1990 Ranger 4.0 4wd/Auto - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 05-13-2015
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1990 Ranger 4.0 4wd/Auto

Mileage 201,000 (HIGH) Well it just cruises down the road at 60mph all day long. But the idle in gear after everything warms up is , well like my 327 after the big cam install. Often I must put in neutral at red lights. This condition is worse in the hot summer. To dat I have run several gumoput thru fuel , new plugs wires, waterpump, power steering pump fuel filter air filter and reaslly nothing has changed. If you have a idea for this olguys old motor please respond.
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Old 05-13-2015
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Clean MAF sensor first thing.

Only use Motorcraft double platinum spark plugs, Autolite if Motorcraft is absolutely not available.
4.0l will eat all other brands in fairly short order.
Ford uses a "waste spark" system, which means 1, 2, 3 spark from center to tip, normal, and 4, 5, 6 spark from tip to center, backwards, which is why you need the double platinum if you want them to last.

Make sure plugs are gaped wide enough, wider gaps are better for low RPM idle and driving, narrower gaps are better for high RPM driving

You could test fuel pressure, could be fuel pressure regulator is not opening enough at idle, high vacuum, so pressure is too high, or it could be leaking fuel into intake, via it's vacuum hose, causing slight flooding at idle.
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Old 05-14-2015
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4.0 idle .

Ron I cleaned that MAF last winter with that pricey spray cleaner. That fuel regulator you talked about. How can fuel go to intake via a vacuum hose? When you sid that it does act just like a carb engine with a stuck choke at idle but no black smoke. I have 7ooo mile autolites in there now. Any more ideas are appreciated thanks Jim
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Old 05-14-2015
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Inside the FPR is a rubber diaphragm/valve, at idle when vacuum is highest and fuel demand lowest the vacuum in the hose pulls open the diaphragm allowing fuel to flow into the Return Fuel line and back to the fuel tank.

If that rubber diaphragm leaks.......................fuel is sucked into intake via the vacuum hose.


Fuel injected engines can't use a Choke Plate but a cold engine still needs extra fuel, advanced spark and high idle to run until it warms up.
Computer uses an ECT(engine coolant temp) sensor to decide if engine needs to be "choked" and runs engine that way until it warms up.

ECT is a 2 wire Sensor, engine will also have a 1 wire Sender used for dash board temp gauge, these 2 separate devices are usually located near each other on lower intake by thermostat housing.

When you start engine cold idle should go up to 1,500+ RPMs, the drop down to about 1,100 RPMs.
As engine warms up idle should start going down to warm engine target RPM of 750 with automatic, 650 with manual trans.
If that happens then ECT sensor is working OK.
When in choke mode the computer ignores the O2 sensor, it is running engine rich on purpose, and O2 sensor won't work until it gets up to 650degF in any case.
This is called Open Loop operation for the computer, it has no feedback to adjust fuel mix.
After engine warms up computer switches to Closed Loop operation, it is getting feedback from O2 sensor, so you have a closed loop, maf>>computer>>O2>>computer

On the upper intake the is a 2 wire IAT(intake air temp) sensor(also called ACT sensor), colder air is denser than warmer air so requires more fuel per liter of air, IAT sensor determines air temp in intake and computer adjusts fuel mix accordingly.
If it is reporting colder than it should you would run slightly richer, BUT............O2 sensor should correct for this, it is the final word on Lean or Rich.

Good site here to read up on sensors and testing them: http://oldfuelinjection.packrad.net/page10.html
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