Where is the diagnostic connector located on a 1991 Jeep wrangler 2.5L? i want to check for codes.

There are several things that could cause this problem - having a Diagnostic scan and code check should be the first step.
There are just too many things to check without that scan done - you could throw a lot of money at it and still be D.O.A.

55 Completion of diagnostic code display

It just says that it completed the diagnostic check (get one everytime you start your vehicle)

You must use a short jumper wire to ground the white diagnostic connector. This terminal is part of the Assembly Line Diagnostic Link (ALDL), located under the dash near the ECM. This is under the passenger side glove box behind the kick panel.

To activate the diagnostic codes, jump terminals 1 and 3 (the two outer terminals of the white connector). Then turn the ignition switch to on (engine not running). The check engine light will flash a code of 12 three times and then display the stored trouble codes.

To clear the codes, remove the ECM fuse for at least 10 seconds.

Under hood, driver side, lower engine area, front of distributor, mounted in engine block.

It looks looks this;

You might check the fan,if it has a thermostatic clutch on it.
It should spin up with the same speed as the engine.And free wheel when the engine is first started. If it looks lazy,and just barely turning, in the heat of being stoped in traffic it may be whats causing the overheating.

Start with the basics...meaning check the spark plug wires and spark plugs. While under the hood, inspect all hoses for a vacuum leak(s). Then turn the ignition key on-off-on-off- on...leave in the on position and write down the number of flashes the check engine light indicates....2 flashes = #2....pause.....3 flashes = #3.
Let start from here. Are you able to perform engine repairs or diagnostics???

see if your engine has a ECT sensor (temp sensor that delivers information to your ECM) they are installed on the intake side of the cylinder head, it usually has 2 cables, have it checked replace if necesary

Diagnostic Connector To read Diagnostic Trouble Codes (DTC's), the test connector for the electronic engine control system must be used. The test connector is more commonly referred to as the Data Link Connector (DLC). On all Jeep models through 1990, the DLC is a 6-pin and a 15-pin connector located under the hood on the right side of the engine compartment. On 1991–93 models, the DLC is located under the hood on the left side of the engine compartment near the powertrain control module. On 1994–98 vehicles, the DLC is located in the passenger compartment, under the driver side dashboard.

Data Link Connector (DLC) location — Jeep models through 1990


Please don't forget to rate and let me know if this helped you.

Greg

So, when the engine is hot, it won't work. Do you have any idea what the ambient temperature is?
The causes might be 1) vapor lock; 2) percolation; or 3) PCV malfunctioning. This is all explained in a brief diagnostic article at
http://yourhandymanzone.com/Your_Handyman_Zone_How_To_Pages_Vehicle_Car_Zone_Cars_Trucks_Vans_Engine_System_Engine_Stalls_When_Hot.htm

The Starter relay is bad.