Technical Resonance at ~2800 RPM

[fiatritmo]

On my 2002 Stilo 1.6 I've recently had the gearbox changed (I purchased a used, low mileage one at a scrappy), and I got a new clutch installed while I was at it. The reason I had the gearbox changed was a wobble on acceleration at around 70-100 km/h, that several mechanics diagnosed as a differential issue. Unfortunately, the wobble became a lot worse with the new box in place, so I ended up having to change the drive shafts as well (the old ones were probably not reassembled the way they were originally fit to the car, so I shelled out for two brand new drive shafts just to get it sorted once and for all). Thankfully, this finally cured the wobble, but all of a sudden I was left with another problem that I didn't anticipate at all.

It turns out that the car now has developed a resonance at around 2800-3000 RPM. To my untrained ear, it sounds like an exhaust drone or something, but I've had mechanics look at the exhaust (which, apart from the cat, was replaced just 3 months ago), and they find nothing wrong. They haven't managed to find anything else wrong with the car, either, so I've been told that I probably have to live with the car behaving this way.

The resonance seems to be RPM dependent, and is noticeable when I drive the car, not so much when I just rev it when it's standing still (though I think one can hear it slightly even then, but I'm not quite sure). It occurs in any gear, but outside of the mentioned RPM range it's fine. I believe this problem was not present when the gearbox and clutch were changed, but occurred first after the drive shafts were replaced one week later.

So my question is, does anyone know what the problem might be? My guess is that something must have happened to the car when the drive shafts were replaced, as new drive shafts themselves could not cause these kinds of problems. Or could they?

I have a slight suspicion that the engine mounts may be the culprit, but again, I don't see how the engine mounts would be affected by a drive shaft replacement.

Does anyone have any suggestions? It's possible to live with this problem, but I'm asking because I don't like leaving symptoms unattended, in case they are caused by an underlying problem that may turn out to be expensive to fix if I just ignore it. Not to mention the fact that since I've spent more than £6500 on maintenance on the car the past 6 months or so, I really want a car that's working properly...

 

[StiloBoy]

Check the heat shield under the bonnet bolted onto the manifold. I bet its loose at one of the bolts.

My 1.2 appears to have not had its heatshield fitted properly at build of the car. and it has moved about and mad the bolt holes bigger so it then pings off the bolts and vibrates. I cured this by fitting great big repair washers so that its clamped tight.

But over time due to heat expansion and cooling contraction it needs the bolts tightened up.

Before the sheild works fully loose it resonates, I've just noticed mine doing this again, mine resonates about 3000rpm as you've noted.

By the way not sure how a garage diagnosed the gearbox as causing the wobble. I'd have thought it would be something further out from the gearbox.

Wheel balance, discs, drive shafts, suspension in that order is what I'd have checked.

 

[fiatritmo]

Thanks for the tip. I had a look at what I think is the heat shield, and I found a nut that doesn't seem to be tightened. See the attached picture. Could this be the culprit?

 

[StiloBoy]

Looks like the other side is snapped off too.

Does the heat shield move if you pull on it? It should be rock solid attached to the manifold!

If it moves even a fraction tighten up the bolts (carefully you don't want to shear any off) then take it for a test drive and report back!

 

[fiatritmo]

I tightened the bolt, but it didn't seem to make a difference. And the shield doesn't seem to move at all when I pull on it. So perhaps the cause of the resonance problem lies elsewhere

 

[fiatritmo]

There are three bolts on the top of the heat shield, does anyone know if there are any bolts further down at the catalyst? Should the heatshield somehow be "mounted" to the catalyst?

 

[almci1975]

The heatshield is in two parts. The bit you see has bolts into the manifold . But there is three that join both of them together further down then two that bolt on to the downpipe on the exhaust. That is a jtd version don't know if your one is similar?

 

[fiatritmo]

Just thought about something - the new drive shafts that were installed were presumably aftermarket items, and probably solid rather than hollow (which the original ones are as far as I know). Could the aftermarket drive shafts themselves be the cause of the resonance I'm experiencing?

Or what about worn or damaged engine mounts?

 

[12[X]]

Feasible, it is also possible that a change in overall frequency/harmonics is aggravating a different issue; cannot really give you any solid info (not had that issue) but would imagine that, if caused by an engine mount , the problem would be more detectable at the rpm in question while decelerating with the engine (i.e., let go of the pedal and let the RPM drop – feels any different?) and even more pronounced if you rev while stationary (i.e., the shafts are not moving..) but it can be many different things.. get a stethoscope and start poking ;-)

Or live with the fault as, given it is serious, it will get worse over time

 

[fiatritmo]

Form what I've observed the resonance pretty much only seems to manifest itself when the car is driving (more on acceleration than on deceleration), and not when stationary. The car does seem to be "shaking" a bit when the engine is starting (which is why I'm suspecting the engine mounts) as well as having a slight wobble when accelerating from standstill, and some vibration in the steering wheel when the car is accelerating at higher speeds.

 

[12[X]]

this is as far as I can take you (based on the info provided - your engine mounts should be fine) someone more knowledgeable may be able to help you further. Final note: if it were my car.. feedback through steering (under the circumstances) is not normal (i.e., could be/become dangerous) and would prompt me to 1) balance + align the wheels (asap) and (if the feedback is still there) remove and inspect the new parts (as they could have been wrong and/or incorrectly fitted i.e., problem did not exist before).

 

[fiatritmo]

I've actually experienced the vibration in the steering wheel before, and it was cured back then (early this year) by a new gearbox mount, but the vibration now returned when the entire gearbox was replaced, so I suspect the gearbox mount has not been properly installed after the gearbox change.

 

[percydnb]

I have exactly the same symptom. Any news on this?

 

[Ralf S.]

My money would be on the gearbox mounts.

If the engine moves around a lot at idle (someone else blipping the throttle) then that's not a good sign. If the vibration/shunting is worse when the car is accelerating (particularly just after changing gear) then it's more likely to be the gearbox mount.

There's a big engine/gearbox mount under the battery that holds the weight of the gearbox (bolts against the inner wing) and a bone-shaped mount that supports the bottom/rear of the differential housing.

The good news is that the bone is more likely to fail.. but is also cheaper to buy and pretty simple to fit.


Ralf S.