98 F150 4.6l 2v NON-COP 266K w Cyl 6 Misfire under load, slightly elevated Idle

Love your videos-you explain things very well and I just registered so I can watch and learn more and ask questions. I have already learned that I need a scope... lol. Watching you explain the Firing Line and Spark Line made sense immediately. Hope you can help me-I am very frustrated tracking down the cause of this Misfire.
Sounds like a slight miss at idle, only DTC is P0306 (and a pending P0306). I don't see an code while in P or N while revving, no matter how hard i run it. I drive 100 yards and Misfire Cyl 6 if I get over 2k rpms. I drove several miles on dirt roads keeping it under 1500 rpm for and hour- no codes. Cycled AC on/off and it kicks up and down normally but the base idle seems 100pms higher than usual. When i get back to the highway and accelerate over 2k within a 100 yds it lacks a little power, then i feel very slight miss (not a dead miss like I have felt before when a coil went bad) and the CEL flashes and my ACTRON CP9135 says its P0306, and 'pending' again. It stops flashing and remains on unless I load it over 2k and it flashes again. Feels like a high idle and a slight vibration at low rpms I get home, clear the codes and can repeat this fault. I pulled plug 6, and thought THIS is IT! compared to new MC plug I was SURE it would solve it. Nope. Neither did a new MC plug wire on #6.
I ohm'd the pass side coil (#6 fires from it) and got .8 and 1.2. repeated test on drivers side coil and got the same reading. not sure about testing output voltage (no scope, just multimeter and test light) so I hooked up a spark tester inline Cyl6 but since the fault doesn't occur while parked, I'm not sure if what I'm seeing ir right or wrong. Didn't see an obvious lack of spark. It flashed as I thought it should but wasn't perfect. Listened to the injectors with a stethoscope and they all sound about the same- all 8 are clicking away.
No other DTCs, no work was done recently except that I had to make a new heavy duty Battery/Starting wiring harness and replaced the original 22 year old OEM starter (Bendix was dragging after disengaging) so the battery was disconnected overnight. I drive it every day, but rarely more than 30 mins at a time so it gets up to correct temp. is it not going into Closed Loop because it has to relearn? I probably have over 50 Drive Cycles since the reset.

When I check the ACTRON CP9135 I/M Monitors I get:
Misfire: Ready
Fuel Sys: Ready
Comprehensive: Ready
Catalyst Not Ready: (hasn't been Ready since I started looking at it after the battery was disconnected overnight 2 weeks ago)
Heated Cat: NA
EVAP SYS: intermittent Ready/Not Ready depending on when I scan it
Sec Air: NA
AC Refrig: NA
O2: intermittent Ready/Not Ready depending on when I scan it
Heated O2: intermittent Ready/Not Ready depending on when I scan it
EGR: intermittent Ready/Not Ready depending on when I scan it

At different times of the drive cycle I expect these to show ready/not ready depending on the system and its function. The only one that has never shown Ready is Catalyst. Not sure what its looking at, since the O2 sensors have shown Ready. But they have also showed Not Ready after a test drive...

Just now when I looked at the monitors to verify the above list, I read the DTCs and found a Pending P0306 (but it has only idled for 5 minutes so far today- no load) with no CEL. WTF? I have read codes before I drove on other days and never had a pending DTC without the hard code and flashing CEL.

Additional Info:
-Once in a while the idle will jump up to 1200-1500 and stay there until I drive (load) or shutdown/restart. Doesn't happen often, but i have noticed it a few times recently.
-Did have an issue where the aftermarket AutoStart was activating out of the blue- like while running already/middle of night/etc. Have not had the issue since the battery/starter job. But I also have not used the Autostart since. Thinking about removing it; but its been hacked in and i don't want to cause other issues so Im leaving it alone for now unless it acts up again.
-I cleaned the #6 Injector connector and wiggled the injector body-no change
-Sprayed water around area looking for vac leak- no change
-last resort was testing separately 1)Lucas injector cleaner in fuel tank- no change over 10 gal fuel burned, and 2)Marvel Mystery Oil in fuel tank and crankcase- no change over 10 more gallons burned. Fuel mileage has decreased but i have no data to back it up. I chalk that up as No change.

