Rough idle has me going crazy

[Green Mesa XJ]

500-550 rpm idle sounds low to me. At least for the 91-92 4.0

Ours normally starts cold at 900-1000 rpm then drops down to 650-700 a few seconds later as it warms up.

Maybe your IAC is going bad. Or maybe you're in the normal range and ours is faulty. I'll see if the FSM has the idle range.

 

Just found out the rough idle my f150 has is because of the O2 sensors are starting to fail. No CEL, apparently the CEL will only come on for a complete failure, only figured it out by watching the voltage readings from the upstream sensors.

Not sure if the O2 would cause the same issue in the old Xj.

 

Almost could think this could be caused a vacuum leak too.

 

 

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[Green Mesa XJ]

Just read through the thread, so this is how the motor ran before the rebuild???

 

 

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[HOrnbrod]

Have you ever checked the ECU 5VDC sensor reference voltage?  A convenient place to check it is at the violet wire of the MAP connector. Check it with the ignition key ON, engine not running and the MAP connector unplugged. Then check it again at the violet wire with the engine running and the MAP connector plugged in by back-probing the connector. In both checks the voltage should be 5V + or - .3VDC.

[SBpunk]

Just read through the thread, so this is how the motor ran before the rebuild???

 

 

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Yes sir. It's a bit better but not much. I've gone through 3 o2 sensors, 3 tps, 2 iac, 2 sets of injectors, 2 sets of spark plugs and a bunch of other stuff. I have a full list at home but short of putting in this new distributor, map sensor and finding another computer it's getting slim on possibilities.

[SBpunk]

Have you ever checked the ECU 5VDC sensor reference voltage? A convenient place to check it is at the violet wire of the MAP connector. Check it with the ignition key ON, engine not running and the MAP connector unplugged. Then check it again at the violet wire with the engine running and the MAP connector plugged in by back-probing the connector. In both checks the voltage should be 5V + or - .3VDC.

I haven't but I will this weekend and report back. Will do that before anything else.

[HOrnbrod]

 

Have you ever checked the ECU 5VDC sensor reference voltage? A convenient place to check it is at the violet wire of the MAP connector. Check it with the ignition key ON, engine not running and the MAP connector unplugged. Then check it again at the violet wire with the engine running and the MAP connector plugged in by back-probing the connector. In both checks the voltage should be 5V + or - .3VDC.

I haven't but I will this weekend and report back. Will do that before anything else.

 

Another thing I'd do that's free and may help is cleaning the 60-pin ECU connector with a good electrical contact cleaner and a toothbrush. Look for any recessed or bent pins also on the ECU connector.

[Green Mesa XJ]

 

Just read through the thread, so this is how the motor ran before the rebuild???

 

 

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Yes sir. It's a bit better but not much. I've gone through 3 o2 sensors, 3 tps, 2 iac, 2 sets of injectors, 2 sets of spark plugs and a bunch of other stuff. I have a full list at home but short of putting in this new distributor, map sensor and finding another computer it's getting slim on possibilities.

ouch! That's a lot of parts to swap test

 

I'm leaning towards wiring problem and/or ecu. But I'd start with some of the checks for map and ecu that have been recommended in the last post or two.

 

 

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[SBpunk]

 

 

Just read through the thread, so this is how the motor ran before the rebuild???

 

 

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Yes sir. It's a bit better but not much. I've gone through 3 o2 sensors, 3 tps, 2 iac, 2 sets of injectors, 2 sets of spark plugs and a bunch of other stuff. I have a full list at home but short of putting in this new distributor, map sensor and finding another computer it's getting slim on possibilities.

ouch! That's a lot of parts to swap test

 

I'm leaning towards wiring problem and/or ecu. But I'd start with some of the checks for map and ecu that have been recommended in the last post or two.

 

 

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Yeah on top of just flat out rebuilding the entire motor. Had low compression on a couple cylinders so it was going to be needed eventually but to drop that much on this damn thing and its still running like crap. If I can figure out a way to keep the idle around 700rpm idle I'll be good. Runs smooth as can be for a few seconds after start up but the idle drops a little then again down to 550ish and starts shaking.

[Green Mesa XJ]

This may sound stupid but how is the alternator? I remember someone had a poor running xj that a new alternator fixed.

 

 

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[Green Mesa XJ]

This might help some, it is how the PCM sets the idle. I would probably check the MAP next then the wiring and ECU/PCM

 

 

IDLE MODE When the engine is at operating temperature, this is a Closed Loop mode. At idle speed, the powertrain control module (PCM) receives inputs from: ² Air conditioning select signal (if equipped) ² Air conditioning request signal (if equipped) ² Battery voltage ² Crankshaft position sensor ² Engine coolant temperature sensor ² Intake manifold air temperature sensor ² Manifold absolute pressure (MAP) sensor ² Throttle position sensor (TPS) ² Camshaft position sensor signal (in the distributor) ² Battery voltage ² Park/neutral switch (gear indicator signal—auto. trans. only) ² Oxygen sensor Based on these inputs, the following occurs: ² Voltage is applied to the fuel injectors with the powertrain control module (PCM). The PCM will then control injection sequence and injector pulse width by turning the ground circuit to each individual injector on and off. ² The PCM monitors the O2S sensor input and adjusts air-fuel ratio by varying injector pulse width. It also adjusts engine idle speed through the idle air control (IAC) motor.  ² The PCM adjusts ignition timing by increasing and decreasing spark advance. ² The PCM operates the A/C compressor clutch through the clutch relay. This happens if A/C has been selected by the vehicle operator and requested by the A/C thermostat. The optional Extended Idle Switch is used to raise the engine idle speed to approximately 1000 rpm. This is when the shifter is in either the Park or Neutral position. A rocker-type 2-wire switch (extended idle switch) is mounted to the instrument panel. This switch will supply a ground circuit to the powertrain control module (PCM). The switch is available only with 4.0L engine when supplied with the optional police package. 

