94 E150 with misfire at idle and high load

Vehicle is 1994 Ford E-150 5.8L V8
Ignition is TFI-IV Push Start (NOT computer controlled dwell)

Symptoms are:
-Misfires and surges at idle,
-no noticeable symptoms while driving at low load (i.e. high rpms, relatively closed throttle, not up a steep hill)
-misfires increase in frequency and intensity as load increases (due to incline AS WELL AS when throttle opening increases)
-barely visible (even in total darkness) blue flashes in and around where spark plugs thread in to block (there is no visible arcing through the wires or boots, which are new). This occurred on all plugs I could get my head in to look for them on.
-No codes

Whole secondary ignition system is new (yes I have been parts swappin pretty hard). This includes the only coil in the system, coil wire, distributor cap and rotor, plug wires and plugs.

Attached photos show secondary ignition waveform on one cylinder, it is typical of all the cylinders I tried. (I have flipped the images for your convenience as on my scope they were upside down). Photo 1 shows what I would consider a normal reading, photo two shows the other reading I was getting. In practice the scope was showing a rapid flashing back and forth between these two wave forms.

(I have uploaded a video although it is sideways and appears to be quite green? but just imagine rapid flashing between the two images...)

Thanks!!
Kiernan

I had similar issue with my Tahoe in the past. I got a p0300 (random misfire) code and started to look at everything I could think of thinking it was a vacuum leak. It turned out, my distributor just needed to be replaced due to the wear on the drive gear.

Try pulling it and inspecting to see if you have the same issue? Just my .02.

Hi guys, i had a similar issue with a E350 5.8L v8.

I wanted to know first the cause of misfire (spark or fuel). I used a gas analyzer, which drove me to a bad O2 sensor. No O2 codes related.

Check for O2 sensor activity. Mine was steady at 0.5 volts

Hi thanks for the reply Randall. I will look in to this today. But I'm thinking I could determine if the gear was worn just by wiggling the rotor and feeling for backlash? Did you have wear that could not be detected this way? Thanks

Guafa: I have previously checked the O2 sensor output, and it shows rapid oscillations that vary across what I recall was a normal voltage range. Also, unplugging the sensor didn't change the symptoms (and I would expect that unplugging it would force it in to open loop and set all fuel trim settings to default, which would correct any misfires due to faulty O2 sensor output). HOWEVER unplugging the O2 sensor ALSO did not throw a code. Could this indicate that the computer is receiving some other signal (maybe zero volts due to short to ground) and interpreting it as O2 sensor output? Will check O2 sensor pin at PCM later today.

Thanks!
Kiernan

Can you post live data? 02 values, MAF, MAP, Baro? Throttle position? Fuel trim (or is it block learn and integrator on that age vehicle)
If you cant read live data take a look at your MAF and see if it's dirty, same with the throttle body.

Clear flood crank? Does it sound consistent or does it sound like a cylinder is dropping out?
what is fuel pressure telling you?
Injector power balance test?


What engine is it?

sorry for all the questions. Itll help us help you.

You can also go for a spin a look for O2 sensor enrichment at wot. That way discard any fuel delivery issue (since misfire is at both idle and wot) and stay focus on spark and air.

John Curtis: Thanks for your interest. While making a few videos of sensor output to post here I noticed strange signals coming from the MAP, but ran out of time. Will investigate that tomorrow!

Guafa: If the MAP sensor turns out to be good, I will investigate O2 response to WOT, although I can only do this at idle because I do not have obd2, so I can only watch through my scope.

Thanks!!

It is proving tough to use the scope and hold my phone to take a video! Here's what I'm seeing.

MAP output is a beautiful square wave with a 5V peak to peak voltage and a frequency of between 160Hz at zero vacuum and 92Hz at ~24inHg (and smooth transition between them).

O2 sensor output seems to to fluctuate between around .3-.5V up to 1.0-1.3V every second or so, and seems to be the same at idle and with some throttle opening. BUT at idle, every time it misses/stumbles, O2 output seems to drop down to the low end. Could indicate a lean misfire?

IN OTHER NEWS I checked the base timing (with SPOUT shorting bar disconnected) and advanced timing (at idle) but was unsure of how to interpret the indicator. The drawing shows the pointer/indicator, which is fastened in two places to the timing chain cover (this is the complex shaped piece with the swoops and small punched hole), the harmonic dampener (the long rectangle) and the arrows show where the cylinder 1 TDC marker appeared with BASE TIMING (No spout) and advanced timing (With Spout at idle). Can any of you tell me how to read this timing pointer?

Thanks!
Kiernan

John Curtis: to answer your other questions: The MAP sensor in this vehicle replaces both MAF and Baro so neither are present. Throttle position sensor is recently replaced. I have simulated a clear flood crank by disabling fuel and there is no noticeable stumble whatsoever (and compression checks good on all cylinders). Fuel pressure is good at the rail under all driving conditions. I don't know of any way to observe fuel trim on this vehicle (it is obd1). Also since it is obd1 I'm not sure if I can do an injector balance test.

Thanks again

Kiernan

MAP output is a beautiful square wave with a 5V peak to peak voltage and a frequency of between 160Hz at zero vacuum and 92Hz at ~24inHg (and smooth transition between them).. What´s your baro pressure (zero vacuum)?

O2 sensor output seems to to fluctuate between around .3-.5V up to 1.0-1.3V every second or so, and seems to be the same at idle and with some throttle opening.. At idle o2 signal should fluctuate from 0.2 to 0.9 volts

Did you already check O2 signal at pcm pin?

I will investigate O2 response to WOT, although I can only do this at idle because I do not have obd2, so I can only watch through my scope.
you can connect a long wire from O2 sensor signal to your scope and watch while you are climbing.

Okay I will try to rig it up so I can watch O2 output while driving today. What should I be looking for when I do that? I am assuming a lean condition following the misfire(s) is mostly just a symptom of the miss itself. But now I am wondering what does an O2 sensor show when it encounters unburned fuel along with the unburned air? Would it show rich instead of lean? Or does the O2 strictly sense oxygen content.

Thanks

At wot, you should see O2 sensor signal rich all the time (something above 0.8 volts)

In order to have enough time to see O2 signal, if it is a manual transmission, start the car in second gear. If it is an automatic transmission, keep throttle on the floor all the time (keep and eye on the road always).

If O2 sensor signal drops below 0.45 volts, that means is getting lean and you have fuel delivery issues

By the way. Yes, O2 sensor strictly sense oxygen content.