How much does cracked exhaust manifold effect mileage?`

When I got my 1990 MJ there was an exhasut leak at the number 6 cylinder. Bone stock I got 21 mpg. I was VERY happy.

Now I think the crack is worse, i have a glasspack instead of the rotted muffler, "Cold air" intake, and a 2" bb

Im lucky to get 16 out of it, used to be if I went wheeling it, it droped to 17mpg, now if I go it, it drops to 13mpg.

Now I got 4.10's running 215s for now, I know that is a factor, but this was happening BEFORE I got the 4.10s

Extra air would make the o2 sensor think the engine is lean and add fuel.

Argee. My mileage dropped off to 16ish with a crack and came right back up (18-20) when I fixed it.

Cold air intake, Glass pack, 2" lift.

ALL of that reduces your mileage, the crack don't help.

Rev

will relocating the incoming air sensor to the cold air box drop mileage?

How would the cold air intake REDUCE mileage? Becuause of the cooler air coming into the motor?

if your "cold air" is doing its job...you should be intaking more air than you were stock. Your O2 sensor will read this as: crap, the air/fuel mixture is too lean (the ratio of air to fuel is off) and ur Heep will now use up a lil more fuel to get the mixture back to normal.

I think thats right. I worded that really bad. But im taking time off from studyin for finals and this is where i go. I swear if i spent half as much time studying as i did on here I'd be a rocket scientist.

A stock air box is gonna want to mix in some heated air from the exhaust manifold area, when the outside air temp. is low. The MAT sensor also has some input on the fuel mix. The MAT sensors inputs to the ECU are programmed to work with the hot air mix, from the stock air box at low temps. If your feeding the MAT colder air than it's designed to read, it seems plausible it might want to widen the pulse a bit, for cold weather operation. Or in other words it may think it's colder than it actually is.
Colder air is denser air, the O2 sensor measures left over oxygen from combustion. Too much left over oxygen and it adds fuel.
You could have more than one issue going on, that affects fuel mileage.
Unburnt fuel is also unused oxygen, which can also fool the O2 into adding more fuel. This happens often at idle.

I have no shielding, its just taking air from the enging compartment. I can see the mat bring in more fuel, but how would tha tbe any differnet then it being cold out? Also, the computer is always going to keep the same 14.7:1 air/fuel ratio. If there is more air coming in, id theoretically (big word) be using less throttle, so it would even out, thawt intake should only really be noticeable at WOT.

then its not a cold air, its a cone.

a cracked manifold can't be good. If you get a a drop i would blame it on a that befor I blame an airfilter