Loud BANG help (please)

[G051999]

Hello all,

New here. Looking for a wee bit of help/knowledge advice..

Car:

Boxster S 986 2002 103k miles manual


What happened:

Driving home from my brother's wedding in Brighton with the missus, taking the South Downs route back and avoiding LDN, I was driving "enthusiastically" (but not ridiculously as the gf doesn't enjoy it to quite the same extent as I do..) when i suddenly felt and heard a rather large one-off BANG from the middle or back of the car.

In a slightly panicked state (as I was on a twisty road with medium amounts of traffic) I naturally slowed down a bit but then found I was free-revving in third and then second gear, before I found some power in first (limp mode?) and managed to pull into a country cottages entrance / side road.

I switched off the engine immediately fearing, as is natural, that it was an IMS fail and the engine was toast.

Called for rescue. Rescue came. To everyone's amazement, the engine turned over, it idled fine, there were no oil leaks anywhere, no metal in oil, no noises at all, and it even engaged first gear and traveled down the path. At this point, I think the mechanic thought I was a bit mad. So did I. My gf is already totally used to me.

Anyway, for fear of doing further "damage" I had it lifted to a specialist near me. They test drove it. "Drives fine." I've driven it since and bar a slight idle concern, it does seem to drive absolutely fine. No noises. Pulls OK (although I've not gone beyond 4500RPM). Clutch engages fine. No slipping. All seems well.

But, as a mechanic said to me: "A car doesn't just fix itself.."


Bit of background:

Car very recently had a major service. Oil Separator had recently gone and was replaced. Had a stall / limp mode incident (presumably due to the AOS) but those issues seemed to have cleared up. Mostly. The car is still (to this day - 2-3 weeks later) idling a little weird. Occasionally it will drop to about 500RPM then surge to 1000 then settle where it should. It isn't dramatic enough to worry me. It doesn't do it all the time. I'm guessing it's a sensor issue and can be addressed as and when I want to address it. - any thoughts on this as a separate issue would be great.. (thinking MAF / TB clean / O2 sensor replacement is due).

Prior to this incident, the car has either misfired or has lacked pull / slipped. Not massively so, but it got me thinking that the clutch might be on its way (after I ruled out alternator / battery issues). There's no smell coming from the clutch unless I rev it massively and ride / slip the clutch, accidentally. Sometimes, instead of snicking into 3rd, it will sort of pop slightly into third like there wasn't a normal engagement (again this only happens at high revs, when hot etc)

However, on the way to Brighton (before the bang), after I'd been through the South Downs, traveling mostly in 2nd and 3rd and driving enthusiastically, when I eventually encountered some Brighton stop start traffic, the clutch felt like it didn't want to play ball. At a busy uphill roundabout approach, the car stalled once and was a bit of a b1tch to move. The clutch was "grabby," the pedal felt light / hot and the signs clearly pointed to a fading clutch. The clutch has felt "light" when hot / used at high revs for a while. Fine. New clutch probably needed, right.

Then I had the bang issue.

It's been booked in for a clutch and flywheel change, as I'm about to embark on another trip (to Edinburgh) and I want a reliable car.


Could the violent bang and limp mode / slipping / over-revving/no power ALL be due to a bad clutch? Or could there be something else wrong that maybe connects all these dots? Potentially engine / tran mount? Is the diff toast? Does this sound like a tranny issues? Wheel bearing? I'm not a mechanic so excuse my lack of knowledge..

The car drives mysteriously well, considering this bang. But as everyone can probably imagine, I'm still pretty uneasy driving it. I don't really trust it.


All thoughts and tales of similar experiences much appreciated!

[oldboxster]

Well, as your mechanic says it doesnt fix itself, sounds like a clutch or something in the gearbox. no way to know until you take it apart. IF it was an IMS then the engine would've self destructed , although Ive never expereinced this failure myself, this is the failure mode i read most about.

[G051999]

Yeah, I'm sure it isn't IMS. I think that is a pretty cataclysmic failure..

I'm hoping something will reveal itself when it's on a ramp for the clutch job.

