We have a code reader. It is not throwing codes. When it first started happening it was random misfire cylinder 8... Husband moved the coil to 6 and the next code it threw was 6 so he replaced that coil. The coil worked for about 2 weeks then went bad. You could really feel that. Code stayed on so he returned and replaced coil. Then it started moving around to 4 then back to 6 so he replaced the other 3 coils. It hasn't thrown a code since but you can still feel the intermittent misfire, especially when sitting at a stop sign and sometimes going uphill. This has been going on for a while now and it's making me crazy. Hubby says he will replace plugs this weekend and if that doesn't fix it, I'm going to have to take it to a dealer. Grrr.. Did I mention I HATE cars.
I don't think I've ever heard of a car actually misfire and not throw a code. Does your scanner show pending codes too? I've had cars that have misfired without the CEL coming on. I'll connect my scanner and there will be pending codes.
Typically, misfires are caused by a few things unless you're getting other codes: spark plugs, coils, injectors, electrical wiring, or ECU. That's pretty much it.
Smart moving the coil around from 8 to 6 to test it. That was going to be the first thing I recommend you doing, but he already did that. It's very obvious when a coil is completely bad because you lose a lot of power. It typically isn't a quick misfire and back to normal.
When was the last time he did the spark plugs? How many miles and years.
Fuel injectors are also a possibility, but it's pretty rare for them to fail. For all three to fail in a small time frame is a very low possibility.
If all of these things don't fix the problem, it's most likely something electrical. Wiring from the ECU to the coils can and do go bad. There could be a short somewhere in the wiring on cylinder 8 causing electrical issues to jump from 8 to 6 to 4. That's a real possibility. The ECU itself can also have a short somewhere and may need to be replaced. When you get into electrical stuff like this, you really need to know what you're doing and can get expensive.
Regardless what the problem is, be careful driving the car if it keeps misfiring. When a car misfires, unless the injectors aren't spraying fuel and that's causing the misfire, fuel is going to leak into your catalytic converters and eventually kill that too. Depending on the car, each cat installed usually runs about $400-$500 each at least. Enough misfires and it's going to kill your cat.