Just this week, my '91 xj started to not start at night. We have a "cold" front this week. During the day it starts fine, but at night or in the morning when its below 75 degrees outside it just will not start. It turns over but doesn't actually start.
I can't think of anything specific things that will cause this, other than the fuel pump or something not running right in the cold.

Thanks in advance for the help

 

I doubt it's the fuel pump. My experience with fuel pumps are the following
1. They just stall out your car whenever they want/where ever they want
2. They go bad when you have just filled the tank.

You can always test the fuel rod unde rthe hood, unscrew the small black cap and just press the valve to see if fuel squirts out.

Could be the battery, a loose connector somewhere (thermal expansion/retraction kinda thing) or the CKP is acting up.

 

But it doesn't seem like the battery because it keeps trying to start but just doesn't. It begins to turn over as usual but doesn't start. When the battery has issues, it doesn't start to turn over. This is why it seems like something with the gas, fuel pump, fuel system, or plugs. But it starts perfectly in the middle of the day. Which makes me think its some problem with something not working when it shrinks when its colder outside

 

Might want to look into the electrical or ignition areas. Condensation can form when things cool down causing an electrical short circuit/ground condition.

 

But what am I looking for? and it's only a difference of about 15 degrees outside

 

It's called the dew point......the 1 degree/temp at which dew/condensation forms at a given barometric pressure and humidity level. Have u done a ignition system tune-up? Checked electrical connections, etc. If anybody knew for sure what the problem is, we'd tell ya. Since the motor cranks over day or night, but only starts/fires during the day (dryer/warmer conditions) and not at night (wetter/cooler conditions), suspect something electrical. Just gotta get under the hood and start diggin'. When it won't start, do u hear the fuel pump hum when ignition switch is in the run/on position (not start position)? Have u pulled the distributor cap and had a look? Gotta have 3 things for a motor to run.....air, fuel and spark......determine what's missing.

 

If its damp when it wont start give a good tuneup. Cap,rotor.wires and plugs. dont by the cheap junk get quality parts.
If its had a recent tune up possibly a coolant temp sensor.
Ron

 

Ron is spot on. A coolant temperature sensor can absolutely create starting problems when either HOT or COLD.

They are inexpensive and take 10 minutes to replace. You can test them, but it's a pain in the donkey. Replace it. I'd do it to rule it in or out.

Be sure you replace the right one as I think you have two. You want to replace the one that sends input signals to the computer, not the one that sends signals to the coolant temperature on the dash gauge.

 

A tuneup may be needed, I've only had the jeep for 3 months and only put on 2,500 miles. Supposedly before I got it, it had plugs and wires replaced. Unfortunately I am at college and don't have the tools to replace everything. It might have to wait till this weekend or next.

 

By the way, it has been fine today starting. I will just try to see if it is getting gas tonight when it's a bit cooler outside.

 

I'm pretty sure it's the fuel pump. It didn't start at work last night and I couldn't hear the fuel pump turn on.

 

I had the same problem before, and tested the voltage on my battery and it was 10.5 (just bought the jeep) it wouldn't start or stall out instantly but started great when hot out. get your battery tested any auto part store will do it free

 

Next time it wont start try tapping map sensor mine did this same thing just on cooler mornings replaced it issue never happend sense..

 

I can check it with a voltmeter. But I'm sure it's fine because it's turning over fine but not starting. But other than the fuel pump not sounding like its starting, I checked the valve on the fuel rail and nothing came out.

 

Which sensor? and where is it located? and wouldn't the fuel pump still pump when I turn the ingition? Because it didn't last night