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craked exhaust and fuel economy

Yes, the crack in the exhaust can adversely effect fuel economy.
 
If the crack is post O2 sensor, I can't imagine it having any effect.
A crack before the O2 sensor would affect closed-loop operation though.
 
QKRTHNU said:
If the crack is post O2 sensor, I can't imagine it having any effect.
A crack before the O2 sensor would affect closed-loop operation though.
Well, since just about every 6-cyl gets cracks BEFORE the O2 sensor, I suppose you've answered your own statement.
old_crack.jpg
 
I dunno... my 89 MJ 4.0-5spd @ 100,000 mi (33"-3.07:1 then 33"-4.56 @ 3-5") and a leaking exhaust gasket got around 18 MPG either way. After swapping in a stock rebuilt motor and a new gasket it got about the same...

My 89 XJ beater 4.0-AW4 auto @200,000 stayed in the 16-19 MPG range going from 31" to 33"/4" lift with 3.55, then to 33" 4.56:1 and 6.5", and then back to 31" again.

My 88 stocker 4.0-AW4 @220,000, 225/75 3.55:1, gets 18-20...

My 90 stocker 4.0-AW4 @220,000 235/75 3:55:1, got similar (IIRC)

NOT COUNTING WHEELING TRIPS, Getting less than 16 average (or beyond 20) over several 'normal' tankfulls with a similar loadout, in any of these older Jeeps, and I'd be looking for "what's wrong?"
 
mine had a crack almost all the way around tube 6 and a pretty large one on tube 5. No noticable change in mileage before vs after welding up the manifold.

I've been getting an average of 15 1/2 MPG for the last year and a 1/4 with a tired 4.0 and 33's. Lately, since the acetone thread, it's been up to the 16 to 17 MPG area.
 
if nothing else cracked exhaust adds to under hood heat = higher engine temps...
 
XJ_ranger said:
if nothing else cracked exhaust adds to under hood heat = higher engine temps...
that's true

I attribute a failed clutch softline to the excess heat in that area. The rubber on the clutch line was major league cracked and brittle.
 
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