My son and I are working on an '89 Lebaron with a 2.5L turbocharged motor. We have a decent Ethos scan tool and are not reading any codes. When cold the motor starts fine and then after running for about 15 seconds begins to run poorly and almost dies when in park and given gas. The car was parked for a long time so we replaced the old gas with fresh gas, checked fuel pressure, replace fuel filter, checked spark plugs, etc. Any thoughts?

Try unplugging temperature sensor connector and then start the car. You can also cycle the ign cylinder 3 times; then turn on and count CEL flashes. That's as much as I know, hope this helps.

Check the map sensor. If the map sensor's vacuum hose isn't getting a solid connection to manifold vacuum, it'll still start and run but will want to run bad and die when you give it gas. All while not throwing a code.

I tried the cycling the key method and did not obtain a code except the 55 end of report code. I am not sure if my son checked the map sensor; however while monitoring the fuel pressure we noticed the motor ran fine while the fuel pressure was maintained at about 50 lbs. however every once in a while the pressure would suddenly drop to about 20 to 30 lbs. and then motor would start to run very bad. My son would pinch the fuel return line and the pressure would jump way up to somewhere between 70 to 80 lbs. which tells me the fuel pump is working fine. I think the problem is with the fuel pressure regulator; however he thinks the problem is an electrical issue to the pump itself. My augment against that idea is when the pressure drops pinching the line causes the pressure to jump up. I guess if the electrical supply to the pump was reduced and the line was pinched it would still cause the pressure to jump up so maybe the issue is in fact the electrical line to the fuel pump. Last night he told me he can feel/hear the pump changing how fast it operates. The pump has been replaced; however maybe the new pump is defective. Any thoughts?

also look very carefully at all the rubber vac hoses- move them a wee bit to see if there is a crack or broken piece!

Should be around 54 psi and varies with boost. Idle for sure needs that and if it does not have it, your regulator or pump is bad. Very sensitive to fuel pressure.

While you're messing with the regulator, replace the feed and return lines running from the firewall to the fuel rail.
Those lines tend to crack with age. If they split and dump fuel on a 1400 degree turbo things get ugly. Plenty of those cars burned because of that.
Be sure to use a high pressure fuel injection hose.

Turns out the problem was rust in the gas tank was getting on the prefilter and restricting the flow of gas. Gas tank was replaced and car runs great!