I've been scouring the forums before I posted this, but I couldn't find anything similar.

My 92 4.0 Auto has been acting funny for the last week. When I'm driving, it would randomly "die" for a second and then come back to life. It was sporadic so I could never really get any other symptoms that might point in a direction for repairs.

Today it did it one time and then was fine. Several hours later, I stopped and filled the tank and I didn't get 100 yards from the gas station and it completely died on me. I had to have it towed home and I've been trying to check different things using my snap-on scan tool.

Here's what I've found so far.
1) No fuel pressure. The pump doesn't kick on at all. I swapped the relay with the A/C relay and still nothing. I can't even force it on with the scan tool.
2) The scan tool can't communicate with the ECU. This bothers me most because I just used the scan tool a few weeks ago and everything worked as it should.
3) I can't check the spark because I don't have anyone to crank the engine for me. (Yes, I'm a loaner)
4) I have power everywhere else in the vehicle.

Back to #2. If the scan tool can't connect to the ECU then I would think the ECU is bad right? I can't think of any other reason why it wouldn't connect and read.

I didn't bother check the CPS or voltage at the fuel pump after I found out the scan tool and ECU weren't talking.

Am I thinking this through correctly? Pick up an ECU first and go from there?

 

Do this first....http://jeep-xj.info/HowtoECUcodes.htm

 

How low on fuel were ya at fill up? Check for current at fuel pump if no codes are present a scanner is not required for basic code retrieval .

 

That returned nothing. I tried it several times and the CEL didn't come on at all. Will the CEL light be one of the lights that come on when you crank it? Because I've never seen it come on.

The low fuel light just came on. Didn't know which wire should have power, but the center wire on the fuel pump plug had just over 6V. Seems low but again, I can't find any specs. Google is not my friend today.

 

Sounds like bulbs burned out and 99 percent of the time when people fill after running that low fuel pump dies because its hot and fuels cold .Do you have a ballast resistor on driver inner fender you can check power in and out or just by pass it should get 12 volts to pump then . Seeing theres current there pump should run even at 6 volts .

 

Yeah, I was thinking the same thing on the voltage. I'll check the resistor.

Still puzzled why the ECU won't communicate with the scan tool now.

 

If I remember correctly bulb completes the circuit.

 

So it keeps getting stranger.

I checked both sides of the resistor. Nothing. Even pulled the connectors and checked directly at the connector and no voltage. How the hell can I have 6v in the back and nothing at the resistor? Although, I didn't check voltage this time so maybe I had nothing back there when I had nothing at the resistor.

I took the dash apart and pulled the CEL and maintenance bulb and swapped them with known good bulbs. They worked, so I know all the bulbs in the dash are good now. Still no CEL when I turn the key to the run position or when I try the on/off three times trick.

 

Key on when checking resistor ?

 

Yes

 

Check for power at relays next.

 

Went to the junk yard today and picked up an ECU. Plugged it in, turned the key and they fuel pump kicked on. Scan tool connected to the ECU and it fired right up.

thanks for the help

 

Follow up post.

It's been just over a week since I replaced the ECU and I noticed that it's running better than it was before AND I had this strange radio problem where if I took the faceplate off, it wouldn't remember what source it was in and I'd have to manually turn the radio back on when the face was put back. And the clock would be off sometimes, but never the same amount.

After the ECU replacement, the radio problem went away.