[SBpunk]

Brand new alternator, starter and optima yellow top with new cables lol

[SBpunk]

Havent really gotten to focus on this lately due to my motor starting to boarderline over heat. Anyway I purchased a new distributor from HonrboD that needs to get installed this weekend (after fishing). Before that I need to get the MAP sensor tested. Every morning she's getting harder and harder to start so somethings going on. Going to have a list of semi easy things this weekend. MAP/ECU testing, distrubutor, water pump, thermostat, AC compressor and crankshaft sensor. Lucky I'm on leave all week next week.

[big66440]

Fuel pressure?

If it's getting harder to start you may want to look into this system, it is possible the hose inside of the tank might be " collapsing" causing a loss in pressure and therefore your bad idle or the pump might be on it's way out . Luckily the fuel pumps on our trucks are easily removable as long as you're below 1/2 a tank.

[SBpunk]

 

Fuel pressure?

If it's getting harder to start you may want to look into this system, it is possible the hose inside of the tank might be " collapsing" causing a loss in pressure and therefore your bad idle or the pump might be on it's way out . Luckily the fuel pumps on our trucks are easily removable as long as you're below 1/2 a tank.

 

Tracked it down. Have a small leak somewhere near the gas tank. Just replaced it not long ago and might not have gotten a good seal. Could be causing the loss in pressure. If I'm lucky its just one of the lines leaking since I just filled it up with 90 grade pure gold...

[big66440]

^^^ Good luck, hope this that fixes it :thumbsup:

[SBpunk]

Ok looking like its something in my wiring harness. Swapped out ECU a few weeks ago and no change. That was pretty much the last part. Going to go through all the sensor wiring and test to see which one is failing between the ECU and the sensor itself. Hoping the service manuals give me what readings to look for. So just an update on parts since its damn near a brand new jeep mechanically...
Motor rebuild
Injectors (stock but new)
Spark plugs, wires, cap, rotor, distributor itself (to replace the camshaft sensor)...
New throttle body, IAC, TPS

MAP
O2 sensors
CPS (AX15)

ECU
Fuses everywhere
Battery / cables / terminals
Alternator
Starter (not sure why that would've helped but it was free)
New grounds

Fuel filter / pressure tested the lines

Finally the vacuum lines have all been tested and/or replaced

So still no luck. I'm getting an 8.8 put in now but all that's left that I can think of Is the wiring harness in the engine bay
 

[Ωhm]

Suggestions:

1. Check throttle plate angle (gap) spec.
2. Pin each injector back to ECU for correct firing order.

[SBpunk]

 

Suggestions:

• Check throttle plate angle (gap) spec.
• Pin each injector back to ECU for correct firing order.

Throttle plate will be an easy check. Not sure how to verify the injectors. I have the test lights for them but if have time to explain or a common way to search for a video/how to I'd appreciate it. Honestly I thought it was the throttle body at first since it almost sounds like it's choking every couple seconds not getting air but I've swapped 2 TBs.

[Ωhm]

1. Disconnect ECU
2. Disconnect each injector and verify continuity (0 ohms) between ECU and each injector.

Don't have the pin outs you need. Check wiring diagrams for correct firing order. 

[HOrnbrod]

I know you have swapped your injectors out. Did you verify the harness wire colors to the ECU against the injector number? They could have been mixed up during the swap if you didn't mark them. Below is an injector wiring diagram for a 99 XJ 4.0, but the injector harness wire colors are the same as your 91.

 

91 Injector - to - ECU pin

Injector #1 - Pin 16

Injector #2 - Pin 15

Injector #3 - Pin 14

Injector #4 - Pin 13

Injector #5 - Pin 38

Injector #6 - Pin 58

[SBpunk]

I know you have swapped your injectors out. Did you verify the harness wire colors to the ECU against the injector number? They could have been mixed up during the swap if you didn't mark them. Below is an injector wiring diagram for a 99 XJ 4.0, but the injector harness wire colors are the same as your 91.

 

 

91 ECU Injector - to - ECU pin

Injector #1 - Pin 16

Injector #2 - Pin 15

Injector #3 - Pin 14

Injector #4 - Pin 13

Injector #5 - Pin 38

Injector #6 - Pin 58

I'll check again but I labeled them (1/3/5) with tape but I haven't  checked since the rebuild. Trust but verify hahaha

[HOrnbrod]

Verify the colors, and check for continuity too as Ωhm said.  :thumbsup: