Engineering Department - wherefore art thou ?

[TripleS]

Since a new exhaust system was fitted to the old 406 Hdi about a month ago, the car has been producing periods of roughness at certain engine speeds, and this is bugging me somewhat.

When I collected the car after the new exhaust system was fitted, I immediately noticed a small increase in vibration at about idling speed, with a more pronounced vibration period at about 1100 rpm, and then again at about 2200 rpm. The roughness was also found to be more pronounced when the car was being accelerated through these engine speeds, and this can clearly be felt through the accelerator pedal and through the floor.

My immediate thought was that this was likely to be caused by some misalignment of the new exhaust, perhaps allowing some part of the system to be touching the underframe, and thus transmitting engine vibration into the body structure. However, this was checked and there appeared to be no such contact, and nor was any problem seen with the flexible mountings.

Incidentally, the work was carried out by the same garage that has done virtually all the work on the car during the past nine years, and they seem very anxious to overcome the problem for me.

The garage technicians are of the opinion that there is a fault with the exhaust system as supplied to them, so they have obtained a new one under warranty and it is due to be fitted this afternoon. This, by the way, is not a genuine Peugeot system, but one from a motor factor, but I don’t see why this should account for the problem we’re having. Anyhow, that’s the way they’re tackling it, and I have to say I can’t see what other explanation there could be.

From my experience back in 1960 with the Austin-Healey Sprite, I well remember that car having a nice sporty exhaust note from new, and that it used to produced a slightly louder sound when being accelerated and the engine speed passed through 1700 rpm. This, however, was purely exhaust noise, i.e. air transmitted sound, as opposed to the mechanical vibration that I’m now getting with the 406.

It is true that the 406 always has produced a vibration period at around 1400 rpm (plus or minus a bit) but I think this is a characteristic of the model, caused by an engine vibration period combining with some natural frequency of the engine mounting system, and causing some resonance to emerge and amplify the disturbance. Certainly it is unwelcome, but it seems to be inherent in the car; and short of renewing the engine mountings (which may have gone a bit soft) I don’t see what can be done about it. Even then we might only shift the problem to a different point in the speed range, so there may be little to be gained.

At the moment we clearly have a new problem with vibration and roughness, and I’m only hoping the new system this afternoon will cure it, otherwise I’m at a loss to know what to do.

Best wishes all,
David.

[Mr Cholmondeley-Warner]

What state are your engine mounts in?

[Gareth]

This, by the way, is not a genuine Peugeot system, but one from a motor factor, but I don’t see why this should account for the problem we’re having.

A can of worms ...

Additional vibrations can be caused by exhaust leaks but with the attention being lavished on it I doubt that's the problem.

I think of the exhaust system as being, in part, a tuned system. It probably is in multiple ways but one way is how the manifold lengths are arranged so that pulses of exhaust gases from different cylinders do not interfere with each other. Another factor is that there is a designed back pressure, without which the engine doesn't perform correctly. Whether the joins between individual parts are smooth or not will vary the amounts of turbulence, as will the exact shape of the bends in the system.

All these could contribute to the differences you're seeing but there is another possible significant difference between an oem system and what is probably a much cheaper after-market alternative, and that is the detail of the construction.

The oem system will probably have benefited from an engineering department testing and measuring various alternatives, as well as using previously discovered design rules. The pipe material may be different to the cheaper alternative, and it may vary in thickness in a different fashion. Here, too, there could be many difference that affect, or not, what happens when the system is fitted to your car.

[TripleS]

What state are your engine mounts in?

Well they are obviously not as firm as they would have been when new, but I can't quantify the extent of the deterioration. There has recently been a marked change in the degree of roughness, so a gradual softening of the engine mountings over a long period probably doesn't really explain the new problem, even if it relates to it in some way.

Best wishes all,
David.

[Mr Cholmondeley-Warner]

Without knowing more, I'd be looking either at the exhaust (as you are doing) or at engine mounts. Vibration transmitted to the chassis pretty much always comes from one of those. You don't say why the exhaust was replaced, but it could be that cause and effect might be the other way around - an engine mount collapsed, leading to stress on the exhaust and a fracture, for example.

Good luck getting it fixed, anyway.

[TripleS]

This, by the way, is not a genuine Peugeot system, but one from a motor factor, but I don’t see why this should account for the problem we’re having.

A can of worms ...

Additional vibrations can be caused by exhaust leaks but with the attention being lavished on it I doubt that's the problem.

I think of the exhaust system as being, in part, a tuned system. It probably is in multiple ways but one way is how the manifold lengths are arranged so that pulses of exhaust gases from different cylinders do not interfere with each other. Another factor is that there is a designed back pressure, without which the engine doesn't perform correctly. Whether the joins between individual parts are smooth or not will vary the amounts of turbulence, as will the exact shape of the bends in the system.

