2003 Ranger misfire has me stumped

I'm working on my Grandpa's 2003 Ranger with a 3.0. Long story short, I seem to have a misfire on cylinder 5 and 6 at all RPMs. Compression, Spark, and Fuel appear to be good. I'm stumped.

Background info:
Grandpa was driving the truck when it overheated on him enough to lock up the engine. Found out the radiator had a massive leak. We got water in it asap and I was able to get it restarted (but running poorly) within 30 minutes. Then got it to my shop a mile down the road.

Several days later I started looking into it. Found very poor compression on 3 cylinders. Ended up pulling the heads to find both heads cracked. Cylinder bores and everything else looked fine. So I got a set of reman heads from NAPA and put on.

Fired it up a couple days ago and it started right up but ran very poorly. I thought I had the spark plug wire order wrong or something, but checked it 3 times and everything is right.

Troubleshooting I've done:
Tested compression on all cylinders and all cylinders are 175-180. Checked spark and had very weak/intermittent spark on several cylinders. Replaced coil and that made everything much better, but still missing. retested spark and now have great spark at all plugs (plugs are brand new as well).

Pulled plug wires at the plugs while idling and think i have it isolated to cylinder 5 and 6 (no change when pulling wires). Pulled upper intake and swapped injector 6 with injector 4 (thinking 4 was working fine). No change in symptoms (cylinder 4 still firing fine). Fuel pressure is steady at 63 when running and leaks back to 50's when off.

There is also a tapping noise coming from the drivers side valve cover. I pulled the valve cover back off and inspected the valve train. a couple rockers have some side to side slop but don't notice any up and down. push rods and lifters all appear fine. Turn over engine by hand and all the valves seem to do what they are supposed to and have the appropriate travel.

Check Engine light is off through the whole process and there are no pending codes.

I'm suspicious I have a bad upstream o2 sensor on bank 2, but don't see how that would cause a misfire on cylinder 5 and 6, while cylinder 4 is fine.

Sorry for writing a book, but I'm stumped and wanted to give as much information as possible. Anything else to check?

Are you sure 5 & 6 plug wires are OK?

Any chance there is coolant leaking into 5 & 6?

Rearanger said:

Are you sure 5 & 6 plug wires are OK?

Any chance there is coolant leaking into 5 & 6?

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I've got great spark at the plugs on 5 and 6. Have also tried a different plug wire on them and it made no difference.

As far as coolant leaking into 5 and 6, I suppose anything is possible. My thoughts are that the good compression numbers rule that out though. Is there anyway I can get coolant into cylinders but still have good compression? Anyway coolant can get into a cylinder through the intake (and only affect 2 cylinders)?

I pulled the plugs from bank 2 a little while ago. cylinder 4 is good and dry. cylinder 5 and 6 are damp and smell like gas. retested spark on those 2 cylinders hooking the plugs back up to the appropriate wire and setting them on the exhaust manifold. Again I had great spark.

I'm down now to thinking it has to be some form of timing issue. Can timing be off somehow that would allow cylinders 1-4 to fire ok and 5 and 6 to miss?

Ok you could be getting good compression on both bit I have had band new and reman heads have bad valves not matting.

The only way to check that is a leak down test.

Check your o2 sensor for that bank see if it's running rich or lean.

Rich not getting good burn

Lean could have a leaking intake gasket.

Or make sure injector is getting power use a node test light on it.

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Double check Coil pack firing order and spark plug wires at cylinder end

Ford uses Waste Spark, inside the coil pack are only 3 coils, each coil fires TWO spark plugs at the same time, 1 spark is "wasted" hence the name "waste spark system"
(this was the very first spark system ever used and your lawn mower still uses it, any engine that uses a magneto uses it)

So since 2 spark plugs spark at the same time they need to be Matched Sets on the coil pack, or you will get misfires.
The match sets are
3 4
2 6
1 5

the 1 2 3 on one side often causes people to do the other side as 4 5 6, which of course would cause misfires, its 5 6 4

Match pairs can be reversed
4 3
2 6
5 1
For example
because both spark at the same time that doesn't matter

But it does matter which coil in the pack each one is on
center coil will always be 2 6 or 6 2
But each end coil will matter, and this usually can be determined by which side the 4 wire connector is on.

Image here: https://static.cargurus.com/images/site/2014/11/23/12/47/pic-8941394052778031213-1600x1200.jpeg

But there are coil packs that reverse 3/4, 1/5 positions as well even with connector in the same place.

