Help before I shoot this dam% jeep

I've got an 89 Cherokee with 4.0L Multiport fuel injection. I'm currently getting 3MPG:disappoin This has happend over the last two weeks. Here's the history.
1. Son driving and engine overheated, he removed the overflow bottle cap and antifreeze blew everywhere.
2. Son adds antifreeze and two days latter beings complaing of poor mileage.
3.. I got a new cap for overflow bottle and spotted two leaking fuel injectors.
4. For the last two years we've lived with a crack in the exhaust manifold, but thinking that it might have a bearing (O2 sensor) on the bad milage we take jeep to MONROE Muffler for a replacement header.
5. They have a rough time with the header but eventually they get it back together.
6. Mileage still sucks.
7. Son and I change plugs, air cleaner, and plug wires.
8. Mileage still sucks.
9. Test Fuel Pressure. With vacume line on 31PSI with line off 39PSI. Just what the manual says.
10. After 21 miles the new sparkplugs look like they have been drug through a coal mine, black and fuzzy.
11. Rechecked all vacume lines can't find any problems.
12. Jeep idles at 700RPM and runs fine, no misses, nothing, except extremely poor gas mileage.
13. Check fuel lines and tank, no leaks.
14 WHAT NEXT?????? HELP

O2 sensor. When they fail, the mixture gets over rich. You might have 2, if so replace them both.

O2 sensor... when they go so does your gas mileage...

Go to an Advance Auto or Auto zone and they will scan it for you for free... They can tell you what one it is updraft or downdraft.... It will really make a difference in the way it runs... good luck

If your O2 sensors are bad the only one you will need to replace is the one closest to the exhaust manifold, the other is just a monitor. (upsteam: closest to your manifold, downstream: furthest away) I beleive that there is only 2 sensors, the newer OBD II had I beleive 4 due to split headers.

My first concern would be to make sure there wasn't any damage done to the engine from the overheat. I would run a compression test ont he cylinders, and check for compression leaking into the coolant system. Also, it overheated for a reason, why? If you still have the old "closed" system I highly recommend changing over to the open system. Its easy and can be done in an afternoon and will takecare of the overheating, unless there is something wrong with the water pump or fan clutch. good luck.

Everyone thanks for the advice on the O2 sensors. I plan on taking the jeep to Advance Auto this weekend to have them put it on a computer. I didn't know they did that. Last night I found a broken vacume line that had been patched, by the guys doing the manifold. The vacume line goes from the intake manifold to the MAP sensor. I got some new stock hard plastic line from Advance Auto and replaced the line. The line was broken in two places. One external that was patched, the second was right at the junction where the line goes into the throttle body.

So after replacing the line I went for a drive. After 10 miles of two line twisty windy hilly road and another 10 miles of 65MPH interstate the gas gauge showed only nominal decline in level. (Prior to this I would have seen a quarter tank reduction in level).

I filled the tank on the way home, reset the tripmeter and gave the jeep back to my son. His instructions, not to put an fuel in or to reset the tripmeter. Tonight I'll pull and clean the plugs and check them this weekend also.

Thanks for everyones help. I'll post back when I find out anything new.
RL

Yup, vacuum leak can play havoc; nice catch.

hey ive got a leak int my exhaust manifold too will that effect my gas mileage greatly? I am going to replace it once it warms up a little, but wondering if i should do it sooner?

I would also run some seafoam in the gas tank to clean those new, now dirty, fuel injectors.

Well the funs not done yet. Last night I drove the Jeep for 45 miles and used a quarter tank of gas. This is double the mileage I was getting, I'm up from 3 to 6 MPG. I called my local Advance Auto and they do not check sensors. So I am going to replace both the MAP and O2 sensors this weekend. One at at time to determine which, if either fixes the problem. If that doesn't do it then I've got my eye on a nice little S10 truck.

Well the funs not done yet. Last night I drove the Jeep for 45 miles and used a quarter tank of gas. This is double the mileage I was getting, I'm up from 3 to 6 MPG. I called my local Advance Auto and they do not check sensors. So I am going to replace both the MAP and O2 sensors this weekend. One at at time to determine which, if either fixes the problem. If that doesn't do it then I've got my eye on a nice little S10 truck.

How many gallons did you use? You can't be sure of the mpg unless you filled up before driving, then drove the miles, then filled up afterward to have an accurate # of gallons used.

To get only 6 mpg, you would have had to use 7.5 gallons, which is almost 1/2 a tank.

Since it fouls the plugs so quick, is it possible that it's a bad fuel pressure regulator? ... I know nothing about the fuel system in Jeeps but I know that caused similar problems on some GMs and Ford's I have seen.

Maybe, probably no. I'd start with the probs/searches related to "Overheated Cherokee/XJ" 1st... then go from there... compression tests as suggested and continue searching/posting... along with the usual tune-up like procedures and then some- just to be sure.

Water is the ENEMY!

~Good Luck To You!!!

My 95 Cherokee also has a leaking exhaust manifold. But it doesn't seem to be bothering the gas mileage, even at 187k miles. My mileage had dropped from 20 down to 16. Mine has the 4.0 engine, auto trans and 4X4. The check engine light had started coming on. Error code was for the O2 sensor. Mine only had 1 sensor, at the discharge of the exhaust manifold. Mileage is now back at 20 mpg and no check engine light.

Currently we are at 12MPG, on a good day. The fuel pressure regulator is right on the money. I don't have an Check Engine Light on. This weekend I do plan on changing out the O2 sensor, couldn't last weekend for family reasons. I'll post with the new results.

Thanks everyone for your suggestions.
RL