Any and all help will be much appreciated, I can't afford to take it anywhere and don't know any good local Diagnosticians; sadly just Parts Changers. Some of these local guys call me for help occasionally, so if Im stuck they are usually not much help besides suggesting unrelated parts to throw at it. I like to fix it right the first time- it's more satisfying and a whole lot cheaper!

I think a quick and easy check without getting too involved would be to swap the coil packs from bank to bank and see if the misfire code follows.
One thing to be aware of, those older OBD 2 Fords are famous for flagging the wrong the cylinder with misfire codes. Could be multiple cylinders or the one before or after #6 in the firing order.
I wouldn't get too involved with what monitors are ready or not ready. That information has never particularly aided me in a misfire diagnosis. Except for in the case of the misfire monitor when relying on mode $06.
Mode $06 test ID 53 may be helpful to determine if other cylinders are missing.

Thanks Noah, I actually did not know about the misreporting cylinder numbers...ouch. I had issues a few years ago with #5 because the thermostat housing leaked into it- gonna change that plug and wire first and see if it solves it. If not I'll change #2, since 2 and 5 are before and after 6 in the firing order. Then I'll change the rest and post my results. If it doesn't solve it I'll swap coils next before running down any more rabbit holes.
I only recently heard of Mode6 data, and the video you posted was the first time i had seen it in action. I don't have the equipment for that, but i wish i did because it seems like an easy way to track the misfire. If I understood correctly, the ECM didn't set a code, but looking in that Mode6 data he was able to see the misfire and its location- So If I were to look at Mode6 on mine would it show 'misreported' cyl 6; or the actual cylinder that has the misfire?

Mode 6 may flag the wrong cylinder as well, so it's not fool proof.

Update: changed #5 plug and wire and got a pending misfire in #6 again during initial warm up. Plug was ugly- but not the cause. On to #2 next!

Update: replaced #2 plug and wire but the #6 misfire is still there. Idles smoother now. Gonna replace #1 next and then swap coils and see what happens.

Update: Swapped coils, fault did NOT follow. Took a bit longer to come back but after a little light driving I'm getting the same P0306 code. I checked ohms on several Fuel Injectors and they all tested at 12.5-12.6. Was really hoping #6 would test bad- but that would be too easy. Gonna try using a test light on Injector 6 next, but am not sure where to "test for voltage at PCM"-actually at the computer? or in the wiring under the hood behind the Power Distribution Block? yikes

I've never got this far... but it ain't gonna fix itself.

Scratching my head... anyone know which is the cheapest scan tool that shows live data? I'm in over my head without it

Also: Shouldn't the Catalyst I/M show Ready at some point? since I've been watching it's the only one that has never shown Ready, At 266K miles maybe it's the beginning of the Converter failure? She revs up quick and smooth up into the 4,000's but takes much longer to return to idle. No lean codes, no vacuum leak that i could find. Could excessive back pressure cause a Cyl 6 misfire?

Again, Thank you to anyone who can help in any way.

Cat monitor is often one of the last to be ready. It doesn't necessarily mean there is a problem with the cats. Back pressure in the exhaust would effect one whole bank or the entire engine equally. Typically not a single cylinder.

I would attempt to isolate the misfire to be certain that it is cylinder 6 that is missing and not another cylinder or even multiple cylinders.



You can perform the test with an assistant in the truck, in gear holding the brake if the misfire isn't present at idle.
Obviously, don't get run over. A wheel chock and a RELIABLE assistant are recommended.

Checking injector operation without specialized equipment will be difficult, but for now I think we can rely on the sound check you've done.



