or crack!!) is still leaking and although you can't hear it over the engine it might be directing the hiss straight back at the firewall, which could be why you're hearing it so clearly inside. If you've got good lambda sensors then the engine will still run with a small intake leak, but would obviously not help the idle.