93 YJ 2.5 5-speed; crank, no start, no check engine light

[93WhyJ]

Hey Folks, hoping to get some advice for my new-to-me YJ.

The issues began with not having a check engine light, no fuel pump prime and hard starts. Then low power, sputtering, back firing and stalling when driving.

Checking the forums, all issues pointed to the PCM, so I removed it, replaced the capacitors with new Panasonic 220uF 25V capacitors.

Now the fuel pump primes on RUN, the starter cranks, but the engine won't fire and I still don't have a check engine light.

I pulled the gauge cluster and checked all of the bulbs, even swapping the working seat-belt light to the check engine spot, just to be sure.

I pulled a spark-plug and have spark, and there is pressure at the fuel rail when checking the Schrader valve, though I haven't put a gauge on test pressure.

I checked the DOME fuse, it was fine, I checked the fuse slot with a volt meter and test light, both tested fine.

I tried swapping the ASD and Horn relays, the connections looked fine, but it made no difference.

I tried connecting an OBD 1 scanner, which says "no connection found"

I'm really hoping for some direction on what to look for next. Are there any other spots to check on the PCM, or anywhere else?

 

[Chris]

Hey Folks, hoping to get some advice for my new-to-me YJ.

The issues began with not having a check engine light, no fuel pump prime and hard starts. Then low power, sputtering, back firing and stalling when driving.

Checking the forums, all issues pointed to the PCM, so I removed it, replaced the capacitors with new Panasonic 220uF 25V capacitors.

Now the fuel pump primes on RUN, the starter cranks, but the engine won't fire and I still don't have a check engine light.

I pulled the gauge cluster and checked all of the bulbs, even swapping the working seat-belt light to the check engine spot, just to be sure.

I pulled a spark-plug and have spark, and there is pressure at the fuel rail when checking the Schrader valve, though I haven't put a gauge on test pressure.

I checked the DOME fuse, it was fine, I checked the fuse slot with a volt meter and test light, both tested fine.

I tried swapping the ASD and Horn relays, the connections looked fine, but it made no difference.

I tried connecting an OBD 1 scanner, which says "no connection found"

I'm really hoping for some direction on what to look for next. Are there any other spots to check on the PCM, or anywhere else?

1. **Verify Ground Connections**: Check the ground connections for the PCM and other engine components. A poor ground could prevent the PCM from functioning correctly and lead to your current issues. Make sure all grounds are clean and secure.

2. **Check Wiring Harness**: Inspect the wiring harness going to the PCM for any signs of damage, corrosion, or loose connections. Pay close attention to the connectors to ensure they are seated properly.

3. **Check the Crank Position Sensor (CPS)**: Since your engine is cranking but not starting, the CPS might be the culprit. If it’s faulty, it can prevent the engine from running, as it provides critical information to the PCM for ignition timing.

4. **Fuel Pressure**: Although you mentioned having pressure at the fuel rail, it’s good to check the actual fuel pressure with a gauge to ensure it’s within specifications. Insufficient fuel pressure could lead to starting issues.

5. **Check for Additional Fuses**: Some Jeep models have hidden fuses for components related to the engine management system. Double-check all fuse boxes and under-hood connectors for any overlooked fuses.

6. **Inspect the Ignition System**: Since you confirmed spark, try replacing the spark plugs and wires if they're old. Sometimes, a weak ignition signal can lead to hard starting and rough running.

7. **System Reset**: Disconnect the battery for a few minutes to reset the PCM. This might help if there’s a temporary glitch.

8. **Use a Different OBD I Scanner**: Since your current scanner isn't connecting, try a different OBD I scanner if possible. There may be compatibility issues with your current unit.

9. **Further PCM Inspection**: If you’re comfortable, you could conduct a more detailed inspection of the PCM (if not done yet). Look for any other signs of damage like burnt traces or components. I'd highly suggest https://wranglerfix.com for PCM testing and replacement.

 

[93WhyJ]

1. **Verify Ground Connections**: Check the ground connections for the PCM and other engine components. A poor ground could prevent the PCM from functioning correctly and lead to your current issues. Make sure all grounds are clean and secure.

2. **Check Wiring Harness**: Inspect the wiring harness going to the PCM for any signs of damage, corrosion, or loose connections. Pay close attention to the connectors to ensure they are seated properly.

3. **Check the Crank Position Sensor (CPS)**: Since your engine is cranking but not starting, the CPS might be the culprit. If it’s faulty, it can prevent the engine from running, as it provides critical information to the PCM for ignition timing.

4. **Fuel Pressure**: Although you mentioned having pressure at the fuel rail, it’s good to check the actual fuel pressure with a gauge to ensure it’s within specifications. Insufficient fuel pressure could lead to starting issues.

5. **Check for Additional Fuses**: Some Jeep models have hidden fuses for components related to the engine management system. Double-check all fuse boxes and under-hood connectors for any overlooked fuses.

6. **Inspect the Ignition System**: Since you confirmed spark, try replacing the spark plugs and wires if they're old. Sometimes, a weak ignition signal can lead to hard starting and rough running.

7. **System Reset**: Disconnect the battery for a few minutes to reset the PCM. This might help if there’s a temporary glitch.

8. **Use a Different OBD I Scanner**: Since your current scanner isn't connecting, try a different OBD I scanner if possible. There may be compatibility issues with your current unit.

