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Bad gas mileage 10-11 MPG

paulhead

I overtighten $#!t
I know a common problem but wouldn't mind some input. 1992 4.0 AW4, 4.88s, 35s. Fuel lines and rails were redone before sold to me so it could pass smog which it did easily. I've had jeeps before and I know it shouldn't be this bad. One thing I've noticed is it's shifting weird likes it's floating between gears once in a while. It was doing this really bad before but I replaced the TPS and it's normal now but still doing it very slightly.
 
What have you done to check the motor(comp check, vacumm, spark plugs, coil, wires, and timing)? I'm getting 16mpg with a high hp stroker!
 
No codes, plugs and wires are fairly new, I guess I could check for vacuum leaks with starter fluid.
 
Check the basics, tune-up, comp, vac leaks, manifold leaks, and then check the O2 sensors. Nearly all XJ's have or will have a cracked exhaust manifold.
 
hows the power feel?
 
Ever pulled the distributor cap and looked inside?

Don't use starting fluid to check for vacuum leaks. It can travel whereas carb cleaner and brake cleaner stay a liquid much longer.
A common issue is loose intake manifold bolts. Snug them up as needed.
The vacuum line from the intake manifold to MAP sensor is critical. any slight leak and the fuel mixture goes richer.
 
Run a can of BG44k through the fuel to clean out the carbon and the injectors. Old clogged injectors is one of the most common things for crappy gas mileage.
 
torque converter clutch not engaging will do it for sure, since without the clutch you are relying on hydraulics alone. if you feel like the trans is shifting weird I would probably start there.

check for fuel leaks / damp fuel lines (no kidding)

use the key trick to pull codes and see if anything there that didn't trip the CEL

are you running a 180 or 165 thermostat? the computer tries to get the engine temp to 195 by running a rich mixture so that the cat converter works efficiently, if temp stays low the computer will forever be dumping unnecessary fuel. O2 sensor can do it too but not usually that aggressive
 
torque converter clutch not engaging will do it for sure, since without the clutch you are relying on hydraulics alone. if you feel like the trans is shifting weird I would probably start there.

check for fuel leaks / damp fuel lines (no kidding)

use the key trick to pull codes and see if anything there that didn't trip the CEL

are you running a 180 or 165 thermostat? the computer tries to get the engine temp to 195 by running a rich mixture so that the cat converter works efficiently, if temp stays low the computer will forever be dumping unnecessary fuel. O2 sensor can do it too but not usually that aggressive

No codes, pretty sure I'm running a 195 thermostat, it's running at operating temp.
 
Ive had my intake man fail to seal multiple times. I would start by using a propane torch off. and searching the engine bay.
or you could use smoke.
whatever floats your boat.
if its a vacuum leak it will show.
 
I didn't see it mentioned, pull the vacuum line to the fuel pressure reg. These are known to slowly lose the diaphragm and start leaking into the vacuum line. If it's gone you will have raw fuel in the line. Good luck getting it figured out!
 
I didn't see it mentioned, pull the vacuum line to the fuel pressure reg. These are known to slowly lose the diaphragm and start leaking into the vacuum line. If it's gone you will have raw fuel in the line. Good luck getting it figured out!

location of pressure regulator?
 
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Also how do you use propane or smoke? Assume propane getting sucked in vacuum leak and increasing idle? Sorry, don't know.
 
I've never had luck using propane or the like to find smaller vacuum leaks. I built a smoke tester for about 50 dollars and love that thing. I found a few very small pinhole leaks using that device.

Here's one example.

https://www.youtube.com/watch?v=Np_duzsaZxI

Here's the one I built which was a little more hack job than others haha. I used a tube of epoxy for all the fittings. If I build another one I'll put the fittings in the side instead of the lid.

https://www.youtube.com/watch?v=29oXZ1sFPEY
 
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So I finally got around to messing with the jeep, meanwhile thank you guys for flooding me with answers. Some suggested that O2 sensors could be bad and not read codes. So I ran am OHM test across the 2 white wires on the sensor and got 3.5 which is below specs. Gonna swap it out and see what happens.
 
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