I would try the coolant temperature sensor. This is supposed to control the richness of the gas mixture as the engine heats up. Beings the engine runs strong at first, I am thinking the sensor is not leaning the mixture down.

Don't confuse this with the temperature gauge sending unit. If you want to see what I am talking about go to www.rockauto.com and dial in your vehicle, then go to cooling system then go to temperature sender/sensor.

The sensor is the one with the gray plastic top. This is what I would try.

 

So does the same event occur whether the engine is cold or warm ??

 

Can't keep it running long enough to get to full temp.

 

That should be your Throttle Air Bypass Valve Sensor (TABVS).
This is an output device controlled from/by the ECM.
(Also known as Idle Speed Control - Bypass Air, ISC-BA)
(Like the modern IAC) GreasyKnuckles was asking about this.
Provides air during engine start around the Throttle Plate.
TABVS/ISC-BA can effect; Idle RPM, Air/Fuel Ratio & Timing. (Engine Stall)

You should take a look at this device, if it closes after your "strong 30 seconds" it will choke of the airflow. You should definitely take it out and clean it.

7-13 ohms across the 2 pins on the sensor. (Solenoid winding)

2 pin connector.
Red wire 10.5-12 Vdc, check to ground.

Other wire is signal from ECM, pin 21 (Grey/White wire maybe)
Probe this wire while engine is running, ground neg meter lead.
Reads 3-11.5vdc depending rpm and engine temp.

Monitor this ECM signal up to the point when your engine stalls.

If you pull the connector off after you start the engine, the rpm will fall or engine will stall.

The original air bypass valve had an external diode in the wiring harness. The replacements came with the Diode built into the device (you can tell by Part No, will have a "D")

There is a Ford Technical Service Bulletin TSB, which states to remove the original diode or cap off before the sensor, when installing a new sensor.

 

Have you checked your EGR to see if it is leaking exhaust into the intake?

greasyknuckles was asking about this as well.

 

So, have you checked the pulse at the injector connectors? 4 injectors fire at the same time, then the other 4, so electrically, it’s possible to be running on 4 cyls.

 

Just doing a mind dump this week, as I think of things, for I am out of town later this week.

Are you tracing fuel wires because you think they are intermittent ?

Fuel Pump Relay - Comes on for 1 sec when key is turned on. Once engine starts and runs, the ECM gets a PIP signal from TFI in distributor, if it is within 1sec intervals, ECM continues to apply power to the Fuel Pump Relay. Once the engine stops, Fuel Pump Relay is turned off.

 

Have you check codes again?

What are u using to check for codes ?

 

Hopefully this might help answer you question, this is just a DRAFT...;

30 seconds is an interesting time-frame; after cold startup (ECM Open Loop Operation), the first 30 sec you should be pulling 15-20 in of vacuum. Then can spread between 7-20 in. for the next 30sec to 2 min, if you stay idling or are using the throttle in park.

Inputs to the ECM (Cold Start)

ECT Sensor - Engine Temp reading is about 3.52 vdc when cold.
(Shorted is read as 0v or Warm by the ECM (lean mixture occurs))

Air Temp Sensor (ACT) - interrogates Intake or Manifold Air Temperature. Air Temp reading is about 3.52 vdc when cold. (Shorted is read as 0v or Warm by the ECA (lean mixture occurs)) This gets gummed up like the EGR & Throttle Air Bypass.

MAP - is sending intake vacuum pressure frequency signals. (0 in. Hg = 152 Hz. To 20 in. Hg = 101 Hz. (you replaced this)

Coil - is providing RPM results.

TPS - Throttle is closed, so the output should be 1.0 vdc on the TPS sensor. (you replaced this)

Injectors - are in Full Rich mode for the first 2 min (30sec + 90sec), unless engine warms up quicker as signaled by the ECT. If warm after the first 30 sec, ECM switches to Closed Loop Operation (then more Sensors are at play). (MFI)

PIP - Generated from the SPOUT in TFI in distributer.

Knock Sensor - (if installed) not activated until temp is 140F. (Retards Timing)


Outputs from ECM (Cold Start)

Spout – Timing signal sent to TFI in distributor

Fuel Pump Relay - Comes on for 1 sec when key is turned on, then as long as the ECA gets a PIP signal from TFI in distributor within 1sec intervals, ECA applies power to the Fuel Pump Relay.

Throttle Air Bypass Valve Solenoid (TABVS) (ISC-BPA)- Is allowing air to bypass the closed throttle butterfly. Full Open after startup (11.5 Vdc). After 30 Sec, ECM uses the Map and RPM signals to determine Valve position.(After Throttle is depressed Bypass Valve closes up some during Acceleration due to Map and RPM input signals) (newer models it’s called IAC)

Inputs to ECM after ECM thinks engine is warm (ECM switches to Closed Loop Operation)

EGR Position Sensor -
PIP -
Knock Sensor -
Throttle Position (TP) Sensor –
Engine Coolant Temperature (ETC) Sensor -
Air Charge Temp (ACT) Sensor -
Manifold Absolute Pressure (MAP) -
Coil -

Outputs from ECM after ECM thinks engine is warm

ERG Solenoid/Emissions Solenoids - circuit is turned off for the first 60 seconds, then allows exhaust flow at higher RPM, but not at idle or WOT (old EGR valves, leak exhaust into the intake at idle)

Emissions Solenoids; TAB / TAD / Purge /

O2 sensor - is ignored for the first 2 min, no signal when cold (giving it time to warm up) Range 0.4 Warm to 0.85vdc Hot.

And;
Injectors -
Spout –
Fuel Pump Relay –
Throttle Air Bypass valve -

 

Assuming your;

Ignition is fine – plugs, wires , cap
Computer is perfect
No vacuum leaks - emissions - PCV
Injectors are perfect
TPS @ 1vdc closed
Air Bypass is working
Coil is good
Fuel is clean and @ 39psi
Distributer module is good.

These three sensors may affect the Cold Start Idle.

ECT Sensor -
Air Temp Sensor (ACT) -
MAP - (You have replaced)