rolling down the boulevard, bucks, snorts, dies. towed to "repair (parts replacement) center". at that time, they changed out the fuel filter, dist. cap, rotor, CPS, camshaft sensor, and after each parts replacement, it ran for a short time (far enough away from the parts replacement emporium to require a tow truck). it threw a code saying the temp. sensor was to blame. I will be changing this out tomorrow, however, when looking at other possible problems, I checked out the fuel pressure. When you first turn the ignition to ON, it rockets to 40psi, then even with the key on, it zeroes out. When cranking, it is 40, then back to on, it zeroes. I was curious if the temp sensor will not allow it to start, which is a problem (won't start now), or could it be a coil, possibly. Filing points, and adjusting carburetors are looking better all the time. Any ideas? All efforts to help will be met with great appreciation. Thanks

 

these are the joys of owning a vehicle that's old enough that a lot of different components are getting to the brink of failure.

sounds like the symptoms I had when my fuel pump was in its last throes. the internal check-valve isn't strong enough to hold pressure anymore. I replaced the pump, but later found an external check-valve that can be installed in the fuel line.

I found it recommended to replace the pump, but the external check valve may: 1) be an easier solution, 2) help keep that pressure off the pump, 3) be another point for varnish to build up and inhibit fuel flow. As I said, I replaced the pump (Advance had the best deal). I'm still trying to decide if the external valve is legit, or snake oil.

I found it recommended to replace the fuel pressure regulator at the same time. That drives up the price, but it, the pump, and the filter are all a "system", that need to work together.

For spark testing, to eliminate that side of the equation, I picked up this tool at O'Reilly: http://www.sears.com/shc/s/p_10153_1...0101019x00001a

just an FYI: due to the age of the jeep, after changing the pump gets it going again (), you might want to remove and flush your injectors. I did this, and found one that wasn't even firing. I just went to the JY and pulled six 703 injectors. (that cost considerably less than one new injector)

when I replaced the pump, that flimsy factory lock ring that, even with a healthy dose of liquid wrench, caused so much frustration, got trashed. NAPA has a replacement that is what the factory part should have been. I did have to take it to the grinding wheel a bit though, to get the tabs tapered enough to start, because the O-ring that came with the new pump was more stout than the factory ring.

I know the prospect of changing the fuel pump sucks on a lot of different levels, but once you knuckle down and get to it, it's not that bad, and doesn't really take that long. Call around ahead of time and be sure the pump is in stock at your parts store. Again, Advance had the best price, and lifetime warranty, but you have to have the new sock filter on the same receipt. When your Jeep is on the road again, you'll (momentarily) forget what a PITA it was.

 

well, the Cherokee Gods are apparently not smiling upon me at this time. don't remember if I mentioned it, but, checked rotor location inside distributor cap, appeared to be correct. noticed new parts throughout vehicle. spent an unreasonable amount of time replacing fuel pump. removal wasn't bad, other than sock didn't come out with the pump. Then the replacement had to be figured out, as it was different. to my surprise, all the parts needed were in the kit. Car Quest does a good job stocking their kits. actually, it was probably a rebox from Bosch, but still a good setup. I wasn't able to replace the pump assy. where it was supposed to be, so I dropped the tank. once on the ground, I could see what was going on. the return line has a locator fixture in the tank. got the tank back in, turned on the key, great pressure, and it didn't drop immediately, so I was stoked! THEN, tried to start it. still wouldn't start. what am I missing? the way it reacts is similar to being too far advanced, or maybe retarded. the camshaft positioning sensor was replaced last week prior to me working on it, and I'm thinking it may have been installed incorrectly, or who knows? my mind keeps drifting towards something in the direction of the main computer. please tell me I'm wrong on the computer thing, and its some goofy thing I haven't thought of yet. Any help would be greatly appreciated. Thanks in advance

 

Your 91 has a ballast resistor (cream colored resistor on the fenderwell). It is there only to reduce noise on the fuel pump circuit.

It is possible that this resistor is heating up and failing. You should rule this in or out before moving onto more complex possibilities.

Get a small piece of wire and "jumper" that resistor, which bypasses it. It is safe to do this; later models don't even have this resistor.

Testing for spark and the "quality" of spark is always recommended with a no-start. Pull a plug, keep it attached to the plug wire, place the plug electrode near a good engine ground, have a buddy crank the engine while you watch. You are looking for a strong, blue, snapping spark. Yellow/orange/white indicates a weak spark, which may not be strong enough to start the engine.

 

I have tried a different coil from a running Cherokee, to no avail. The spark looked pretty good, but didn't ever see a blue spark. The spark did look pretty strong. I am getting fuel pressure nicely with the new pump, and it is holding after the initial pressure charge. Prior to this, with the old one, it would drop to zero like a rock. The ballast resistor problem would show up as a no pressure situation, would it not? I'm just beside myself to figure this out. Thanks for the input. Anyone know what happens if the computer crashes? Thanks again. Appreciate it.

 

A good sign of a failed ECU would be no CEL when first started.

 

I have tried a different coil from a running Cherokee, to no avail. The spark looked pretty good, but didn't ever see a blue spark. The spark did look pretty strong. I am getting fuel pressure nicely with the new pump, and it is holding after the initial pressure charge. Prior to this, with the old one, it would drop to zero like a rock. The ballast resistor problem would show up as a no pressure situation, would it not? I'm just beside myself to figure this out. Thanks for the input. Anyone know what happens if the computer crashes? Thanks again. Appreciate it.

 

Nice double post.

 

sorry about the double post. apparently I'm not much better on the computer than I am trying to fix the Cherokee. Thanks for the idea about the CEL as an indicator for the computer. I know, once an idiot, always an idiot. I'll try to do better, really, I promise. Thanks again.

 

http://www.filestube.com/537d2723635...ual-84-93.html

 

1. Need a BLUE spark.

2. A ballast resistor will not result in no pressure. The ballast resistor by design, is BYPASSED when the key is in the full right/start position. Therefore, you do have fuel pressure and fuel going TO the injectors for a little while.

Take the 5 minutes to bypass the ballast resistor. You don't want to be chasing your tail for something that takes 5 minutes to verify. It may be something else, but this would be absolutely the first thing I would do if I had your XJ in my garage. Simple stuff first. Always.

 

thats weird, my 91 doesnt hav a camshaft sensor. did u ever check that coolant sensor to see if its good? mine are all bad except map sensor,not sure about o2's yet though.

 

my answer for everything: o2 sensors!

 

my buddy's '91 and I are getting really close now. I jumpered out the ballast resistor, to no avail. still won't start. then, I re-verified the distributor on number one along with the dampener. installed new temp sensor a couple of days ago. The only thing I haven't replaced under the hood, is the throttle positioning sensor. I'm going to try to test it in the morning. I even tried to move the distributor wires advanced, then retarded, still no start. I tried a used coil the other day from a running XJ, and the spark looked exactly the same as the one in it. I do appreciate the help. Would a TPS cause a no start? Also, would an O2 sensor cause a no start? CEL works good. thanks

 

All 4.0 engines have a camshaft position sensor. For a 91, it is located inside of the distributor.