Stilo - boomy exhaust/performance loss - Jas
Hello all, I was wondering if anyone could shine some light on my problem.

The car is a Fiat Stilo, 1.6 16v petrol, only 2 months old. The car will run perfect from cold, but started from hot, sometimes (but not always) the exhaust has quite a 'boomy' tone under acceleration, and sometimes from low revs gives off a deep rasping flatulent sound. The boominess sounds embarrassingly like one of those boy-racer exhausts, and it is sometimes loud enough for people in the street to look as I go past. When it is like this, the trip computer doesn't appear to tell me that my fuel economy is down, but the performance is markedly worse - it feels sluggish under acceleration, doesn't like being revved, and is quite hard work to drive. However, once I reach my desired speed and stop accelerating, the boom is -almost- gone, but the ride feels ever so slightly rough, not like when the car is working properly, where it rolls effortlessly along the road, with a very quiet engine tone and exhaust! Even if it is booming, when the car is at rest, the idle is rock solid, and the engine is very quiet. Only under accelation do the problems appear.
I've had the car into both local Fiat garages, and both say they cannot find a problem. One garage said they connected it up to their computer and gave it a good look over before taking it for a spin, and they said it was fine. Indeed, after I got it back it ran perfectly in all conditions for about a week until the problem returned. Maybe they reset the computer back to its defaults, which have changed again, I don't know.
The major headache I have is how can I get it fixed when the car hasn't recorded any faults, and by my bad luck, it decides to behave when they have it? They said I might have to run it for longer until it records a problem, but what if it is never bad enough to be out of spec?

Anyway, could any experts give me an insight into what can physically cause this booming etc. and if there are any techniques I can do (without voiding my warranty!) to narrow the fault down?

Thank you.
Stilo - boomy exhaust/performance loss - Dizzy {P}

I don't know if it helps but I also sometimes give off a deep rasping flatulant sound and I agree that it is very embarrassing, though I haven't yet had people in the street look at me as I go past - they tend to turn the other way.

Sorry about that! I'm far from being an expert on modern ECU controlled engines but your problem sounds to me a bit like a fuelling fault, i.e. a mixture that fails to enrich for acceleration every so often. I hope and expect that there's an expert here who will have a much better answer however.

Whether or not we can suggest answers, I think you should be persist in getting the supplying dealer to accept that there is a problem and to put it right, even if it means that they have to involve Fiat directly. You paid out for a new car so as to be assured of problem-free motoring and good service and that's what you should get.
Stilo - boomy exhaust/performance loss - DL
Intermittent faults are always the worst.

Try leaving it with the garage and seeing if somebody trustworthy woudl drive it for a couple of days or so and see if it manifests itself.
Stilo - boomy exhaust/performance loss - Jas
I was going to use the word \'*****\' but wasn\'t sure if it would be censored!

If it hadn\'t. Mark or I would have done so anyway. DD
Stilo - boomy exhaust/performance loss - Jas
Sorry, I was just trying to give the most obvious description of the sound - no offence meant.

Looking under the car, and from listening carefully with the windows down, the noise seems to be coming from the front exhaust, if that's the right description, which is roughly under the driver's seat. It is particularly throaty from about 1500-2500 revs, after which most of the boom eases. Still only doing it when the car is warmed up.
Could this be an actual problem with the exhaust itself, and if so, could this have an influence on the smooth running of the car?

Thanks again for any ideas.
Stilo - boomy exhaust/performance loss - Mondaywoe
Sounds a bit like an exhaust leak that develops as the system heats up / expands. There's every chance that that could upset the running of the engine. Would it be possible to ask the garage to remove and re-hang the exhaust to see if that makes any difference? Has it got one of those flexible couplings at the manifold? Could be the culprit.

Stilo - boomy exhaust/performance loss - Dizzy {P}

I told you someone would come along with a good suggestion, and Graeme has done just that. Having thought about what he said, I agree with you both that the exhaust system could be the source of the problems.

You said that "only under acceleration do the problems appear". This does sound a bit like a joint or crack opening up as the engine 'twists' under load, as might happen with a fractured flexible exhaust coupling.

Perhaps it doesn't happen with a cold engine because you do not accelerate hard until the engine has warmed, or the engine isn't producing full power anyway as it is still cold. Or, as Graeme said, it doesn't happen until the exhaust system heats up (there's quite a lot of thermal expansion in an exhaust system).

Best wishes for a fast cure, and thanks for entertaining us with your superb description of the exhaust flatulations!
Stilo - boomy exhaust/performance loss - howy686
I've seen some data on catalytic converters which suggests that they can become noisy if partially blocked. Apparently the gasses find their way between casing and core instead of passing through the middle.
In fact I've suffered the noise which you describe for several years with my 16V Ecotec Cavalier, but it's always passed the MOT emission test, and you don't tend to spend £200 unless you're absolutely sure!!!
I can also relate to the 'flatulent' noise from under your feet when accelerating from lowish revs - it's never been very nippy either and certainly has 'off' days.
Recently I've even considered taking the 'cat' off and shaking (gently) in a forward direction to see if any bits fall out - at least I'd be sure then...
Has anyone tried this?

Value my car