I HATE MIS-FIRES!!! Need GRM guru help!

Here's the dealyo, the 2002 Millenium Escort has developed a random mis-fire, beginning about 3 weeks ago. It does it with both the custom tune and the stock tune equally. I loaded both and drove each tune multiple times to verify, so it's not PCM related. So, here is what I do know, and I hope the GRM collective genius can help me fill in the blanks, because it's driving me nuts! A reminder to please take note of what I have, and have not done to this point. So let the hive mind activate, and may the Force be with me!

Starts, warms up, and idles normally

Runs perfectly under light engine load regardless of RPM

Have determined that it is definately engine load sensitive, not RPM sensitive

Light load response and torque production is normal; the lighter the load, the less the mis-firing (down to none if the load is light enough) regardless of engine speed or vehicle speed.

The plugs are Autolite 5144, wires are Denso OEM, O2 sensor is Denso OEM type, all with only 2600 miles/less than two years old.

Coil pack is the original Ford OEM with 286,629 miles.

Fuel filter has 103K on it, but has never had the pump unit out to change the pickup strainer.

Air filter has only 2600 miles.

The cam has NOT jumped time.

The exhaust free and clear.

The condition did not get progressively worse; it simply began. Up until I cleaned the MAF (correctly) yesterday. Before I cleaned it, it would happen happen under moderate to heavy load and at low to middle RPM range, but would then clear out either as the revs came up or after a second or so at WOT at which point WOT would run normally to redline, making full normal power. AFTER I cleaned the MAF it is 5 times worse and now mis-fires at WOT as well; not as bad after some "clearing out" but still it does at WOT now.

It does NOT misfire at all if I load the engine during the cold-start warm-up phase; not torque braking, not 1/3, 2/3 , nor WOT. But, the instant the PCM switches to closed-loop it begins.

I cannot find even the slightest vacuum leak, and I've used a whole can of B12 spray trying to find one.

Throw in the above mentioned "coincidence" that it got WAY worse instantly after cleaning the MAF. I do have access to a high-end shop grade scanner, for those of you who have any "check this or that reading and report back" wisdom.

Annnnd BEGIN!

How do the plugs look?

Misfire, does it pop up a code? Have you scanned it yet? 100k on a fuel filter sounds like its due whether its the problem or not. If it pulls up a cel for missfire on whatever cyl clear it and swap the coil around, ive had one that loved to die at 3400rpm no matter what. To me it sounds like its the maf..im not as smart as some guys here though but considering it got worse after you cleaned it. But you said you cleaned it properly, would you mind divulging deeper into that for me?

My first suspicion would be the ancient original coil. Any specific cylinder misfire codes stored?

The next thing to look in to is lean misfire. The coincidences lean (pun intended) that way. Not sure what the flow rate should be, but watching live data of the MAF should reveal if it's reading air flow correctly.

Yup, you need live data. I strongly suspect your MAF is off since it's load-related. If the OE coil was going out, you'd have constant misfire or be RPM-related.

rslifkin wrote: How do the plugs look?

Insulators and electrodes are all a nice light brown (I only run pure 91 octane/no ethanol gas), except for the side that faces the intake valves which are almost perfectly insulator white. Clean, no deposits or crud.

chiodos wrote: Misfire, does it pop up a code? Have you scanned it yet? 100k on a fuel filter sounds like its due whether its the problem or not. If it pulls up a cel for missfire on whatever cyl clear it and swap the coil around, ive had one that loved to die at 3400rpm no matter what. To me it sounds like its the maf..im not as smart as some guys here though but considering it got worse after you cleaned it. But you said you cleaned it properly, would you mind divulging deeper into that for me?

I used the CCR MAF cleaner spray (although B12 is probably just as safe and cheaper). Hosed the hotwires down, let it stand a minute, repeat three times let dry. I never physically touch MAF hotwires with anything, ever.

What MAF voltages should I look for when I run scanner? If it helps anyone, this is a standard Ford EEC-V PCM. Also, this is the original MAF, now with 286K on it.

