Bad Cat for sdpatt

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sdpatt

Sr. SHO Engr.
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Pulling into the garage today I heard the distinct clanking rattle of a catalytic converter gone bad. Even my wife heard it inside the house and asked me what it was. On the way home, I had noticed a different note to the exhaust and a big drop in available power. I have been through this before with the SHO Shop Y-pipe where both cats were dead. I was disappointed that it may have happened again to the Performance Plus Y-pipe.

I called one of the few muffler guys in the area that can do MIG welding on my all stainless exhaust system. He had repaired a broken weld for me before. He took my ailing SHO in at 5:00pm. We found two chunks of ceramic doing a very good job blocking the exhaust flow. One at the exit of the rear bank cat and the other at the entrance to the resonator in the Borla cat-back.

The chunk was removed from the cat-back by lowering and shaking the pipe. The cat was cut out and replaced with a Mustang GT targeted hi-flow cat. The engine was returned to its free-breathing best.

The photos below are of the old cat and the chunks and dust that came out of it and the exhaust piping. A closer look at the cat showed that it was most likely the victim of a road debris impact. There was a noticeable dent on the bottom of the casing along with a series of scratches that may have come from contact with part of the road.

Since the SHO is still running at 255,117 miles after three separate cats have turned to marbles and dust, it may be another piece of data that says the cat dust does not get sucked back into the engine. At least not on the 3.0L that does not have the exhaust gas recirculation system.

Happy to be back on the road,
Scott

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9

94ShoOff

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How old was the y-pipe?

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Steve
Shooff94small

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sdpatt

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Originally posted by 94ShoOff:
How old was the y-pipe?

I installed the Y-pipe on October 30, 1999 at 223,564 miles. That comes out to 831 days and 31,537 miles, but it didn't die of natural causes. The matrix in the front bank cat looked intact and very white.

Scott
 

SilverSHO

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I've always wondered myself how dust from the converters could get sucked back into the engine. There's always a positive exhaust flow through the cats, even when you let off the gas. It's not like it's pulling air back in through the exhaust valves. The exhaust going through the pipes goes slower when you let off, but it doesn't completely reverse its flow. Besides, wouldn't any dust from the catalyst be blown back through the exhaust system if you buzzed the engine up to about 6500 RPM and suddenly let off?

When I cut off my old stock mufflers, I found pieces of blackened, melted catalyst in them. I also pulled a wire mesh thingie out of the rear split where the pipe breaks off to the mufflers, along with a significant quantity of more melted catalyst. I'd had no idea my Y-pipe had been so utterly destroyed as it was getting red-hot.

I've even dumped pieces of broken ceramic out of my new mufflers. I heard the left one rattling, so I took it off and sure enough, a roughly golf ball-sized chunk of catalyst fell out. Figures.

Okay, I feel better now.
smile.gif


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Jeremy Hohn

Guest
Well, I'm glad you are keeping the white beauty intact! Sorry to hear about that happening to a pp y-pipe. I hope mine doesn't catch what yours had when I get over there!!
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Jeremy
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1993 atx 126k miles
ENGINE:
-SHOSHOP underdrive pulleys
-SHOSHOP cold air induction
-SHOSHOP 80mm MAF
-Ted Breaux LPM
-Performance Plus y-pipe
-SHOSHOP catback exhaust with Magnaflow chrome tips
-Redline Water Wetter
-fresh 60k (SDPATT)
TRANNY:
-Rebuilt 94/95 axod tranny with trans-go kit and Redline Type 3 ATX fluid
BRAKES:
KVR cadmium plated and cross drilled rotors with performance friction pads
APPEARANCE:
-Colgan hood cover with SHO lettering
-Lund blackouts front and back
-Cincysho white face gauges
-black body filled s-h-o lettering
-Blazer fog lights
-PIAA high output headlamp bulbs (fratperformance.com)
-fish-tank bulbs installed
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Denny

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Yeah! I was talking to an engineering major friend of mine and he said it wouldn't be possible to have ceramic material get sucked back into the engine. We both agreed that it could possibly happen on an EGR system which we five speeds do not have! WHOOHOOOO!

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Stock? Aftermarket clutch? BAD clutch?
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Coming soon:Major suspension repairs
80mm MAF
Ah yes, the feeling of an open throttlebody


1992 3.8
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RIP
 

FordLover

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Originally posted by SilverSHO:
I've always wondered myself how dust from the converters could get sucked back into the engine. There's always a positive exhaust flow through the cats, even when you let off the gas. It's not like it's pulling air back in through the exhaust valves. The exhaust going through the pipes goes slower when you let off, but it doesn't completely reverse its flow. Besides, wouldn't any dust from the catalyst be blown back through the exhaust system if you buzzed the engine up to about 6500 RPM and suddenly let off?

