Hello together, Cadillac Eldorado BJ 1990 4.5 L V8 PFI It runs both cold and warm only on 5 of 8 cylinders. Thereby 1x cylinders from the front bench does not run, and 2x from the rear. Candles and ignition cables are quite new. Before that it stood about 1 year. No smoke (neither blue nor black) recognizable.What can the problem be?