99 with L6 4.0 drops cylinder at idle
 

I have a 99 with the L6, it seems to be dropping cylinder at idle, it recovers, then drops over and over. Runs fine otherwise.
I have recently changed plugs, but don't know if it did this prior. It is my daughters truck, I don't drive it. Thoughts?

It does not have service light or any active codes.

If you read threads on here, you'll see that this is common problem, at least with some years. I've had a '97 w/ L6 for 5+ months and it does that. (I've got other issues I've been addressing and have let the stumbling idle go for now.)

From what I've read, the prime suspect is low fuel pressure, or fluctuating fuel pressure. These have in-the-tank strainers, not in-the-fuel-line filters like sensible vehicles do (mine anyway, don't know about '99, but I think it's from '97 and up that do), so as you can imagine, it is a fairly major pain in the neck to change them, thus they don't get changed as often as they should to prevent problems. You can go to most of the major auto parts chain stores and borrow a fuel pressure gauge. Take a gas can with you to catch the drain-off and just check it in their parking lot. You shouldn't have to "loan-a-tool", i.e., buy it and return it, they'll just hold onto your license or credit card.

Another culprit is a dirty idle air control valve. These are cleanable by people with mild mechanical aptitude, thought you should search threads first to read the precautions. There is a piece that you don't want to move while you are wiping it off. On the L6, it is much easier to get to if you remove the whole throttle body.

There are any number of electrical issues that can cause the stumbling idle, a loose ground connection, a loose wiring connector into the PCM, cracked solder joints inside the PCM, corroded connector pins in the wiring connectors to the PCM, a wiring short on one of the engine sensors, loose connections on a relay in the engine electrical box, and so on.

Fuel pressure is the least hassle to check, though correcting it is a pain, or more expensive than it should be if you pay someone else to do it.

I should add, my "dropping a cylinder" or "stumbling at idle" issue was definitely noticeable, it happened frequently. Then I developed the "dying randomly while driving" problem, with some nasty side effects. I luckily noticed that the 3 wires going into the wiring harness connector to the Crankshaft Position Sensor were bare for about 1/2" and were apparently touching. I put electrical tape around each one and it fired right up and hasn't had the dying problem since, plus, the idle stumble is much less noticeable and much less frequent. I'm doing the fuel pump and cleaning inside the tank next.

Thanks. What should fuel pressure be?

According to my manual, '93 to '96 have an inline filter, '97 to '99 have the in-tank filter, and 2000 and up have the inline filter.

Great, I have the failed experiment.

Think I'm going to cut a hole in the floor, rivet a couple of tabs to the bottom side that the cut-out piece can rest on, then screw it down. A little duct tape should seal it up.

Of course, I'll probably learn that taking the carpet loose is more of a pain than dropping the tank.

In this case it doesn't sound like it could be the fuel filter, when more fuel is needed at acceleration, it does fine. How does the coil system work on these? Does each plug have its own on the rail? If it was the idle air control valve, wouldn't it throw code(s)? I really hope it's not bad wire somewhere, I hate tracing that crap down. Thanks for the help.

Under the coil pack, each plug has an insulator boot and a spring contact. One of these boots may be shorting out to ground, or "flashing over" as it's called. Try replacing all the boots and see if this cures your problem. It's a lot easier than pulling the pump.