F355 Exhaust Reassembly - Ferrari Life
 
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post #1 of 12 Old 10-20-2015, 09:33 PM Thread Starter
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Exhaust Reassembly

Do you guys uses exhaust sealant on your connections between the headers and the cats and the muffler.

Mine seem to leak a bit when cold but much less when hot.

The reason i ask is my short term and long term fuel trims are high compared to others but not high enough to trigger CEL
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post #2 of 12 Old 10-21-2015, 02:41 AM
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Nope. But those composite gaskets are less than wonderful - perhaps replace them?
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post #3 of 12 Old 10-21-2015, 04:01 AM
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Replace the exhaust donuts with OEM parts and get new hardware. Often times, the bolts were replaced in the past with ones that are too long. Hence, even when tightened, the nuts bottom on the non-threaded part of the bolt, limiting clamping action. Verify that the slip collars have not moved back, as this too can limit sealing of the donuts. If you have any leaks at the headers flanges, the car will never run correctly. Just because the CEL is off does not mean that all is OK. LTFT is an indicator of problems brewing...

This 355 had high fuel trim and noises when cold. New donuts and hardware resulted in near zero ST and LTFT. Repairs were verified with a smoke test.
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post #4 of 12 Old 10-21-2015, 09:49 PM Thread Starter
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Thanks Dave good advice as always. The springs in the photo should they be compressed almost all the way? I maybe have left them a
Little to loose.
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post #5 of 12 Old 10-22-2015, 04:40 AM
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Quote:
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Thanks Dave good advice as always. The springs in the photo should they be compressed almost all the way? I maybe have left them a
Little to loose.
Perhaps. Take a half turn or so and see if the leak stops...Short of physical damage to the donuts, it's not a very common place for a leak.

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post #6 of 12 Old 10-22-2015, 02:47 PM Thread Starter
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Just to add some good info for others my fuel trims i think were about 4 and 8 for long term bank 1 and 2 respectively. From what i have read others are getting 2. So thats quite a ways off.
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post #7 of 12 Old 10-26-2015, 04:30 AM
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Smoke test the intake. I did and was very surprised at the leaks I found.
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post #8 of 12 Old 10-27-2015, 10:30 AM
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Quote:
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Perhaps. Take a half turn or so and see if the leak stops...Short of physical damage to the donuts, it's not a very common place for a leak.


Dave,


Are you saying the bolts should only be tightened until the springs are fully compressed but not more? The bolts can be tightened until the springs are fully compressed and then even more against the collars which will compress the donuts more than by spring action alone. What's your advice?


Jerry
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post #9 of 12 Old 10-27-2015, 10:32 AM
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Dave,


Are you saying the bolts should only be tightened until the springs are fully compressed but not more? The bolts can be tightened until the springs are fully compressed and then even more against the collars which will compress the donuts more than by spring action alone. What's your advice?


Jerry

Just to be clear, I'm talking about fresh new donuts, bolts, springs, et al here as part of a new installation.


Jerry
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post #10 of 12 Old 10-27-2015, 10:48 AM
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Jerry, if you compress the springs to a solid height, you defeat their purpose. The springs should provide the clamp load and allow for float in the connection so it's not so rigid.
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post #11 of 12 Old 10-27-2015, 11:39 AM
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Quote:
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Are you saying the bolts should only be tightened until the springs are fully compressed but not more? The bolts can be tightened until the springs are fully compressed and then even more against the collars which will compress the donuts more than by spring action alone. What's your advice?
Jerry
In theory, you can't over-tighten the hardware, as there's a fixed length collar (inside the spring) preventing that exact situation. However, I have replaced many of those collars due to crush and/or simply being MIA. So, snug everything thing down...and if there's a leak, take another half turn. (The springs are typically reusable, the collars maybe if you can get them off the old bolt, new bolts, washers (if not frozen to the bolts/missing) and nuts.) As with most conical-type seals, you really don't need much clamping force to make a good seal. More is not better here...which really get's to Dave's point. If the springs are truly bound, you've really defeated the purpose the the spring-based expansion/shock absorber system that Ferrari so cleverly designed.

I stock a massive inventory of Ferrari semi-specific hardware, as this is one area I only want to deal with once. Unless the donuts are beat up, cracked or otherwise visually damaged, I've had little issue with reusing them. YMMV

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post #12 of 12 Old 10-30-2015, 01:25 PM
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I had a cat start rattling on my 5.2 and after changing the catalytic converter I checked it with my smoke checker a redline tech plus..

This is on cold exhaust, nice and tight.
I found leaks at the metal on metal fittings at the cat to muffler conections and at the bypass valve. No leaks at my donuts all new ones and no header leaks.
I took it apart and looked it all over and now back together its still leaking at these spots.

I made sure they are flat flush double checked it all.

Please Tell me when its cold that smoke leaking at these metal to metal conections are cceptable because the little bit of brain i have says yes when it heats up it seals those areas better.

I was thinking about checking it while runing like i did aircraft and feel.for any exhaust leaks as you find them pretty quickly.

Thanks
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