Had a turbo muffler installed with 138K. Some performance gain but I thought the mileage would increase too.
Starting to suspect the CAT may be plugged? Does anyone make a test pipe I can swap in to see if the mileage increases?
If not, what's a good way to tell if the CAT is plugged or restricting the exhaust?

New HG & ARP Studs
GDE Eco Tune FT
Turbo Muffler
K&N Filter
Racor 245
Lift Pump

You should be pulling up codes if your cat is bad..... Mileage won't increase with a muffler mod....

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2000 Cherokee Trail Hawk
3.5" R.E Superflex Lift, R.E Quick Disconnects, R.E Rear Driveshaft, SYE, OME Steering Stabilizer, ARB Diff Covers, JW Speaker Black Heated Headlights, Mickey Thompson 16x8 wheels, 245/75r16 Toyo Open Country AT's, 15 lb Powertank

2003 KJ Sport (BLACK) 230,000+Miles (Decommissioned)

I dont think that is true for the diesels. I dont believe there are any sensors around the cat for it to know if its bad.

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2006 CRD - GTB2056 turbo by Dieselguy86, Eco Trans Tune, Lift Pump, Week's, HDS Tstat, Racor Filter, ARP's, OME 790's+Top Plate, JBA 2.5", JBA UCA, Moab's+265/75R16, ARB Bull Bar, 4.10's, TrueTracs

Only true way is to remove it and drive without it. There is a bolt together flange in front of the CAT that can be temporary disconnected and moved to one side for a test. Just be sure and wire the CAT up with some safety wire while driving to keep it from falling down and contacting the ground. It will be a little noisy while testing!
As others stated, there are no sensors before or after the CAT on a Jeep CRD! That is what makes it so easy to permanently remove it if you are not subject to tailpipe inspections...

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Supporting Vendor and Moderator of LOST
05 Jeep Liberty CRD Limited
Ironman Springs/Bilstein/Shocks
Yeti StgIV Hot Tune
Week's BatteryTray
No FCV/EGR
Samcos/ProVent
SunCoast/Transgo
Carter Intank-pmp
2mic.Sec.Fuel Filter
Flowmaster/NO CAT
V6Airbox/noVH
GM11 Bld.fan/HDClutch
IronrockArms/wwdieselMount
98 Dodge Cummins 24 Valve

Anyone know a source for a replacement cat? Hate stealerships.

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I've heard tell that a good cleaning procedure for the CAT (which is not really a catalytic converter but a soot trap) would be to disconnect the muffler at the flange just downstream of it and ram a suitable length--maybe 2 - 2 1/2 feet-- of 3/4" galvanized pipe into it repeatedly to "dislodge" the collection of crud.

****WARNING**** This method may also destroy the innards of the thing and allow exhaust gasses to be expelled out the tailpipe without hindrance. I neither recommend nor condone it.

Otherwise, a good Italian tune job might work.

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2006 CRD Limited Dk Khaki
InMotion S-II, GDE TCM, Cummins Lift Pump, Mishimoto CAC hoses
Cooper Discoverer ATP LT245/75, Rola roof basket, JCR S3 sliders
OME lift+JBA UCA's, custom front hitch receiver, Mile Marker winch
All J Products rear cargo shelf
Sips biodiesel as if it were 12 y/o Scotch