Newbie here! I have a Chevy P30 step van. Nick named big foot. It has a 5.7l on propane. It has developed an issue.. Intermittently it has a stumble, the SES light comes on, and it rev's up like it was put in neutral. It idles high, stumbles down, then goes back to normal, and SES goes out. It stores no codes. Occasionally it will idle high in park, then stumble down to almost stall, then go up again.
So far what has been done are: new TPS(gm part), new ECT(gm part), new plugs, new cap, new rotor, and a tie rod... (pretty sure the tie rod is not related...lol)

Having said all that- eliminating the following potential items- fuel pump,fuel filter, and injectors- as they have been removed for the propane. (i did change the propane fuel filter...)

HHEELLPP.... when she runs right, she runs like a CHAMP!!!!!!

Trans 4L60E Rebuilt 2 yrs ago...
235,000 kilometers

thats all i can think of... look forward to your help!

 

im researching a similar issue, seems most people are saying its the torque converter unlocking and locking, but so far ive seen no clear consensus on the cause.. i have seen alot of threads saying dirty or out of position idle air control valve, im taking mine apart tomorrow.. gl