My cng truck runs fine on nat gas,when switched back to unleaded gas truck stalls,what is the fault ,how is the two computers controlled,what deos the switch turn off.

The first thing you need to do here is have the computer control system scanned for fault codes and an have a engine running test done to see what is going on in the automatic idle speed control system.

P1388 JEEP - ASD or CNG (compressed Gas) Relay Control Circuit Possible causes - Faulty ASD Relay (most common cause)
- Faulty CNG Relay
- ASD or CNG Relay harness is open or shorted
- ASD or CNG Relay circuit poor electrical connection
- ASD or CNG Relay poor ground or supply voltage condition

P1399 is a related to the P1388 failure.

You may be losing fuel pressure or have a hard fault in the electronics of the engine control system. Do this, have the fuel pressure checked and have a fault code test run on the computer system. If all is ok then the ignition starter switch is suspect as well and the MAF sensor which will not set a fault when it fails.

Mass Airflow sensor is attached to the air breather system usually near the air filter. Its usaully a square box looking thing with a wiring harness on it. Here is a pic of one below, but keep in mind this is not a pic of one from a intrepid

Hi!!
At the pump, when it says Regular, it means Regular Unleaded Gasoline, the one that says Super, is Super Unleaded Gasoline, the only difference between the two is the octane rating. Both will work just fine on your vehicle.
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Replace the fuel filter on van and check the spark plugs for a fouling condition. If the van in question has not been serviced in a while you might think of doing it to correct your problems. Check all the filters and replace spark plugs. Hope this will help you.

Sounds like it may have been a flood car


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If the car is running that bad the check engine light should be on and code/s stored in the pcm along with a snapshot of what is going on at the time the code is set. A possible solution is to check the Mass Airflow Sensor or the MAP (manifold absolute pressure) sensor depending on which your vehicle is equipped with. The engine may run until the engine computer switches to closed loop fuel control (means its warmed up and the pcm is in control of the fuel trim) and then stall upon accel. Be sure you have checked all the basics first.

clean your air filter and try it .... normally on CNG at low Rpms it has unstable running

Regular unleaded (87 octane) is the correct grade.