Car shakes while driving/idling. 6th cylinder issue?

So the car drives well besides this. I had a mechanic and my husbands dad who is a mechanical engineer look at the car. The mechanic told us it was a spark plug issue. We replaced all the spark plugs and wiring and the issue was still going on. When we took it to his dad, he tested if it was electrical, it isn't. It sparks and fires well… So his dad thought it was a mechanical issue. But the mechanic tested it for mechanical failure and said it was not that. So there's nothing mechanical OR electrical wrong with the 6th cylinder… But there's something wrong with the 6th cylinder to make it shake. It also is slow to respond to getting up to higher speeds… Like if I floor it at 50mph it takes 2 solid min to get up to 70mph.

I have a 1999 ford ranger v6 rwd 100k miles. Does anyone have any ideas what the issue is?

Well, it's not your sixth cylinder. You should change the fuel filter and check the air filter first. If new filters don't fix your issue, you should have the fuel pump pressure tested. You might want to clean the throttle body too. Is the check engine light on? When my truck ran rough like that, I had a vacuum leak in one of the lines that ran over to the vapor purge valve solenoid.

Combustion requires 4 basic things: Air, Fuel, proper compression of the air/fuel mixture, and an ignition source.

The spark plug is your ignition source. If you've installed new plugs & wires and verified that you have good spark, then its safe to assume that's not your problem.

That leaves fuel and air. The most likely scenario is that fuel is not getting to the cylinder, possibly because of a clogged fuel injector or a clog somewhere in the fuel line to that cylinder.

A clogged fuel filter and/or failing fuel pump could also cause a lack of fuel, but generally that would affect all cylinders equally and the engine would struggle at high speeds & high loads, or would not run at all. If you're certain that all 5 of the other cylinders are firing properly then its unlikely to be either of these issues.

So I would check the injector on that 6th cylinder and either clean it out or replace it. If it does end up being a dirty or clogged injector then I'd replace all 6, the others might not be bad enough to malfunction yet, but if one of them is clogged then all of them are probably dirty.

If its not a fuel issue, then that leaves air or compression. Air problems such as a dirty air filter would usually affect all the cylinders unless there was some sort of obstruction in the manifold that was only blocking that one. That's doubtful.

So if its not the spark plug or fuel injector, then it could be the valves or valve seals. If they are leaking then you aren't getting compression and you'll get weak combustion or possibly no combustion at all. The repair is to fix the valve seals. Its also possible that your camshaft or push rods are messed up and the valves aren't operating properly, but if that happens on a 19 year old truck its time to go car shopping because the repair would cost more than the truck. That's unlikely though.

Replace the rotor, distributor cap and the plug wires (if you have not done so). CHECK which cylinder is misfiring by doing an rpm drop test. When the engine is running and at operating temperature, pull the spark plug cap one by one and check for an rpm drop. The cylinder that does not have a noticeable rpm drop is a misfiring cylinder.

When you find the misfiring cylinder/s, CHECK for spark at that cylinder with a spark meter. CHECK if the injector is firing or getting command to fire with a noid light. CHECK for compression at that cylinder with a gauge.