All these could contribute to the differences you're seeing but there is another possible significant difference between an oem system and what is probably a much cheaper after-market alternative, and that is the detail of the construction.

The oem system will probably have benefited from an engineering department testing and measuring various alternatives, as well as using previously discovered design rules. The pipe material may be different to the cheaper alternative, and it may vary in thickness in a different fashion. Here, too, there could be many difference that affect, or not, what happens when the system is fitted to your car.

Thanks for that, Gareth.

I've no doubt there are various factors that could affect the issue here, and I've maybe not twigged them all, by any means. Even so, I'm inclined to suspect that the increased roughness is more than can be accounted for by the way the system 'breathes', i.e. gas flow behaviour. I do appreciate that in certain powerful cars with large engines and noisy exhaust systems (like some TVR models, for example) the air transmitted disturbances can feel to be hitting you almost in a mechanical sense, but the low powered 406 with much smaller gas flow rates hardly seems likely to be capable of that. Incidentally, it used to be clear that exhaust systems manufactured from copper tubing would produce very different exhaust notes compared with steel, but even then it was heard, rather than felt.

The overall effect in this case is as if there is now a direct contact between the engine/transmission unit and the body underframe at some point, and the new exhaust system was naturally first suspect - but so far we haven't found the precise cause; but I sure hope we soon can.

Best wishes all,
David.

[TripleS]

Without knowing more, I'd be looking either at the exhaust (as you are doing) or at engine mounts. Vibration transmitted to the chassis pretty much always comes from one of those. You don't say why the exhaust was replaced, but it could be that cause and effect might be the other way around - an engine mount collapsed, leading to stress on the exhaust and a fracture, for example.

Good luck getting it fixed, anyway.

The exhaust system (which was last renewed about four or five years ago), had been looking pretty rusty for a while, and then recently it started to leak a bit, hence the need for replacement.

There is no sign of any engine mountings having collapsed, but the rubber components are somewhat softer than they would have been originally. If I become convinced that renewing them will give an improvement in overall refinement, I'll certainly consider having them changed, but as I said earlier it might only alter the point at which vibrations become emphasised.

Best wishes all,
David.

[TripleS]

May I ask if the system it replaced was aftermarket or OEM? Gareth raises some interesting points. My feeling is that there may be some internal part of the system which is loose & reverbrating. Although, as you say, they're probably not involved, if your engine mounts are original it could do no harm to change them; they deteriorate not only with mileage but also with age. (Well we all do, don't we)?

Some folk might actually improve with age, at least in some ways. It's maybe not all bad news.

Anyhow, the car was on its original exhaust system until about four or five years ago, at which point another genuine Peugeot system was fitted. At the time I felt that was extremely expensive, mostly due to the wretched catalytic converter.

The one now being fitted is supposedly one of the better quality aftermarket systems, so I just hope it turns out right this time.

Best wishes all,
David.

[Mr Cholmondeley-Warner]

You've had to have two complete exhaust systems (including cat) in 5 years? I call that very bad luck

[fungus]

You've had to have two complete exhaust systems (including cat) in 5 years? I call that very bad luck

I have to agree there. The wifes 306 hdi is ten years old, and is still on its original exhaust. The milage is fairly low though, being 69K.

We did experience some rough idling back in the winter, but I just adjusted the idling speed a fraction, and that's cured the problem. Our mechanic told me that I should not have fiddled as it's all tied in to engine management system.

[TripleS]

You've had to have two complete exhaust systems (including cat) in 5 years? I call that very bad luck

I have to agree there. The wifes 306 hdi is ten years old, and is still on its original exhaust. The milage is fairly low though, being 69K.

We did experience some rough idling back in the winter, but I just adjusted the idling speed a fraction, and that's cured the problem. Our mechanic told me that I should not have fiddled as it's all tied in to engine management system.

Well I don't know what would be regarded as a normal lifespan for a mild steel exhaust system these days, but in my case they each seem to have lasted about five years.

The catalytic converter, by the way, is an integral part of the front pipe assembly, so I've tended to replace the whole system, rather than mess about doing one part this year, only to have to renew another bit fairly soon afterwards. I've generally found that when the system looks generally grotty and starts to leak, it's not worth doing it on a piecemeal basis. You sometimes find that what looked like a fairly decent bit falls apart when you starting dismantling things.

Anyhow, an update:

Another complete new system from the same manufacturer/supplier was fitted yesterday afternoon, and I'm told it appeared to fit together better than the previous one, but it still has not solved the problem. The periods of engine roughness can still be felt just as before, so the mystery remains.

Having again discussed the matter at some length with the garage people, I'm wondering if the problem lies in the area of the mounting that is located just downstream of the manifold outlet. This is described as a metal/rubber assembly that bolts to the subframe and fits around the front pipe and incorporates a heat shield. Maybe there is some unintended metal to metal contact in that area, effectively by-passing the rubber cushioning elements of the assembly. I think we need to have a more careful look in the area of that mounting.

