Technical Noisy exhaust around 2000rpm, only after engine is up to temp

[gadb]

Hello all,

Have an odd noisy exhaust problem. A mechanic fitted a non-oem exhaust to my 500 a few years back (not the front bit that includes the cat, but the middle (inc flexi joint) and end bit. It's not been trouble free I must say - flexi joint went after about 14 months, which I cut out and replaced. However it's been making a bit too much noise lately, but strangely, only once the engine has come up to temperature and generally only noticeable around 2000rpm. I thought exhausts tended to be noisy when they engine was cold rather than hot. I'm slowly working my way along it with the soap spray gun, compressor stuck in tail pipe, but wondered if anyone knows where I'm likely to find the problem? I guess if it only becomes noticeable once things are hot, the soap gun won't do me much good and I don't have a smoke machine. Don't have any issues with power from the engine and no errors apparent from the dash. The exhaust looks in pretty good shape from the outside (garaged when not in use and I do maybe only about 4000 miles/year).

Any ideas? Cheers!

 

[John202020]

Is the front pipe to gearbox clamp still there? If not it could be the lower manifold bolts

 

[varesecrazy]

When cold / just started..

Put a rag over your hand while lying at the rear of the car

Place the padded hand over the end of the exhaust..

Do you see / hear smoke coming out of other joins..?

 

[jrkitching]

Is the front pipe to gearbox clamp still there

That one's not the original!

Here's an OEM bracket.

It's in two parts; one attaches to the maniverter, the other part attaches to the gearbox. It's the part attached to the maniverter that usually corrodes, OEM replacements are readily available and reasonably priced. The part that attaches to the gearbox is harder to find, although fortunately it is usually OK.

But any combination of brackets and clamps which firmly holds the pipe in position will do just fine. It's important to secure the pipe properly at this point, or the exhaust will overstress the attachment at the manifold and may break the studs and crack the maniverter, and they're not cheap!

When refitting, covering it with anything that's greasy and reasonably heat resistant may help delay the inevitable.

 

[gadb]

Is the front pipe to gearbox clamp still there? If not it could be the lower manifold bolts

Thanks all. Had a look last night and although my clamp looks a bit different to yours (I note prev comments about it often needing to be replaced), it seems pretty solid. I gave the exhaust a good yank and it didn't budge. There are small leaks at the entry to the flexi-joint (a hideous bolt on affair for my sins) and around a weld around a sensor attachment hole just after the cat. I have a stick welder which tends to blow holes in things that are thin and was considering getting one of those cheap flux core welders from Lidl to seal things up, as they look a bit more delicate. I imagine welding with the sensor in place would be foolhardy and don't know how easy the sensor would be to remove and then re-attach after. These leaks are small though and I suspect that's not the source of the problem. Although perhaps things get significantly worse once everything heats up. I've yet to look past that sensor, at the cat and manifold. Cheers.

 

[gadb]

After a bit of fiddling about, thought I'd update this thread. I plugged all the leaks I could find on the underside (from tail pipe to the start of the cat) - there were several minor leaks identified with the air compressor / soapy water technique. This unfortunately included a large leak I created myself when I blew through the pipe near the oxygen sensor below the cat with my stick welder (facepalm).
Alas, noise still there. I noticed a crack in the rear rubber exhaust mount, so got a new one. After much puffing and panting, I replaced that.
Alas, noise still there.
I don't know why they don't put stainless exhausts on all cars as a matter of course.
I'll look again when my "fix-it" battery is recharged.

 

[steveras57]

Could you reiterate what type of noise your talking about? is it just louder than normal?, or a blowing noise, or a rattle etc! might give us some ideas on pinning it down!, have to say aftermarket IE- non -Fiat exhausts often cause changes!- mine made the car very" raspy" just off idle but was ok over about 1000rpm, on a friends car the flex joint broke up inside causing a clatter at certain revs! describe the noise and situations that cause it in more detail and we might get closer to the problem!.
PS have you checked that the manifold heat shield is still attached buy ALL its bolts? and there are no broken manifold studs are there?.

 

[gadb]

Could you reiterate what type of noise your talking about? is it just louder than normal?, or a blowing noise, or a rattle etc! might give us some ideas on pinning it down!, have to say aftermarket IE- non -Fiat exhausts often cause changes!- mine made the car very" raspy" just off idle but was ok over about 1000rpm, on a friends car the flex joint broke up inside causing a clatter at certain revs! describe the noise and situations that cause it in more detail and we might get closer to the problem!.
PS have you checked that the manifold heat shield is still attached buy ALL its bolts? and there are no broken manifold studs are there?.

Cheers. It's a sort of boom around 1800 - 1900 rpm (this rpm seems to be a magic range for my 500 where any initially hard to notice noises get loud / resonate). Doesn't really sound like blowing, although conceivable. Manifold heat shield seems pretty solid, although I haven't checked all bolts - will take a look. You can't hear it when you first start up, but once the car gets warm (halfway to normal working temp) it's there. Maybe 6 years ago, I asked a garage to replace the exhaust before I knew anything about cars at all and they said they used non-oem parts to get "better value for money". They put it on squint (tail pipe is a "roundtangle" shape rather than a circle so it was pretty noticeable) and the flexi-joint went within 18 months (and about 5000 miles) so I replaced that. And yeah it always sounded louder than the original (which was literally falling apart from rust). From the outside, the exhaust looks not too bad, but the welds are pretty crap I think. It's really not done that many miles and the car is garaged in winter. I guess I should probably check the problem areas again with the compressor / soapy water after all the wiggling that went on replacing the rear rubber mount.

 

[steveras57]

Good idea ! as the noise is not there until the car warms up a bit its obviously due to some thing expanding!( either causing it to touch something else and vibrate!) or a hole/gap opening up allowing gas escape!, does not sound like broken metal as you would expect that to make the noise hot or cold?
Might be worth LIGHTLY tapping the cat with a rubber mallet or the like just to make sure nothings loose and rattling about in there either!

 

[gadb]

Another update - tried putting smoke through after driving around for 10 minutes and hearing the boom. Smoke machine was a home made variety (mineral oil, rags and soldering iron element) and seemed to produce plenty of smoke (I stuck the outlet in the tail pipe) but couldn't spot any smoke emerging anywhere either along the underside or in the engine bay. Perhaps the smoke wasn't able to make it along the exhaust due to insufficient pressure (about 1 bar from my compressor). Anyway, it's back to the drawing board.