Hi everybody, I'm totally stumped as I have done many things to try to remedy this dreaded code. After searching the vast interwebs for solutions it seems many others have not been able to fix this or they never posted their fix. When I drive the freeway I can feel the truck shudder and misfire when im in 4th and 5th gear then the light will come on after about 80 miles from reseting the ECU. Occasionally the CEL will flash which is alarming because I heard this is very damaging to the cats. Street driving is fine. Rock solid idle and I get about 15mpg( body lift, suspension lift, light bar, tubular bumper, and 33x12.50 tires)

Before this code it was throwing a CEL for EGR valve something or other. So I changed out the EGR solenoid, and DPFE sensor.

Other parts I have changed recently include:
Spark Plugs (NGK)
Wires (NGK)
New Coil
PCV Valve
fuel filter

Compression check has each cylinder coming out between 150-160psi.

I do notice im losing quite a bit of oil (1 qt every ~125miles). There is a tiny drop collecting at the bellhousing but its not puddling at all in the street. Also during the compression test I noticed there was some oil on the threads of spark plug #1. Not sure if thats relevant though.

I want to do a leak down test and see whats going on with the head but I dont have access to a air compressor. I'm debating on whether or not to take the intake off and switch the injectors from #6 to another one and see if the misfire moves to another cylinder.


Any suggestions would be greatly appreciated as this is my daily to get to school and Im currently unemployed so I cant afford a tech to check it out. Thank you.

It could be an injector causing your problem but you should be able to tell by looking at the spark plugs. Also I would switch my spark plugs back to either Motorcraft or autolites.

__________________
1999 Ranger 4.0 and extreme Snap-on tool fetish

Beenusing NGK wires and plugs exclusively since ownership, 5 years now. Better low end performance, and smoother especially near the top end of the RPM band in all gears. There wires, lowest resistance of any found for my OHV. FAR better than stock which admittedly would be my 2nd choice over anything else.

Causes
"A code P0306 may mean that one or more of the following has happened:
- Defective ignition coil(s)
- Bad spark plug(s), spark plug wires, spark plug boots
- Faulty fuel injector(s)
- Malfunctioning fuel delivery system (fuel pump, fuel pump relay, fuel injectors, or fuel filter)
- Major engine vacuum leak
- EGR valve stuck in the wide open position
- Clogged EGR ports

Diagnosis & Repair Steps
A diagnostic scanner, digital volt/ohmmeter (DVOM), and a reliable vehicle information source will be required to diagnose a stored (or pending) code P0306. Begin your diagnosis with a visual inspection of the affected ignition coil, spark plug, and spark plug boot Components that are contaminated with liquid (oil, engine coolant, or water) must be cleaned or replaced If the recommended maintenance interval demands (all) spark plug replacement, this is a good time to do it Inspect primary wiring and connectors, for the ignition coil in question, and make repairs as necessary Key-on-engine-running (KOER), listen for the presence of a large vacuum leak and make repairs if necessary If lean exhaust codes or fuel supply codes accompany the misfire code, they should be diagnosed and repaired first All EGR valve position codes should be rectified prior to diagnosing a misfire code Insufficient EGR flow codes should be addressed before diagnosing this code

Read more at: https://www.obd-codes.com/p0306
Copyright OBD-Codes.com

I think Swifty is close with a faulty/clogged injector. Your oil leak could mean a head gasket issue which would be verified by a leakdown test of course allowing air into the cylinder? Is the oil getting into the wire boot itself causing an issue? Did you check that #6 plug for damage and the GAP?

.

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I figured a blown headgasket would show up with low compression during my test? Someone on the other ranger forum suggested I run this procedure posted below. Im busy with school until the weekend but Ill report what I get this saturday.

"Yes, flashing CEL means bad mis-fire in progress, and computer would shut down #6 injector at that time to prevent unburned fuel from going out exhaust and into Cats

If you have tried a new spark plug in #6 and check firing order on coil pack
3 4
2 6
1 5
Front

5 6 4 side gets me every time, lol

Then I would test injector, not hard to do
All fuel injection computers have a Clear Flooded Engine routine
Ford(and most of the others) use this to enter Clear Flooded Engine

Turn key on(RUN)
Press gas pedal down to the floor(hold it DOWN) all the way
Turn key to START
Engine will crank but should not start or even fire

Fuel injectors are shut off but spark is still on

If engine starts you could have a leaking #6 injector, or a stretched throttle cable, google: Ranger throttle cable mod

So engine didn't start, which is good
Now that we know fuel injectors can be shut off, lets see if they are all working
(just FYI, is use this Clear Flooded Engine every morning when I cold start my 4.0l with 350k on it, I get oil pumped up to the top then release gas pedal for it to start so I don't get a dry start)

Now disconnect coil pack's 4 wire connector, so no spark
Crank the engine a few times, but don't push down on gas pedal

Pull out #5 and #6 spark plugs, both should be equally WET with fuel, if #6 is dry or drier, then fuel injector is clogged up, replace it

reconnect coil and start engine
Turn engine off
Do same test, no spark, and use Clear Flooded engine
Pull out #5 and #6, both should be dry, if #6 is wet then you have a leaking injector


Your compression is good, leak down test won't tell you anything more
But the loss of oil and #6 misfire could mean bad valve guide seal
#6 is at the back of engine so #6 valve guides are usually covered with oil
Intake vacuum can suck the oil down the valve stem where it is sucked into the cylinder and ends up fouling the spark plug over time
What did the old #6 spark plug look like?

Old test for failing valve guides was to go down a hill in a lower gear using the engine as the brake to slow you down, this creates very high Vacuum in the intake, so will really suck in the oil if guides are failing, then at bottom of the hill or after at least a minute or two of engine braking, step on the gas and watch for tell tale puff of black/grey smoke out the tail pipe from burning oil in the cylinders
"