1996 ranger 2.3 misfire/idle issue

[jkeile]

I'm completely stumped. I have a 1996 ranger with a 2.3 showing cylinder 4 misfire. It also has strange idle issues. Sometimes it idles fine but after being at idle for 30 seconds or so it almost stalls out and runs as it i have dropped a cylinder. Drivability is never affected, always runs great when driving on interstate running 80 runs like a top. Then i can stop and a light and my idle will race up to 3 grand and stay there like the throttle is stuck even while shifting gears. Have to shut truck off to stop the high idle. I have replaced the iac,both coils, plugs and wires and cleaned and checked for vacuum leaks. Hope someone can shed some light on this for me. Its all just so random. It has 140 on the clock and all of this just randomly startedand i wish it would just go as it came.

 

[RonD]

Welcome to TRS

Well, there is only one thing that can increase engine RPMs, increased air flow.
You can add more fuel and all you will get is a too rich mix and flooded engine, lol, gas don't make a car go, air does.

So throttle plate or IAC Valve.

Now a '96 manual trans will have hold and shift RPM software, so if you are moving and shifting gears the RPMs should hold or increase a bit(no load with clutch pedal in) for a few seconds before dropping down if you keep holding clutch pedal in.
And RPMs should stay above 1,100 until speedometer is below 5MPH, that's another software add on.
These both lower emissions because RPMs don't drop down too much.

So throttle plate or IAC Valve, both control air into engine so both control RPMs.
Throttle plate would be easiest to check, strictly a mechanical device.
Check that plate isn't loose inside intake(throttle body), and that the Return Spring can still snap throttle closed quickly, hard to stop that air flow at 3,000rpm, need a good strong spring.
Generally feel the throttle opening and closing, smooth operation no binding or stickiness.
If you have cruise control disconnect that cable from throttle linkage and drive a few days that way.

IAC valve
"New" used to mean "tested and works"
"New" now means "you test it, we warranty it"

So don't put alot of trust in "new" anything anymore.

Since you replaced the IAC Valve did you do a computer reboot?
Swing by an auto parts store and have them read the codes, write them down, and then reset/reboot the computer.
IAC Valve is a "learned" control, takes a few days, drive cycles, for computer to "learn" new IAC Valves settings.

Vacuum leak, small one and near cylinder 4, could be causing Lean mix at idle.
Easy to test for
After engine is warmed up and idling, unplug IAC Valve, it will close and idle should drop down to 500rpm, or engine may even stall, either is good it means no vacuum leak.
If idle stays at 700rpm or higher then there is extra air coming in from somewhere, air is RPMs

 

[jkeile]

Thanks for all of the information. Will certainly be looking into each of those possibilities first thing in the morning. When the idle is increased it stays surged up at 3 grand even while shifting. I also have a code reader and i can clear the misfire code and drive the truck to my destination with no loght but my cel is on upon restart. Its just from one extreme to another and will run without any issues for a day or 2 then I get the surging rpms or rough choppy idle. I will check your recommend areas and let you know the outcome. Appreciate your input.