Single tire chirp with downshift
 

Out for a drive today and noticed my rear passenger side tire started chirping with downshifts. And not "sporty" downshifts, just regular driving, maybe slowing down for a turn in the road. If I double clutch it doesn't happen.

I also noticed a rather strong smell, something I couldn't place. (Not burning oil, tranny fluid, burnt clutch or tire rubber). If I had to describe it, I'd say scrambled eggs, but less sulphur-y. When I got home the engine bay looked totally normal, no leaks anywhere. 20 minutes later, the floor underneath is perfectly dry.

What could this be? The only thing that came to mind is the limited slip - I've only had this car about 4 years, and it's my first experience with LSD.

Are you sure it isn't your fan belt ?

Sulphur smell could be an overcooked battery. Faulty VR will do that.

http://forums.pelicanparts.com/uploa...1591328691.jpg

fan belt is still there, and there was no light on the dash.

How loose is it ?

Don't rely on the lights. Use a voltmeter.

Thrust bearing?

Belt tension feels good, about 1/2" deflection. Voltage looks good too, 13.1 with the engine off, 13.75 at idle, 13.9 at 2000rpm (without headlights, a/c or anything), measuring at the battery terminals.

You might have better results measuring the voltage as you're driving, not stationary. When I had a bad VR, volts were good engine off, at idle and 2000-2500 rpm. I only saw it get into the danger zone after I started driving the car - half way down the block it jumped to 16V. I just measured it at the cigarette lighter with a pigtail I made from a 12 volt accessory cord and my meter.

I believe that when the regulator goes bad it'll be high voltage, not low.

I've got a cigarette voltage meter coming today from Amazon. But none of that would explain the tire chirp on the downshift would it? I'm worried there's something going on in the differential, and maybe the smell was related to that.

If you're going straight, the differential part isn't doing anything. So if you're thinking the diff is essentially momentarily stopping the right rear tire, it doesn't make much sense mechanically. If a diff fully locks, it keeps both rears turning at the same rate, it doesn't stop one side.

Just out of curiosity, have you checked your CV joint bolts? Or, have you jacked the car up and wiggled the tire? Have you turned the RH rear by hand to make sure you don't have a sticking caliper or e-brake mechanism?

Maybe it's pulled head studs.

I haven't done any of that, but I will. I know one of the calipers does stick from time to time, and I have the parts to rebuild it (it's on my list for this summer), but would that actually grip the brake disc enough to actually lock the tire up momentarily, and only during a downshift?

Though now that you mention it, I wonder if it was only happening in turns. I'm going to take it for a spin today and test that.

If its dragging the extra drag could overcome the tires grip.

Even at 60 kmh?

The emergency brake is assisted by rotation so that could be it but you should see a bunch of heat coming off of it.

Doesn't the e-brake only lock the driver's side wheel?

Both sides.

A bit of a weird one.

Some thoughts:

Can you get it to chirp by coming off the power and then going back on without shifting?

The grip of the tire is so great I can not imagine that the sound is really coming from the contact patch.

I'd look at drive shafts, suspension mounting points, engine mounting points etc

Use an IR thermometer to compare temps side to side to see if anything is dragging.

For it to smell, something is getting hot.

I don't think it was doing that, but I can't say with 100% certainty.

I'll look at those, for sure.

If double clutching eliminates the problem, could be sticking clutch.

Wouldn't that affect both wheels, not just one?

Clutch release bearing - mine made same noise- thought it was the belt.

Just got back from a test drive, VR looks fine - steady at 14v on the highway, down to 13.7 with headlights, 13.5 with headlights and AC. No spikes at all, on the highway or city streets. No smell at all. Couldn't get the tire to lock up on downshifts, either going straight or in a curve. Haven't had a chance to jack it up and get under it yet.

Porsches never act up on the test drive!

Any roadkill marks on the tire :D ?

TBH, I'm wondering if it wasn't something like that that just got stuck somewhere, burned up, and is now gone.

Yes. If minimized when double-clutching, the clutch disk, if oil-fouled, will tend to grab instead of typically slip when torque continues from engine to gearbox.

If so, check rear main and/or input shaft seals.

Sherwood

A good thought. If it is the throw out bearing, it will become more obvious as time goes by.

Ok, I'll keep an eye (and ear) on that.

See if you can (re) create the sound other ways to isolate the cause. Or just wait to see if it appears in other scenarios, or changes with time.

For example, if it is the throwout bearing, it ought to make the sound any time you depress the clutch and not just on downshifting. Try downshifting earlier or later than usual to see if the sound changes, or perhaps it may reveal that the timing of the sound with downshifting was coincidental, etc.