Misfire on 4 and 5

Hey guys,

I'm trying to help my dad diagnose his 2003 Ranger EDGE 3.0L 5-speed. He's the original owner, and it has 251,000 miles on it. He takes very good garage-kept care of all of his vehicles and this truck is no exception. It'd be very easy to assume this truck was much newer based on its looks and *usual* running condition.

The truck has progressively developed a stumble which seems to be more pronounced at or near idle and during acceleration at lower RPMs (it smooths out a lot once the RPMs get above 2000). It seems to run nice and smooth at highway speeds, but loss of power and stumbling is noticeable while trying to accelerate at higher (~60+mph) speeds.

CEL scanner says misfire at startup (P0316) and on cylinders 4 and 5 (P0300, P0304, P0305). To verify the misfire is isolated to cyls 4 and 5, I have cleared the codes and sure enough they came back the exact same codes. The CEL light occasionally flashes while driving - even while cruise control is set on the highway.

My dad's "trusty" mechanic has replaced the plugs, wires, and ignition coil, and hasn't been able to isolate the source of the problem.

I have replaced the IAC, TPS, and even the camshaft synchronizer. No change. I have also verified the order of the plug wires. Some background on my experience: I am by no means a mechanic myself, but have performed many successful vehicle-repair-related jobs such as transmission repair, transmission replacement, cylinder head replacement, and the like (I'm not afraid to delve into big projects after I've done extensive preparation and have factory service manuals available - I like to do things "by the book" and don't like to tighten any bolt without a torque wrench lol)

A couple other weird possibly-not-related nuances: speedometer needle "jumps" at idle; oil pressure gauge needle "jumps" at idle; and probably most relevant: I've recently noticed the throttle hangs intermittently. The throttle hang became most noticeable recently when I drove it thru a drive-thru fast food establishment. I would creep forward in first gear, but when I would release the accelerator and engage the clutch, the engine speed would creep up to 3000RPM. It would correct itself back down to the normal ~700RPM after a few seconds, and tapping the accelerator *seemed* to assist it back down (but not sure that had anything to do with it).

My planned next steps are to test compression on 4 and 5 (the mechanic supposedly did this and didn't report any negative results), and to test fuel pressure. I doubt pressure at the rail will be the problem though, because the misfire is only happening on 4 and 5.

Anything special and/or related to cyls 4 and 5 that I could look for? They're right next to each other, and I'd not like to assume the worst, lol, if ya know what I mean. Any and all [constructive] help is appreciated! ;-)

place a vacuum gauge on the intake someplace. determine if its steady or pulsing.
this is just gathering more useful info, no diagnosis yet.

well, maybe: about the needles jumping around, when symptoms are present turn the fan on hi, and headlights on. do the lights flicker with the needles jumping?

Yes, do a compression test and look to see if 4 and 5 are different (lower). It's possible that you have a head gasket failure between cylinders. Remember to have all the plugs out when you do the test.

It could also be iffy injectors and those two just happened to fail at the same time.

Make sure you don't have any vacuum leaks. Look for small hidden cracks in the intake snorkel; they sometimes will fail in an intermittent way.

Finally, make sure the tone ring on the crank pulley isn't damaged. Not likely, but another cause of a specific misfire that you haven't covered.

As for the gauges, that's an electrical issue, perhaps a sketchy voltage regulator? Your oil needle, if stock, is switch driven and shouldn't move once running. If it's modded (like mine) then it will move, and at least in my case it's very twitchy like that. Either way, if the other gauges are jumping too, either you have an odd electrical loading issue or your power supply (Alternator /voltage regulator) is malfunctioning. Or perhaps you have a bad chassis-to-block ground and the engine moving under load is making it worse.

Edit- Do check the grounds, when they go bad they will cause all sorts of trouble, from iffy gauges to odd CELs to "random" misfires.