No Crank nightmare

1996 XJ, 4.0, Auto, 160k miles

On a hot engine, shut jeep down for about 30 minutes or so. Jeep will not crank. Absolutely no sounds are heard from starter when this 'no start' condition occurs. Relays in the engine electrical panel / bus do produce audible clicks on attempt to start. All dash lights operate normally, headlights do no dim if key turned to crank. Problem is intermittent. I cannot reproduce reliably. Jeep cranks and starts with no issues after about an hour and a half of sitting [on all occurrences to date].

Battery is two years old and seems strong. Battery terminals are 2 years old and clean. Battery leads are also new and heavy gauge [yea - two years old, heh].

What I have tried so far:
1] I cleaned the NSS as reverse lights were not working [for a few years]. Reverse lights are now working. Starts in park & neutral when the no-start condition is not occurring. No-start problem reoccurred three days after NSS cleaning.

2] Cleaned the electrical connections to starter as they were coated in oil from leaking oil filter relocation assembly [o-rings have been ordered to resolve this]. Problem reoccurred.

3] Tried pulling the starter relay from under-hood electrical panel / bus and tapping it, replace... does not resolve issue.

Any ideas on the most likely causes of this issue? Maybe a bad starter solenoid? I'm thinking the solenoid as I'm thinking temperature is a possible factor as I seem to remember solenoids getting temp sensitive as they age.

thanks for your time.

Is 2 years ago when you last cleaned and connected the battery? If so I'd revisit that first. Have you tested the battery? It sounds like a bad connection either there or in the ground(s). Possibly just that simple.

NSS assumed good. Not that.

There was a thread here about an oil soaked starter failing, couldn't find it. You'd do well to really clean things up on that side of the engine and shine up all the contacts.

weak battery connection is my guess. clean the terminals.

Ok, put the key in the lock cylinder, turn the key to START, key effects ignition switch, ignition switch sends 12 volts to starter relay, if the transmission is in PARK or NEUTRAL and the NSS is functioning as intended, then the NSS provides a ground to the starter relay and the energized starter relay forwards 12 volts to the starter solenoid, the starter solenoid engages and the starter cranks the engine
over.

Find the missing piece.

Crawl under the rig and give the starter a couple of good whacks with a large hammer. The Bendix actuator can get corroded and stick.

I am waiting for a re-occurrence and will try the hitting the starter, also will bring my voltage meter to check voltage at the starter [if I have someone available to operate the ignition switch]. Thanks for the advise.

Good advice above. Always simple stuff first like connections, verifying voltage, etc.

If this problem is only happening with a hot engine, my gut feel is that the starter itself may be to blame.

Here is how I would approach it if it were mine. WHEN IT FAILS TO CRANK: With the key in the ON position, put a meter directly on the starter itself. If you do have approx. full battery voltage at the starter when you do this, the starter itself is almost assuredly the culprit.

However, if you don't have good voltage at the starter, you need to start working your way backwards to find out why.

Thanks for all the advice to date.

Today I performed several hot starts with no re-occurrences of the No Crank situation, but I noticed something new... each hot start resulted in very slow cranking, almost like the battery was going dead. I checked the battery voltage with the Jeep off / key out and with a hot engine bay, it reads 12.8 volts.

I'm going to check voltage at the starter on the next no crank re-occurrence as I am now riding with a voltage meter [and small hammer] on board.

If the starter turns out to be the issue, I'd like to go for an upgrade for max reliability and longevity. Which is best: PowerMax or Mean Green?

I would take the battery in and have it load tested.

Other issues could be poor main cables or a failing starter-solenoid.

I have replaced the o-rings in the oil filter relocation kit, cleaned up all oil. Having very slow cranking issues on a hot engine, but no failures to start so I have not been able to do a electrical test on failure yet. I'm leaning towards thinking it's a failing starter or solenoid.