Maddening electrical problem (stalls and no-start)

[DougSkinner]

I’m having a frustrating problem that I THINK is likely electrical. This is a very consistent (intermittent) problem. (Stick with me)

1999 TJ 4.0 Auto

Cold - ALWAYS starts, no problem.

Once at normal operating temp, and running, it will run all day with NO problem at all.

In between, I have a seemingly random stall/no start issue. Maybe a minute after starting, maybe 10, it WILL stall. It might start right back up, or it might be anywhere up to 5 minutes of no start. It will do this maybe once, maybe 3-4 times. It MIGHT stall out, but start back before the engine quits turning. (Tachometer always drops to 0 when it’s not firing, and then back up when it starts firing again)

Once I go through this a while, it eventually gets to a point that it is good and warmed up, and runs fine. It WILL NOT stall again until I stop and let it cool down, and then it WILL repeat the same process all over again.

I’m working on trying to troubleshoot voltages and resistance at various points, but during the trouble points, it SEEMS so random and intermittent that it’s hard.

Any ideas?

 

[DougSkinner]

Let me add one piece to the puzzle. ORIGINALLY, I had a problem where it would randomly? stall when hot, and not start until it cooled down some. My first guess was the coil. It was AFTER replacing the coil, that the problem evolved into what it is now… This new problem just doesn’t seem like a “bad new coil” to me. Am I just being stupid there?

 

[AndyG]

Did you replace all the coil and plug wires

 

[pagrey]

Might be a failing ignition switch. Sometimes it'll just catch as you release the key or if you wiggle the key it'll die. The tach dropping might be a sign of this, the ignition circuit might be loosing power when you turn the key to crank.

 

[DougSkinner]

Did not replace the coil/plug wires YET. Again, just didn’t SEEM to fit the failure mode, although nothing else really seems to, so…

 

[DougSkinner]

Update: Crank Position Sensor

The problem got worse. On one hand, it made for a BAD trip home last night, but on the other hand, it got a little easier to diagnose. Swapped the CPS, and it APPEARS to have the problem cleared up. I’ll be 100% convinced once I’ve gone another day or two without incident.