94 4.0 misfire under load

Brad474747 · May 10, 2016

I've got a misfire from my 4.0 when it's under load. we haven't been able to determine what cylinder. It idles just fine but I've noticed it happens more at operating temperature, and they seem to be much rougher at operating temperature too.
We've checked fuel pressure and it was within spec, it didn't change at all when these misfires happened
We checked plugs and replaced one that was mismatched (1 autolite and 5 motorcraft) the autolite was improperly gapped, the rest looked fine. (they have about 4000 miles on them as well)
Wires appear to be fine, I'm going to test the resistance tomorrow. they only have about 4000 miles on them but I'm being pushed to replace them under warranty and to do the plugs too.
Is there anything else that I may be overlooking?

RonD · May 10, 2016

I would run a can of Seafoam, or similar, in the gas tank to clean out injectors.
If an injector has lower flow it can cause misfires under load, but won't effect idle.

Coil pack can be tested with OHM meter as well, it can't tell if coil is good, it can only tell you if one coil in the pack is bad.
And since you don't get the misfires at idle I don't think testing it will show a problem, but it is free to test

Each of the 3 primary's should test as .6 to 1.3 OHMs
But all 3 should be very close to the same

Same with secondary's, 7,000-11,000 OHMs, all fairly close to the same.

If you pull out all the spark plugs look at the tip of each plug, in your description blackish color means spark issue, if all plugs look the same then fuel mix is the problem.

Blackish color can also mean overly rich mix, and if all spark plugs show a bit of color then time to clean the MAF sensor and replace O2 sensors.
O2 sensors are good for 150k miles, after that they tend to cause Richer fuel mix.

O2 sensors are not used until engine warms up.

Also have you noticed a drop in MPG, obviously misfires don't help MPG, lol, but dropping MPG over the last few months before the misfires started could indicate MAF/O2 issues starting.

30-40psi fuel pressure is important, holding fuel pressure after engine is shut off is also important, fuel pressure should hold above 20psi for a few months, but you can just test it for 10min.

to make sure there are no leaks at valves or injectors in the system.

Brad474747 · May 10, 2016

We didnt test fuel pressure after engine shut off, may do that tonight when I test the coil pack. I drove it to work today, when I started it cold the idle stumbled a little bit, but it didn't persist after it warmed up.
I haven't noticed a dip in mpg, and the plugs were a uniform brown color across all 6 cylinders
I'll update later with an actual fuel pressure number and coil pack results

Brad474747 · May 11, 2016

Okay I've tested fuel pressure again, it held at 30psi at idle, couldn't test under load cause I don't have an extra set of eyes right now, killed the engine and after about 13 minutes it dropped down to 12psi, just checked again as I'm writing, after probably 15 minutes total it's dropped down to 10psi

Couldn't find a how to on testing the coil pack but decided to test resistance on each plug wire

I was testing at 20k ohm setting on the multimeter
Cylinder 1 tests at 0.90
Cylinder 2 tests at 1.14
Cylinder 3 tests at 0.54
Cylinder 4 tests at 7.10
Cylinder 5 tests at 1.30
Cylinder 6 tests at 1.05

RonD · May 11, 2016

3,000 ohms per foot of wire is about right for "distributed resistance" spark plug wires
Yours look like 1,000 ohms per foot which is fine, but............

On 4.0l the coil pack is right above #3 so shortest wire and 540ohms

#4 would be longest wire, although that does look high at 7,100 ohms compared to the other wires, this one looks like a stock Ford wire.

There are 3 types of spark plug wires
Distributed resistance are used on most Fords and Chevys, they provide less radio interference for the least amount of money, these are 3,000-10,000 ohms per foot

MAG are used on japanese vehicles, more expensive, lower resistance, they use a "shield wire' to block radio interference.

And Resistor wires, they actually have a resistor in the spark plug boot, mostly european

It looks like you have a mix of wires, when looking at #4 wire.
Most important thing is that all wires are close to the same resistance per foot.

You can buy 500 ohm per foot wires, and no issues using them but they ALL need to be the same 500 per foot to get reliable spark, especially on Waste spark system

I would change the #4 wire, it is connected with #3 wire in series on the same coil in the coil pack, and electricity flows on the "path of least resistance" so #4 is probably missing under load since #3 is getting most of the voltage
#4 has over 10 times the resistance of #3

Coil pack test here: http://www.autozone.com/repairinfo/repairguide/repairGuideContent.jsp?pageId=0900c15280077cd2

Ford coil pack Primarys are fine up to 1.5 OHMs, as long as all 3 coils in the pack are close to the same

Brad474747 · May 12, 2016

Seafoam didn't fix the issue, neither did plug wires... I haven't been able to test the coil pack today cause it's pouring down rain.
Is there anything else this may be?

enjr44 · May 13, 2016

That fuel pressure seems ok at idle, though a little low. Leaking down in 15 minutes is bad. Should stay up there for a very long time. Something is leaking. Pressure regulator, injector or pump check valve. If the pressure drops during load you will get random missing.

94 4.0 misfire under load

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