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2.9L Hard Start Hot

hondakillah

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City, State
Washington State
Year, Model & Trim Level
1994 Limited
1988 Ford Bronco II 2.9L V6
New: FPR, O2, Lower Intake Gasket, Cap, Rotor, Wires, Plugs. Injectors have been cleaned, and do not leak. KOEO Fuel Pressure 35. Vacuum gauge suggests healthy valves and rings.

The engine fires up fine when cold, and idles around 750 RPM. As it starts to warm, it slowly works its way up to 800-850. When the computer reads the engine approaching temperature, the IAC valve kicks open, the engine stumbles, and the computer closes the valve. It does this a couple of times until it finds it's balance. Once it does, the engine idles at 1250 RPM. When I rev the engine, in a futile attempt to kick down the idle, it's a slow set back down to 1250 RPM.

My issue seems like a vacuum leak problem, but I can't find a reasonable place that could be a vacuum leak. That goes to the wayside with the opening of the IAC, and the increased RPMs due to extra air from there; this makes it seem like the computer reading something wrong (thus the new O2 sensor). I bench tested the old O2 sensor, and compared readings to one I had laying around. The old O2 sensor didn't fluctuate as fast as the other one, and figured it wouldn't hurt to replace that one.

Once the engine is warm, and then turned off, I can not restart it until it has cooled enough for whatever to let me restart it with the proper fuel/air ratio. Unplugging the IAC does not help in this situation.

The EGR port into the intake was plugged, but when it was all apart, I cleaned it out.

Where might I be over looking in regards to something that may create a vacuum leak? Is there a sensor I should inspect that may cause false readings to the computer?
 
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hondakillah

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Update:
I went and tested the MAP sensor.
It has steady on voltage. The signal sits at 2.64 volts.
When I add 20 inHG of vacuum to the sensor alone, and plug the port on the engine, I can get the engine to start. But when the sensor is plugged into the engine, I cannot get the engine to start.
I would like to verify that this is indicative of the MAP sensor going bad, as it is unable to hand a fluctuation of vacuum, even though it holds pressure just fine.
 
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hondakillah

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1994 Limited
Update:
When I add vacuum, artificially, to the MAP sensor, I can get the engine to start, warm. Approximately 20 inHg to the sensor, and the engine naturally sits around 18 inHg at 850ish RPM, with a vibrating needle, but not fluctuating.

When I unplug an extra hose, and the RPMs rise to about 1200, engine vacuum rises to 20 inHg, but I get irregular drops of 3-5 inHg. The book describes these as either sticking valves or a misfire. Seeing as I don't see the drops without the increased RPMs, nor does it happen at a regular and consistent basis, I do not suspect valves, but an ignition issue. While cranking the warm engine, there is negatable vacuum, but again, when I tell the computer there's vacuum, she fires up just fine. I can only think the computer is doing some timing advancement.

With the shortening bar removed, the engine is set to fire at 10* BTDC.

I forgot to add at the beginning, the TFI-V module on the back of the Dizzy has been replaced as well. I don't see the stator as bad because I have spark, and can get the engine started.
3 cylinders are producing fantastic compression. The other 3 are less than stellar, but still within spec.
 
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hondakillah

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1994 Limited
Solved!

It was the ECT sender to the ECU that was bad.

The sensor was registering ~3M ohms cold, 2.6M ohms hot.

I had a spare one laying around that registers at ~33k ohms cold and 22K ohms hot.

The increased resistance told the computer was practically an ice cube and the air was super dense, so advance the timing. And even when the engine was actually warm, the resistance was still below minimum standards for the engine to be out of warm up phase, so every start was a "cold start."
 
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