2.3 Stalls during warmup

[Trippy]

My 1994 Ranger with a 2.3 Manual will stall and sometimes die during the warm up procedure if I hit the gas pedal, forcing me to have to warm it up to operating temperature before driving it. After it reaches operating temperature it has no issues as far as I can tell.
The previous owner abused it terribly and destroyed the head and block beyond repair. I'm not a fan of shooting the parts cannon at vehicles. All the components that were replaced failed a test or were damaged previously.
For Reference I'm using a Snap-On Solus Ultra with the EEC-iv adapter.

What I have replaced so far (Relevant to Drivability Concern):
-New Engine (Reman ATK-DFD4)
-ICM (BWD)
-DPFE w/connector (Motorcraft)
-EGR Valve (Standard)
-02 Sensor (Denso)
-ECT w/connector (Motorcraft)
-Thermostat (Motorcraft)
-Ignition Coils (Motorcraft)
-Spark Plugs (Motorcraft)
-Spark Plug Wires (Motorcraft)
-IAC (Standard)
-DPFE Hoses (Gates)
-All intake and Exhaust Gaskets (Felpro)
-Fuel Pump (Spectra)
-Fuel Filter (Fram)
-PCV (Motorcraft)
-All new exhaust parts except the exhaust manifold.

Serviced items:
-Cleaned and tested MAF (Both with Multimeter and Scan Tool)
-Cleaned Throttle Body and all Passages
-Tested TPS (Both with Multimeter and Scan Tool)
-EVR (Tested resistance and Signal with Multimeter)

Suspect Items:
-IAT
It reads 50F at start up and slowly creeps to 104 after engine reaches operating temp. Likes to hang out about 1.5-2 volts above ECT. According to AllData that is still within spec.

I have sprayed all around the intake manifold, hoses and components attached to it to see if I can find a leak.

I have performed the KOER procedure and no codes are found. Wiggle test shows that connectors are good.

Vacuum will bounce around between 20 and 15 during the warm up procedure and will stabilize after warm up at around 18.

Sorry for the long post, I try to be thorough. Maybe someone can chime in and guide me in the right direction.
Thank you for taking the time to read this really long post

 

[RonD]

Welcome to TRS

All gasoline engines need to be "choked" when cold, fuel injection doesn't change that
Cold engine will stall if you give it "gas"(air really) when not enough "choke" is applied

So the PCM(engine computer) has a choke mode
Key on boots up PCM
PCM opens IAC Valve all the way for start up, always, warm or cold
PCM looks at coolant temp, via ECT sensor, if under 140degF it will set choke mode, Rich fuel mix

Choke mode is high idle, Rich fuel mix and advanced spark timing for the Rich fuel mix

So on cold starts, without touching gas pedal, engine should REV(IAC Valve open all the way) then RPMs should drop down to about 1,100 as PCM close IAC Valve a bit, assuming 70degF temp, warmer the temp the lower the idle, so at 40degF it may idle at 1,300
This means PCM is in choke mode
As engine warms up PCM will lower idle RPMs by closing IAC Valve bit by bit, until it gets to "target idle" 1994 4cyl would be about 750 manual trans, 800 automatic

If this is not happening then IAC Valve, ECT sensor or PCM is the problem

The bouncing vacuum may be PCV Valve, plug that hose and see if bouncing stops
If not then plug each vacuum port on the intake one at a time, including power brake hose, until you find the leaking device

IAT(intake air temp) sensor doesn't have alot of effect on PCM, so unlikely to cause described issue

And about "new" parts, grain of salt
New now means NEVER TESTED, not "it works"

 

[Trippy]

Ron,

Thank you for taking the time to answer.

I went out and hooked up the scan tool and observed the rpm go up to 1300 and slowly come down to around 750-770.

ECT voltage and temperature seem to move accordingly.

I took out the new IAC to bench test it. Resistances were within spec and you could hear the solenoid when 12v were applied. Installed back on and after the engine warmed up I attempted to unplug the IAC and the engine would stumble. During the warm up procedure the scan tool was showing that the IAC was supposed to be at 35-40% duty cycle. Not sure if that is normal once the initial "choke" has ended. IAC hangs around 23-26% after warm.

Bench tested the new EGR just in case. Everything tested good. DPFE is switching voltage when the EGR in manually vacuumed. Don't think that the DPFE can cause this issue, but tested nonetheless.

Followed the procedure for isolating the possible vacuum leak from the hoses, but no luck there. Everything seems to be sealed. Sprayed around intake manifold and gaskets, no luck there either.

Tested Fuel Pressure at the rail with gauge. It was within spec with a slight increase when the FPR hose was removed. No sign of leaks from FPR.

TPS has always had a smooth curve when this is happening.

If the throttle is not touched during the warm up no issues are present.

Does the information exonerate the PCM, ECT and IAC? Or are there any test (outside of a breakout box) that can be performed to the PCM?

Side Note- I use to be a Ford Tech a few years back. It was always interesting watching people as you explain to them that the "gas" pedal is actually an "air" pedal . This truck really makes me miss bi-directional communication .

 

[RonD]

"I went out and hooked up the scan tool and observed the rpm go up to 1300 and slowly come down to around 750-770."

