YJ NO Spark No Fuel 4.0 based engine

[jsawduste]

The other day the YJ 4.8 (based on a 4.0) stroker quit running. Acted like it was runing out of gas.

Now it has no spark and no fuel delivery.

The ECU needs to see a crank signal within 3 seconds or it shuts the fuel pump down. I will have flow during key up (prime) and a few seconds on cranking. So that tells me the ECU is not seeing a crank signal OR since it fires the ignition/fuel pump by grounding the signal. It is not grounding.

Here is where I am at/have checked.

40 psi at the rail.

Jumping circut 30 and 87 in the underhood box (fuel pump ECU relay) powers the pump up.

The other two circuts are staying hot all the time in the FP relay system

Same with the auto shut down circut. Stays hot all the time.

Crank Sensor is reading 190 +/- ohms. Right on spec.

Cleaned and checked the grounds in the engine harness.

Cleaned the ECU connector.

Swapped relays around.

No tach on cranking.

Engine spins over fine but will not fire.

Seems the ECU is ethier not seeing a crank signal or is not processing the signal to ground.

Any thoughts ?


Thanks !!

[actionpaintball]

90% of the time it will be your crank position sensor OR wiring...then I would move onto the PMC/computer.


1. Check the 8-volt power supply to the Cam Angle Sensor (CMP) and Crank Angle Sensor (CKP).

2. Unplug them (cam and crank) and the Vehicle Speed Sensor (VSS) and then turn the key on.

3. If 8 volts is now present, start plugging each of the sensors in again, to see which one is shorted out.


Potential Causes:
Crank Angle Sensor (CKP)
Cam Angle Sensor (CMP)
Vehicle Speed Sensor (VSS)
Powertrain Control Module (PCM)
Wiring

A PCM shut-down, due to a short to ground on the 8 volt power supply. The sensor may not come back to life, after removing the short, unless the key is cycled off and then on.

Also, The Automatic Shut Down (ASD) relay is powered up when the Powertrain Control Module (PCM) sees the signal from the CKP sensor toggling.



Another check:
1. Check the 8-volt power supply to the Camshaft Position (CMP) sensor and Crankshaft Position (CKP) sensor.

2. If OK, check the Grey/Black wire at the CKP sensor for a 0-5 volt square wave signal when cranking.

3. If not a 0-5 volt square wave signal, the CKP sensor is faulty.

[jsawduste]

Originally Posted by actionpaintball

90% of the time it will be your crank position sensor OR wiring...then I would move onto the PMC/computer.


1. Check the 8-volt power supply to the Cam Angle Sensor (CMP) and Crank Angle Sensor (CKP).

2. Unplug them (cam and crank) and the Vehicle Speed Sensor (VSS) and then turn the key on.

3. If 8 volts is now present, start plugging each of the sensors in again, to see which one is shorted out.


Potential Causes:
Crank Angle Sensor (CKP)
Cam Angle Sensor (CMP)
Vehicle Speed Sensor (VSS)
Powertrain Control Module (PCM)
Wiring

A PCM shut-down, due to a short to ground on the 8 volt power supply. The sensor may not come back to life, after removing the short, unless the key is cycled off and then on.

Also, The Automatic Shut Down (ASD) relay is powered up when the Powertrain Control Module (PCM) sees the signal from the CKP sensor toggling.



Another check:
1. Check the 8-volt power supply to the Camshaft Position (CMP) sensor and Crankshaft Position (CKP) sensor.

2. If OK, check the Grey/Black wire at the CKP sensor for a 0-5 volt square wave signal when cranking.

3. If not a 0-5 volt square wave signal, the CKP sensor is faulty.

Thanks ActionPB.

I can go through and check what you have mentioned. Thanks !!

One question I am a bit confused on. You mention an 8 volt power supply that powers up the CKS, CPS. Does that 8 volt supply come off the ECU ? Sorry but I am a bit lost as to where that 8 volts comes from. IE what is the 8 volt power supply and where can I find it ? DUH

[apeters89]

does the 4.0 computer shut off fuel, etc. if there's no oil pressure? I know some engines do. Might be worth checking out.

[actionpaintball]

8 volts are supplied from the computer.

[jsawduste]

Originally Posted by apeters89

does the 4.0 computer shut off fuel, etc. if there's no oil pressure? I know some engines do. Might be worth checking out.

Wondered about that also. The engine is fresh with only a couple thousand mikes on it. Will pull the dist cap and make sure the rotor is turning.

Thanks APB. Will get the VOM back out and do some probing.

[jsawduste]

Originally Posted by actionpaintball

8 volts are supplied from the computer.

Thanks !!

[apeters89]

Originally Posted by jsawduste

Wondered about that also. The engine is fresh with only a couple thousand mikes on it. Will pull the dist cap and make sure the rotor is turning.

Thanks APB. Will get the VOM back out and do some probing.

could be as simple as a sending unit?

[jsawduste]

Turns out it ate a dist. gear. Had a bronze one turning the hi pressure melling pump. Cleaned the teeth off it.

Thanks to all !!

[apeters89]

Originally Posted by jsawduste

Turns out it ate a dist. gear. Had a bronze one turning the hi pressure melling pump. Cleaned the teeth off it.

Thanks to all !!

[actionpaintball]

Thats an oddball-glad you found the problem

(noted)