Svo gurus-- help !

[Laredo]

...I know I keep harping on the coinciding sudden RPM drop (shown on tach)-- like cutting the key off. I can't help but think that there is an electronic/electrical source that is common to the miss and sudden tach drop. Anyone know where the tach feed comes from ?

Just to clarify - it sounds like now we're talking about 2 different symptoms here with the additional engine cutting out like the key shutting off.
Let's start there and DIAGNOSE the problem rather than guessing or before swapping and/or replacing parts.

First, we can "divide and conquer" your primary ignition system (pre and post coil - which is you actual "load" in the primary circuit).
"Pre" is the power side of the coil. This will include your power supply, ignition switch, and wiring feeding 12v power to the coil. The "post" is the ground side - switched on/off by your TFI module.
For test purposes, run two long wires (different color to make identification easy) from the each of the + and - terminals of your coil into passenger compartment. Grab your digital volt meter and measure from your coil + to ground. Ignition "on" should be relatively steady charging voltage. Now go drive the car (with an assistant) until the car cuts out. Watch the power side voltage. Did it cut out or drop during the engine falter? If yes, lets's start there. Where is your power side voltage going? If the voltage remained steady - good! We can now focus on the ground (control) side of the circuit. Swap your Pos lead of the volt meter over the the ground side of the coil. You are now measuring "dwell" voltage. This will be less than 12V, proportional to the "on/off" time of the coil being grounded. Eg - 50% dwell will equal 6v. This will vary up and down as your dwell changes. But....here's the important part - it should not cut out or fluctuate sharply. Drive the car again and watch for sharp fluctuations to 0v or 12V when you symptom occurs. If you see this on the negative side, we can start to narrow down your issue to module, pick up coil, wiring etc on the control side of the coil circuit. If you're not seeing abnormalities here, look elsewhere - fuel cutting out, secondary ignition, etc.

[Rickrey]

Thanks Laredo-- l feel the problem is in the primary ignition, somewhere. I got out my old service manuals yesterday and, as would be expected, the feed for the tach comes off the primary coil wire-- negative side. I will try your method to narrow the problem.

[Meaty]

It's unclear to me, are you running the ford Thick film module on the distributor? I have had multiple WEIRD misses with a bad TFI module.

Not to beat a dead horse. (But I will) The TFI has other failure modes than just fail/work. I have chased a low RPM intermittent miss in a 2.3T, and did all the same things you did (replaced everything in the engine bay). Ultimately, when I dissected the bad TFI, there was a internal ground wire that was broken. In some operational modes it made a solid ground, and sometimes it did not. I believe the TFI pulls the signal from the Hall-effect and send it to the coil and tach.

[Rickrey]

Meaty-- I hear you. I had a TFI failure on a low milage SVO in the late 1980's-- quit completely at a stoplight. This is what I have been told by the professional techs-- works or doesn't work. BUT, also, I had a very weird problem on another SVO that was fixed by switching out the distributor-- likely the TFI module. I have been dealing with a home improvement project, but will start in on the SVO in the next couple days. I will report back soon. Thanks all.

[SVOeric]

Don't forget the failure mode on the TFI where it locks timing at 20deg ignition timing.

[GAboySVO]

What year model of SVO do you have? If it is an 84, then you need to check your TFI pigtail and engine harness wiring insulation. Try the wiggle test on the wire harness and see if you can get the rpm drop while idling. What is your fuel pressure doing at speed?

[gbeaird]

What year model of SVO do you have? If it is an 84, then you need to check your TFI pigtail and engine harness wiring insulation. Try the wiggle test on the wire harness and see if you can get the rpm drop while idling. What is your fuel pressure doing at speed?

IF it's an 84, just closely inspect the wiring insulation for cracks. If you see cracks in the insulation, you could have sections broken off somewhere and grounding out at really odd times. I had an 84 with this affliction, and it stranded me twice. It would have done it a third time, except I got out, moved the wiring around (not sure what I was looking for, but was trying to find some evidence of failure). Because the car would invariably start after getting dropped in the driveway by the tow company, I got back in, turned the key and the car started right up.

Ford made (makes?) a replacement TFI and distributor pigtail because of this issue.

[Rickrey]

Thanks-- It's an '86. Still in house project mode, but will be back to the SVO soon. I'll let you know what I find.

[Rickrey]

OK Gentlemens-- First, I replaced the ignition module with a new Motorcraft unit-- No Help. Second, I checked for proper ignition advance(with engine speed)-- all OK. Third, I pulled the spark plugs and regapped to a tight 0.030". This did make difference-- the high speed miss moved up in the RPM scale a bit and the misses are not quite as violent. Still has multiple misses above 5000 RPM. I am now going to follow Laredo's plan and use my voltmeter. I will report back soon.

[Laredo]

If ignition checks out ok, pull the valve cover a inspect closely for a broken valve spring.