How long between 1,300 and 750RPM?
Should be 5 to 8 minutes for full warm up

If it just dropped down to 750 within a 30-60seconds then no Choke


TPS acts as the accelerator pump did in a carb setup
MAF sensor is too slow to react to a quick increase in air flow, so engine would stumble on acceleration if only using MAF
TPS gives the PCM the "heads up" to open injectors longer, ASAP, for the rush of incoming air.
You should see short term fuel trims jump to +15 when you press down gas(air) pedal

 

[Trippy]

Waiting for it to cool down completely now to time it in order to give an accurate answer. I didn't time it earlier, but it felt like the decrease in rpm was spaced out and not sudden. Should it stay at 1300 for 5 to 8 minutes and then drop or is it a stepped decrease in that time frame?
Is that STFT number to be observed during the "choke" or once it reaches operating temp?

 

[RonD]

STFT during choke when you press down on gas pedal, PCM should respond by squirting extra fuel into the intake instantly, +15 or more
But it should do that anytime gas pedal is pressed down, squirt in that extra fuel

RPMs should go above 1,500 at startup, IAC Valve is open all the way, then drop quickly to say 1,100 as PCM closes IAC valve a bit
Then over the next 5-8 minutes RPMs will drop slowly to 750
Drop is solely based on ECT temp, 2.5volt cold, then .7-.6volts warmed up

 

[Trippy]

Ron,

Thank you for taking the time to look into this issue. Your input has been awesome.

Started with a completely cold engine this morning.
-RPM went up to around 1800 at first
-Within 10 seconds it dropped to around 1500
-20 seconds later it dropped to 1300
-It held at 1300 for about 30 seconds
-Moved to around 900 and held that for about 1:30
-Held 850 for about 2:30 minutes
-Finally stabilized at around 750-770

It was kind of tricky to accurately watch the scan tool and the timer, but my best estimate is that it reached 750-770 mark at about 5 minutes. ECT was around 187F/.7v at that point. ECT voltage seem to move accordingly to temperature.

STFT will increase up to +30 when the throttle is moved to around 2000 RPM. The curious thing is that it will stay static at +30 for about 10 seconds. Is that normal behavior or a slow response? STFT will usually fluctuate between -5 to +10 when engine is warm at idle.

 

[RonD]

Yes that all looks normal, including the STFT

Not sure why there is a hesitation when cold?

+ fuel trims are also used to keep Cat converters hot, and Richer mix also cools cylinders a bit as RPMs are going down
Idle STFT would usually be in the +5 to +10, warm, this is also done to keep cylinders cooler and Cats HOT

When cruising along -5 to +5 is expected, but..........you have to include LTFT in that, if LTFT +5 then STFT would actually be 0 to +10 not -5 to +5

LTFT is used to offset engine systems getting older, i.e. smaller air leaks and lower fuel pressure
So PCM doesn't have to relearn the system every time it boots up

 

[Trippy]

I drove it for about 40 miles today and it was idling really good when warm. The fact that it only happens when the engine is cold and the throttle is touched makes me think that is either TPS or MAF related. The vacuum readings make me think that a mechanical component (intake gaskets or valve seals) is at fault. A head scratcher for sure . Under the " new now means never tested" I'm going to compression test the new engine to make sure they didn't fudge the rebuild. If it was a timing issue it would manifest itself all the time. PO abused it heavily and there is evidence that it was driven through deep water several times. Thinking about taking a look at the ECM connector. Are there any particular problematic areas I should be looking at?

 

[Trippy]

Just to keep the thread in the loop. I removed the ECM to inspect it and found nothing obvious wrong with it. Capacitors look intact. Actually looks pretty clean in there compared to the rest of the truck. I was expecting to find mud inside the connector. I'm running the F47F-ADB, cheapest one of the bunch, which makes me happy in case I have to replace it in the future.

After putting it all back together I started it and noticed smoke coming from the exhaust manifold area. I had ignored such things before because when putting on the new engine a lot of chemicals got spilled and I figured It was all of that burning off. While it was still cool I sprayed it with soapy water to confirm that it is indeed leaking just above the collector. That might explain the whole "gets better when warm" thing because some of the leaks will seal up after heating up.

Could that be throwing off both the 02 and the DPFE? Could that give me the bad vacuum readings? Performed KOER three times after and no faults were found.

 

[RonD]

Exhaust leaks before the O2 will certainly "throw it off", exhaust runs with negative pressure so actually sucks in air between puffs of exhaust coming out, causing O2 to show leaner voltage, just slightly lower MPG, no engine running issues

And thats only when O2 is being used, after its warmed up

Can't see that effecting DPFE or intake vacuum

 

[Trippy]

Compression numbers:
#4-165
#3-165
#2-170
#1-170

One thing I noticed is that the thermostat may be stuck opened as the upper hose seems to warming up and pressurizing before it should. Seems like a long shot, but could that be causing this?

Fuel pressure stays in specification when running (37 psi) and when vacuum hose is removed (42 psi). But, it leaks down to about 10 psi when the truck is turned off. Blocked the return hose and it holds the pressure when the return hose is blocked. No signs of fuel leaks at the pressure regulator. All the injector resistances are testing within specification and you can hear them clicking. Took them off to bench test and all of them are spraying.

Where to test next?

I have found a bunch of threads in different forums with similar complaints. I'm determined to find what this issue is. I have access to a full shop with most diagnostic tools. All I'm missing is a breakout box and a NGS