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4.0L OHV & SOHC V6 Tech General discussion of 4.0L OHV and SOHC V6 Ford Ranger engines.

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Old 08-12-2016
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I am: David Franklin
Join Date: Oct 2011
Location: Corvallis
Vehicle: 1997 Ford Ranger XLT
Drive Type: 4x4
Engine: 4.0
Posts: 25
Total Props: 0
High idle and slow acceleration

I am starting to see a high idle and sometimes slow acceleration. This seems to only happen on hot days.
The high idle is constant...At a stop light, i'll be at 2k and then it gradually comes down to about 1k-1100.
The sluggish acceleration is a new one. Ill start driving at a green light, and it will feel like the truck is sluggish...ill give it some gas, and then all of a sudden...its like the throttle opened up.
I have cleaned the MAF and the IAC...Is it time for new parts?
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Old 08-13-2016
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I am: Ron Dean
Join Date: Jan 2014
Location: Vancouver, BC
Vehicle: 1994 Ford Ranger
Drive Type: 4x4
Engine: 4.0
Posts: 4,765
Total Props: 88
After engine is warmed up to normal operating temp let it idle and unplug the 2 wire connector on the IAC Valve.
IAC Valve will close all the way and idle should drop down to 500rpm or engine may even stall, either is good, it means no vacuum leaks.

If idle stays above 700 then you have a vacuum leak.
Start removing hoses from upper intake manifold and plugging that port with your finger, when idle drops down you found the hose/device with the leak.

A vacuum leak usually will cause CEL(check engine light) to come on and a Lean code but not always.

Get a volt meter and a sewing pin.
On the upper intake near the front is the TPS(throttle position sensor)
It will have 3 wires connected.
Push the sewing pin into the middle wire, to test voltage.
Set volt meter to DC volts, 20 vDC if available
Remove throttle linkage cover so you can open the throttle manually
Turn on the key, leave engine off
Ground the meter to engine or battery Negative
Touch red meter probe to the sewing pin
You should see .69-.99volts, with throttle closed all the way, under 1 volt
Now slowly open throttle manually and watch voltage
Volts should steadily go up, no dropping or jumping up
At full throttle, middle wire should show above 4.5volts.

TPS can wear out, it isn't common but it is a moving part.
MAF sensor tells computer the amount of air coming into the engine, so computer can set the correct fuel mix, and when you open the throttle the computer does "see" the increase in air flow, but there is a slight delay.
The TPS is there to give the computer an instant "heads up" that more or less fuel is needed, so if TPS is not within spec voltage then throttle response would be sluggish.
Failed TPS would for sure turn on CEL and set a code, but failing TPS might not.

And the computer also compares MAF sensor air flow with TPS position and will set a code if they don't match up, i.e. if MAF air flow increases but TPS voltage doesn't change or doesn't change enough, and the reverse, if TPS voltage increases but MAF sensor air flow doesn't or doesn't increase enough.

I assume you don't have CEL on, since you don't mention that.
Does the CEL work?
It should come on when key is turned on and then go off after engine starts.
If it is working I would still go by an auto parts store and see if there are any codes in memory, they read the codes for free.
WRITE THE CODES DOWN, "I think code was............" is worse than no code at all.
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