gyrator930

Ive been driving the trip from my college KC to Chicago for a few years now. My xj usually got a solid 18-19 highway. Last summer i swapped in a 62.5mm throttle body and intake manifold. Now i cant get any better than 15.5. would a bored out tb really drop gas mileage like this?

hankthetank

More air = more fuel to keep stoichiometric mixture.

kyledye25

What year XJ? And what year of XJ did the intake manifold come off of? I read some where that swapping a new 4.0 manifold to an older 4.0 won't actually help. There's more to it then just simply swapping the manifolds. I believe it has something to do with size of the ports. Dont match up or something. That could be part of the issue. But the bigger throttle body means more air flow. In turn means more gas. So there's that too.

cruiser54

Something else is the cause. I've used bigger throttle bodies on 4.0s before with no reduction in mileage.

N20jeep

Vacuum leak

Or you're just on the throttle more

CharlesS

When you swapped the parts did you disconnect the battery cables and touch the cables together to remove the voltage from the ECM? This defaults the ECM to factory settings.
After 50 starts, warmups and stops the memory will be retuned to a new set of engine air/fuel adjustments....
When I replaced the single hole fuel injectors in my 1998 with the newer style 4 hole injectors I wiped the ECM memory so it would relearn a new set of engine air/fuel adjustments.

CharlesS

Good point......
Test for vacuum leaks with a spray bottle of water. With the engine running spray the water near the intake manifold gasket and then the throttlebody gasket. If the engine idle changes...; there is a good chance of a vacuum leak.

gyrator930

Its a 2001, and the manifold came off of a '99, which unless I'm mistaken, they are the exact same manifold.

There was one more mod I did which was new rear leafs. This set the rear of the xj an inch or 2 higher than the front so it sits with a slight nose-down stance. Any chance this would be the cause?

Veeb0rg

Personally I prefer carb cleaner for this. Water possibly into the intake makes me a bit nervous.

Qroberts7

X2 don't use water, your just asking to get it in your engine by doing this. Use carb cleaner or another petroleum based product that would be compressible should it happen to get in the engine

cruiser54

A little water doesn't hurt anything. You're not driving it into a pond or lake. It will help release a teeny bit of carbon at the most. Old school way of removing carbon deposits was to slowly pour water in the carb.

Diesel Dan

this^

weaselguys

The 2000 - 2001 year 4.0 had a different head and intake than 1999 down , but it should still work , how do your spark plugs look it could be running lean from increased air flow making less power so you are pushing it harder and using more fuel. I plan on adding the big bore throttle body 2 but only after doing an injector swap for more flow -stock injectors only flow so much.

cruiser54

A bigger bore TB can be added without upgrading injectors. There's alot of headroom in the stock injectors to add more fuel as needed.