12d • Cummins 12 valve P-pump
Repair Guessing Game!
1997 Dodge Ram 3500 – 12-Valve Cummins (P7100) – Need Help Diagnosing
I've been chasing a fuel-related issue for a while now, and here's the timeline.
Original Symptoms
- Truck would randomly stall when put into reverse.
- Replacing the fuel filter would temporarily fix the problem.
What I Tried
- Replaced the mechanical lift pump.
- The truck improved but would still stall occasionally.
- Increased the idle speed with the idle screw, and that seemed to solve the problem for a while.
Problem Returned
Eventually, the same symptoms came back. I repeated the previous repairs, but this time nothing fixed it.
FASS Conversion
Thinking the factory fuel system was the culprit, I:
- Deleted the factory fuel filter housing.
- Removed the mechanical lift pump.
- Installed a block-off plate.
- Installed a FASS fuel system.
Current Symptoms
- The FASS primes with the key and supplies fuel to the P7100.
- The truck will not start on diesel alone.
- It starts immediately with a small amount of starting fluid.
- Once started, it will sometimes idle on its own, but other times it needs throttle to stay running.
- Without starting fluid, it only cranks—there's no attempt to fire.
After spending a lot of time troubleshooting, I finally decided to take it to a diesel shop for a proper diagnosis.
Based on this timeline, what would your diagnosis be?
I'll update this post with the shop's findings once they call me.
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Repair Guessing Game!
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