My v8 jeep laredo will start for 3 to 5 seconds then shut off? Why?

It sounds like you may have a security system problem. Beginning in 1998, selected Chrysler products began rolling off the assembly lines with a new advanced security system known as the Sentry Key Immobilizer System (SKIS). Most WJ Grand Cherokees are equipped with this feature. For more info see this link: http://www.wjjeeps.com/sentry.htm. During a successful start there should be an indicator on the dash with a key. If the indicator is not there, the Jeep Sentry Key Immobilizer Module (SKIM) will shut down the engine after about two seconds. Apparently, problems with the SKIM are somewhat common. The best solution is to remove the SKIM module and replace the PCM (engine computer). The SKIM module permanently alters the PCM so if you install a new PCM you will have troubles in the future and if you remove just the SKIM, the Jeep will never start. A new SKIM costs about $65 and a new PCM is about $850 or you should be able to get a good used PCM is about $75 and you can install it yourself. If you are worried about the SKIM continuing to cause problems it can be removed, but a new/refurbished PCM will still need to be installed. Here is a link to an article about the SKIM for reference. http://articles.jeepforum.com/SKIM_%22Sentry_Key_Immobillizer_Modul e%22_install