Scan it and watch maf readings i think that's your problem

It's not that you cleaned it, the MAF was going out before. Again, you need live data to look at your flow rate and fuel trims in both open and closed loop.

Javelin wrote: It's not that you cleaned it, the MAF was going out before. Again, you need live data to look at your flow rate and fuel trims in both open and closed loop.

Ima borrow the scanner and report back, soonest.

http://easyautodiagnostics.com/ford/1.9L-2.0L/maf-sensor-tests-1

Oh, and NO CEL. Will check for pending codes while I have the scanner.

Only code is P1506 IAC overspeed error in the Pending File (which is odd, since it idles dead on the OEM idle speed), but that doesn't account for the misfire. Checking fuel trims and MAF voltages next.

You got 286k out of an original coil pack???

Knurled wrote: You got 286k out of an original coil pack???

Yup! I might have suspected it, but that doesn't account for running better at higher RPM vs lower, since the saturation time decreases with rpm. Or, could this all be from the coil pack???

OK guys, this scanner is giving figures in % instead of voltage. However, I think the Calculated Load should be higher than 23% at idle/neutral and 35% idle/in drive, and the grams/second bottoms out at 1.47 but is suspiciously steady. Nor do I think the load should jump from 23% to 41% when the radiator fan comes on. Short term FT is running between 0 and 3-4%, long term FT is in the low 20% and fairly steady. And, I don't get any weird numbers when brake torquing it into the misfire zone.

Next step?

A weak coil could definitely cause it. As you increase throttle, cylinder pressure goes up, so a weak coil may not be able to fire the plugs as well under those conditions. It's possible it's breaking up a bit all the time at high rpm (even under light load), but just isn't obvious until you get on it harder.

Miss under load is usually weak spark, suspect coil pack. You could put it on an adjustable gap tester to confirm.

Welp, my "next door neighbor" who owns the used car lot across the street (they do all of their own repair/refurb work in-house) is also thinking weak coil pack, based on my descriptions and his own experience with used late-model Fords. OK Hive, based on my descriptions/scanner readings, was the worse running after cleaning the MAF just genuine coincidence, and are all signs pointing to a dying coil pack???

Looks like even a good coil pack (Standard) is under $50, might as well replace it for ruling in/out and preventative maintenance at this point.

I still think it's the MAF, and it's been going out for awhile based on the LT 20% fuel trims...

If it's missing a little under light throttle, that'll make the LT fuel trims high as well. Misfires make the O2 sensor read lean due to the extra oxygen, so it would end up adding fuel to make it read correct (which would in reality mean running rich).

The fact that it runs fine while cold points me in the direction of the MAF instead of coil pack. The temp sensor will tell it to add more fuel when cold, masking a lean condition that shows up when warm.

Boost_Crazy wrote: The fact that it runs fine while cold points me in the direction of the MAF instead of coil pack. The temp sensor will tell it to add more fuel when cold, masking a lean condition that shows up when warm.

That could point to either. A lean mix is harder to fire, so when it's cold and running richer, it might be just enough to fire with a weak spark. Flaky coils also tend to get worse with heat.

Javelin wrote: Looks like even a good coil pack (Standard) is under $50, might as well replace it for ruling in/out and preventative maintenance at this point. I still think it's the MAF, and it's been going out for awhile based on the LT 20% fuel trims...

Point taken on the coil pack, since it IS 286,000 miles and 14 years old; I just now ordered a new jen-you-ine Motorcraft unit from Rock Auto for $48.56 shipped.

Now, tell me more about what you are seeing in the 20% LT fuel trims. What should it be, if all were as it should be? Incedentally, the scanner was still showing the MAF at 1.47 grams/second in Key On Engine Off, the very same number it bottomed out steady at Idle Neutral. Shouldn't it read zero if there is zero airflow?

Ideally, LT fuel trim should be close to 0. In most cases, it'll end up a little off, but not more than a couple percent.