Fixing a pernicious exhaust leak (help!)

[SirBrass]

I don't have a garage to put the car up on Jackstands with (apartment dweller). So, can't do that.

I'm just going to take it to an exhaust shop on Friday & see what they can do for me.

On phone I am. Therefore, if mistakes I make, Frank Oz blame you should.

[mrk1]

So a leak down stream of the sensors is causing a mixture problem? And the leak is so small it only shows with smoke. I would use a sealant and a fresh gasket. Setup times suck but got to what you got to do.

[SirBrass]

I am using a fresh gasket.

On phone I am. Therefore, if mistakes I make, Frank Oz blame you should.

[Mad1723]

Maybe the flange is not completely straight on either the JDL or the Nameless part. It could explain why even with a new gasket it leaks.

[s2d4]

Originally Posted by mrk1

So a leak down stream of the sensors is causing a mixture problem?

This sounds weird to me as well....

[nlowell]

I have the same nameless pipe but matched up to the Open Flash Header. At WOT I'm pushing 18:1 AFR below 4k indicating a leak according to Shiv but can't find one. Interested to see what you find out on yours. I think I'm going to remove the header and check the flanges for leak marks.

When are you taking it to an exhaust shop?

[SirBrass]

Originally Posted by Mad1723

Maybe the flange is not completely straight on either the JDL or the Nameless part. It could explain why even with a new gasket it leaks.

I think it's on the JDL, which I bought used (but good condition). The Nameless pipe was new.

And to note, the logs don't definitively show a leak, but there are some weird "why is it going to - 3 there?" points. Nothing in afr was really indicating a leak.

It was hearing it after multiple pulls, and seeing the leak on a smoke test that proved the leak was there.

On phone I am. Therefore, if mistakes I make, Frank Oz blame you should.

[xjohnx]

Originally Posted by nlowell

I have the same nameless pipe but matched up to the Open Flash Header. At WOT I'm pushing 18:1 AFR below 4k indicating a leak according to Shiv but can't find one.

i'm still confused as to how a post sensor leak could be throwing your AFR off that drastically. And the leak only occurs below 4k, not past it? Seems odd to me.

[SirBrass]

@nelsmar sent me a new log late last night (primarily to turn off the ability for the rear 02 sensor to throw a code, since it finally sent P0420 last evening after many days of pulls and driving and not doing jack squat), and this morning I logged my drive to work and did 2 pulls. The second one was the real shocker. more than 5 minutes into the log, 3rd gear, did a WOT hard pull from 1.6krpm to right about redline, on a slight uphill (one of the only onramps along my route with the length and lack of traffic to let me do this safely). NO fine ignition correction, and the early "just getting on the freeway" 4th gear pull only dropped to -1 (my tuner prefers to shoot for 0 all round, not just "better than -3").

Now, I didn't do any more pulls, but this is really interesting. MAYBE (speculation here) the exhaust leak sealed itself? My tuner would know better.

Attached is a log graph (not zoomed in), and the appropriate pull areas circled in red.

[mrk1]

Originally Posted by SirBrass

I think it's on the JDL, which I bought used (but good condition). The Nameless pipe was new.

And to note, the logs don't definitively show a leak, but there are some weird "why is it going to - 3 there?" points. Nothing in afr was really indicating a leak.

It was hearing it after multiple pulls, and seeing the leak on a smoke test that proved the leak was there.

On phone I am. Therefore, if mistakes I make, Frank Oz blame you should.

I would try dropping the headers and looking at the gaskets there for signs of a leak.

[SirBrass]

Originally Posted by mrk1

I would try dropping the headers and looking at the gaskets there for signs of a leak.

None there, confirmed. Smoke test didn't reveal them, nor did logs, nor did sound.


From this morning: here's jpgs of the 1st and 2nd pulls, showing fine ignition correction, maf voltage, and engine speed

[bluesman]

Originally Posted by SirBrass

MAYBE (speculation here) the exhaust leak sealed itself?

That's certainly possible, and if you lucked out this way I'd count my blessings and forget about it. But it's a rare leak that fixes itself, and those that do are usually just "plugged" by oxidation etc and will recur.

This is why I say that the best approach is to disassemble, inspect, clean, reinspect and (especially if there's no visible abnormality) measure everything. There are rare leaks that are intermittent, e.g. tiny cracks at odd angles and locations that only open up with heat expansion, vibration at a specific frequency, twisting under load etc.

Something's keeping the mating surfaces from fully mating and sealing. It could be imperfection in the mating surface(s), malalignment of the mating parts, distortion in one (or, less likely, more than one) of the parts etc. Your original message seems to say that this installation had a tiny leak from day one, so whatever's wrong has been wrong from the start. The used component could have been damaged during original installation or removal, or it may have been altered a bit because of problems on the original owner's car.

As a general rule, if the same parts go together without a problem on other cars and your car's not damaged or altered in any way, they should go together without a problem on your car too. I don't believe in ghosts.

[SirBrass]

Originally Posted by bluesman

That's certainly possible, and if you lucked out this way I'd count my blessings and forget about it. But it's a rare leak that fixes itself, and those that do are usually just "plugged" by oxidation etc and will recur.

That's what I'm afraid of. But.... what oxidation? This is Arizona. It's dry as hell here! The only real condensation to create oxidation with drains out perfectly fine through the a/c condensate tube (I should know... I got dripped on plenty when removing the fiberglass under tray).

Your other comments echo my own concerns pretty well, and that's why I'm going to still take it to a professional custom exhaust shop on Friday. If they feel they need to remove the header and front pipe/overpipe and measure, I'll pay. I'm not doing it again unless I HAVE to. Putting those in was a royal pain. Reversing all the work and time I spent just to fix one TINY little leak that is intermittent only just feels ridiculous, and if it must be done, then I'd rather it be done by those who do this kind of thing (specifically exhausts) for a living.

[bluesman]

Originally Posted by SirBrass

That's what I'm afraid of. But.... what oxidation? This is Arizona. It's dry as hell here! The only real condensation to create oxidation with drains out perfectly fine through the a/c condensate tube (I should know... I got dripped on plenty when removing the fiberglass under tray).

Your other comments echo my own concerns pretty well, and that's why I'm going to still take it to a professional custom exhaust shop on Friday. If they feel they need to remove the header and front pipe/overpipe and measure, I'll pay. I'm not doing it again unless I HAVE to. Putting those in was a royal pain. Reversing all the work and time I spent just to fix one TINY little leak that is intermittent only just feels ridiculous, and if it must be done, then I'd rather it be done by those who do this kind of thing (specifically exhausts) for a living.

I've come to feel the same way. I hate working on street cars - they're dirty underneath and not designed for easy maintenance. Of course, I'm shopping for a SS exhaust for my FRS right now and will put it on myself. But lying on a concrete garage floor on my back under a car perched on ramps and jack stands while juggling a dangling exhaust system is simply a pain in the @ss.

BTW, water is a major component of the exhaust mixture from an internal combustion engine - it's the commonest reason for exhaust system rust-through, which usually starts from the inside. It condenses out of the gas mixture onto the cold pipes when first starting up, and if you don't drive long enough to let the system, heat up fully and vaporize it, that water can do significant damage over time. 304's not going to be a problem, but if the flanges on your system are 409 (which is common even when the rest of the system is 304), they'll oxidize readily and rapidly.