Hey mate! Reaching out from Australia - I’ve got a 91 Isuzu(Holden) TF rodeo 2.6l petrol - the engine had shat itself (worn pistons rings) and so I bought a replacement engine - which is in it now - which is a 93 model engine - everything’s gone in and the car starts and drives - however - I seem to have the same issue I was having with previous engine - which leads me to believe maybe the issue has something to do with the intake? 
when accelerating and stopping, instead of dropping in rpms when clutching in to change gears , the rpms shoot up - even when in neutral? It’ll go straight back to normal if I give the accelerator a quick push? 

but also, I’m getting a bit of black soot from tailpipe ? The engine oil got pretty dirty pretty quickly when we first filled it up after installing the new/used engine... got a new oil filter for it coming today though... but yeah I’ve only driven it for abt a week since fitting this engine... and last night I had to top up oil as it just reached the low line ...

oh also.. stopped on way home to get a quick feed at local shops after a 20min drive on high way.. car was off for abt 5 mins I’d say? But when I went to start again, although it started fine - I could hear a couple clicks which sounded like what you’d hear when battery is flat as?? 

please tell me there’s hope and I didn’t buy a dud engine ?! Haha