Lookup the 1" rear hood spacers. They go on the hinges and raise the rear of the hood almost like a cowl. Independent tests shows a 20* temp drop.
Got mine off ebay for $15. 5 min install
Rear of the hood is high pressure area:
This means that at speed, you will be pressurizing the engine bay (bad). It will help at lower speed.
I've had problems with temps for quite some time with my XJ with the winch and extra coolers (trans/PS) in front of the radiator. I'm running a new stock radiator, new hoses, 33% coolant/DI water mix (antifreeze has less heat capacity than pure water) with water wetter, ZJ clutch fan, new 97+ auxiliary fan, rubber hose to fill the gaps between the fans and radiator, etc, etc, etc. On hot days at elevation (all summer for me in the San Diego County mountains), my XJ will regularly run at about 215-220*F if under load, and I've had to turn the AC off and run the heater on the trail to keep temps in check. Since this is a family hobby, I'd like to keep things as comfortable as possible, so running the heater when it's over 90*F outside is unacceptable.
Having had excellent experiences with hood venting on racecars, I bought a set of these:
http://rodlouvers.com/Pair-of-Angled-11-Louvered-panels-4-P1036801.aspx
Installed them 6" from the header panel (closest to the front I could get to keep out of the hood supports and also look right), 1/2" away from the crease line running diagonally on either side. Vents were installed so they point straight backwards for better airflow at speed.
A quick day of testing showed air conditioning output on a ~85* day is 5* cooler, engine coolant temps on the freeway and idling around town are more than 10* cooler (I'm now actually using the thermostat, so can't go any lower), radiator temps are WAY down now (approx 130* on an ~85* day), and the vents allow a surprising amount of air to escape both on the freeway (tuft testing) and stopped.
Other things I've noticed:
1. Less air coming out from underneath and wrapping around the front of the Jeep when stopped (kinda like VRS on a helicopter).
2. Less heat on the floorboards.
3. Less stupid hot air blowing under the jeep into the open windows.
4. There's a lot of heat under the hood of an XJ. Despite 130* radiator temps (again, thermostat is closed), air leaving the vents is 160-180*.
5. Placing the vents where I did put them over the battery and the airbox, so a lot less water on engine sensors.
I'll try to get some videos of tuft testing, etc when I get a chance and post it up. Overall, I think hood vents are a worthy mod if you run in hot temps.