2.5 runs rough

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Bull84

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Nov 15, 2023
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Location
Maine
1989 Jeep YJ 2.5 runs rough and fluctuates idle speed I’ve done
New throttle body
Plugs
Wires
Checked spark
WD 40 to check vacuum leaks
Blows black smoke
Runs fine at high RPM
Anyone have anything else I should check I’m thinking cat may be next
 
Bull84 said:
1989 Jeep YJ 2.5 runs rough and fluctuates idle speed I’ve done
New throttle body
Plugs
Wires
Checked spark
WD 40 to check vacuum leaks
Blows black smoke
Runs fine at high RPM
Anyone have anything else I should check I’m thinking cat may be next
Click to expand...

is that the TBI yes, or carb?(4.2L)
it is not MPI (newer engines)
I ask as no 2 jeeps seem the same after 3 engines swaps in 30 year.
its rich is all we know now. black means unburned fuel. rich as pig in fact.(incomplete combustion)

The FSM shows TBI new. throttle body injection system and loves to fail. the FPR is is first. it must regulate fuel pressure fully.
read FSM chapter 14 page 5 and 6
fueling chapter.
(14-15 psi) and how to do it and set it and pass fail it.
if too high PSI, check fuel return line for blockage
if not that the regulator is shot. and more read page 6 it nails the tests.
good luck
did you buy a rich jeep
or it just now failed.
history maters and PO gremlins, endless. see folks block return lines to make it run weak pump or FPR shot.
see it too many times.
 
other possibles and tests to find it ( im not guessing it is asking to do tests)
  • FPR bad,
  • return line pinched/clogged, fuel line to tank. see 95psi , its failed. a gauge called a fuel pressure gauge. all shops have it.
  • injector leaks like mad. bad. or seals related to it.
  • bad spark,or skipped the 60k mile tune up? (filters, cap,rotor, hv wire set, plugs/?
  • coolant leaking in to 1 cylinder, and misfires like crazy (compression tests fail or leak down test) spark plug tip green or dripping in fuel, oil or antifreeze.
  • 02 sensor shorted. (reads 0vdc all the time , fuel trim goes max rich and over injects and black smoke. and CAT glows red hot if is still there.
  • fuel pump put in is wrong one and is for V8 engine?,(gee seen crazy things) and over drives the FPR causing FPR to fail. hard. even 10 new FPR., seen other put external pumps here off Ford Crown VIC. (fail it will)
  • To find the pump fail deal we do the shunt test (ASE tests, standard) we block the return line and see correct pressure. for shunt mode. tests. 95psi normal. the FPR has an adjustment screw. base of TBI, 14.5psi at idle set.
  • Injector leak down test, too, finds leaking injector. TBI. but not on this TBI engine, it goes to 0psi key off. (unlike most cars ever made TBI)
  • page 14-6 in the FSM, chapter 14. read only pages marked for car model 81 and engine 2.5 TBI okay? factory service manual free.
  • a mixture of wrong parts for this car or EFI system. :cry:
 
it won't be vacuum leaks! (assumes FPR is working, if vacuum path fails to FPR or MAP all bests off)
this jeep engine EFI is called the Speed Density system , so that all vacuum leaks cause racing fast RPM at idle and full % HP power and not rich ever.!
vacuum leaks on this engine work just like you stepping on the throttle pedal which is only and air pedal (vacuum leak by you)

this is not a MAF engine say some Toyota, called air density system. at all.
forgot maf inducted engines.


WE test the map and FPR both, first. if engine is assumed good and is a huge age related, thing to assume.
if the map fails to get vacuum to it the ECM can not compute fuel rates now.
The map sensor high (high vac HG inches) causes low injections rates and if VAC is stuck low ( at low Hg inches ) the injection rates are lower.
and all Hg reading between.
The FPR must also not be dead, stuck high or fails to regulate at all speeds and engine loads. all.
here are the mAP readings
i made nice exel graph. as vacuum drops as you near WOT The hg goes low and voltage rises to 4v or more. I made this off my JK.
vacuum is about 20 at idle and for SURE NOT BOUNCING, oops a vacuum gauge finds that.
MAP-sensor-output-real.jpg



chapter 14, your 89, map page 48 states

The voltage
should drop to 1.5 - 2.1 volts with a hot, neutral idle
speed condition.
Test ECU terminal 33 for the same voltage described
above to verify the wire harness condition. Repair as
necessary.
Test the MAP sensor supply voltage at the sensor
connector terminal C with the ignition ON.
The voltage (keyon engine off)
should be 5 volts (±0.5V). Five volts (±0.5V) should also
be at terminal 16 of the ECU wire harness connector.
Repair or replace the wire harness as necessary.

,

and now a simple free test,
unplug the 02 sensor and drive. no more smoke black?
this year car the 02 is too powerful, cured by USA laws 1996,
that means if 02 fails and sticks at 0vdc, it goes way way too rich.
so unplug it.
also exhaust leaks any where near the 02 bung, will cause huge rich conditions. (air is 20 times more oxygen than any o2 sensor can stand)
at its tip running. cracks happen.

last the parting shot.

engine good, spark good, then last is all fueling tests.
ASE.
 
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