But you would really notice if that was happening, more likely a no start that a misfire

But check the 5 and 6 wires from plug to coil pack, then check them again, then have someone else check them, lol

Checked spark plug wire order on plugs and at coil multiple times and the order is correct.

performed a leak down test on cylinders 5 and 6 and have 4-5% leakage (100 psi to 95-96 psi) That sounds good to me.

When I start the truck it switches to closed loop almost immediately. My short term fuel trims are both high, but not pegged (indicating a lean condition?)
22% on bank 1
15% on bank 2

after running for a few minutes those numbers dropped a little bit to:
16.3% on bank 1
9.5% on bank 2

Long term fuel trims are still at 0. I haven't been able to run it very long yet as I haven't had a chance to get it outside.

I think I've got a bad upstream 02 sensor on bank 2. It's voltage is steady at 0.05v. Bank 1 is switching between around .3v and .75v. Downstream is switching between 0.1v and 0.65v. Even if I had a bad 02 sensor, would it be able to cause a misfire condition on 2 cylinders?

I'm struggling with both fuel trims being positive double digits. Bank 1 seems to run ok and even if I had a vacuum leak, wouldn't that cause a stumbling engine and not a steady misfire? I tried checking for vacuum leaks by spraying carb cleaner around everything. I couldn't detect any change at all.

I tried running it with all the lights in the shop off and looking closely for any spark jumping around. When I tested for spark, I had the wires out of their normal position. So i was thinking, maybe with everything tucked back where it goes I had a couple bad wires arcing. I couldn't see any sign of a problem, but I'm about ready to go get a new set of wires just to rule that out.

Another way to check for vacuum leaks is to unplug the IAC while the engine is running. It should either die, or switch to a very low (like 500 rpm) idle. If it doesn't do one or the other, there is a vacuum leak somewhere. Mine switches to a very low idle, almost dies.

I had tried that earlier. The truck dies instantly when I unplug the IAC

Ok, New plug wires made no difference. But I finally got it outside today and as soon as I put it in gear I stumbled on a huge piece of the puzzle.

Put my foot on the brake pedal and I had no power brakes (should have checked that or put a vacuum gauge on it much sooner). So obviously I have a big vacuum leak. I've tried propane and carb cleaner around the top of the engine with no luck finding a leak earlier.

Given the circumstances, I'm going to assume that I've got a lower intake manifold gasket problem. Is there any other way I could test that out before tearing it back down? Anything else that could cause these symptoms that I haven't thought of?

mdnelson86 said:

So obviously I have a big vacuum leak.

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I think with a vac leak that big you'd get no idle. As well your LTFTs would not be zero. As well you state STFTs are double digit, but they are not static numbers, but you have no codes.

mdnelson86 said:

Ok, New plug wires made no difference. But I finally got it outside today and as soon as I put it in gear I stumbled on a huge piece of the puzzle.

Put my foot on the brake pedal and I had no power brakes (should have checked that or put a vacuum gauge on it much sooner). So obviously I have a big vacuum leak. I've tried propane and carb cleaner around the top of the engine with no luck finding a leak earlier.

Given the circumstances, I'm going to assume that I've got a lower intake manifold gasket problem. Is there any other way I could test that out before tearing it back down? Anything else that could cause these symptoms that I haven't thought of?

Click to expand...

What if the vacuum leak IS the power brake booster, check it first

Rearanger said:

I think with a vac leak that big you'd get no idle. As well your LTFTs would not be zero. As well you state STFTs are double digit, but they are not static numbers, but you have no codes.

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I'm not sure what to think. It barely runs and is obviously misfiring, yet throws no codes. I still can't understand how the short term fuel trims can stay in upper double digits and long term trims are still at 0.

I'm about to pull my hair out on this project.

mdnelson86 said:

I still can't understand how the short term fuel trims can stay in upper double digits and long term trims are still at 0.

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How are you reading STFTs? Graphing program or just digital output?

Rearanger said:

How are you reading STFTs? Graphing program or just digital output?

Click to expand...

At the time of my earlier post I was using a simple scanner that just gave a number value. I just got a better unit that does graphing.

Short term Fuel trims start off around that mid 20% range at idle. Engine at 2500 rpm they both drop at first but then climb back up to about the same range.

After running the engine for a while at various speeds my short fuel trims changed a lot. Bank 1 settled on a range from 8-18% and bank 2 switched to the negative side at -16 to -9%

Long term trims are changing, but they've stayed within 1.5% of 0.

I think I'm more confused now than ever.

STFTs should fluctuate from rich to lean. This is a function for CAT efficiency established by the ECU program.

Something is wrong if you don't get peaks (rich) and valleys (lean) for STFT.