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kye · Posted: Fri Oct 01, 2004 3:45 pm Post subject: loud pop near redline.. oops!

I was out investigating where redline in 2nd gear was to clarify my dyno plots, and I got to about ~6200rpm, and there was a loud pop.
I pushed in the clutch, and the car went back to idling and I coasted and pulled over to stop. it was idling fine, so I limped carefully into a carpark and popped the hood.
it's idling nicely, but I can now hear a lot more engine noise.. that 'clacky' sound that farm machinery makes in poor countries. it appears to be coming from near the turbo, so I wonder if i've blown a waistgate seal or something.
on the way back here to work I gave it a little throttle and I gave it a bit of throttle and got it up to about 5lb of boost before backing off, and it seemed to build boost about the same speed as normal, so it doesn't appear to be a failure there.

sticking my hand under the car near the turbo it seems to be pulsing air from somewhere, and it's not the thermofan (which wouldn't pulse anyway).

i'll have a look tonight to see if I can see what's now got a hole in it, but is this sounding familiar to anyone?

I am sure testing and finding the weakpoints in this car!

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it's funny, but when they said "Anything is Possible", that's exactly what they meant....

Khal · Posted: Fri Oct 01, 2004 4:07 pm Post subject:

Rough guess... it sounds to me like you've blown an exhaust gasket.

Hopefully that's all it is. Very easy to fix (I've done it a couple of times)

When the engine's cold (and the exhaust pipe!), start it and cup your hand around the flange where the exhaust manifold joins the exhaust pipe... if you can feel exhaust gas "pfutting" out of the flange, then it's just the gasket there (like that word? "pfutting"? Doug Adams would be proud!)

It could also be a gasket where the turbo bolts into the exhaust manifold/pipe. Probably was just old and couldn't take the pressure any more

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'80 924 Turbo

kye · Posted: Fri Oct 01, 2004 4:49 pm Post subject:

that makes sense, and seemed to be in that general area.
I was thinking it was unlikely that i'd blow a hole in anything metal! i'll check that tonight.. thanks

_________________
it's funny, but when they said "Anything is Possible", that's exactly what they meant....

Smoothie · Posted: Fri Oct 01, 2004 5:42 pm Post subject:

The "loud pop" has me thinking the large o-ring at the outlet of the turbo compressor's let loose. If it's the case, you'd also have some bucking and hesitation somewhere close to 5 psi boost along with a sound like an exhaust leak - that would be air escaping from that joint. Rattling can be coming from there also with the o-ring gone.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Smoothie · Posted: Sat Oct 02, 2004 3:38 am Post subject:

Another thing occurs to me - If the o-ring were completely gone, you'd have the bucking and hesitation all the time since the air:fuel would be off at all times. If the bracket that holds the lower pressure pipe to the block became disconnected and the o-ring's still intact, that's the situation that'll cause bucking/hesitation under mid-level boost as the pipes can be together at no-boost and low-boost, then get pushed apart when it reaches a higher boost pressure. -So since you don't have running problems at lower boost and below-boost levels, I'd guess the bracket's come off, the o-ring's still there and the joint should seperate at higher boost levels resulting in the bucking and air-escaping sound. (If this is the actual problem at all.) The bracket, if it's loose, might account for the clacking sound.
Then again, a "clacking" sound suggests something happening with the turbos' recirc. valve.

One other scenario - If you have a rev-limiting rotor, it cut the spark at 6200rpm which then let some unburnt fuel into the exhaust and it ignitied there, accounting for your "pop". That scenario alone wouldn't account for the pulsing air and clanging sound though - which brings us back to Khal's scenario of a blown exhaust gasket.

_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

kye · Posted: Sun Oct 03, 2004 10:49 pm Post subject:

a small adventure under the car with a stick of insense (to see the air movement) revealed the approx 1cm round hole I have in one of the pipes down there, which is letting all the noise out!
it's the pipe that's parallel to the exhaust pipe, and I believe is for letting the blow off valve vent back into the system..? that's what it looks like in the manual anyway.

that would explain why I can get full boost (verified that since my last post) and why it doesn't hurt performance.
the idea about the excess fuel getting passed through causing a backfire and thus making the hole seems likely, although would that have caused a hole in the blow off valve? I know the valve would be open by those revvs, so maybe some pressure found it's way in there?

the spot it blew out looked thin around the hole and was flaky, indicating rust or wear at that spot.
closer inspection of the pipe reveals that it's been welded before in other places, so maybe it's just a case of a quick weld? not that I know how to weld or have gear or know anyone that does, but it's an easy thing to say!

what do you think the best approach to fixing it would be?
obviously I can get it welded up, or I could get a replacement pipe. I have the facilities to solder - would that work? or would it get hot enough to melt the solder again?
i've also heard about these things you can get to fix exhaust holes.. you wrap some fabric type stuff around the hole and then the heat from things seales it around the pipe and seals up the hole. sounds easy, but I have no idea about cost or their downsides.

opinions?
_________________
it's funny, but when they said "Anything is Possible", that's exactly what they meant....

Paul · Posted: Mon Oct 04, 2004 1:05 am Post subject:

On US cars that pipe (the one that runs from the outlet side of the wastegate to the CAT) is made of stainless steel, so I doubt it rusted.

I usually use my MIG and weld it up with some stainless wire.

Khal · Posted: Mon Oct 04, 2004 10:52 am Post subject:

kye, I've used that wrapping tape. It's not great, a temporary solution at best. And with the heat and pressure on that pipe, it's not likely to last long, if it holds at all -I think it's meant for temporarily repairing small holes in a muffler, not the actual exhaust header/manifold or pipes.

If the hole's not too big, you could just have it welded (solder definitely won't hold) Any exhaust shop should be able to do that for you.

Of course, if you have the means, I'm sure it's better to just replace the pipe.
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'80 924 Turbo