'84 Bronco II 4.0L wont cold idle

jollykreiger · Aug 22, 2015

'84 Bronco II 4.0L wont cold idle randomly Bogs down

So I just picked up this bronco II with what appears to be a first gen explorer 4.0 setup, all I know for sure is that it has the aluminum upper like the 1st gens Ive seen and not the plastic ones like my 1997 was.

It runs pretty good, the only real problem is that when you go to start it without it warmed up, it will not idle. It almost does, but will die.

As a note the only thing that isnt complete is the exhaust. right now its open header. Right where the cat would be is open so its loud. Would that be enough loss in back pressure to cause this.

RonD · Aug 22, 2015

No 4-stroke engine runs better with back pressure, that is a myth that persists.

So no, not having exhaust pipe wouldn't be the problem.

The computer uses the ECT(engine coolant temp) sensor to set idle and "choke" level.
When you first turn on the key the computer gets the coolant temp from the ECT sensor, this is a TWO WIRE SENSOR located near the thermostat housing on lower intake, top front.
There is a ONE WIRE SENDER used by the dash board temp gauge located near the ECT sensor, they look similar.

If Computer "sees" coolant is cold, under 190degF, it will use the IAC(idle air control) Valve, located on upper intake, to set higher idle, 1,100rpm, and also run a Rich air:fuel mix with advanced spark timing(choke mode)
As ECT sensor shows coolant warming up the idle will start dropping, when fully warmed up(190degF) idle should be 650rpm for manual trans, 750rpm for automatic

ECT sensor is about $7, many just change it, but you can test it with a Volt Meter.

IAC Valve can be removed and cleaned, it could be stuck.

jollykreiger · Sep 11, 2015

So that part as it turns out is $35-$45, but if mine is indeed bad, couldn't I just plug in a new one and let it hang there. and if mine is bad, then it would cold idle correctly (if the motor is cold)

jollykreiger · Sep 23, 2015

So an update, I mostly put an exhaust on it and the cold idle problem is better but still not completly resolved.

As another note I took it on about a 6 hour round trip hunting trip over the weekend.

It ran good even with the trailer on the freeway. I got up to 85 going up some of those hills following people and not paying attention. But I did notice a few more symptoms.

If I have it at full throttle it will always pull and run fine but I can feel it almost surging like im lifting a little and then going back to full throttle. It doesn't do it all the time and its not really bad, but it still does this.

The other thing is if I am just trying to maintain a speed, any speed at any time. It will start to bog down like the timing just got thrown off or Im running out of gas, (I have had both happen to me.. Loose dizzy in case you were wondering)... Then I will throttle more and more while still loosing speed until I reach full throttle and it will take off like a bat outta hell.
It has done it 5 min after starting and 1 hour into the trip. Randomly as far as I can tell.

I'm pretty sure its not the fuel pump, as I can run it at WOT as long as I want and it will never fall on its face.
I have also disconnected the Map sensor and it will run the same way.
I tested my TPS and got a good reading throughout the range.

Im not sure where to go next? Any Ideas

'84 Bronco II 4.0L wont cold idle

jollykreiger

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