4.0 ohv chasing mis for 1 year

[Dirt Devil]

Ive been chasing a mis for almost a year now on my 2000 4.0 Ranger. I have replaced the plugs, wires, fuel filter, intake manifold gaskets, plenum gaskets and have done a compression test.

It only misses at 1k rpms. No power loss, no sputter or rough drive. Pulls a 306 for cylinder 6 and lean bank 1 & 2. The injector was swapped with another and it still holds to #6. Inspection is due this month and i am in need of help.

I have searched google and two other forums and haven't been able to get a clear idea.

Any help is appreciated.

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[RonD]

Welcome to TRS

Then don't drive at 1k RPMs.................Thanks Groucho


Try swapping Spark plug wires 2 and 6 on the coil pack to see if misfire moves to #2, P0302

Ford uses Waste spark so 3 and 4, 2 and 6, 1 and 5, each share 1 coil in the coil pack
So both spark plugs fire/spark at the same time.
This is why the coil pack spark plugs wires are arranged so oddly
3 4
2 6
1 5
Front

The 5 6 4 side always gets miswired by me
So you can swap wires for 1 and 5, 2 and 6, and 3 and 4

Compression doesn't come and go so that can be taken off the table
Problem with injector shouldn't be RPM specific, i.e. just misses at 1,000RPM
Spark....well swap the wires and see

A misfire dumps unburned oxygen into the exhaust and O2 only sees Oxygen so that could cause Lean code but ONLY on bank 2, #6 cylinders bank

Lean on Bank 1 and 2 at the same time usually means upper intake vacuum leak.
Which is easy to test for
After engine is warmed up and idling, unplug the wires on the IAC Valve
When IAC Valve closes RPMs should drop to 500 or engine may stall, either is good it means no vacuum leaks
If idle stays 700 or higher then there is a leak.

Dirty or failing MAF sensor could also cause dual Lean codes
Low fuel pressure as well, 2000 Ranger runs 60-70psi fuel pressure

Lean code does not mean the engine is actually running Lean
Lean code means the computer is having to open the fuel injectors longer than it calculated
Computer is adding the extra fuel based on O2 sensor feedback, it is just letting driver know there is a problem with the calculations.

Back to #6 misfire, does #6 spark plug tell you anything?
Blackish color
Pull 6 and 5 spark plugs and compare the tips

 

[stang99x]

I would also lean toward it being in the coil pack

 

[Dirt Devil]

I forgot to add that I have replaced the coil pack the same time the plugs and wires were changed.

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[RonD]

Inspect the 4 wire connector on the coil pack, the end red wire supplies the 12volt when key is on
The other 3 wires are the grounds for each coil in the pack, center one will be for the 2/6 coil.

Long shot but worth a look.
Ford move the PCM(engine computer) to the upper center of the firewall, good location except it can get water intrusion, lol, they added a cover for it in later years.

Disconnect battery
Remove the PCM connector
Pin numbering is here: http://mechdb.com/images/0/0e/Ford_EEC5_pinout.png

Pin use is here: http://s1006.photobucket.com/user/dickbragg/media/EEC Pinouts/1997BirdECMpinout2of2.gif.html

But go by wire color as well
You will see pin 99 is for fuel injector #6, test that wire, between PCM and injector, and move wire around while testing, making sure it is OK

Pin 52 is Coil Driver #2, test that wire as well

Look for corrosion in connector or loose pins

Long shot as said, but running out of things to look at

Could be internal PCM issue, not sure if that can be tested

But...............you could get an old time timing light and connect it to #6 spark plug wire
Start engine and watch strobe flashing, strobe flashes at each spark pulse.

Increase RPM to the "bad spot" and see if strobe becomes erratic so spark plug is not getting correct spark timing at that RPM
If that happens then it would indicate bad coil in pack, bad wire to pcm or bad pcm

There is a similar test you can do for the #6 fuel injector
You need a Noid Light, it attaches between #6 injector and its wire connector
Similar to strobe setup it flashes at each injector pulse
Raise RPMs to bad spot and see if pulse becomes erratic


Another long shot is the Fuel Rail's Pulse Damper
When Ford increased fuel pressure to 60-70psi with no Return there could be a problem with pressure waves building up in fuel rail as injectors open and close.
When an injector closes there is a recoil wave when flow suddenly stops, these waves could multiple and re-enforce themselves when reflected back at the end of the fuel rail, so 65psi at an injector might jump up to 80psi or drop to 50psi depending wave status above it when it opened.
So there is a Pulse damper at the end of the fuel rail, basically just a rubber diaphragm that absorbs pressure waves so they can't multiply.
It will have a vacuum hose attached for safety reasons, in case it leaks fuel it won't drip down on exhaust or ??

Now say this is bad, not even sure they can go bad, lol, leak yes, but not sure about "bad".
And at 1,000RPM a pressure wave valley is hitting #6 injector causing low flow, so misfire but doesn't effect it at any other RPM

It's a stretch

 

[Dirt Devil]

Thank for your in depth input! Itll be another few days before i can dig into it but ill try my hand at all of it. Update in the near future

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[Dirt Devil]

So testing the harness pins didnt show me anything out of the ordinary. I replaced the maf with one from a u pull and the mis went away in the cold morning. Came back after the day became warm.

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