Misfire P0306

[deneggo]

Hi everybody, I'm new to this forum, and I am desperate! I would appreciate any help in solving this mysterious misfire in cylinder 6. I have a 1996 4.0 pushrod V6 Explorer Sport with manual transmission, best car I ever had. It had developed this misfire a few months ago and I've been chasing it ever since. When starting cold - runs great, then warms up and begins to shake. On the freeway runs great, no loss of power, no weird noises or smells. Once I exit the freeway and slow dow and stop - misfire code comes up on the OBD II reader. Fuel consumption has increased. I'm not a mechanic, but had to learn a few things. I understand that a misfire can have 3 primary causes: spark, fuel/air, and compression issues. Here's what I've done so far, please let me know what I'm missing:

New coil pack, wires, spark plugs. Checked it with the ohm meter, plugged the inline ignition spark checker - all ok. When the engine is on I unplugged the wires from the coil pack one by one - all make a noticeable difference except #6 . Swapped the spark plugs - no change.

Compression test: 1-5 140 PSI, 6 - 135 PSI. Didn't do the wet test to check the rings, but not burning any oil so it's probably ok.

Swapped fuel injectors - same, checked the electrical impulse with noid light - ok.
Swapped the computer - same thing. Used PCM from the same year/model but auto transmission - got a bunch of codes related to transmission issues as well as the P0306. So I figured it's not the PCM, noid light confirms.

Changed all the sensors: O2, Mass Air Flow, Crank/Camshaft position sensors, Engine Coolant Temp, Intake Air Temp, Throttle Position sensor, EGR backpressure sensor, Idle Air Control valve, EGR vacuum solenoid, and EGR valve. There is no knock sensor or I would have changed it too.

Changed all the gaskets: Upper and lower intake manifold/plenum, head gaskets. While at it, I swapped the hydraulic lifters, reseated the valves, changed the valve guide oil seals.

Changed all the vacuum hoses, checked for vacuum leaks with propane torch - nothing. Anyways, the misfire is specific to cylinder #6 , so I don't think its a vacuum leak.

Fuel pressure test - unable to perform due to the stripped schrader valve, but when pocking at it - the fuel squirts right out. Besides, the fuel pump and the fuel pressure valve have been replaced. The mechanic who replaced it could not tell me why my car had trouble starting on a hill, facing up or down. Maybe there is a clue here. The fuel supply line is right next to the fuel injector #6 - could it have something to do with it?

Checked inside the fuel rail - clean. Checked the flexible fuel hoses - clean.

What else? I poured a gallon of lacquer thinner into the fuel tank, maybe the cat converter was clogged - burned through it and no change. Poured a can of seafoam into the tank too - no change. I don't think its the cat because there is no rotten egg smell or loss of power at high speeds.

Possible clue: 5th gear went out for a while, so I was driving on fwys on 4th, maybe it did something to the car? Repaired the transmission and drove fine for some time before the check engine light came on and the misfire started.

I believe that's it. Any ideas? I'm hoping for any clue. After having done all this work I am very curious as to what could be causing this misfire. One last idea is a worn out cam lobe. I am not willing to take out the engine - I don't have those kinds of tools or the expertise, and besides, my wife will probably leave me. Please help!

 

[BrooklynBay]

Welcome to this forum! Which brand did you use for the plugs & cables? Did you check the spark gap of the plugs prior to installation?

 

[deneggo]

I used Autolite Platinum for the spark plugs and I have no idea what kind of cables I got. They were installed by the transmission shop less than a year ago upon my request. I'm totally new to the world of auto repair and did not think about these things right away. After having studied the forums I would do it differently. But since I swapped the spark plugs between cylinders and checked the resistance on the cables with ohm meter and the inline ignition spark checker I didn't think much of it. Do you think I should look into it still? I totally would if you think there is a reason...

 

[BrooklynBay]

The two tools in this link are good for comparing spark voltage in each cable: http://www.explorerforum.com/forums/showpost.php?p=3009013&postcount=16

 

[Turdle]

Wow. You seem to have covered all the bases.

Man, something in my gut says the fuel injector wiring harness has a bad connection.

Both sides have 12v until fuel pulse time, when one side switches low (0v)

If there happened to be bare wire touching in that injector wiring it may cause the 0v side to stay high, might be induced by vibration ( bump in the road)

 

[RomeovilleIL]

Try swapping the plug wire with another to see if the misfire moves. Even though it may be testing ok, there may be an issue in the boot allowing the spark to jump to the block and not fire the plug.

