98 2.5 misfire issue. I'm lost

[Joey_deluca]

Ok so here's the deal. I have a 1998 ranger 2.5. Has 107k on it and I have a misfire issue. It was running great then started misfiring really bad out of nowhere but would clear up and act like nothing was wrong. Then come back and misfire at a red light, but when I go it would clear up around 20-30 mph. Now I thought this would be a simple issue and have exhausted all of my knowledge and many others to try and fix it. Here's what tests have been done and what was changed.

Compression test - returns good at 175 psi
Leak down test - results good less than 10% drop
New wires - ohms within range
Another new set of wires - again within range
New plugs - gapped and recheck multiple times
Checked coils - ohms within spec and firing on all but cylinder 1 on rear most coil pack
Swapped coil pack - nothing changed
Swapped injectors - nothing changed
Noid light on all injectors - pulsing constant
Smoke test for vacuum leak - no leaks

Now I'm lost, I ran out of ideas on what to test next. I may be forgetting something that I did as another test but I'm open to any ideas. If someone can give me a clue as to what it could be. Thank you

 

[Mark_88]

Check the codes...there has got to be codes...even though no CEL has come on there is probably something sitting on the computer.

This issue has come up a few times over the years...for a few different reasons...like MAF sensor is out of spec or O2 sensor...could be fuel pump is wonky...but don't change any more parts until you've at least read the codes.

 

[Joey_deluca]

I knew I was forgetting things to post. I am getting the code P0304 (cylinder 4 misfire) and thats it. No other codes pending no other codes came up since misfire started. The fuel pressure is sitting at a solid 65 psi at the fuel rail and stays constant through driving at 64-65 no dropping or bouncing. O2 sensor was replaced around 1000-2000 miles before misfire started.

 

[RonD]

Spark plug can do that.

O2 sensor isn't used at idle, computer sets a rich mix per factory and long term fuel trim data.

Does it misfire on cold start idle, 1,100rpm?
or only after warm up and at idle, 750rpm?


Yes see if you can get a code that will ID a specific cylinder, P030X, X being the misfiring cylinder.
If you get two cylinder misfire codes then swapping injectors may have IDed the problem but unfortunately you won't know which was first and second.


EDIT:
Pull spark plugs on #4 and #3 see if they tell you lean or rich for #4
Idle has a high vacuum, if there is a bad valve guide seal you can get extra oil being sucked in, fouling the idle air/fuel mix in #4
Spark plug should show that.

After warm up unplug IAC Valve, idle should drop to 500rpm, or engine may even stall, if idle stays higher than 500rpms then there is a vacuum leak, probably by #4 intake port

 

[Joey_deluca]

Spark plug can do that.

O2 sensor isn't used at idle, computer sets a rich mix per factory and long term fuel trim data.

Does it misfire on cold start idle, 1,100rpm?
or only after warm up and at idle, 750rpm?


Yes see if you can get a code that will ID a specific cylinder, P030X, X being the misfiring cylinder.
If you get two cylinder misfire codes then swapping injectors may have IDed the problem but unfortunately you won't know which was first and second

It misses at warm idle and when a load is first put on the engine (taking off from a light) also when I swapped injectors it stayed a misfire in cylinder 4. No new code so the miss didn't move with the injector. I have driven on it to see if a pending code would pop up but no such luck.


Edit:
I pulled #4 and #2, and #4 has a darker look to it than number 2 which is very light brownish hue to it. Also when I unplug the iac at warm idle it is on the verge of stalling out.

 

[RonD]

IAC Valve test pretty much rules out vacuum leak.

Darker color on #4 spark plug means richer mix or oil, but..........misfires cause the darker color as well.

Quick test for fuel issue(wiring not injector) would be the Clear Flooded engine test.
With engine warmed up
Turn on the key
Press gas pedal to the floor, and hold it down(this turns off fuel injectors but spark stays on)
Crank engine, try to start it
It should not fire at all, just crank
If it fires intermittently then it is getting fuel from somewhere

No misfires when cold is puzzling, computer runs a richer mix and higher idle(choke mode) when engine is cold.
So the misfire only comes as fuel mix is leaned out and rpms are lower.
Which would point to vacuum leak near #4, i.e. cold it gets enough fuel, warm mix with added air causes lean misfire.

