01 4.6 Persistant #3 misfire.
 

My friend has an 01 Expedition 4x4 with the 4.6 liter engine. Over Christmas, he took a road trip up to Boise. On the way, the CEL came on and his engine started missing, idling rough. When he got back, I pulled the codes. #1 misfire and #3 misfire. I told him I thought it was caused by moisture, so I suggested changing the plugs, and testing the two COPs. Both of the little coily-springy thingys in the COP "wires" were covered in gross black dust, while none of the other ones we pulled were. So we changed the spark plugs, and bought two new coils, then reset the trouble codes. The engine was running smoother. On the way home, he called me up and told me that the CEL was back on, and the engine was idling rough again. So I pulled the codes again. The #3 misfire was back.

OK, so the miss is coming from the fuel injector side of things. Since I was working outside, in the freezing cold, and I didn't have a pressure guage (plus erratic fuel pressure would cause random misfires, no just one cylinder, right?), I swapped the #4 injector and the #3 to see if I could get the problem to migrate to #4. I reset the CEL and told him to drive it a bit, and call me back if it came back on.

Today he calls me back and informed me that the CEL was back on, we pulled the code, and lo and behold it's the #3 misfire again. All I can think to do next is check compression.

What am I missing?

Yes a compression test would be a good idea. Other than that, new parts can be bad. Try swapping the spark plug and/or COP between #1 and #3 to see if the misfire moves.

Speaking of the COPs, how do you test them? I checked the resistance across both the primary and secondary circuits, but the other test I read about didn't have a how-to. So when I tested the old COPs, I could only test the resistances, and the two old ones both were within the same range as the rest. (I changed them anyway, because their springy-thingys were miscolored.)

Resistance testing a coil puts a minuscle amount of current through the coil windings. If the coil has a definite open or a short you will be able to detect it with a resistance test.

But for a marginal or intermitiant coil issue, you really need to watch while under load, the current flow through the coil primary. For this you need an amp clamp and a digital storage oscilloscope to capture the current wave form.

For example, suppose there is a clean break in the coil's internal winding. The resistance test would show no continuity through the primary side of the coil and you could declare the coil as bad. Now suppose there was a weak spot in the coil's primary winding. It may be able to handle the micro amps of current the ohm meter sends through the coil's winding and the coil would appear to be good. But if you put the normal amount of current, maybe five to ten amps through the winding, the weak spot would break down and you would see the fluctuation in the currents wave form.

So to put it another way - the resistance test can show a coil is bad, but it can't prove that it is good.

Cool, thanks. I'll go switch the #1 and #3 COPs and see if the prob migrates. If so, I'm not too impressed with the quality of the parts my friend got.

One more thing. I was talking with an aquantance of mine about this problem, and he suggested that the fuel injector connector (on the wiring harness) could be causing a miss. He said that sometimes the connector will overheat, causing a higher than normal resistance.

Would a noid light pick up on this?

If that's the problem, then it is not a very common one on these vehicles.

I tried the noid light, the injector is definitely getting the signal.

I also tried to do a compression test, but I'm not to confident in the accuracy of my gauge. I tested 1-4 cylinders, and I came up with ~100 PSI on 1,2 and 4, but only ~60 PSI on 3. I'm not sure I got a good seal, though, because I only had my cheap tester with no ultra-long addaptor to get down into the deep sparkplug holes on this engine. I had to fish the hose down the hole, then tighten by hand since I couldn't get a wrench or socket down the hole with the hose in the way. The pressure built quickly, and stayed on the 4 tests I did.

Anyway, I'm thinking there's maybe 3 probable causes for this miss:

#1 Piston rings on the offending cylinder.
#2 Bad piston on said cylinder.
#3 stuck or very slightly burnt valve.

I'm skeptical of #1 and #2, because there are no noticable blow-by gasses coming out of the oil filler neck if you remove the oil cap while the engine is running.

I'm going to try running some SeaFoam in the fuel and hope that it's just a stuck valve.

Here's to hoping!

It would be quite a coincidence that the one cylinder you didn't get a good seal on, is the one cylinder that is misfiring. :rolleyes:

Yup. Indeed.

03 expi misfire
 

I also recently replaced all the plugs and COP's on a V8 @ the firehouse and low and behold had 2 new issues. 1) The sleeve or threaded female portion of the intake, where the small bolt for the #5 COP is secured, was shot and came right out after a test drive, screw with COP and threaded sleave all still attached. Apparently it had been rigged before with some sort of RV sealant. Simply fixed by gently drilling through the hole and using a 3/16 threaded rod with nuts on both ends to secure the COP. However, the code that it through, after the complete ignition tune up, was a p0307. Motorcraft plugs were used and ebay COP's. After reading some other posts either here or in the super duty forums I came across a thread stating that just because a new plug was installed doesn't mean it is fully functional as it is implied when purchased. Sure enough, when the #7 plug was replaced with another new motorcraft plug...whaaalllaaahhh, runs like a top. AZ allowed me to return the bad plug and acted like it has happened before.