I think ok4450 is right. Have a compression test done. If the compression is good, then their diagnosis is wrong.
If they sprayed injector cleaner into the intake and the vacuum readings smoothed out, it may be because if a fuel problem, not a compression problem…if the injectors on that side of the engine are sticking, then not enough fuel is reaching the cylinders, causing the misfires. When they sprayed the injector cleaner into the intake, the injector cleaner burned like gasoline, making up for the gas that the injectors weren’t supplying, and the vacuum readings smoothed out.
Not saying that’s definitely what happened, but I believe that’s a more likely explanatiion than theirs. If the injector cleaner smoothed the vacuum readings by magically removing carbon buildup, why did the vacuum readings go rough again as soon as the spray stopped? The carbon that disappeared momentarily suddenly came right back? I’m not buying it.
I would do two things:
- Have the compression checked.
- If compression on those 3 cyls is bad, have the valves adjusted, then recheck compression.