And manual transmission. 213,000 miles
Back history (very little):
test drove this truck about 3-4 weeks ago, ran fine.

My buddy brought me the truck on Saturday.
From the first time I drove it on Saturday, it ran rough. So much so that it threw a code within minutes.
P1131.
I contacted my buddy and he said it started running rough a couple miles from my house.

It belonged to his FIL who likely didn’t really service anything that wasn’t broke. None of that preventive 50,000 and 100,000 mile stuff.

I know the code is related to to the upstream O2 sensor.
My research indicates it could be a number of things.

It runs the same, like on 3 cyl, no matter the cold/warm state, or the speed.

So far I’ve pumped propane at all the vac lines I could identify and intake manifold joints, sprayed out the IAC and MAF.
I’ve also unplugged each coil and found it runs the same either way.
It had less than 1/4 tank of gas, which had been in there a while. I put about 6gal in it and a bottle of HEET.

One other thing, in case it’s related; the temp gage will barely come off the bottom line above the “C” on the gage.

I plan to check the MAF I/O voltage this evening.

*edited* Another thing im seeing, which is why I looked for vac leaks, is “hanging idle” when shifting, although I’m learning that some of that is expected.

Any additional insight you can give would be appreciated.

 

Welcome to the forum

Always good to post the code definition you are using with the code
P1131 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 1

Lean does not mean engine is running Lean, it means the O2 sensor is seeing too much oxygen in the exhaust on that bank, in the case of a 4cyl its only bank

From your description you have a misfire occurring
This code is more likely to be cause BY a misfire, that to cause a misfire

Since O2s can only see oxygen and when a cylinder misfires no oxygen is burned up so ends up in exhaust manifold, that may be the cause of the P1131 code

But............there should also be a misfire code, P030X, X being 1 thru 4 to ID the cylinder that misfiring on the engine, P0316 is a random misfire code which comes up if several cylinders are misfiring and computer can't ID a specific one

The dual spark plugs fire at the same time, I would continue driving it with one coil pack unplugged to see if you can get it to throw a P030X code
Then switch coil packs and see if it throws a different P030X
If it an injector issue then same cylinder would always misfire

Fords use Waste spark, so 2 spark plugs share one coil in a coil pack
Cylinder 1/4 are on 1 coil, and 2/3 are on the other coil on each coil pack
If a coil fails engine would barely run on one coil pack, only 2 cylinders would be firing

With on 1 coil pack in use, and using insulated pliers/gloves you could pull off 1 spark plug wire from the coil pack at a time to see if that spark plug makes a difference

Ultimately I think you will need to pull all 8 spark plugs and have a look at the tips to see what they tell you, before spending any money on other things


Previous owner may have put in a lower temp thermostat, 192deg is what Ford used, many people put in a 180deg thinking "cooler is better" but it actually costs them more money since best MPG occurs when coolant is above 190deg, which is why most car makers switches to 190deg t-stats back in the late 1970's
Temp gauge should be above 1/3 but under 1/2 after full warm up, 210deg is 1/2 on Fords
Unlikely to cause the misfires

 

thanks for that “unburned oxygen” from misfire info.
Makes perfect sense, particularly given how it runs.

I think ill pull the exhaust side plugs and have a look this evening and check compression while im at it, just to see overall health of the engine.
When I have more daylight, I’ll pull the intake side as well.

Another thing im seeing, which is why I looked for vac leaks, is “hanging idle” when shifting, although I’m learning that some of that is expected.

 

Yes, emission requirements for manual transmissions call for RPMs to hold high when shifting if speedometer is above 5MPH
If, while driving, you push in the clutch pedal and hold it down for 7 seconds or so, the RPMs should drop to about 1,100, that would mean the computer was holding RPMs high for emissions reasons
If RPM do not drop to 1,100 then could be another issue
Under 5MPH RPMs can go down to idle levels, in a 1999 4cyl that might be 750-800, with warmed up engine

After engine is warmed up and idling, you can unplug the IAC Valve, it will close and RPMs should drop to 500 or so, engine barely running, or engine may even stall, either is GOOD, it means no vacuum leaks

 

i did unplug IAC yesterday while idling and the idle dropped a little, bit I doubt it was down to 500. I’ll try it again when I get the plugs back in.

