I have a 1996 boniville that we have an idel isue with .We swaped out the engine for a used one for a customer. He says it ran good before.

the idel will go up and down till it finaly stalls.

I am thinking it is a vacume leek but i can not find one external.
Itake the oil fill cap off and dont seem to have any trace of one internal either.
We have changed all the switches crank,cam,maf.iac,module, checked all grounds changed the wire harness betwen the crank and module still the same.
We have no codes. and when we put the code reader/scanner it shows it has negitive timming. starts off around -6 and goes to -18.
what next?????

clean the iac seat and area? timing seems low, havent checked my bonnie but the yukon and monte are like 20 at idle and go into the 30'*. check fuel pressure and fuel trims? check boot on the tb that connects the evap purge and fpr vacuum hoses for cracks?

Negative timing? Is it possible the timing chain is out of position?

i think some scanners just display weird. i sort of assume that means 6 to 18 btdc