Follow the Data or Trust your Gut?
Last week my 1988 XJ off road Jeep played a trick on me. I needed to move it out of the way so I could get my tractor out. It started with an extended crank. This Jeep normally stars quickly so this was odd.
It was stumbling and running rough so I decided to take the long way around to it s parking spot. About half way round the driveway, the Jeep almost died but out of the corner of my eye I saw the REM indicate crazy numbers for the engine RPM. Like 9000 or so. The factory tach was showing 500 to 600 at the time.
I finally got it around to its parking place and shut it off. After grading the driveway, I decided to take a look at the Jeep. Now it refused to start. The REM was showing zero RPM and would put an asterisk beside the number if I continued cranking.
So I hooked up a tow rope and drug it into the shop.
I noticed the wires to the Crank Shaft Position sensor were melted form being too close to the exhaust so I decided to check the sensor output. My scope showed a trace that matched the one int he RENIX manual exactly.
I then suspected the J10 connector so I pulled the seat, laid in the floor and pulled the ECU out where I could get to the wires. I hooked up the the Crank Sensor pins and got the same proper trace.
I figured I must have a bad ECU at this point. I have been wanting to swap to a 1990 ECU for a while so I jumped on EBAY and Got one on order. I also ordered a new CPS just in case.
Once the replacement ECU arrived, I plugged in with it laying in the floor board and gave the starter a spin. I got the exact same symptoms. No start and zero RPMs on the REM display.
The new CPS arrived the next day so I set myself to the task of changing it. This is not easy on the particular Cherokee as I have modified the cross member for extra clearance that makes it impossible to get a tool on the upper CPS bolt.
I lowered the trans enough for access and swapped out the sensor. I plugged it in and expected the engine to start. But no go. Still the same zero RPMs on the REM.
Next I decided to go back to basic and check spark and fuel. I put an inline spark checker on the coil wire and saw flashes as I spun the starter. That meant the CPS was actually working.
Next I connected the fuel pressure gauge. Zero PSI! I had found the problem. I squired a blast of starter fluid in the intake pipe and spun the starter. I got a quick fire and then stall as the vapor quickly burned off.
I could hear the fuel pump run so I then set out to find what the trouble was. Before going much further, I decided to drop in an extra gallon of gas that is normally set aside for the mini bike. The fuel pump made a different sound and this time the engine fired right up.
It was out of gas the whole time. I guess now I need to work on the gas gauge.