Exhaust noise on ZX

[the_weaver]

I've just had a new exhaust fitted on my ZX 1.9D, for the MOT. I took the car for a test drive, and heard a rattle from the exhaust at about 70 mph. It sounded like I'd hit a speed which set off a resonant frequency.

I also noticed a sound like a cover missing on the inlet manifold side. When I got home, I had a quick look and noticed that the large diameter plastic pipe on the air filter housing had come adrift. That might explain the inlet manifold sounds. I've put the pipe back in place, but I haven't been for another test drive yet.

When I switched the engine off, I heard a metallic clank. It happens almost every time I switch the engine off. I expect it's the new exhaust, it didn't happen before. I think the movement of the engine, when it's switched off, is getting transferred to the exhaust.

I'm going to inspect the exhaust tomorrow, and I'll be taking it back to the garage, if it's the source of the noise. However, I would like to arm myself with some information, before I go back, so I can confront them with some facts, rather than just a general "it's noisy" complaint.

The exhaust has got a flexible coupling. Is this flexible coupling meant to stop movement in the engine from being transferred to the exhaust? If so, it may be source of the problem. Is there any way to check that it's working properly? Is there a test I can do by levering the exhaust sections apart, to check that there is some springiness in there? Is the spring coupling just meant to stop the rotation of the engine from being transferred to the exhaust, or is the spring coupling actually meant to have forward-rearward springiness in it.

I'm not sure I understand how the spring coupling works. I can think of two explanations for how it works. Can somebody tell me which of these two explanations is correct:-

Explanation 1 - It's a gasket which can rotate. The springs serve to clamp the gasket, whilst allowing a bit of rotation.

Explanation 2 - As above - "It's a gasket which can rotate", but in addition, the springs also act as a buffer, to stop movement in the engine from making clanking noises in the exhaust.


One other thing I noticed, is that the tail pipe of the new exhaust, doesn't stick out at the back, as much as the old one did. This could be explained, by the fact that it's a replacement exhaust, from a motor-factor. However, it could mean that the exhaust is sitting a bit too far forward.

Are there any common fitting problems that would cause the exhaust to clank? Are there mistakes that mechanics often make when fitting the flexible coupling? I remember a mechanic, in an exhaust place, once telling me that they used to add washers, to space out, or adjust, the flexible couplings, to compensate for the fact that the metal in replacement exhausts was half as thick as the metal in Citroen exhausts. I think some flexible coupling kits have a choice of three screw lengths, probably to suit different exhaust thicknesses. I suppose if you use the wrong screw, then the coupling wouldn't work properly.

Paul

[DickieG]

In a nutshell it appears that the new exhaust is hitting the body somewhere, it may be possible to sort it out by simply bending one of the support brackets for the rear box to gain a larger clearance between the exhaust and car body. The other tweak that may be necessary is for the joint between the back box and the section ahead of it to be released slightly and the joint position adjusted, both of these jobs should take no more than 5 minutes to do.

The manifold to downpipe joint is sprung loaded to allow for the tilting of the engine whenever the amount of drive being applied is varied and coarse gear changes, without the joint it would put excess stress on the exhaust causing metal fatigue as happened to the original Mini where exhausts were prone to fracture. Such joints are not necessary on rear wheel drive cars as with rear wheel drive the engine only twists side to side which doesn't put as much stress on the exhaust compared with front wheel drive/transverse engines.

The new exhaust being a pattern part more than likely explains the shorter length, however it should still clear the rear bumper so that exhaust gases are not being pushed directly onto the bumper as that may (due to the heat of exhaust gases) on a long journey cause the bumper to distort, not to mention possibly causing additional resonance and potentially a loss of performance by restricting the free flow of exhaust gases.

[citronut]

check there is a gap between the spring coils, or even if they have fitted the spring back in place,

if there is no gap there wont be any flex/give were its spozed to,

the other thing is if the jointed sections have been fitted at the corect oriantation/s,

if not the exhaust can foul on the body work,


regards malcolm

[the_weaver]

Thanks for the replies.