If anyone else has experienced similar please share

[crod]

Not sure if we are talking about the same thing but years ago I had something similar happening with my 2001 S. I was stopped at a light and when I got the green I started moving and when shifting to 2nd I heard this loud 'bang', almost like if something on the transmission had dropped or even like if I did not shift properly (mine is a manual as well of course) and I had to coast to the side of the road. Turned everything off, sort of scared. Waited like a minute, discussing with my son what could that be. We decided to start the car again and everything started 100% like normal. Car drove just fine after that and I have never heard that sound again.
Now that you mentioned your issue it got me thinking if that noise had anything to do with the metal piece we got into the combustion chamber years later (may be two years later). But considering how long later we had the issue I am almost certain these two events were not related at all.

CR

[G051999]

That sounds nasty. Did it lead to engine fail? How did you diagnose it?

[judd]

could be the clutch release bearing partially sticking on shaft ,personally id have the box out for a look at the clutch

[crod]

Originally Posted by G051999

That sounds nasty. Did it lead to engine fail? How did you diagnose it?

No idea really if that was the event that led to a failure down the road. Again, this was like two years before I got the misfire on Cylinder 5 that led us to rebuild the engine. Given how far apart these two incidents were I do believe the first one had nothing to do with the second one.

CR

[G051999]

Thanks Judd, I'll mention this when I go for a new clutch

Crod, yeah I'm glad DME throws no misfire codes so far.. How old / how many miles was yours when it went?

[judd]

no probs hope you get to the bottom of it ,glad to help

[Benjamin]

Was it a bang like a backfire or firecracker going off, or more of a thud like maybe something kicked up on the road and contacted the skid plate? Rough idling could mean a bad MAF. If the MAF isn't working correctly, you could have excess unburned fuel getting into the cats. Once the cats and the exhaust get hot enough, bang. Just a theory that could fit with your symptoms. Does the CEL come on when you first turn the key? (ruling out a burned out bulb).

[rene525d]

I had an experience with that loud bang and the accessories belt had slipped momentarily. Turned off and back again and all was fine. Belt was visibly worn at 60k miles and replaced it.

[G051999]

Hi Benjamin, thanks for the reply

It was more a thud, like a guttural mechanical thud than a crack - plus you could feel it rather than with a backfire where it's more audible than anything you can feel

Tbh my immediate reaction was IMS due to how it felt and the loss of in-gear power

As for CELs, I currently don't have one no. I've had intermittent CELs in the last three months. I've had manifold bolts and coil packs done, but I have a feeling either the MAF needs cleaning / replacing or it's ICV, TB, inlet plenum or O2 sensor related. My Porsche specialist preferred to rule out the gas leak from the manifold bolts and the "corroded" coil packs before replacing sensors

I will ask the next garage (who do a lot of Boxster clutches) to clean out everything and maybe run the car with MAF unplugged to see if I can get rid of this idle issue

It does occasionally backfire a little actually. I have the Oettle bypass box on it and you can hear the unburnt fuel burbling - it's ace!

I think the root of this might be something to do with fact the gearshift feels "loose" when hot. The fact it's weird going into 3rd sometimes. In a way, it's really frustrating that everything seems fine. I'd almost prefer there was a problem that could be rectified and I could move on!

[G051999]

PS.. I did also have a pretty bad AOS fail that spewed out thick plumes of white smoke, so I have a feeling that was causing the CELs - it definitely caused the previous stalling / limp mode episode I had.

In saying that, with this occasional/slight idle issue, I wouldn't be surprised if I saw another CEL soon

[G051999]

rene, I'll defo get them to inspect the belt - hopefully this would have been done at the major service, only a month ago..

[BIGJake111]

Sounds similar to the thud i got when my rear left axle sheered the bolts conecting it to the cv joint.. where then the car free revved... however my car was not going anywhere after that haha.. so i have no idea.

[crod]

Mine really did not 'go' per se. But had to tear it down to fix it and as we were already there I said let's do the whole thing and be done for at least 100,000 miles.
Car had 56,000 miles when I got the misfire.

CR