Any help on where to start and how to test would be greatly appreciated.

I drive a 97 4.0 ranger and recently it has been acting up. I first noticed that my rpms were slightly bouncing up and down and thought nothing of it.

On my way to school the other day, I was at a red light and it just stalled on me out of nowhere and the battery light was on. It turned back on and was running perfectly fine the rest of the day.

Here comes next morning and same thing but worse. Slowing down in traffic, the rpm would drop to below 700 and then fluctuate like crazy until it caught itself or just stalled.

I already looked for major vacuum leaks, replaced my IAC, cleaned my MAF, and checked my battery. It acts up worse in the cold mornings but by the time it warms up, a snap of the acceleration makes it want to stall.

I will be going to my buddies place to hook it up to a scanner and get all the necessary info off of it. Is there anything I should be looking for, or a single part I should check?

 

Have them check ECT(engine coolant temp) sensor on Live data make sure it is warmed up to 180+ degF
And then cools down under 140degF after it sits, engine off, for 20-30min
If ECT sensor is "telling" computer engine is always warmed up, even when it is cold, then engine will run poorly cold, rough and stalling, and it will not set a code

Check IAC Voltage, IAC gets full 12v with key on so opens all the way, the computer controls the IAC's Ground.
Computer Pulses this Ground to lower the voltage which closes the IAC Valve so computer can set Target Idle RPMs
Computer is letting engine stall because it can't control IAC, IAC control is very fast and very precise, within 25RPM of target
It should set a code if it can't control IAC Valve

Check brake booster for vacuum leak, often over looked, but when you stop you do use the brakes so............could cause engine to stall if it is leaking

Always good to include what transmission you have, manual or automatic
With an automatic you could have a torque converter issue or TCC lock issue so torque converter is not going to full disengage every time you stop, causing a varying drag on engine and stalling
Try shifting to Neutral before stopping

 

I have an automatic 5r55e I believe, it is a five speed but the fifth is overdrive.

And speaking of transmissions, I listened to it last night and there is a quiet clunking sound coming from under the truck and I could feel it in my feet every time it would fluctuate coming to a stop.

 

Try shifting to neutral and coasting when slowing down to see if engine seems normal
That will take transmission issue off the table or tell you it is to blame for the stalling

4R55E would be in a 1997 Ranger 4.0l, 4-speed

5R55E was used in 1998 and up Rangers with 4.0, but late 1997 could have one

These both use the engine computer software and shift solenoids in the transmission to get full shift control



Which one you have doesn't really matter, exactly the same transmission, Ford added a "faux" 2nd gear to the 4R to make it a 5R by using software in the computer
4R shifted from 1st(2.47 ratio) to 2nd(1.47 ratio) but 1st had to go higher in RPMs because of the 1.0 ratio change so it didn't bog down in 2nd

So for the 5R they stayed in 1st and turned on OverDrive which gives a combined 1.86 ratio, 2.47(1st) x .75(OD) = 1.8525 ratio, closer ratios give smoother shifts; then shifted to 2nd(which is 3rd on 5R) 1.47 ratio
Very clever changing the 4-speed to a 5-speed by a software change
4R uses
1st: 2.47
2nd: 1.47
3rd: 1.00
4th: 0.75

5R uses
1st: 2.47
2nd: 1.86 <<< 1st + OD
3rd: 1.47
4th: 1.00
5th: 0.75

 

Alright so I hooked it up and went to live data and clicked on everything that showed up to getthe readings.




There were more but those sensors did not register and either maxed out the value or went to the minimum.

I do have lean codes on bank 1 and 2, so I cleaned the MAF sensor and deleted the codes to check if they'd come back later, but nothing yet. Also my vacuum is at around 28 at idle.

My fuel pressure was at 30 running and it was at 10 key on engine off. The voltage going into the IAC was above 12 so that brings that out and the ECT sensor cooled down to exactly 140 after letting it sit after it reached 180+

I haven't had a chance to test the shifting into neutral yet but I will ASAP.
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850rpm idle is high after warm up


MAF should have about 4 Grams per second, G/S, with a 4 Liter engine under 1,000RPM. so that is OK, a little low, so cleaning would help

IAC should only have 12volts before starting engine
Unplug it after warm up and RPMs should drop to 500 or engine should stall either is GOOD
If RPMs stay high then you have a vacuum leak

O2 sensor shouldn't read 0 volts with engine warmed up, 192deg and idling at 850rpm, so something wrong with that

You need to test your vacuum gauge
Only time engine should go above 21" vacuum is when using the engine as a brake when coasting down hill, and I don't think you were doing that when you read 28" vacuum
In Death valley you might get to 22" or 23"

 

​​​​​Cleaned the MAF sensor and cleared the codes but they came back today. I unplugged the IAC and it did go below 500 rpm. I checked vacuum again at idle and this time it was in between 23 and 24 at idle and went low on acceleration. Coasting did make it go up above 25.

I will be checking the O2 sensors tomorrow after class to see if anything is out of the ordinary, if I'm not mistaken, I should get a voltage generated between 4 and 8, right?

I tried the coasting in neutral and idle went above 1000 and only bounced once then seemed normal until I put it back in drive.

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SOLVED
I got tired of trying to figure it out myself so I took it to a shop and found out my intake manifold gasket is leaking upper and lower.

Time to replace it I guess

 

What is IAC AND WHAT DOES IT DO ?

 

Idle air control, I think it monitors the speed of air coming in and uses it for the amount of fuel coming in