Best wishes all,
Dave.

[quintaton]

I experienced roughness after hitting a large deer which ran into the road at full chat from behind a high wall. I never even had a chance to hit the brakes, and the impact at 55mph was quite something. Fortunately, the deer was killed instantly, and with it, the front end of my Laguna Turbo.

For some time after, I experienced roughness in the exhaust note, and thought that the impact may have damaged the system; the noise and vibration coming from behind the front bulkhead.

Believe it or not, the source of the noise turned out to be a heat-shield which had loosened on impact with the deer, but not at the front of the exhaust system. In fact, it was at the rear, and the vibration was coming right through the exhaust and sounding as if it was a front vibration.

As a musician, I note that two engine speeds are mentined, at 1100rpm and 2200rpm, and straight away, that tells me something, because the harmonics created will be exactly an octave apart in frequency. That leads me to believe that this may well be a loose baffle, (not properly welded in place in the silencer box or boxes), or a loose heat-shield. If it's more of a sonic type of boom, then it may just be poor design. I know that my old Honda Accord sounded quite different when fitted with non-standard exhausts.

Q

[TripleS]

I experienced roughness after hitting a large deer which ran into the road at full chat from behind a high wall. I never even had a chance to hit the brakes, and the impact at 55mph was quite something. Fortunately, the deer was killed instantly, and with it, the front end of my Laguna Turbo.

For some time after, I experienced roughness in the exhaust note, and thought that the impact may have damaged the system; the noise and vibration coming from behind the front bulkhead.

Believe it or not, the source of the noise turned out to be a heat-shield which had loosened on impact with the deer, but not at the front of the exhaust system. In fact, it was at the rear, and the vibration was coming right through the exhaust and sounding as if it was a front vibration.

As a musician, I note that two engine speeds are mentined, at 1100rpm and 2200rpm, and straight away, that tells me something, because the harmonics created will be exactly an octave apart in frequency. That leads me to believe that this may well be a loose baffle, (not properly welded in place in the silencer box or boxes), or a loose heat-shield. If it's more of a sonic type of boom, then it may just be poor design. I know that my old Honda Accord sounded quite different when fitted with non-standard exhausts.

Q

Thank you for that. I think Gareth mentioned the possibility that the new system might have problems with mating parts - pipe joints etc. - that are not fitting together correctly and thus producing noise and disturbance of some kind, but these have now been examined quite carefully, and we can't see anything wrong with them.

I think we've now established that heat shields are not the problem; and your reference to a possible loose baffle is a fair point, although in the case of heat shields and loose baffles I would expect those to produce a tinny rattle as they are lightweight elements made from fairly thin metal. What we actually have is a sort of resonance produced by vibrations of heavier masses, like the engine unit, with some sort of amplifying effects thrown in. Come to think of it, I suppose the mere act of fitting a new, relatively rigid, exhaust system will have changed the damping effects on the engine, compared with the degree of restraint provided by the old system. It does seem to be, as Gareth said, a can of worms. I just wish we could get the bloody lid back on it!

Anyhow, for our next move I'm probably going to have the engine stabiliser bush replaced, in the hope that the new bush will cushion the vibrations better than the existing component, which has clearly softened a good deal and now permits a greater range of engine movement. Having said that, I'm still a bit anxious in case it merely shifts the vibration periods to different points in the engine speed range - presumably higher up the speed scale. Of course if it gets shoved up the speed range sufficiently, it might not bother me, given that I seldom use more than about 2500/3000 rpm.

Best wishes all,
Dave.

[WhoseGeneration]

Of course could be any number of things, which you appear to be investigating.
However, the Peugeot NVH folks worked on the OEM stuff.

[quintaton]

i'm an ex-organ-builder......bit of an expert on pipes and noises.

This is what you do to decide if it is a sonic problem or a mechanical construction problem.

Take a solid ball (or two), and ram them into the exhausts, having drilled big holes through them. Now try the vehicle in a safe place (away from, other people and glass windows) on a light throttle. If it is an acoustic problem, the vibration will have shifted to a different engine speed. (You can do something similar with heavy metal mesh scrunched together, but it's less scientific).

Another way is to press the palm of the hand against the end of the exhaust to create back-pressure.

Does the vibration disappear? (Obviously, the vehicle has to be parked, or you will do yourself damage running after the vehicle at 20mph in a crouched position, and in any event, you would probably be pulled for "inappropriate behaviour" on the Queen's Highway).

If it turns out to be a mechanical/poor baffle/badly made exhaust scenario, take a hammer and smash it into the silence box(es) a few times. It's amazing how that often "cures" most vibrations.

I don't know why, I just imagined all this as a scene from "The Simpsons," with Homer running behind, and Marj driving the car.

Q