Won’t start when engine is warmed up

[Traveler]

My 1987 MJ with 4.0 starts fine when cold, but won’t start after I’ve driven it. If I leave it to cool down, it starts right up. Suggestions?

[eaglescout526]

Crankshaft position sensor is heat soaked or the coil is. A common problem that seems to pop up when the parts are old. But don’t trust those aftermarket CPS’s though. They can be crap out of the box. 

[Traveler]

Recommended source for a quality CPS?

[eaglescout526]

Its a crap shoot honestly. When I worked at a dodge dealer, there was still some floating around. They could be gone, they could still be there.

 

But you want to make sure that any aftermarket sensor outputs .03 VAC or higher.

[Traveler]

This Comanche is new to me. I’m trying to test things before “firing the parts cannon” as Cruizer would say. I’m pretty sure the problem is fuel-related. I know I need a new sending unit/fuel pump assembly because my gauge reads 1/4 tank all the time and the sock filter on mine looks like a Hershey bar melted inside it. The fuel in the tank is clean. Also, the pressure at the fuel rail is 31psi at idle.

I’m assuming I have to unhook the wires on the CPS to test it?

Looking for a thread on how to test the coil…

Thanks for your help

[eaglescout526]

No problem. You dont have to unplug the CPS to test it, you can backprobe it so the voltmeter leads stay in contact or have someone help you if you unplug it and test it. The VAC can only be found during cranking. 

 

I think 31psi at idle is normal. With the regulator vac line unplugged you should get 39psi.