Re: Diagnosing 4.0l rough idle
As I understand it then, it feels like the miss is coming from just one cylinder, -- not all of them at once, mainly hot, at idle only, goes away above idle, starts fine. And it tends to come and go?
You used a timing light to check plug wires, and you didn't find a vacuum leak with propane.
Q-- When you put a small amount of propane in the throat, did it "really like it" or not have much effect. If it "Really liked it" indicates there's still a vacuum leak somewhere.
If the EGR was leaking, slightly open from carbon, it would act like a vacuum leak the way it feels, but propane doesn't help. The exhaust gases recycled by the EGR valve don't contain enough oxygen for the propane to combine with, so you don't get much, if any reaction like a genuine vacuum leak. An EGR problem feels, looks, acts like a vacuum leak, but isn't! It's "dead" air.
The timing light method - the light will flash even if the plug wire's pretty bad. Completely open it won't, and it won't tell you if the plug is actually firing or firing properly.
I'd toss in another set of plugs (sometimes a plug cracks on the side, spark jumps the wrong way and doesn't ignite the fire properly.) Make sure you use the proper plugs for that engine - check the service manual. So often the so-called "trick" plugs cause problems, like plats, splits, multi's, etc, or the "one size fits all" types like Accel, Bosch, reds, blues etc. They are made to sell, not to use. The engineers designed it around a specific plug, use it. -- If the problem's still there you now have a spare set.
Then a set of wires - unfortunately there really isn't a good way to test them without a scope. Testing by Ohmmeter might indicate one's bad, but often will fool you. A break in the wire still has a carbon track across the gap - the ohmmeter "sees" the carbon track and reads fine, yet it cannot carry the amp or so of the ignition spark.
Cheap way - get one plug and one wire -- swap them around - lots of work though. Some day you'll need full sets anyway.
If it feels like it's just one cylinder (like thump thump thump) look for something that can only affect one cylinder (not too many things that can do that.) If it seems like it's several cylinders, look for something common to all cylinders. Cap rotor, coil wire, coil etc. I rather doubt it's caused by the coil or coil wire though, as it runs fine at higher speeds and under severe load.
You can isolate it to one cylinder by pulling one plug wire at a time and listening for the least amount of change. A good one will drop the R's. Listen close, often a computer system will compensate for the RPM drop, so the change will only be momentary.
Sometimes you'll find when you pull one cylinder's wire, RPM's go UP! Sounds goofy, but that's often an EGR problem, or ignition crossfire.
Not much, if any chage indicates you've got the right one.
So Mr. Sherlock Holmes, get out that magnifying glass and go searching - let us know what you find.