When I cut off my old stock mufflers, I found pieces of blackened, melted catalyst in them. I also pulled a wire mesh thingie out of the rear split where the pipe breaks off to the mufflers, along with a significant quantity of more melted catalyst. I'd had no idea my Y-pipe had been so utterly destroyed as it was getting red-hot.

I've even dumped pieces of broken ceramic out of my new mufflers. I heard the left one rattling, so I took it off and sure enough, a roughly golf ball-sized chunk of catalyst fell out. Figures.

Okay, I feel better now.
smile.gif



I tend to agree...I remember friends being afraid of water getting in the tail pipe of my truck while retreiving a boat at a terrible ramp. They didn't believe me that exhaust is not an intake...and always has positive displacement, except when the motor is off.
-Martin



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drivinhard

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Trust me it does. Probably under high rpm's if you are off the gas. The original motor in the SR71 suffered a shortened life due to this. Friend who had it before me bought it. All of the sudden, power went into the toilet, few days later it was using a qt of oil every 75 miles. Blowing smoke out the exhaust. When I put the 3.2L in there and I pulled the 3.0L out and pulled it apart, Some pretty serious cylinder wall scaring going on.

On another note, my beater '93 had a trashed set of cats, and same as scott, plugged the resonator (I replaced that), and some material even made it's way to one of the mufflers. And is still in there
wink.gif



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SilverSHO

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Originally posted by drivinhard:
Trust me it does. Probably under high rpm's if you are off the gas.

Yeah, that seems like it might cause it. A sudden drop in exhaust gas velocity (and hence less exhaust gas pressure in the pipes) would cause that kind of problem.

It's hard to imagine it happening, but think of it this way. When you hold a bottle of shampoo upside-down and squeeze it, shampoo comes out of it. If you abruptly stop squeezing the bottle, you can see some of the shampoo that's already outside the bottle get sucked back up into it. Granted, you can't squeeze the exhaust pipes like that, but it's basically the same principle. Running the engine up to 7K is like squeezing the bottle; letting off quickly is like releasing pressure on the bottle. There's still some positive shampoo flow--what drops into your hand after you stop squeezing--but some shampoo gets sucked back into the bottle. So, along those same lines, it is possible for some catalyst material to get sucked back into the engine.

It's not the best analogy, but for 1 in the morning, it's still pretty damn good.
smile.gif
 

sdpatt

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The SHO engine is not a shampoo bottle. When the shampoo bottle is released, the plastic container expands to create a slight vacuum. This is what pulls the highly viscous shampoo back into the bottle.

Thinking about it farther, when the throttle is closed on the SHO engine, the idle air control valve still provides a path for air to enter the intake and cylinders. The EEC also cuts fuel flow to the injectors so that only the air in the intake enters the cylinders. The cylinder fill is at less than atmospheric pressure so there is at a slight vacuum in the cylinder. Depending on the timing of the exhaust valve opening, when the exhaust valve opens, there may be a slight "inhale" through the exhaust valves. The overall airflow through the engine is still in the forward direction, but that little inhale could possibly draw in some thing close to the exhaust port.

Also remember that the cat dust would have to climb up the 18" or so from the level of the cats to the entrance to the exhaust ports. Hopefully, this elevation difference will protect the engine from the debris in the exhaust system. The still forward air flow through the engine even at closed throttle from high rpm should still keep the dust at or downstream of the cats. It may also depend on how much high rpm engine braking you use (not a good idea). I won't say that it's impossible to get cat dust into the engine, but after losing three cats, I haven't **** a SHO yet. Knock on wood.

Scott

[This message has been edited by sdpatt (edited 02-11-2002).]
 

SilverSHO

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The SHO engine is not a shampoo bottle. When the shampoo bottle is released, the plastic container expands to create a slight vacuum. This is what pulls the highly viscous shampoo back into the bottle.

Scott...that did occur to me about halfway through what I wrote, and I was going to mention the vacuum thing , but it started getting really long and involved, so I just left it at what it was. Besides, I figured it wouldn't look too good if I debunked my own theory. And I'm still trying to figure out how I came up with the whole thing.
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sdpatt

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You noticed that I didn't say it was impossible? I will no longer say that it can't happen, just that it hasn't happened to me. Again, knock on wood.

Scott
 

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