Problem on startup after engine is warm.

ok first of all u did not say make , year, model, fuel type(injection or carb)
but i would say that if is injection, then u have a leaky injector.

99 XJ with a 4.0 liter.

ya i still would think its a leaky injector, a leaky injector is an injector that leaks fuel , and the reason u dont have a problem when its cold is cause, by that time the fuel has evaporated.

its probly to later for injection cleaner, if i remeber there is a place that will check ur injectors(u take them out) from none or little charge.

How much are new injectors?

This seems to be a common jeep symptom, and there seem to be a multitude of possible causes. In the case of my '96, it's a crimped wire to the MAP sensor.

I figured this out, oh, a couple years ago when unplugging and replugging the sensor fixed the problem for a couple weeks. I've never respliced the wire, just lived with the problem, and it took until a couple weeks ago before it ever threw a MAP code.

First thing to do is check for codes. Not sure the procedure on a '99, but you should be able to search for it, or someone will jump in this thread. Also could take it to autozone they usually check codes for free.

If no codes are thrown, it can get expensive if you start replacing injectors and sensors that you don't really need to fix - may want to consider finding a source of used parts or someone that can do live scanning - or if you have a laptop, getting the software and doing it yourself.

If you search for injectors you will find there are 2 or 3 vendors that XJ owners seem to like. Cost seems to be between 150 and 200.

If it is your injectors here'e some info on them and some swap info...Thanks to Dr. Dyno

Model Year, ...Part #, ......Colour, .....Fuel Pressure, ...Static Flow,
'87-'90, ........53003956, ...Black, ........39psi, .................18.6lb/hr
'91-'93, ........33007127, ...Brown, .......39psi, .................21.0lb/hr
'94-'95, ........53030343, ...Tan, ...........39psi, .................21.0lb/hr
'96-'99, ........53030778, ...Grey, .........49psi, .................23.2lb/hr
'00-'01, ........04854181, ...Blue tip, .....49psi, .................22.5lb/hr

The injector flow rate varies as the square root of the pressure drop across the injector. The stock '91-'95 4.0 injectors are rated at 22lb/hr @ 43psi fuel pressure, so at 39psi they flow ( sq. rt.(39/43) x 22.0 = 21.0 ).
For stock/modified 4.0 engines or stroker build-ups the following injectors can be installed. The flow rates are:

Ford Motorsport 19.0lb/hr @ 39psi (Part no. FMS-M9593-C302)
Ford Motorsport 24.0lb/hr @ 39psi (Part no. FMS-M9593-A302)
Ford Motorsport 30.0lb/hr @ 39psi (Part no. FMS-M9593-B302)
Chevy LT1 24.0lb/hr @ 43.5psi (Part no. 17124248)
Chevy LT4 28.0lb/hr @ 43.5psi (Part no. 17124251)
'98 Chevy LS1 25.2lb/hr @ 58.0psi (Part no. 12533952)
'99-'00 Chevy LS1 26.2lb/hr @ 58.0psi (Part no. 12456154)
'01-'02 Chevy LS1/LS6 28.6lb/hr @ 58.0psi (Part no. 12482704)
Accel 19.2lb/hr @ 44.1psi (ACC-150119)
Accel 21.1lb/hr @ 44.1psi (ACC-150121)
Accel 24.4lb/hr @ 44.1psi (ACC-150124)
Accel 25.6lb/hr @ 44.1psi (ACC-150126)
Accel 29.4lb/hr @ 44.1psi (ACC-150130)

The '95 Jeep 5.2 ZJ/Dodge 5.9 injectors (Part no. 53030262) are rated to flow 24.6lb/hr at 39psi.

The selection of injector size will depend on the estimated horsepower output, brake specific fuel consumption BSFC (assume 0.5), no. of cylinders, and the injector duty cycle (assume 80% or 0.8). The formula is:

Injector size (lb/hr) = (horsepower x 0.5)/(no.of cylinders x 0.8)