i cant use lava cells as fuel?
wtf?
i put them in cargo manager, pull them into the engine slot and they just stay there - not being used.
using ftb ultimate reload latest stable release(not beta)

To use lava as a power source you need to have either a Thermal Engine or an Advanced Thermal Engine

sorry but it does indeed have a thermal engine, it just doesnt intake the lava cells i left in cargo manager, i made sure to make it output 1 lava cell with advanced track

also if it helps, whenever it stops it "attempts" to take it, as in - i see the progress bar but then it just leaves without the item !

Can you show an image of the cargo manager setup and one of the cart gui?
Also what version of Steve's Carts is in the modpack?

Sorry, I no longer play the game but upon looking @ https://www.feed-the-beast.com/projects/ftb-ultimate-reloaded/relations/dependencies?page=4

Looks like it's using the latest one for 1.12.1 - uhm... I was trying to use tech reborn lava cells in the cargo manager - explicitly telling the manager to GIVE THE CART one single lava cell but it didn't work - the cart was stopping to refill - the animation showing progress of the cart "doing the action" but it just didn't intake any items at all - note that the items that the cart outputted had no issues.

Could be FTBUtils claimed chunks messing with your mod? Maybe? Not sure.
I also tried using multiple engines not just one and the issue didn't go away. Also another fun thing shift+clicking the cells wouldn't work, and if you do it with multiple storages it will go in either the internal one or other inventories/engine slots.