i have a 91 jimmy with a 6.2 in it and at idle in gear it has a dead miss, when you crack a line to trace missing cylinder all the cylinders drop the same, any ideas?

How's the blow by? Have you changed the filter(s) lately?

I recentley changed the fuel filter and air filter, has a little blow by compression is good all the way around

Single or true dual exhaust? White smoke coming out the pipe(s)? What do you call good compression? Should be in the neighbor hood of 400 psi and even. Could be a bad injector, got spares?

all the cylinders were around 320 psi i put a set of injectors out of a 6.5 and nothing changed still missing true dual exhaust and black smoke at idle

black smoke at idle? you have a injector with very low pop pressure or is stuck open.

black smoke at idle? you have a injector with very low pop pressure or is stuck open.
....or leaking nozzle

it ran the same with the 6.2 injectors, and nothing changed with the 6.5 injectors

if you have a dead miss. when cracking the fuellines you GOT to have at least one cyl that does not respond, or the IP has an issue,alternating the miss between cylinders

320 psi is kind of low.

Press test taken hot or cold?

the compression was done hot and you get the same rpm drop when you crack a injector line for all cylinders

that don't make sense, pop test injectors, check timing chain for wear.
Compression should be no less than 375/380. got to find out if rings or valves are the culprit.

you say,miss at idle and in gear. what about in neutral or park?