87 XJ is hard to start when cold..

Alright, I'm trying to diagnose a little problem my friend is having with his rig so he can be more confident in driving it to Alaska soon. The rig is an 87 Cherokee, 4.0, AW4, has a recently replaced long block (receipts from the previous owner), and seems to run pretty well except for a little bit of lifter noise which we will also be trying to fix (but that's a whole 'nother issue in itself).

The problem is, whenever the Jeep has sat long enough to become completely cold (few hours, days, whatever), it is VERY hard to start and requires constant attention with the throttle until it warms up to keep it from dieing. It cranks over and will sputter a bit the first time, maybe fire a bit and make some noise, then you turn it over and again and you can get it to fire up and run for a few seconds and then it will die if you don't keep touching the throttle, then after a few more tries of this, eventually you can get it going smoothly. After that and while it's warm, there are no issues with starting.

Here is what we've done so far. The previous owner replaced the coolant temp sensor on the side of the block, thinking it wasn't operating properly and the ECM didn't know the engine was cold, but said it had no effect. Also, the previous owner replaced the ballast resistor for the fuel pump on the inner fender, which did not fix the problem. We did a ghetto leak-down test on the fuel system, which basically consisted of letting the rig sit for a few days and then pushing the little relief valve on the fuel rail... fuel sprayed out pretty well, so it seems to be holding there. When you turn the key, you can hear the fuel pump prime and then cut off after a few seconds, so it seems to be getting the fuel it needs.

What else could this be? We're going to try another coolant temp sensor tomorrow because that was my first instinct when we encountered the problem, as I've had this happen before in other vehicles. I'm not sure the ballast resistor has anything to do with how much fuel the pump will push right away or not? Where does it fit into the equation.

Anybody have any suggestions? Seems like a simple problem to fix, but it's frustrating because we're out of ideas.

First of all, I think they replaced the wrong coolant temp sensor. If I recall properly, the one that sends info to the engine is the one up front on the thermostat housing. The one in the back is just for the gauge. You'd better double check that, because I might be remembering wrong, but make sure you get the right one. If in doubt, unplug one and see what the gauge does. If it drops the gauge, or if shorting it pins the gauge, it's the wrong one.

It sounds as if the fuel pump is all right. When it leaks down, it will cause a long start, but it should run fine once it's going. I guess the next question is, can you tell whether it's running too rich or too lean when it's starting up? If too lean, the temp sensor might be a good candidate, since it is what sends the signal to the ECU to richen up the mixture when you start. It sounds from the description more lean than rich, but if too rich, you might want to check for an injector not closing properly. That can cause rough running, etc. until the accumulated gas burns off, after which it will run fine.

The ballast resistor shouldn't have any effect, because it's bypassed when you start, but I'm surprised there is one in an 87. My 87 had none.

Just a thought to throw out there, could the NSS starting to go cause a problem at all or no? When it was cold here in the winter, was a little harder to start with my '90 but giving it a wee bit of gas always fixed that up fine. Only other problems I've ever had starting was to do with my NSS being fookered. Took it off and cleaned it and has been fine every since. Like I said. not really sure what to look for, but just a thought.
Hope you get it worked out ok.

There is no coolant sensor on the thermostat housing of a Renix XJ that I've ever seen. No need to shotgun troubleshoot temp sensors, just use a DVM to check.

Forget the NSS, that's for no-crank condition. Won't idle cold? What's up with the IAC/TPS? Is IAC actually working? Has the TB/intake been cleaned recently? Check the small vac tubing from TB to MAP and make sure the entire vacuum system is tight.

Oh yeah, qualify the injector connectors carefully. They hold moisture and get pretty funky. Weirdest thing is that faulty/corroded injector connectors seem to give me more trouble when cool/humid.