 

[Joey_deluca]

IAC Valve test pretty much rules out vacuum leak.

Darker color on #4 spark plug means richer mix or oil, but..........misfires cause the darker color as well.

Quick test for fuel issue(wiring not injector) would be the Clear Flooded engine test.
With engine warmed up
Turn on the key
Press gas pedal to the floor, and hold it down(this turns off fuel injectors but spark stays on)
Crank engine, try to start it
It should not fire at all, just crank
If it fires intermittently then it is getting fuel from somewhere

No misfires when cold is puzzling, computer runs a richer mix and higher idle(choke mode) when engine is cold.
So the misfire only comes as fuel mix is leaned out and rpms are lower.
Which would point to vacuum leak near #4, i.e. cold it gets enough fuel, warm mix with added air causes lean misfire.

I actually started it today cold start and it misfired right when I start it, I guess I may have never noticed it before because of the higher rpm not shaking the truck as bad. So it's a miss at all temp range but I guess just more noticeable at lower rpm.

Also it doesn't look like oil fouling on the plug, it just looks rich from the miss, it's not sooty or ashy it's just a darker brown no deposits at all but a little glossy look to it.

 

[RonD]

One other thing since we are looking at non-obvious reasons, and it is a long shot
When Ford raised the fuel pressure to above 60psi they(and others) run into the problem of pressure waves forming in the fuel rail, as an injector opens and closes it creates a pulse wave in the rail, with several injectors doing it the waves can reflect and get in sync which can cause low pressure or high pressure at an injector port when it opens, so less fuel or more fuel will flow into intake.
A Pulse Damper was added to the end of the fuel rail to prevent this, it is basically a rubber diaphragm that absorbs the waves without reflecting them.
It has a vacuum hose attached for safety reasons, if it leaks fuel will be sucked into intake not start a fire.
Don't know how you would test this?

You might see a slight fuel pressure pulse at idle, but not sure.

For spark:
Disconnect intake side coil pack, the exhaust side coil pack is used for starting engine, intake side starts working above 500 rpms.
Hook up an old style timing light to #4 spark plug, exhaust side
Start engine and see if spark pulse is missing when engine does.

ICM(ignition control module) is in the computer(PCM) so not much you can check on it

Chances of you have 2 coil packs that are marginal for #4 are close to 0 but not 0, lol, so...........

 

[Joey_deluca]

I thought the dual spark system was used where the front coil (one going to the intake side of the engine) was fired on the exhaust stroke of the cylinder, and the back coil was used on the compression stroke. Is this not true? Could you explain the dual coil pack system to me? Because I may be fighting an uphill battle if I have the wrong information about what fires when.

 

[RonD]

No, it is not true, but it is popular, have read it before

Exhaust side coil pack is the only one used when starter motor is turning.
Intake side coil starts working after engine starts.
There are only 2 coils inside each coil pack.
The 2 spark plugs on each coil are wired in series for the "matched cylinders"
1 and 4 are a matched pair
2 and 3 are the other matched pair

Match pair means both pistons are at TDC at the same time, one on power stroke the other just finishing exhaust stroke, this balances the engine and crank.

The waste spark is because of the series wiring, so when computer fires #1 spark plug, #4 spark plug also fires, and when computer fires #4, #1 also fires.
Same for #2 and #3
So 1 spark is wasted on the exhaust stroke of its matched pair

This is the same for both coil packs.

The series wiring is why you should use double platinum spark plugs.
One spark plug on a coil sparks from center to tip, the other sparks from tip to center, so both surfaces need the platinum.
There was another popular "myth" of swapping spark plug wires around so coil packs matched each other, wouldn't matter, if it did make a difference then I would check the spark plugs

Only sparking on exhaust stroke:
Ford added the dual spark plugs to get lower emissions, a more complete burn, because of the waste spark system both spark plugs fire on power and exhaust strokes, no spark plug just fires on exhaust stroke, because its matched pair would be on power stroke, there is no way for them to do that with waste spark.