My 1991 Cherokee started stumbling at traffic lights. Shifting to neutral would calm it down some but it was getting annoying. Then it started to die and was hard to restart. So no more daily driving until I got it fixed.
I pulled the codes with the key trick and got only the 12 and 55. codes. I hooked up my Actron scan tool and all the readings looked normal. I could see the injector pulse width and O2 readings changing just like they should. Occasionally the O2 voltage would go to zero just as the engine died so I began to suspect a fuel problem.
I hooked up my Hantek oscilloscope and monitored the fuel injector current and voltage. Everything looked normal. I watched the CPS and saw that there were occasional missing pulses so I replaced it. No change.
I monitored the fuel pressure and hooked up an inductive timing light. The ignition pulse continued to flash all the way to a stop when the engine would die and the fuel pressure was right on the factory specs between 31 and 40 psi.
I noticed that the stumble would get worse as the engine warmed up. And some days it would hardly stumble at all. The problem was very erratic and unpredictable. But it died too often to drive in traffic as sometimes it was hard to restart.
I pulled a sample of fuel and noticed a tiny bit of water in the first sample. I pumped full through the system until I got no more water in the sample jar. I took the Jeep for a test drive and it stalled before I got out of the driveway. And it was very hard to restart.
I added some Isoheat dewatering alcohol to the tank. The engine ran horribly and then would not start. I thought I had killed the fuel pump. But the fuel pressure was still right on the mark and had good flow when pulling a sample.
I decided to give up for a while and work on some other projects. When O reached in the drawer for a mother tool, I notice my spark strength tested gauge. I decided to hook it up just to see. I hooked up a switch to bypass the state relay and hooked the spark tester to the coil wire. I got no spark. I finally dialed it all the way down and got a feeble yellow spark.
I made a quick trip to town and picked up a new coil just before the store closed. I bolted it in place and tried the tester. There was a nice big blue spark. I reconnected the coil wore to the distributor and reinstalled the starter relay.
BAM the engine fired right off and after clearing out all the unburned fuel from the testing it settled to a nice stable idle. I was super excited. I took it for a test drive and no more stumble. The engine felt a bit peppier so the coil has likely been going bad for a while before I noticed.