I pulled all the plugs and found one (#2)

to have a pretty huge gap and “black” vs other 3 were tan.

I also checked compression and found 3 cylinders at 135-140 and #4 at 125.

 

The compression is within 10% so OK, but it is on the low side, expected is 160psi or so, but could just be the gauge or crank speed(battery/starter)

Yes #2 would be the misfire, lol, looks more like oil than fuel fouling but hard to tell, so valve guide seal maybe
#4 looks like a different spark plug brand, so previous own may have swapped it out, also looks darker

As long as idle dropped when you unplugged the IAC Valve, that means computer was setting the idle at that RPM, so no vacuum leak
Some one may have adjusted the "anti-diesel screw" on the throttle linkage, it looks like an idle screw, but every one knows fuel injection can't use an idle screw, no idler Jets, lol
But nothing to worry about ..................YET

I would get the spark plugs sorted out first

Only use regular copper or DOUBLE Platinum, single platinum is a waste of money because you have a Waste Spark system

 

just got back from the store with some double platinum plugs.

I just checked my old plugs plugs and they ranged from .051” to .067”( the black plug)
ill check the gapbon these new plugs and run them in with some anti siege and call it a night.
I’ll check the other side next day or two

 

Should be running at least 25% better in that one cylinder starts working again

 

well.....it’s a little little better with the 4 good plugs. It’s smoother above 2500 rpm but still has that miss from idle to 2500. Engine speed may have just made it feel smoother. 40mph in 3rd gear (about 3000 rpm IIRC) was pretty smooth
I still have the front coil unhooked and will have to leave it that way until I get more plugs and a complete wire set. I didn’t grab the boot correctly on 2 plug wires and I yanked the connector out of the boot. Ruined 2 plug wires. I robbed 2 off the intake side so I could test it and get to work tomorrow.

 

Another interesting observation: before I left work yesterday, I cleared the codes because I had the pending codes due to having unplugged the IAC and then the MAF, while it was running. The CEL has not turned back on in 30 miles, yet it still runs rough and with a miss.
Could I be having an injector problem on one cylinder? I’ve been thinking spark problem, but maybe it could be fuel related?

 

Injector issues are few and far between but still possible, after seeing the spark plugs on one side I would stick with spark as the main issue at this time
And possibly leaking intake valve guide seals on 2 and 4, just have to wait and see after a few miles you can pull #2 again and have a look at the tip

So Lean code hasn't returned?

 

Not yet. Only made a trip home 13 miles, before plug change, and afterward, a trip around the block and back to work this morning

 

very interesting finding just now. I don’t know why I didn’t try this earlier. I unlocked each spark pug wire at the rear coil (only one hooked up at the moment). I cranked the truck and started lifting wires off the coil. I found no spark output for #3, the “black” plug.
I swapped coil packs and #3 still doesn’t arc when I lift the wire at the coil. It no wonder it feels like a dead miss.
What’s going on here?

 

#2 shouldn't be sparking either, they share one coil in the pack, but are wired in series

Try this, reverse #2 and #3 wires on the coil pack and see if the #2 is now the misfire, idle doesn't change when you lift off the wire

In your model there are 2 coil inside each coil pack, one fires 1 and 4, the other fires 2 and 3 spark plugs
This has to do with how the 4cyl engine is balanced
1 and 4 are a matched pair they are both at TDC at the same time, then 2 and 3 are at TDC 180deg later(crank rotation)

So the one coil fires both 2 and 3 spark plugs at the same time, one of these cylinders will be on compression stroke so spark will ignite air/fuel mix, the other cylinder will be on exhaust stroke so spark is Wasted, so its called a "waste spark" system, first spark system ever used on 4-stroke engines and very reliable

Anyway the 2 spark plugs are hooked up in series wiring, so electrical spark pulse looks like this as it travels thru the two spark plugs: https://mymiata.paladinmicro.com/SparkCoilLg.jpg

It travels from one coil tower to a spark plug, and the spark jumps from center to tip(normal spark), then it travels thru the head metal to the other spark plug and jumps from tip to center(reverse spark) and then back to the other coil tower, circuit complete

So you can reverse the 2 and 3 wires, or reverse the 1 and 4 wires shouldn't matter at all unless there is something wrong

 

i also need to swap at the plug right?

i have arc’ing from the other three. There is no arc whatsoever on #3, no matter which coil pack I use on that set of plugs/wires.