I had a look at the exhaust today, with the car on ramps. The exhaust is touching the heat shield, on one side (nearside). It's the heat shield under the floor, near where the handbrake lever is. The exhaust has got two boxes. It's the first box (approximately in the middle, not the rear box) that's touching the heat shield. I've taken the car back to the garage, and they're going to adjust the exhaust mounts. It looks like the exhaust box is not quite centered, and it's a bit too high (too close to the floor).

I think this may be the cause of the rattle I heard at speed. I'm not so sure that it's the cause of the clanking noise, that I get when I switch the engine off. It looks like the exhaust and heat shield are able to slide past each other easily enough, so I can't see why it would cause a clanking noise. However, I will give the garage a chance to adjust things, and see if they can fix it.

I told the chap in the garage that the new exhaust sounded different to the old exhaust. It sounds tinny. The sound of the engine is different. I can hear a sound, when I'm driving, that seems to come from a particular place in the engine bay. It sounds like I'm hearing the internal sounds of the engine, but clearer than I could before. When I'm driving, the sound seems to come from the area of the exhaust spring coupling/downpipe. I can't see or hear any exhaust leaks in the engine bay however, and I can't smell exhaust smoke. The mechanic said that all new exhausts sound tinny at first, because old exhausts have a coating of soot on the inside, which damps the sound out. It sounds like a plausible explanation. Has anybody heard this explanation before, or had experience of this themselves? I suppose the other explanation could be "cheap exhaust".

Paul

[Xsarahdi]

Hi Paul,

We had an exhaust put on our zx 1.9d a few years ago and we suffered the same problems as you. The exhaust used to knock when going round some corners which was cured by adjusting the rubber mounts at the rear and it also knocked when turning the engine off however it still does sometimes - I have just learnt to live with it.

Regarding the different sound, again I have had th exact same thing. It sounded more tinny/hollow and possibly slighty louder (even compared to the old exhaust that had small holes in) than before and still does a few years later. The sound hasn't improved with any coating of carbon unfortunately

To be honest the original exhaust was much better and I wish I had have just patched up the few small holes it had in it as it was only on the central pipe section, all silencers were still perfect for its age. The cheap ones really don't match the originals.

Regards
Mark

[the_weaver]

Mark

Thanks for the reply. It's very interesting to hear that you've had the same problems as me. I was starting to get the feeling that "cheap exhaust" was the root cause. At least I can now make more informed decisions anyway. At some point, I suppose I'll have to stop taking it back to the garage, and start getting used to the new sound of my exhaust. I wonder if it's possible to have sound-deadening on the outside of the exhaust. Like the anti-drum pads stuck on the inside of door skins.

Paul

[citronut]

i think if you stick those pads on your exhaust you will get a contiuous bitch you mean (bitchumen) smell as the exhaust gets hot,

if the flexi coupling is not tight enough it might only leak/blow as you accelarate/under load, you may not hear the blow with head under bonnet whilst car stationary,
try bliping the throtle whilst head under bonnet,

also quite offten the exhaust centres will fit a fibre sealing ring, insteated of the correct more expencive wovan wire ring,

the cheaper one will break up in no time at all as its not ment for a moveing/flexing joint situation,

regards malcolm

[myglaren]

Mark

Thanks for the reply. It's very interesting to hear that you've had the same problems as me. I was starting to get the feeling that "cheap exhaust" was the root cause. At least I can now make more informed decisions anyway. At some point, I suppose I'll have to stop taking it back to the garage, and start getting used to the new sound of my exhaust. I wonder if it's possible to have sound-deadening on the outside of the exhaust. Like the anti-drum pads stuck on the inside of door skins.

Paul

May be possible to use fibreglass matting as it will be heat resistant. No fun to work with though.

[the_weaver]

Thanks for the replies. The garage adjusted the exhaust mounts this morning. I've taken it for a test drive and all the problems have been fixed. The rattle at 70 mph has gone. The clank when the engine is switched off has gone. The tinny sound has gone. The strange sounds of the engine's internals have gone. The exhaust is quieter and I'm happy with the sound now. It sounds just like the old exhaust, or maybe quieter than the old one. The difference is amazing. I suppose the sound of the exhaust was being amplified because it was touching the body. The tinny sound may have been the sound of the heat shield vibrating. It is a large thin sheet of steel which would probably make a lot of noise. Now I understand why exhausts are hung from rubber mounts.