My bad miss clicked the wrong button in chat for shop hours. Do have a question if you could please answer. I am from Ardmore OK.

My grandson has a 05 trailblazer 4.2l 291K miles getting a p0304 and fuel gauge code because it was not working. So he changed all the plugs and coils and changed the fuel pump. (Note in 2 years he is going to votec to be a mechanic helping him get a head start best I can on my fixed budget) this fixed the p0304 code and the fuel gauge code now get a P0300 with ruff idle. I got a bluedriver reader you suggested I like it still learning all the features. So he changed the water pump since it was going out and changed the oil and filter, new anti freeze and thermostat the oil one looked like it never been changed, Put new idler pulleys and tensioner arm on because of a weird sound can’t locate in the lower riders side of the motor towards the front when ruff idling. Now he is taking the intake manifold off to change the seals and get to the fuel injectors to check and clean them. Have not found any vacuum leaks with the cigar trick.

If this does not fix the issue of bad idling at stop signs or in park like a small idle surge what could be the issue and should he check next? I fear a maijor issue hoping for simple fix  I know you in Texas where? Thank you love your channel.