9. **Further PCM Inspection**: If you’re comfortable, you could conduct a more detailed inspection of the PCM (if not done yet). Look for any other signs of damage like burnt traces or components. I'd highly suggest https://wranglerfix.com for PCM testing and replacement.

Thanks for the in-depth response, I managed to get it to start last night, turns out my fuel gauge is not accurate, $14 in fuel tool me from 1/4 tank (actually empty) to 7/8 full, I then boosted the battery to 13V and it came to life!

I still don't have a check engine light, so I'm going to follow the other steps listed and go from there.

I'm planning on a tune up, new ignition components, sensors, jeep-cables big-7 kit and a fresh battery. I was reading that if your battery isn't supplying 13v or greater, it can do funky things with the PCM.

I'll update with my findings for anyone that comes across this in the future.

 

[Plastic Fury]

Thanks for the in-depth response, I managed to get it to start last night, turns out my fuel gauge is not accurate, $14 in fuel tool me from 1/4 tank (actually empty) to 7/8 full, I then boosted the battery to 13V and it came to life!

I still don't have a check engine light, so I'm going to follow the other steps listed and go from there.

I'm planning on a tune up, new ignition components, sensors, jeep-cables big-7 kit and a fresh battery. I was reading that if your battery isn't supplying 13v or greater, it can do funky things with the PCM.

I'll update with my findings for anyone that comes across this in the future.

I definitely recommend the Big 7 kit. That was the very first thing I did to my YJ when I got it. The battery cables were corroded and the ground strap from the cylinder head to the firewall was almost severed. Rather then spend the next few years chasing down old grounds and other possible corroded wires, I opted for that kit and just swapped it all out. Cleaned all grounds while doing so and used dielectric grease on the connections. Well worth it to me.

Stupid question but have you checked the bulb in the instrument panel for your check engine light? Could be burned out or could just have been removed by a previous owner. I've seen that before lol. Worth a quick look if you haven't done so just to confirm the bulb is working.

 

[93WhyJ]

I definitely recommend the Big 7 kit. That was the very first thing I did to my YJ when I got it. The battery cables were corroded and the ground strap from the cylinder head to the firewall was almost severed. Rather then spend the next few years chasing down old grounds and other possible corroded wires, I opted for that kit and just swapped it all out. Cleaned all grounds while doing so and used dielectric grease on the connections. Well worth it to me.

Stupid question but have you checked the bulb in the instrument panel for your check engine light? Could be burned out or could just have been removed by a previous owner. I've seen that before lol. Worth a quick look if you haven't done so just to confirm the bulb is working.

Awesome! That's good to know, the kit seems to be well reviewed. It feels like a worthwhile investment to me.

And yep, I checked the bulb, and even swapped it with the only known working one, the seatbelt warning light. Good tip though, I'm really hoping the lights not currently working is something simple like that, haha

 

[89redyj]

Battery ok? At rest VDC across the terminals should read 12.6 minimum. Alternator output should be 13.7VDC to 14.8VDC with very small tolerances allowed. Also, remember the the regulator is inside the alternator to convert ACV to DCV.

But remember, voltage 'good voltage' does not indicate that the amperage needed to fire-up the starter then start the engine is in the battery. Many times a '12 VDC' battery of all sizes will not amp up and that is due to sulfation of the lead plates in a wet type battery. Think corrosion on the plates. My Shcumacher charger will go into a de-sulfication mode and takes about 10 hours for it to charge as it is removing the scale on the plates over that same period.

If your battery is not permanently sealed, look in the six (6) holes for each plate. Wear some goggles and disposable nitrile gloves and check on the sulfuric acid and distilled water mix? Top off with distilled water. I have added about 1/4 cup of sulfuric acid to a cup of distilled water to try and resuscitate a MC battery, riding mower battery and the Die Hard in my '89 2.5L YJ. Most of the times it works and I can get another year out of the battery but watch your voltage gauge. If it does not work, the battery is gooched and must be replaced.

I gleeped this from the Internet:

Common Signs of a Bad Voltage Regulator:

• Dead Battery: The most obvious sign. A bad regulator may not be charging the battery properly, leading to a dead or rapidly failing battery.
  
  • Dim or Flickering Headlights: As the voltage regulator struggles to maintain a stable output, headlights and other lights may dim or flicker.
    
    
  • Irregular Engine Behavior: The engine may stall, misfire, or have difficulty starting.
    
    
  • Burning Smell: A burning or rubbery smell can indicate a failing alternator and its regulator.
    
    
  • Battery Warning Light: The battery warning light on the dashboard may illuminate or blink.
    
    
  • Excessive Battery Voltage: A bad regulator might cause the voltage to rise above the normal 14.5V, potentially damaging other electrical components.
    
    
  • Battery Corrosion: Corrosion on the battery terminals, especially positive, can indicate overcharging.
    

Other Potential Indicators:

• Difficulty Starting: If your car is slow to start or needs multiple attempts to start, it could be a sign of a failing alternator and its regulator.
  
  • Frequent Stalling: A malfunctioning alternator may not provide enough power to keep the engine running, leading to frequent stalling.
    
    
  • Flickering Instrument Cluster: The instrument cluster lights may flicker or dim.

rsa (retired electronics engineer)