KentF New Reader
11/29/17 9:28 p.m.

I took my 2002 V6 Mustang autocrosser to a local dyno shop tonight to get a new baseline for the next set of modifications. On the first run something went very wrong. The car idled and ran very poorly. Exhaust smelled like sulfur. It made less than half the power it should. Just a few minutes earlier it was running fine.

Since this was just a basic load test the dyno guys could not troubleshoot much and anyway it was closing time. When they were unhooking the car one of the technicians noted gray dust on the floor near one of the exhausts. They suggested one or both of the ceramic cats may have let go suddenly and that these things can fail in this manner due to overheating, etc. They suggested that if that proved to be the problem I should replace them with metallic core cats.

On the drive home the car was definitely struggling, idling poorly, no power.

On inspection at home I noted gray sand like dust in both exhaust ports (see photo below). I removed the cats tonight and they appear intact. I can see light through them. However, some testing with the electric leaf blower indicates they have a lot of back pressure. The rest of the exhaust flows freely with the leaf blower.

These are 2.5” Magniflow ceramic catalytic converters. They have been through two autocross seasons (about 220 runs) and several thousand miles. The car has ceramic coated long tubes just upstream. All O2 sensors are in place upstream and down. It has not thrown any codes.

I plan on sticking in some cheater pipes temporarily to prove out if this is the problem or not.

Question: Do ceramic cats fail in this manner? In this relatively short time frame? What causes it?

Question: Are metallic cats the way to go? Any down side with those? Recommendations?

Any other suggestions besides leaving the cheater pipes in?

Gray sand in tail pipe

¯\_(ツ)_/¯ UltraDork
11/30/17 6:17 a.m.

Is it possible that they touched down at some point?  I have found that ceramic cats really, really don't like hitting the ground or other things very hard.

KentF New Reader
11/30/17 7:20 a.m.

In reply to ¯\_(ツ)_/¯ :

It is possible they touched down. However, no external signs of distress on the cat heat shields other than a few scrapes I put there. I watched them load it on the dyno via video camera and saw nothing wrong. They would have to really wack into something to get both cats. I have sand coming out both sides in roughly equal amounts. The mesh in the cats is light gray, no carbon or ash build up. Looks almost new and as I said, you can see light shining through. yet they appear to be breaking down. the smell of sulfur is also a clue. Been a long time since I smelled that coming from an exhaust.  For reference the build log for this car is at:

Any thoughts on how this has happened?

¯\_(ツ)_/¯ UltraDork
11/30/17 7:30 a.m.

Looking at the thread, is it possible the cats shed some bits which have ended up in the 180deg mufflers and are clogging things now?  If you're taking the exhaust apart anyway I would give those a thorough check too.  Do the cats rattle/does the core move around in them?  You might try contacting Magnaflow to get their take on how long those should last in a racing application.

KentF New Reader
11/30/17 9:49 p.m.

Issue with the car found: Ignition coil and three of the six wires bad. This caused serious misfiring which pounded the heck, both physically and chemically out of the cats during the dyno tests.

The trigger was when the technician clipped the probe wire on the #cylinder wire. This moved the wire slightly breaking an already very bad connection. The top three ignition wires, in the caps on the coil,  were very badly corroded and two of them broke off when I checked them.  They look find down at the plugs. All of the arcing and sparking probably wrecked the coil.

I removed the cats for inspection, they appear fine. The car ran better but only slightly which caused me to go back after the ignition.


dj06482 SuperDork
11/30/17 10:42 p.m.

I'm surprised it didn't throw any codes.  I had a COP go bad on my Mustang a few months ago, and it threw a code on the third drive cycle after I noticed the issue (2nd drive cycle was just to move the car in the driveway, third was literally just a startup).

KentF New Reader
12/11/17 9:57 p.m.

I was surprised I did not get miss fire or O2 codes also. Did not run it for long.


Keith Tanner
Keith Tanner MegaDork
12/11/17 11:23 p.m.

One of the fastest ways to kill a cat functionally is a misfire. They really, really don't like raw fuel. Hopefully yours survived.

FYI, ceramic cats can to fail quickly. They basically break into chunks. If you're lucky, you spit them out. If not, the chunks jam into place sideways and block the exhaust.

I've taken out some metal core cats. They have a more entertaining failure mode - in one of mine, the metal core came loose of the housing and started to bounce around. It just compacted and got smaller and smaller in diameter until it basically became a metal plug about half the diameter of the body. That's about the diameter of the exit pipe. It's a weird one. We had enough chemical failures of metal core cats that we stopped selling them and went back to ceramic in our products, unfortunately.

KentF New Reader
12/14/17 6:26 p.m.

Cat Failure Part 2 - 

In Response to Keith Tanner - I am not sure if my ceramic cats are any good or not any more. When the engine went into a bad fire mode due to bad wires it was basically running on three cylinders and misfiring on the other three. Rotten Eggs smell and as you saw from photos above it appears the coatings were begin knocked off as sand that appeared in the tale pipes and on the dyno shop floor. The cats appeared intact and you can see daylight through them but I have decided to try metal core units from Vibrant. You might have some input on the test data below.

For what it is worth (which may not be much). Flow test of partly damaged (but physically intact) ceramic cats verses new, smaller metal cats:

From earlier tests hooking up my little electric leaf blower to the MAF (to do some testing on the new CAI ) I know that the blower puts about about 11 to 12 lb/min of air when the battery is fresh. The car moves about 23 to 24 lb/min at WOT using the same MAF. So the leaf blower moves about as much air as one exhaust pipe (although this is cold air and that is hot exhaust gas - this is just a comparison)

Here is my cat flow test rig with a makeshift water manometer.


The original Magnaflow ceramics had a back pressure of about 4.5 inches of H20.
The new Vibrant metal units had a back pressure of 1.5 inches of H20.

This test is only valid as a comparison of the used, possibly damaged, units verses the new. Also the metal core units are 1/2 the length of the ceramic units.

It is not a fair test. But, assuming I don't have O2 alarms going off after install I am glad I replaced the old units. We will see. 

Does anyone have any insight on what typical back pressure numbers might be expected from ceramic cats like this?


Keith Tanner
Keith Tanner MegaDork
12/14/17 6:39 p.m.

Metal core units are much lower restriction than ceramic.

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