Hi everyone! My trusted Volvo mechanic is having great difficulty in fixing this problem for me. 1991 740 Base with LH system: It starts easily, runs well, very smooth on highway. Just a slight vibration at idle. You can feel engine "misfire" or "hiccup", or as if "one injector didn't squirt" or "had no spark one time", feels like it "bucks once or a couple times", that causes this interruption. Any speed, anytime, though seems only when car is warm. This is at highway speeds where best felt. It is not the AC cycling when I use it. I have chemically cleaned the fuel injectors. No codes from the on-board diagnostics module under hood. Mechanic has replaced: Pre-Pump, (Main has less than 30k miles on it), crank position sensor, Suppression relay, newer used distributor, new cap, rotor, and wires, New fuel pump relay, engine temp sensor, Ignition control unit. I am heartbroken spending all I have, but I really don't want to drive anything else! Love older Volvos!!!
He doesn't think it's any wiring issue, but I historically don't understand wires anyway and have refused to learn. I do not see any bad looking or worn, or missing insulation on any wires anywhere underneath or in engine compartment. I am bummed out and don't know what to do or when to consider "giving up". The engine is immaculate with no oil leaks and engine compartment is very clean! Anybody have an ideas what to do? Thank You most sincerely!