Got a stumper for you…

97 F150 4.6. Engine has a slight miss at idle and at tip in. Seems to disappear up around 2000 RPM and on, release the throttle and tip in again before it drops below 2000 and misfire does not seem to be there. Pull the connector from the injector on cylinder 5 at idle and it seemed to have no effect, any other cylinder will make the rough idle even rougher.

Here's what I've done:
1. Swapped the two coil packs
2. Swapped spark plug with Cylinder 6
3. Swapped plug wire with Cylinder 6
4. Swapped injector with cylinder 3

It runs a steady 18 to 20 in vacuum and I've performed a smoke test on all circuits individually and the entire vacuum system as a whole. Compression test on cyl 5 came back with 190, Cylinder 6 compression was 180. When I pulled #5 spark plug, it had gone from grey Ash color it was in cylinder 6, to black and was slightly wet - tells me there's an incomplete burn at idle, I surmise that it's not getting enough Air at idle and under low RPM acceleration.

The computer has no idea there's a problem. I do hear what seems to be a backfire through the throttle body occasionally on Startup and definitely when I do a hard bump of the throttle from idle.

Not a whisper from the valve train, although tonight I'm going to take the valve cover off and just check to see if the intake valve is opening and closing all the way.

Thoughts?

 

Going out on a limb here... the intake valve isn't opening every time? Check for burrs and such on the intake valve? Or pehaps carbon buildup on the intake valve? but your compression test gave good numbers. I'm not sure. How long have you had your spark plugs for? I just changed mine at 50k and they appeared to be over due for a replacement. They were stock motorcraft. If you're almost due for a plug change, you might try changing them all and just see.

Just some thoughts. Chew on it, or don't you know the problem better than I do.

 

Idle Air Control Valve? Its on top of the throttle body

 

Not sure how long the plugs have been in there, they are the correct Motorcraft number and gapped at .054. They have plenty of electrode left on them and look to be in pretty decent shape. I suppose I got myself stuck in the box of only looking for things that could affect just one cylinder, which is why I Ruled the IAC out. I don't recall for sure, but knowing the way that I work, when I took the throttle body off clean out the EGR ports, it certainly seems like I would have cleaned out the IAC as well. That said, I would recall if I had tested the IAC for smooth movement, and I didn't.

I'm the kind of guy that likes to give everybody all the information, but doesn't like to throw out red herrings for people to get distracted by. I do recall at one point that that cylinder had water in the plug Valley and I suppose that, as unlikely as it should be, it's possible that it got down into the threads and caused some Rust which is now causing a bad connection. Also, someone had put anti seize on the plugs before they put them in and now I have basically what you get when you use WD-40 as a lubricant in places that you shouldn't: crud...

 

An odd thing that I do recall, is that it took 17 Strokes to get up to the final compression number on both of the cylinders that I tested. At the time I chalked it up to the fact that I had to use an extension on the compression tester to get it in the hole because the threads on my hose weren't long enough to hit the threads in the head. But when I look at the end of that extension, it has the same size hole as my hose does and therefore shouldn't have caused a problem.

 

it had gone from grey Ash color it was in cylinder 6, to black and was slightly wet - tells me there's an incomplete burn at idle, I surmise that it's not getting enough Air at idle and under low RPM acceleration...

or its telling you that you have a leaking injector and running too much fuel into the cylinder ?

or its telling you that you don't have a constant spark available at that cylinder ? ( bad plug / coil / wire)

 

I swapped the injector, plug and plug wire with other cylinders. I also took all of the injectors from Bank One and performed a leak and spray test on each of them and for good measure ran a mixture of seafoam and gas through them. My next approach is, however, to investigate Crossfire from another plug wire, as they all run on top of each other.

 

You need what's called a noid light and connect it across the 2 wires for the #5 injector. This light should pulse on and off with the injector, if it does not you either have a break in the circuit or the injector driver in the ECM is blown.

 

It was late when I wrote the original message, and I couldn't remember all of the things that I had done, but I did test the injector circuit with a light and also confirmed the injector was firing using my stethoscope.