My '97 4.0 AWD has developed a slight miss under load.
It idles so smoothly that I sometimes touch the throttle a little just to make sure it didn't die.
I had a major miss about a month ago and I replaced the plugs and wires. One of the wires was completly dead. Could this have damged the coil pack?
I also have a check engine light that comes on every now and then, code P0171 ( lean bank 1 ).
Would this be an indication of an injector going bad?
I did pull the P0171 again when it was missing today, even though the check engine light hadn't come on.
The van was running a bit rougher than I wanted it to when I bought it, so I have relaced the O2 sensor, the IAC, the engine coolant temp sensor (that is when the idle got so smooth) plugs and wires, and cleaned and recleaned the MAF.

I am just running out of ideas.

 

You have done an excellent job of attacking the usual culprits. Since the miss shows up under load, a vacuum leak or leaking manifold would generate this type of problem. I would suspect the manifold gasket as this has repeatedly shown up on later model 4.0 engines. Someone posted the other day their manifold bolts were loose enough to turn out by hand. Often just retorquing the bolts will solve the problem. The use of some mild threadlocker would be advisable.

The coil pack should not be harmed by a faulty plug wire. While it is always possible to have a faulty injector, generally they are pretty much bulletproof affairs. The OBD stores the last generated code, so you may just be reading the previous one.

 

aerocolorado,

I should have explained that I had already cleared the codes before I read the P0171 the last time.
I do agree, it is looking more and more like a vacuum leak to me. The CE light ( and resultant P0171 ) only comes on when the van is going through the warm up cycle when the temp seems to get to what I would consider running temp. And then, it only happens at a particular throttle position. So it would seem that the manifold might be passing air just at a right set of conditions.

Now as far as the miss goes. I read on the explorer forum that Bosch "double platinum" plugs were not too happy in the 4.0 engines. Guess what plugs I put in about a month and a half ago? Yep. Bosch DP.

So I put in a real set of Motorcraft PPs today and the miss is gone. ( of course my 'lovely' Ford dealer raped me on the price. )

Upon inspecting the Bosch plugs with a very high power eyepiece, you can actually see when the center electrode has deposits from the side electrodes. ( I guess thats what you call them.) In any case, the center electrodes are actually mushroom shaped from the metal that has transfered during the "wasted spark" phase. I guess the build up would almost have to lower the voltage of the spark, so while the spark is not really hot enough to keep the engine from missing, the coilpack is still firing and the ECM sees that it is firing so you don't get a misfire code.

Thanks for all the info, I am sure I will have to tackle the intake manifold bolts sooner or later.