Raspy exhaust note

[dan 244]

Hi, I have a recently aquired 940 2.3lpt and it has a definite more pronounced low note when revved, than is normal. It is most pronounced at around 2500rpm. The last owner struggled with a vibrating sort of noise at about 50mph, and changed the prop bearing, the universal joint and had the prop balanced. Also the clutch changed, but I think it’s more engine related. Particularly as it has this underlying deep smooth exhaust note at 2500rpm, which ties up with 50mph too. The previous owner to the last guy had changed the timing belt.
So I’m wondering if it’s possible for the belt to be say a tooth out so as exhaust ports are not opening at the correct point to let gasses escape, causing a low tone noise.
The noise when running at 50mph sounds like if you was in 5th gear and slowed down to 30mph and accelerated.
Any comments appreciated.

[AndyV7o]

A number if things here, is the exhaust comfortably clearing everything? If it touches the body anywhere, or can make contact when running, this will cause such things.
Secondly is the exhaust a pattern-part? Some pattern part exhausts use different silencer internals which can make odd noises, sometimes the tubing material/bore/bends/stresses can cause resonances, and so-on.
Then are there definitely no manifold leaks at either end, or cracks etc.
Are engine mounts all in proper order, a failed one can transfer engine vibrations or allow engine to touch body, etc.

All things to eliminate before going much further as these are the most likely causes. If you've eliminated these already, then ignore me!

[Laird Scooby]

Does it go a bit better than the other one Dan?

I'm going to take a wild guess here that someone has knocked the innards out of the cat, either because it blocked up and it was a case of having to so the car ran or because they wanted more grunt and economy.

If not, then check everything Andy has suggested - he saved me a lot of typing!

[dan 244]

Originally Posted by AndyV7o

A number if things here, is the exhaust comfortably clearing everything? If it touches the body anywhere, or can make contact when running, this will cause such things.
Secondly is the exhaust a pattern-part? Some pattern part exhausts use different silencer internals which can make odd noises, sometimes the tubing material/bore/bends/stresses can cause resonances, and so-on.
Then are there definitely no manifold leaks at either end, or cracks etc.
Are engine mounts all in proper order, a failed one can transfer engine vibrations or allow engine to touch body, etc.

All things to eliminate before going much further as these are the most likely causes. If you've eliminated these already, then ignore me!

Thanks. Can’t see any obvious leak. Would definitely say exhaust is a pattern part. Engine mounts look to be ok, I will think about changing them and the exhaust.

[dan 244]

Originally Posted by Laird Scooby

Does it go a bit better than the other one Dan?

I'm going to take a wild guess here that someone has knocked the innards out of the cat, either because it blocked up and it was a case of having to so the car ran or because they wanted more grunt and economy.

If not, then check everything Andy has suggested - he saved me a lot of typing!

That’s interesting, the car goes like mad! Wouldn’t say economy is any better than the last one, which it should being a manual, but yes it goes very well. Didn’t know you could do that to the cat. Would it affect the emissions? It just passed the mot? How can I tell? I suppose it would need removing to have a look.

[AndyV7o]

Yeah you can gut them, illegal for cars after august 1992, but if its done well enough that mot tester is totally unaware and emissions are passed then it can be got away with. Before that date cats werent mandatory. However mot rules have just changed and I -think- it might now be if the car came out the factory with it fitted, any signs of tampering is an instant fail. But if its pre aug 92 and has the non-cat pipe, I doubt theyd even give it a moments though, and if they did, theyd have to prove somehow that it didnt leave factory like that which isnt worth their while so long as emissions are within spec for the year it was made.

[Laird Scooby]

Originally Posted by dan 244

That’s interesting, the car goes like mad! Wouldn’t say economy is any better than the last one, which it should being a manual, but yes it goes very well. Didn’t know you could do that to the cat. Would it affect the emissions? It just passed the mot? How can I tell? I suppose it would need removing to have a look.

Quick test is crawl underneath and tap the cat with your fingers - if it sounds hollow then it probably is.

Many cars will still pass the emissions if the engine is well maintained without the cat. I've had two 7xxs (previous 740GLE and current 760GLE) where the tester has put them back on the ramp to try and find the cat because the emissions have been so good. Neither had a cat from new and don't need one either.
Also i used to run my 827 Sterling without the cat through the year, just replacing it for the purpose of passing the MoT because it was fitted from new (although in fairness, it fell into the "grace period" where it still doesn't need one) and one year i forgot to refit the cat. It passed the emissions but he failed it as it was obviously a decat pipe that was fitted.