More on the non starting 1990 Civic

Yesterday afternoon I was able to rule out the ignitor.
Bought an AmPro "High Energy Ignition Spark Tester" for
$7 at Advance Auto. The spark was as strong as the spark
from a running 1991 Civic.
The tester is a hollow plastic tube with an adjustable,
calibrated gap. with a "S",stnadard, and "E", electronic.
This little gem showed a friend' Chevy had a bad coil.

Sunday afternoon 2 weeks ago the engine would not start. ECM
showed error 16, an injector "failure". this had happened
at odd intervals before, but the car would
always start after waiting a few minutes.
Our frined ordered a replacement, and I cleaned and
made partial repairs the original ECM and got the car
running. I contiued the repair effort, and pulled the
PGM-FI relay and resoldered it. One the next atempt to
start the engine, it almost caught but "burped", made an
odd sound I have never heard before.
I thought that the engine had jumped timing, but extensive
tests and close examinaiton have pretty much ruled that out.
When you crank the engine it almost starts, but blows back
through the EACV hose. I can't be certain without another
test, but I think it is not blowing back through the throttle
body. We had teh accelerator wide open and I wedged the "choke"
plate into the open position, but it started rainng so I was a
little rushed and need to do that test again.
The original ECM and the rebuilt unit both work in my Civic.
[while my paper work says my car is a 1991 "model year", it has many
1990 features, darn near is identical to our friend's except for color]

I have some maybe simple if stupid quesitons.
Is the firing order and distributor plug assignment the
same in 90 and 91?
Does the 1990 Civic have an EGR, and if so is it controlled
by the ECM, and would a EGR that failed in the open position
cause a blowback? (I use this term instead of backfire as
it really does not sound like any backfire I have ever heard
before)
What does the "Tandom Valve Control Solenoid" do, and what
would a failure cause?
Can a Electric Air Control Valve" (EACV) failure cause?

I intend to verify every wire, test every sensor, test every
solenoide, see it the TDC and CRANK coils produce the same
voltage that my car does.
And jsut fro grins I have borrowed a dual channel scope so I
can see if the ignitor is waiting before firing. Unlikely but
I am grasping at straws here.
Does anyone have any ideas on what else to test?

Terry