 

[deneggo]

Ok, thank you for your suggestions. I inspected the fuel injector wiring harness - no evidence of bare wires. I got a new set of spark plugs and I gapped them 0.054-inch (Autolite APP103, equivalent to Motorcraft AGSF-22PP as recommended by the Chilton manual). I also got a set of brand new wires (Duralast). I made sure to apply the dielectric grease too. The most amazing thing happened: the engine runs terribly, and there is a loud ping noise coming from under the car, specifically from the catalytic converter. The engine shakes as if there are multiple misfires, and the noise from the cat converter sounds like it's clogged up. After turning off the engine the cat continues to make this weird noise as if small pebbles or sand are being sprinkled on sheet metal... Goes on for minutes after the shut off. However, after I unscrewed both O2 sensors between the exhaust manifold and the cat converter nothing changed, except that it became loud like an old tractor. OBD II reader gives no codes (although I haven't driven it, just idling on the driveway). I'm thinking that a lot of air must be trapped in the fuel system since I had taken off the the fuel rail and the flexible hoses to clean. I've been reading about purging the air out, I tried turning the ignition key to let the fuel pump do its thing for 20 seconds and repeating it 8-10 times. Pressed the Schrader valve, fuel burst out. Still, same thing. What is happening? Is it a bad cat converter? Why didn't it behave this bad before I put in the new wires and spark plugs? Do I need to keep purging the air? Can air bubbles do this? Or do I need an exorcist?

 

[lobo411]

The most amazing thing happened: the engine runs terribly, and there is a loud ping noise coming from under the car, specifically from the catalytic converter. The engine shakes as if there are multiple misfires, and the noise from the cat converter sounds like it's clogged up.

That sounds like you messed up the firing order. Double check that you have the plugs and coils wired correctly. It's not at all logical, and half the time when I mess with the plug wiring I end up with exactly the situation you describe (running horribly, backfires, smells like gas, no codes) until I fix the wiring.

 

[deneggo]

You're absolutely right, Lobo411, I did mess up the firing order. I corrected the problem, all those issues I just mentioned went away. And that's the good news. The bad news is that after I went for a test drive and got off the freeway - P0306 pending code came back on. So, I'm back to square one, only now I know for sure it's not the wires or the spark plugs.

Here's what I'm thinking: if the problem is specific to the cylinder 6, then it cannot be a systemic issue that would affect all the cylinders, such as vacuum leak, or fuel delivery. It has to be something that is particular to that one cylinder. We ruled out the spark and the compression. We ruled out the fuel injector (I swapped them between cylinders). It's probably safe to rule out the valves, the push rods and the lifters - all have been reworked and rechecked. What's left? Weak springs?(I never measured them, they looked ok to the naked eye) Wiring harness to the fuel injector? (noid light ruled it out too...) Also, the code is pending (it was pending for the most part, became full code with CEL after I replaced the lifters, now back to pending, no CEL), and "pending" means it's not happening consistently. Cam lobe? It would be consistent and worse at high speeds. Is this logical? What am I missing?

 

[Turdle]

What is left is the wiring to that injector-or possibly not quite enough fuel pressure-flow.

A cheap , round dial type air pressure gauge can be used to read fuel pressure BTW. With engine running just push it onto the valve, you might see some fuel leak out when you remove it but you should be safe. I have performed this quick check often.


Do you have a vacuum gauge? if you did you could check for "pulsing" vacuum which would indicate a valvetrain issue.

 

[Turdle]

Quoted from a "cylinder 4&6 misfire thread

looks like it was the injectors...took my dmm and got a reading of between 13.9-14.8 on all the injectors except #4 and #6 which were like 8.2 ohms and 6.4 ohms respectively. went to the JY pulled two that were at 14.1 ohms and will be installing them tomorrow along with the new PCV valve, new plugs (because why not at this point), new upper and lower intake manifold gaskets and we'll see where that gets me

I know you stated you swapped injectors, but this does deserve looking into.

 

[torqlox]

I know this is a 4-month old thread but I'm curious if this was ever solved. I too have a 00" 5-Speed Sport OHV with a diabolical P0306 along with a P0171 that just will not go away. Thanks for providing us with updates.

 

[torqlox]

Nothing at all???

 

[Ruano]

Hi, I have the same problem P0306, tomorrow I will try with all the advice I read from this thread. Any Other advice?