2008 jeep liberty remote start problem

I would definitely use the Tach signal instead of voltage sense.To me tach signal is solid."ole blake"

Did you use a immobilizer bypass module?

Hmm, 5 seconds doesn't sound right. Are you sure is isn't more like 1.5-2 seconds, or 10 seconds? Runs for 1-2 seconds, engine dies, ignition stays on for a short time: Probably the transponder bypass isn't right. Temporarily unplug your bypass, insert a good key almost all the way into the ignition, and try again. Runs 10 seconds, shuts off, and does the same thing again twice Go into programming, and change the voltage threshold to the >.5 volts setting.

It could be closer to 10.   I set the voltage threshold to >.5 and that didn't fix it. 

There will be a red flashing light in the cluster when it dies..... Check that your transponder ring is positioned properly, and that it is actually programmed. To eliminate a setting issue, unplug your antenna ring, stick a key in the ignition, and then remote start. If it starts successfully, you have a transponder issue.Wherever I go, that is where I end up......

Hmm, I've never had to hook up tach on the new-style Chryslers. Is the air conditioner on during remote start? (Leaving the climate controls on defrots turns the AC on too.) Try making sure the AC is off. Also, try this: Turn ignition on, turn blower on high, rear defroster on, high beam headlights on. Leave it like this for maybe 5 minutes. That should lower the battery's voltage a little bit. Then, turn all the accessories back off, turn the car off, and try the remote starter again. P.S. You don't have the car hooked up to a battery charger, do you? That'll mess up voltage sensing for sure.