94 Ranger 2.3l Rought Idle Nightmare

[Donny Burger]

i bought a 94 ford ranger 2.3l, 5 speed, and 2wd. it was in a small fender bender and the price was cheap for a truck with only 125k on it. the idle is rough and low but drives fine down the road without a problem. i have been trying to figure this out all summer. its my 4th ranger and I'd rather fix it then try to sell it.

so far I've replaced all this:

- throttle positioning sensor
- air idle control
- temp sensor
- o2 sensor
- all 8 plugs and wires
- the front coil pack ( hard a crack in the plastic)
- fuel injector on the first cylinder ( didn't make a difference in idle when i unplugged it like the other 3 so i thought it was bad. this didn't change anything still doesn't make a difference when i unplug it)
- fuel filter
- oil change

i was told to check for vacuum leaks by spray crab cleaner on the lines that didn't find anything. i don't know what to check next and i wanna fix it and keep it cause its cheaper then a car payment. i got to much in it now to give up. no fluid leak of any kind and can't see any signs of the oil and coolant mixing. everyone i talk to has a different idea on what the problem is but no one can pin point it.

 

[RonD]

First, after engine is warmed up, and idling(should be around 700rpm), unplug the IAC valve, idle should drop to 500rpm or engine may even stall either is fine, it means no vacuum leaks.
If idle stays high then you do have a vacuum leak.

'94 computer may be using Batch fire for the injectors, so 2 injectors open at the same time, sequential is when only 1 injector opens at a time, with Batch fire there can be enough fuel left in the intake to keep a cylinder firing even if it's injector is unhooked.
Injectors get 12volts when key is on, computer grounds each injector to open it.
So a shorted injector wire to computer could cause injector to open when it shouldn't.
To test for this you can use the "clear flooded engine" routine.
Turn key on
Press gas pedal to the floor and hold it there
Crank engine
Engine should not start or fire, fuel injectors are disabled but spark is still on.

When computer sees above 4.5volts from TPS(throttle position sensor) and no signal from CKP(crank position) sensor, it will enter "clear flooded engine" routine, this turns off fuel injectors but leaves spark on.
If you release the gas pedal injectors will start up.

So if engine fires with this test then fuel is getting into intake from somewhere, could be injector or Fuel Pressure Regulators vacuum line.

FYI, pretty much every fuel injected vehicle has this same routine, it isn't a Ford only "thing", lol.


Do you have an EGR system?
If so there could be a little carbon build up that is holding open the EGR valve a bit, this fouls up idle but would run OK with more fuel/air added, i.e. RPMs above idle.