Automobile(s): 58 Coupe Deville, 77 Coupe deville, 90 Deville
Re: 4.9L missing
I have a 90 4.5 with similar problems. There are so many things to check it would be hard to list them all here. You could start with and inspection of the vacuum lines and then cap, wires and plugs, fuel pressure, use noid lights to check that the injectors are getting a signal. Compression check also. I have done all of this stuff to my 4.5 and it still misses!!! Good luck with GMs finest, those plugs are a real bear to change. I know because I have changed mine 3times in the last month, if I had a Cadillac engineer handy this afternoon I would have beat him to death with a crowbar! Or worse yet made him change those plugs.
Well, if you think you have a 1 cylinder misfire, first step is to isolate the cylinder.
The easiest way is to use the onboard diagnostics to do the cylinder balance test. Do you know how?
Otherwise, use the classic method. Engine off, open the hood and pull a spark plug wire from the distributor cap. Set the wire back on the cap terminal put don't seat it all the way. Just let it set on the terminal so it will be easy to pull off again. Do so for the other 7 wires.
Fire up the engine and start pulling those plug wires, one by one, listening for a change in how the engine runs. The one that makes the least or no difference is the affected cylinder.
After isolating the cylinder using either method, go on by checking for spark, fuel, and compression on that cylinder.