1990 cadillac deville 4.5l runs rough and missfires. Checked fuel pressure and for vac. leaks, both ok. have replaced plugs, wires, cap, rotor, performed injection service. no results. Hooked to scope and diag. bad distributor. repaced distributo, still have same problem. What next?

Is the check engine light blinking if it is it means random missfire,does it have a check engine light if so go to local auto parts store and get it scanned,what kind of truck is it,more info

Check fuel pressure and fuel delivery. MAP sensor also needs to be checked.

you may have bad or very dirty fuel injectors causing too much fuel into the cylinders causing rough running and smell of gas. do you have a fuel leak or too high fuel pressure.

have you check converter clug and o2 sensor
vacuum leak check leak with w40 or else
rotor,cap.quality of fuel
hope it helps

Probably not. If it was, the engine would run rough at all speeds and the check engine light would be on.
Have you checked for vac leaks ?

Just a thought,pull the dist.cap and make sure the rotor has
no side to side play ,thinking maybe a bad upper bushing in
the dist.Also,if the EGR valve has carbon and soot built up
on the piston it will make it idle really poor ,but run ok.
Might pull the EGR and clean it with intake cleaner.
Hope this helps.

Hello! when you say you checked the MAP sensor what do you mean? Did you take voltage readings at idle ? A vacuum leak is the likely culprit, especially to the MAP...The minor shake at idle being the major clue...Send a comment...guru..saailer

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

Oxygen sensor only report the condition of the engine the rough running condition can range from a worn sparkplug wires to worn sparkplugs to valve problems to injector problems. with the cadillac deville a tune up should be the first attempt to smooth the rough idle with the inspection of the cap and rotor cap and rotor do have a tendancy to arc internally and create misfire so does sparkplug wire that can leak spark voltage to ground and create misfire. the spark plug wire especially if they are brittle and hard can create issues

Here are a couple of guesses for things to check: vacuum leak (low vacuum pressure), worn distributor cap or rotor, incorrect timing, clogged fuel injectors, malfunctioning fuel pump (low fuel pressure), ignition module on its way out. I hope these suggestions help you out.

Was the TPS adjusted to 1.5 volts or checked for that voltage reading beforehand?