Ford 2.0 SOHC random misfire

I'm looking for some direction on what my next step in troubleshooting should be.

This is 1999 Ford Escort with a CVH 4-cylinder SOHC engine
The issue is an intermittent misfire. It can be felt at idle, but it isn't consistent. Sometimes it happens at the time of a 1-2 shift, and causes a shudder. I don't notice it during 2-3 and 3-4 shifts.

-- NO DTCs are stored
-- Ford power balance test via scan tool has shown misfires on every cylinder.

-- Long term fuel trim at idle is about +10. Short term varies between +2 and +4.
-- upping RPM to around 2,000 causes the long term fuel trim to drop to about +4.6
-- it doesn't seem to happen on a cold engine. I don't feel any misfires when the engine is completely cold and the transmission shifts 1-2 smoothly. It starts shifting funny on some shifts within a couple of minutes after a cold start.
-- I don't think it's happening at higher RPMs

I know Mr. Danner would say vacuum leak.

I have checked all around the engine using the water test and also attempted propane. I can't hear any change in idle with either of these tests. I have tested by disconnecting all of the vacuum lines that I can find from the intake (and plugged them) and then checked to see if the short term fuel trim goes negative, and I don't see any difference. I have looked at the ignition primary with my picoscope. One thing I don't fully understand is how the waveforms should look with a throttle snap. They look different under a throttle snap than at idle (aside from the double firing that Ford does at idle). I'll attach screenshots of it when I did the snap throttle and at idle.

I tested to see if the system responds to the addition of fuel by injecting propane before the MAF sensor, and that causes the short term fuel trims to go negative.
What should be my next test?
Idle:



Snap:

Your next step, because there are no DTC's is to check the monitors in Mode 6 for misfires.

I don't see a mode 6 option in my software (Hickock NGS), but here's a screenshot of the power balance, which counts misfires. As you can see, misfires show up on every cylinder if I let it collect data for a couple of minutes.

On a hot idling engine, in Park, is that software logging misfires?

Does it log misfires @3000 rpm in PARK?

What Year is this vehicle?

Seems interesting that any P300 codes aren't setting.

Drivetrain issues can log misfires on an engine.

Just because it's easy and because it happens between 1st and 2nd (high load)
i would check spark on all cylinders with an adjustable spark tester.
I have weeded out many misfires using that tool on coils that test "good" using less dynamic methods

www.amazon.com/dp/B00IXMA1RY/?ref=exp_scannerdanner_dp_vv_d

Scanner Danner has this one on the tool page (profits to charity)
you should be able t wind the screw out to 35-40 without losing spark

I have a Lisle adjustable spark tester, and it's handy tool. I didn't try it yet because an ignition misfire wasn't making any sense. This is a waste spark system with two coils. The fact that it was misfiring on all four cylinders in a random pattern really seemed to point me away from that.

I realized today that I had missed a vacuum line, the purge line from the charcoal tank, because it connects to the bottom of the throttle body on this car. I tested disconnecting this line, but still no difference in fuel trim. I also took a vacuum reading and I was getting a good, steady 20 inches of vacuum.

Satisfied that there were no vacuum leaks, I tried disconnecting the oxygen sensor, and the car ran quite a bit worse. The misfires increased. Obviously, the oxygen sensor was helping the situation.

Next, I decided to disconnect the mass air flow sensor. I started the car and it seemed to run a little better. The Ford service manual says when attempting this test, you should drive the car around for 20 minutes. I drove it around for close to 20 minutes and noticed that it seemed to be shifting better. Additionally, it was more responsive to the accelerator pedal. A slight hesitation that I had been noticing was gone. After stopping and letting it idle for a minute, I started the power balance test again, and found zero misfires. I also didn't feel any.

I am now convinced that the mass air flow sensor was causing the misfires on this car.

I should add that my total fuel trims after running with the MAF disconnected were between -4 and -9 at idle. That's a change of 23%!

I'm going to put my scope on the MAF signal wire and see what I get.

I though MAFs usually got worse with higher RPMs but that's not the case here. This has a good air filter and I've cleaned the MAF in the past with CRC so I don't think it's dirty. I'll have to come up with some tests for this thing and compare to the replacement.

I'm going to order a new sensor. Ford is ridiculously priced but I need one that will be accurate so I think I'll gamble on a Delphi because I found a good price on one of those and Delphi makes parts for GM.

Scoped and also looked more at the MAF readings.

Scope shows some spikes and dips. They are as much as 1 volt between a peak and one of the valleys. I now need to test to see whether this is caused by the sensor, the PCM, or something else in the car, such as ignition system or alternator, or possibly a bad ground.

MAF readings seem reasonable for the size of the engine -- about 2.58 g/sec at idle. One thing I noticed when I graphed it is that they are going up and down by just under 10 percent. All this leads me to believe the spikes and dips are the cause of the random misfires -- if the computer recalculates fuel trim based on one of those spikes or dips, it's going to cause a cylinder to misfire, and it will probably be somewhat random.

I cancelled my MAF sensor order this morning because I realized I hadn't done these necessary tests yet. I'm not going to be a parts changer!

well this gets interesting. here's an A/C ripple check. i don't think this looks good. next is a driving test with the alternator disconnected.

I did a test with the alternator disconnected, and i still had the same issues. Even though I think the alternator is starting to have issues, it doesn't appear that it is causing this MAF issue/misfire.

I ordered the Delphi MAF sensor and I'll report back how that goes.

That alternator trace looks like something is pulling voltage down pretty hard. Might just be the alternator or could be something else.

please keep us updated