BII won't idle

[jlswindle]

I bought a Bronco II and it won't idle. It does start right up and I can keep it going for a little bit if I feather the throttle. There's no power behind it either. I cleaned the throttle body, but sea foam gas treatment into the fuel and even gave it 4 gallons of fresh gas. I have good spark, the firing order is correct however the plugs do look worn. I was told a new fuel pump was installed but that the truck has been sitting for about 6 months..
Any ideas will be appreciated.

Thanks
Jeff

 

[RonD]

Welcome to the forum


Couple of things to check.
On the upper intake is the IAC(idle air control), fuel injected engines use a "controlled" vacuum leak to set the idle level, the IAC is a valve that opens and closes to allow air into the intake(vacuum leak), it is controlled by the computer.
Cold engine idle is usually 1,000rpm and warm engine idle 750rpm, those are preset levels in the computer, the computer tries to achieve those rpms by opening and closing the IAC.
The IAC can get dirty or sticky or quit working altogether, it is the large cylinder on the side of the upper intake, it has an electrical connector, you can unbolt it and clean it and even test it.
When first starting the engine the RPM should go up to 1,500rpm, sometimes a little higher, then drop down to 1,000(cold) or 750(warm), that is the computer testing the IAC, if this doesn't happen then IAC is stuck(or broken) or wiring has a problem.
IAC problem would not cause a lack of power.


Fuel injected engines can not use a regular "choke", no jets, so they use a temp sensor.
The ECT(engine coolant temp) sensor is a 2 wire sensor used only by the computer, there is a 1 wire sender used for the dash board temp gauge, they look similar but are not the same.
The ECT sensor tells the computer the engine is cold, the computer then switches to a rich fuel/air mix, advanced timing and increased idle.
If the ECT sensor is reporting that engine is warm all the time, then cold engine would not idle well and would lack power until it warmed up.
If ECT failed in this way the engine would run fine when warm, only cold engine would be effected.


Also check all vacuum lines for cracks and brittleness, especially the MAP sensor vacuum line.
The MAP sensor is usually on the passenger side firewall, sometimes hidden behind the wiring harness, follow the vacuum line to find it, also unplug it's electrical connector and make sure it is clean and dry.
MAP sensor plays a big role in the fuel/air mix the computer uses, it tells the computer the load the engine is under so could give the computer the wrong info if vacuum line is leaking.

Worn plugs need to be replaced, how old are they?

One other thing, anytime you work on a computer controlled engine, you must reset the computer for it to "see" any changes you might have made.
You can use a scan tool for this but most just unhook the negative battery cable for 5 minutes, this will cause the computer to "reboot" and then relearn any changes you might have made.
Some forget to do this and end up replacing sensor after sensor trying to find a problem, when they fixed it the first time but never "told" the computer to look for the change.

 

[Lil-Pony]

+++1 ^
everything ron said should be checked, swap those plugs inspect your cap and rotor, swap the wires if they look older, and a basic tune up, also check your 02 sensor.
02 sensor is a big part of power, for it actually measures how much 02 is in the exhaust after each exhaust stroke... if this is off it can cause a rich condition to the point of killing the motor

 

[ab_slack]

All of the above.

Also, my experience is limited but had some things recently with my B2 that might be relevant. Obviously there can be many causes.

I would mention there is a jumper plug in the wiring harness that one pulls when setting the ignition timing. If for some reason that jumper plug is missing RPMs will drop and won't stay at idle if idle had been on the low side to start. It is a simple item to check make sure is there and connection isn't bad. Also with this plug removed I don't believe there will be any ignition timing advance which would certainly cause low power.

+++1 ^
02 sensor is a big part of power, for it actually measures how much 02 is in the exhaust after each exhaust stroke... if this is off it can cause a rich condition to the point of killing the motor

As I recently learned. A quick diagnostic on the O2 sensor is to simply unplug it and see what kind of difference it made.

My BII was idling fine, just had some driving oddities and low fuel economy. Unplugging the sensor made essentially no difference except it was more consistent. Pretty much that indicated that it wasn't really using the sensor anyway. Replacing the sensor, my fuel economy jumped up. It is not hard to imagine an O2 sensor not working in some other way would affect idling. Unplugging the sensor is a quick way to eliminate a variable even if it won't run the best without the sensor.

6 months doesn't seem too long, but depending on climate, could there be moisture in the fuel? I have had this new kind of gas go stale to the point my lawnmower wouldn't run from the last fill in the fall till the next time in spring. I learned that stabil works wonders.