Hello all,

I have a 1999 ranger 3.0 4x4 5 speed with 170,000 miles. It has a month old battery. Two weeks ago it recently started having engine issues. The 6th cylinder is miss firing. I changed the spark plugs and wires when the check engine light came on with this code. I still get a 6th cylinder miss fire check engine light. The truck starts right up, runs fine for about 10 minutes. Then as soon as my truck gets up to the warm running temp, the 6th cylinder starts to miss fire and have terrible acceleration and rough running. I thought that it might be a bad coolant temperature sensor, so I changed that. The truck ran fine for a day, then the next day it went back to miss firing after 10 minutes of running. If anyone has any ideas as to what might be wrong, please help! I didn't want to keep going out and buying parts and not getting the problem fixed. Thats why I am here. Thanks in advance.

 

Had this on my old 2002 3.0. It could be a bad valve.

 

Welcome to the forum.

I would think a bad valve would show itself cold or warm, but still a possibility.

A cylinder/engine needs 3 things to fire
Spark, at the right time
Fuel, in the right mix with air
Compression, to heat up the fuel enough for spark to ignite it

Compression is a mechanical operation, so doesn't tend to change, it can be tested first to either find that it is the problem or take it off the table as the problem.

Spark and fuel problems can be intermittent, so not easy to test.
Couple of free tests.
Ford uses a waste spark system, so two cylinders share 1 coil in the 3 coil pack.
2 and 6 share the same coil
very unlikely that #6 looses spark but not #2, but it could happen
So on the coil pack, swap #2 and #6 positions
3 4
2 6
1 5
front

Change to
3 4
6 2
1 5
front

3.0l also used
1 5
2 6
3 4
Front

But you get the idea, 3 and 4, 2 and 6, and 1 and 5 are the "Match sets" of cylinders that spark at the same time

If you start to get P0302, misfire #2, then coil pack is bad

Fuel test
Cold engine
Unplug the 4 wire connector on coil pack, you want a no start
Crank engine a couple of times, count to 3, slowly, each time, turn key OFF between cranks

Pull out spark plugs #4, #5, #6
Check for the amount of fuel on each, you are look for #6 having less fuel on it, showing weak or clogged injector.

When engine is running cold, computer runs a Rich mix, so a problem injector may be giving enough fuel to keep #6 firing.
As engine warms up computer changes to Leaner, more economical mix, if there is an injector problem then that's when it will show up.

A lower intake vacuum leak at #6 can show same running symptom, but long shot

You can test fuel injectors with ohm meter, Fords should test between 11-18 OHMs, that can show a bad injector, out of range, but doesn't say if injector is 100% working
Coils and injectors can both be tested with OHM meter but it can only show if one is bad outright, it can't test if it is "good' because static tests are not definitive with the components

 

Thanks for all the info!
Will it be ok to change the plug wires for my 13 mile trip home after work?
I will try the fuel test this weekend and take pictures of my results.
Also I drove about 40 miles this morning making a few stops. During the first 10 or so miles the truck was misfiring. The last 30 miles of driving the truck ran perfectly fine and the check engine light went off.

 

Yes, you can swap them anytime

So engine was misfiring cold?
Then OK when warm?

So now opposite of first post/symptoms?

 

Well I let it idle for a couple of minutes this morning in the drive way. It drove fine for a few minutes, then once it got warm it started to misfire again.

 

Also after I drove the first 10 or so miles I was at my first stop with my truck off for like 15 minutes. Then all driving after this the truck ran fine.

 

Hardest problem to track down is when it "ain't broke" just failing, lol.

Could be loose connection, I would swap spark plug wire positions for 2 and 6
And if possible pull off #6 fuel injector's electrical connection, make sure it looks "clean and dry" then put it back on, if you have an OHM Meter handy I would test injector, but if not just reconnect wires

 

I'll swap those tonight and see what happens. I will also test the electrical connection, I think my Dad has an OHM meter that I can use.

 

Since compression is either good or bad it wouldn't be intermittent, that can be taken off the table....for the most part

Spark or fuel is the culprit, you just have to narrow it down to one or the other