 

Did I also need to swap at the plugs? By your description, it doesn’t sound like it.

i have arc’ing from the other three. There is no arc whatsoever on #3, no matter which coil pack I use on that set of plugs/wires.

I just swapped #2 & 3 at the coil only and still no arc’ing from the #3 coil terminal.

 

No you don't need to swap spark plugs just the wires at the coil pack

After swapping positions of 2 and 3 wire on the coil the engine ran the same?

So #2 would have been misfiring at this time not #3, because its the #2 wire thats not "arcing"

If so that would mean #2 and #3 spark plugs and wires are OK, the problem would be in the coil pack, BOTH of them

Get an OHM meter and test either coil pack
Set meter to 20k ohms
Put one probe in #2 spark plug wire contact, put other probe in #3 spark plug wire contact, should be 7,000-10,000 ohm if memory serves

You can do same test on the 1 and 4 coil, to see what it has, should be same for 2 and 3
And the same on BOTH coil packs

Also if a coil pack is out give it a good look for cracks


Since you have no history on the vehicle previous owner may have swapped coil packs around and the condition of that #3 spark plug took out the other coil pack the same way it did the first one
Which is why both are having the same problem, so test them

 

About 13.4k ohms on all 2-3 and 1-4 tests (both coils)

 

Then I am at a loss as to why the coils are not sparking all 4 plugs

Did you say you swapped over the wires to other coil packs 3 wire connector, so you are running exhaust side spark plugs on intake sides spark signals?

 

i just moved the intake side coil to the exhaust side swapped positions) so I could hook up the exhaust side connector and plug wires.

I did just go out and pull the plug wire off #3 plug and put in a spare plug and layer it on the engine to ground it. It does have an apparent spark, but does look a bit weak, and still shows “no arcing” when I lift that same wire off the coil. That must be a really weak spark. I also swapped the wires from the #2 plug and coil, over to the #3 position. No change.

At this his point I may as well change both coils and all plug wires I like to find a correct problems rather than throw parts at problems.
Diagnosis points toward coil, but they both behave the same way and the odds of the exact failure at the same time on 2 duplicate (redundant to a degree) are about astronomical.

 

That's good it had a spark

The failure of the coil could be caused by a spark plug or wire issue, so swapping coils may have fixed it temporarily and then caused the swapped in coil to fail in the same manner, so you then have 2 coil packs with same failure
Long shot, agreed

You may not get the "arc" because spark plugs are wired in series, if #3 connection is the "-" side of the series then it has no reason to "arc"
Series wiring seen here: https://cimg7.ibsrv.net/gimg/www.mia...24a3b83e2a.jpg

But you said both #1 and #4 both "arced" when wire was lifted?

 

yes, 1, 4 and 2 all arc real strong. I’ll try to post a short video.

 

Don't need a video

You can test the primary coil on each coil pack although unless its bad, it can't test if its good because the ohms are so low

In the 2 coil pack units the connectors center pin is the "+" for both coils
Each outside pin is the "-" for the coil on that side
Set OHM meter for lowest setting, 100 ohms maybe
Test ohms between + and - on each coil, expected is .6 to 1.4 ohms, but should be close to the same for both

Also I assume all the spark plug wires look like the same brand?
You can test those as well, rule of thumb is 11,000 ohms per foot, so 2ft wire would show 22,000ohms

Use a high OHM setting on meter of course

 

I checked all “+/-“ connections on both coil packs. All 4 readings were 0.5 - 0.6 ohms.

I checked #2 and 3 plug wires because it was quick and easy.
#2 (about 12” long) was 6.78k ohms
#3 (about 16” long) was 7.08k ohms.

I do have one wire wire that appears to have a different boot. It was either #2 or 3, but I’ve also swapped those around with no change in how it runs.

 

Wires are fine, as are coil packs

I am at a loss