1991 Ford Ranger High Idle

[91 blue ranger]

Hello, I have a 1991 Ford ranger that has a high idle. Checked for vacuum leaks and found no leaks. When I start the truck i revs up to 2,000 rpms, and stays there for 5 seconds then goes down to 1,500 rpms, then after a while longer goes to 1,000. I replaced the IAC but that did not help. If I rev the engine (warm or cold) it always goes up to 1,800 to 2,000 rpms stays there for 5 seconds or longer. Then drops to 1,200 then to 1,000. If I am driving the rpms will not come down from 2,000 rpms. If I am moving and push in the clutch it will stay at 2,000 rpms until i come to a complete stop. I replaced the transmission with a M5ODR1-HD ( so it does not have the neutral sensing switch). If I am parked and very slowly press on the gas petal it revs up to 1,800 2,000 rpms, ( even tough I only pressed the gas petal in so that it should have gone up only 100 rpms. If i unplug the IAC it idles around 1000 and if you rev it it falls nicely.
Also some time i have to push on the gas petal to make it start.

Any help is greatly appreciated.

Thanks, Solly

 

[PetroleumJunkie412]

Was your truck an automatic?

 

[91 blue ranger]

No is was not. I bought it and it needed a transmission but that transmission was a manual. And the door sticker said is was a manual form factory.

 

[PetroleumJunkie412]

Copy. Almost sounds like you have a vss giving up the ghost, or you have an automatic ecm in it.

Don't know much about your year. May have to wait for a more knowledgeable person.

 

[91 blue ranger]

I replaced the vvs and when I unplug the two wires going to it the truck still idles high when diving but comes down after 5 or more seconds.

 

[91 blue ranger]

Oh forgot to mention, there is no Check Engine Light.

 

[RonD]

After engine is warmed up fully, and idling
Unplug IAC Valve

RPMs should drop to 500 or so, or engine can stumble and die, either is GOOD

if not then look on the throttle cable linkage for a screw, its the anti-diesel screw, but "looks like" an idle screw
With IAC Valve still unplugged and engine idling high
Turn the screw counter clockwise(unscrew) a 1/2 turn
If RPMs start to drop keep turning it, until engine is barely running, its re-set, it was messed with before you got it
If RPMs do not change then turn it back 1/2 turn, this isn't your problem

If adjusting the anti-diesel screw didn't lower RPMs then you have an air leak, period(long shots below)
You just need to find it.
With IAC Valve still unplugged you will need to unplug each vacuum hose from upper intake and plug that port with your finger to see if RPMs drop
Start with PCV Valve, it should be mostly closed at idle, pull off hose and plug with finger


As far as the higher RPMs when shifting, that is normal
Its an emissions thing
The computer uses the IAC Valve to hold RPMs high when shifting, but only for 5-10seconds
And RPMs will stay high until speed is below 5MPH
Idling engines are BIG polluters, idling engines have to be run Rich to prevent overheating

Just as a heads up
There is a Throttle Position Sensor(TPS) on opposite side of throttle cable linkage
If this has been replaced with non-motorcraft sensor then it can HOLD OPEN the throttle plate a bit causing the higher idle
You can pull off this sensor and restart warm engine to see if RPMs are lower with IAC Valve unplugged
If so you may need to drill out the sensors bolt holes, elongate them, so sensor can be installed without holding throttle plate open

Wrong IAC Valve
Motorcraft or Hitachi IAC Valves are the only ones that work on Rangers
So when fully closed(unplugged) another brand may be letting more air in that one of the above brands, so higher idle

 

[91 blue ranger]

Thanks ronD. The TPS is still original Will test what you said and report back.

 

[91 blue ranger]

How high should it idle when moving

 

[91 blue ranger]

Drove around with the old iac on and it worked better.
Now if I rev it the idle is around 1,500 rpm not moving and then will slowly drop. But if I rev it longer and at 2,500 rpm it falls to 1,500 rpm and but takes longer to fall.

 

[91 blue ranger]

I put the old iac base on with the new iac motor on the truck. I also adjusted the anti-diesel a little and it runs much better. It does not idle at 850 more like 1,100 or 1,000 but at least it does not idle at 2,000 when shifting.
Thanks guys.

 

[Monkeyboy]

Check your TP voltage closed throttle for approx .95V key on. I have seen in the past incorect TP values causing just this. RonD has good advice to follow above for sure. I have been a ford tech for many yrs. I would chase the TP sensor if it were mine. Don't mess with throttle plate adj screw settings, they don't need to be changed, you will probably compound the issue with that. Sensors fail and change value.

 

[91 blue ranger]

Ok will check tps. What should it be on the high side when the throttle is open? And I already messed with the screw will that make the tps read wrong?

 

[Monkeyboy]

Ok will check tps. What should it be on the high side when the throttle is open? And I already messed with the screw will that make the tps read wrong?

Should be colse to 5V Wide open but that end of the sensor never wears out, its always the lower part of the potentiometer inside that wears out. Try and put the adj screw bact where it ws if you can remeber the turns taken in or out and then check the TP value, adjust the sensor to near 1V but not a full 1V, .95-.99 is good. Disconnect the battery for a moment too, it seems to help reset the pcm, I know that sounds wacked for an older pcm but it does have an effect in some cases to help relearn TP base value. It's been a few yrs since Ive had such an issue, I was in a ford dealer for 23yrs and those older engine just don't come in anymore so I'm going on memory but the TP sensor did create just what you're experiencing. Go around ith a voltmeter and just check all sensor values key on engine off. You have a MAP sensor too right? Not Mas Air Flow in the intake duct right? Can't remember exaclty what MAP should be KOEO but the interweb should be able to get that for you.

 

[91 blue ranger]