This car is in a garage, driven a couple times per week.

Suddenly, yesterday while driving, - Runs rough.  Idle, dies.  Idle in N dies.
I did manage to drive it home, Seemed to run ok down the road, not acting like starved for fuel.
In N, press the gas and hold, the RPM goes up, [example 2500].  
Then after a second or 2 drops about 500, and the roughness returns. 
Does this regardless of what RPM you get initially - goes up, then drops back.
Does this hot or cold.

Capped the vacuum tap on the passenger side, no effect.  This eliminates EGR, Evap, HVAC controls as possible culprits.
Capped the vacuum tap for the brake booster, no effect.
Distributor cap & rotor look normal, not loose.
Timing belts are good, not jumped.
Swapped the MAF, no effect.
Swapped the IAC valve, no effect.
CTS has nominal normal readings.
Fuel pressure normal 22-23 PSI.
Ignition coil & igniter swapped, no effect.

ECU swapped - Still runs crap / same, but doesn't quite die at idle in N

Running out of ideas....

I haven't troubleshooted that problem on one of those, so just some random ideas...

Does fuel pressure go up to 35+ when you give it gas, and stay there as the rpm drops?

Did you recently get gas, or other chance of contaminated fuel?  Sample a bit into a clear container from the pressure test port if not sure.

Does the exhaust coming out the rear feel normal, with distinct pulses per cylinder as it idles?

Does it feel like all cylinders are weak, or like one or two cylinders are cutting out?

Coolant level and condition normal?

 

Coolant normal.  Fuel pressure stays steady at 22 PSI, just like my other EA82 SPFI wagon.  Never moves vs throttle position or load.

No recent fuel up.  The roughness seems more random than cylinder specific. I'll try to check the exhaust pulses tomorrow.

I also swapped the distributor since the original post, no change.

I'm thinking of the mpfi version.  they were still doing spfi in 93?  Shows how many of those I've worked on.  lol  Sorry, my bad.

egr valve stuck open?

 

SPFI in 93, I think I read to 94.

I checked the EGR valve after the OP, and yes, it is working normally.  Took it off to check that it wasn't stuck open, etc.

 

Have you checked the values and calibration of the TPS?

I'm not as familiar with the autos as I am with the manuals. Isn't there a drop down circuit that senses when you go in gear and it'll drop the idle? 

More things checked - 
Spark plug wires, checked ok, but swapped anyways, no effect.
TPS switch and pot both are withing normal settings / ranges.

One other thing I forgot to put in the OP, it starts right up no problem - no hard start.

 

I've had a wire break inside the engine harness to the IAC before. I've also had several connectors have poor terminal connection. Then again, I would typically get a CEL but not all the time. 

Since you have no hard starts I don't suspect a wiring issue with the IAC. The fact that you've swapped a majority of the typical suspect components and it still acts the same leads me to think an intermittent connection somewhere.

The IAC has no effect at open throttle, so it's easy to rule out anything to do with it.

Yeah, I figured the IAC shouldn't bother open throttle.  I did have a wagon years ago that had a broken wire in the IAC circuit years ago.  It ran fine, but died if I let the pedal go to idle.  

I also put a voltmeter on the 12V powered side of the ignition coil, and it was steady except for slight variations because the alternator rpm varying.

I found the problem!   Swapped throttle body top 2 layers.  This include fuel pressure reg, IAC valve, and Injector.  Runs fine now.  
Since the regulator measured good, and the IAC was swapped earlier, I'm going with the injector is bad.

I think there's a hidden filter in the injector.  You might try seeing if it's got a chunk of decaying fuel line or something stuck in it, that might happen to the new injector.

ok, Ill look at the old one.

Checked the filter in the injector, it is clean.

For any of those interested Rockauto has 4 NEW SPFI injectors in stock under an '86 GL. 

They're $400 USD. That's a very low price compared to some eBay sellers. 

Edited March 2Mar 2 by SuspiciousPizza
Those injectors will fit all SPFI vehicles

Glad you got it fixed!

A very interesting thread and timely too. I was just about ready to post up my own appeal for help with my 87 DL SPFI 5-speed that has nearly the same symptoms. It starts as easily as it ever has, over and over but will not idle or run at a steady throttle position. It runs great if under acceleration but I have to keep blipping the throttle to keep it running. I'll review the info here first and make sure I haven't missed anything before I start my own thread.