Assuming that the code is correct, since the symptom hasn't changed despite moving the coil pack and changing the plug and wire, I think a compression test of all the cylinders would be a worth while test.
You can disable the fuel pump and crank the engine while listening for any unevenness.




I can't find a Scanner Danner video of a traditional compression test with a gauge, but this Eric The Car Guy video will do.


You may be able to borrow a compression gauge from a local parts store if you don't have access to one.

A vacuum gauge on the intake may or may not be helpful considering the miss is not present at idle.
That test is shown near the beginning of this video


If nothing else, that's gotta be a couple hours of quality training videos

Rob L wrote: Scratching my head... anyone know which is the cheapest scan tool that shows live data? I'm in over my head without it

Here's a recommendation I wrote a couple weeks back. Since you've got a Ford with an "interesting" problem definitely go with something that'll run Forscan. It's basically dealer level software which is free for Windows and cheap for the phone apps.

Matt T wrote: A USB or bluetooth adapter + app(s) is the better value at $100 and under. Much more capable than the handheld code readers. Link to the Forscan compatibility list below.

forscan.org/forum/viewtopic.php?t=6142

Personally I've got an Obdlink MX+ which is on the upper end of the price range for these things. I went with it because I wanted something that'd work with iOS which limits your options on hardware and apps. When I bought mine the only other iOS options were wi-fi adapters with terrible reviews.

It works fine with Forscan. Also the app that comes with it will pull OE codes and data from Ford, Toyota, and newer nissans. OE codes only on gm, usually a bunch of 'em :lol: Generic OBD only for chrysler.

Update: bought BlueDriver and can now see the fuel trims and o2 sensors. Still getting misfires in cylinder 6; no other codes. Bank 2 stft seems ok but the ltft is adding about 19%. Ruled out spark, and have sprayed all around the intake while watching stft but only saw a minor % change and the rpm never increased. I think I ruled out vacuum leak. Tried it with Brakleen one day and water the other time with no change will post a screenshot from BlueDriver if anyone can make sense of it

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Update: did a flood clear crank and it sounded great- smooth consistent cranking both times I tried it. Ruled out compression issue.
Started immediately afterward and still showing Misfire cyl 6, so I unplugged the #6 injector and there was no change- the engine is still vibrating exactly as it was before (misfiring)! I checked the plug and there is battery voltage on one pin, gonna check the switched ground side after I watch more video so I don’t do more damage due to ignorance. From what I now understand, If there is a pulsing ground, that means the injector is bad because it’s receiving correct inputs. If not I’ll dive deeper into the harness and PCM.
Thanks for listening and I hope to post a FIX next!

Looks like you're on the right track!
The positive fuel trim in bank two supports a lean misfire on 6 in my opinion. I would follow up with an injector balance test, but if you don't have the capability you could consider swapping #6 injector with a known good. It is kind of a lot of work though, and not something I would do it I had another method to check it.
Keep up the good work and keep us posted!

Update: replaced #6 injector, fixed Misfire issue! Had a little trouble getting the fuel rail seated- the #5 injector didn’t fully seat and sprayed fuel all over, but just a little wiggling it snapped in. Runs MUCH better now.
However, as I was test driving I got a P1506 code High Duty Cycle in the IAC. When I rev it ramps up smooth but on the way down it hesitates around 1500 rpm for a second or two, then down to ‘normal feeling’ 820rpm.-if the AC is ON. If the AC is OFF, the idle rests around 975 rom.
Kind of the opposite of what should happen, right? I did clean the IAC and visually checked the connector but nothing obviously wrong.
I also noticed the LTFT numbers are higher now and the B1S2 B2S2 O2 Sensors read quite differently.

In the third screenshot the AC is ON, then I turn it OFF and see the drop in Engine Load% and the uptick on the RPM line and the S2 O2 Sensors respond. Then I turn AC back ON, and see the corresponding Load % increase and downtick on the RPM line.
Guess it’s better than a misfire.
Fixed one problem and found another— or two.
At it again tomorrow