4.0 RPM's dropping

[American1990]

Hey guys, I got my hands on a Fresh 4.0 with just over 100k so this engine is NO where near the end of it life, and it been well maintained . Just got the swap done, but it idling a little high, not when i first start up but once I give it some gas it wants to stay up around 1k RPM, at first it wanted to stay at 2.2k RPM but I messed with the idle controller to get it to idle at normal RPM range, but like I said once you give it gas it wants to stay at 1k. Also, when I'm cruising everything fine but when I put it in neutral while driving the RPM drop and shut back up 2 or three times, then it stays at 1k RPM. The Engine has E3 spark plugs, could these "advanced" spark plugs cause this? Going to try cleaning the MAF and switching out spark plug wires, spark plugs, and ect.. tomorrow just looking for some ideas that i might not be thinking about. Any advice is much appreciated thanks.

 

[RonD]

OK, assuming a 1990 ranger from your name.
And I assume you swapped all your original sensors to the newer 4.0l, so computer wouldn't have to relearn new sensor.
And you are using the original computer.

Reads like a vacuum leak, check PCV valve and it's hose first.

"but I messed with the idle controller to get it to idle at normal RPM range"
?? this is controlled by the computer, so nothing to "mess with", IAC can and should be cleaned if there are idling issues, but not sure what "messed with" means

Check throttle cable and cruise cable(if so equipped) either can hold throttle plate open, or delay it's closing.
Disconnect both from throttle plate and operate throttle manually, make sure spring is returning it.
Start engine and check idle with no cables attached, REV engine a few times manually and make sure throttle closes quickly when released.

 

[American1990]

1989 Ranger, but all the wiring from a 1992 Ford ranger 4.0, under dash, engine compartment, everything. The engine from a 1993 Ranger, but i switched out all the wiring on the engine to the 1992 4.0 harness. The truck starts fine, once you give it some gas it wants to idle at 1k RPM. And when you let off the gas while driving and put it in neutral the RPMS drop and go back up two or three time depending on what RPM your at.

 

[RonD]

It still reads like a vacuum leak.
Yes, clean the MAF and check the air tube from MAF to intake, an air leak in this tube IS a vacuum leak.

The computer is trying to set a stable idle using the IAC valve but for some reason it can not, this usually means there is "unreported" air coming in.
Computer reads the RPM from the crank sensor, and it adjusts the IAC to set idle RPM at 1,000 cold or 750 warm, so it is not a complicated system as far as setting the idle.

All the air coming into the engine needs to pass thru the MAF or computer is getting bad data.
PCV Valve is "reported" air, IF it is working correctly, if it's valve inside is not stable then idle can't be either.

I assume the Computer is the correct model for the transmission?

 

[American1990]

Yes, the computer is the right one for transmission. Out of curiosity, on the intake the tree of vacuum lines are any of them really required? Could I cap them all of?

 

[SenorNoob]

Well, there is one for the fuel pressure regulator and I'm going to assume you want to keep power brakes. Other than that it shouldn't be vital unless you have an automatic transmission. Although everything else there is something you SHOULD have running. It all only increases MPG.

 

[American1990]

When i disconnect the Idle Air control valve isn't the engine suppose to idle low? or crappy? When i disconnect mine nothing happens, I even had that and the TPS disconnected and nothing, still idles at 1k RPM.

 

[American1990]

Bump

 

[truckerdawg]

If your sure there are no vacuum leaks and the 4.0 runs smooth with good power thru the entire rpm range and appropriate mpg then one idea is maybe adjust the idle on the throttle body. Pay attention to how many turns you make in case this makes no difference so that you can turn it back. IAC- not needed. I know I will get disagreements here, but I worked on Beech Mtn, NC all winter where temps have been regularly below zero and single digit highs without the IAC with no problems cranking and idling. There is a thread on explorerforum how to do this.