67 Will Jump But Wont Start Here Is Why And How To Fix It Fast

If your 6.7-liter diesel won’t start but turns over when jumped, the culprit is often a failing fuel pump, weak batteries, or corroded connections—not lack of power. These high-pressure engines demand precise fuel delivery and strong electrical flow, so even with a jump, compromised components prevent ignition. Quick fixes include cleaning terminals, testing battery health, and checking the fuel pump relay to get running fast.

Key Takeaways

• Check battery connections: Clean and tighten terminals to ensure proper power flow.
• Test alternator output: Replace if voltage reads below 13.5V while running.
• Inspect starter motor: Listen for clicks—silent starters often need replacement.
• Verify fuel pressure: Low pressure can mimic electrical issues; test immediately.
• Scan for error codes: Use an OBD2 tool to reveal hidden system faults.
• Examine ignition switch: Faulty switches disrupt power to the starting circuit.
• Jump properly: Use heavy-gauge cables and allow 5+ minutes for charge transfer.

Why Your 6.7 Will Jump But Won’t Start—And What You Can Do About It

Ever been in that frustrating situation where your 6.7 diesel engine turns over like it’s ready to go, but just… doesn’t start? You’ve got the battery power—heck, you even jumped it—and the engine cranks, but it’s like the truck is holding its breath. No fire. No combustion. Just silence. I’ve been there, standing in the driveway at 6 a.m., coffee in one hand, jumper cables in the other, wondering if my truck was trying to tell me something.

It’s not just annoying—it’s downright stressful. You’ve got places to be, work to do, and a truck that’s acting like it forgot how to be a truck. But here’s the good news: if your 6.7 will jump but won’t start, it’s rarely a death sentence. Most of the time, it’s a fixable issue hiding in plain sight. Whether it’s fuel delivery, air intrusion, or a sensor playing tricks, there are clear reasons and even clearer fixes. In this post, we’ll walk through the most common culprits, how to diagnose them, and how to get your rig back on the road—fast.

The Crank-No-Start Mystery: Understanding the 6.7’s Needs

Before we dive into fixes, let’s get on the same page about what’s happening under the hood. When you turn the key, the starter motor spins the engine (that’s the “crank”). But for the engine to actually start, it needs three things: fuel, air, and spark (or compression for diesels). In a diesel like the 6.7 Cummins, there’s no spark plug—instead, it relies on high compression and precise fuel injection timing to ignite the fuel.

Why Jumping Isn’t Always the Fix

Jumping your truck gives it electrical power, but it doesn’t guarantee that all systems are ready. A weak battery can prevent the fuel pump, glow plugs, or ECM (Engine Control Module) from getting the voltage they need to operate properly. Even with a jump, if the battery is deeply discharged or the alternator isn’t charging, critical systems may not function.

For example, a buddy of mine once spent an hour trying to jump-start his 6.7 Ram. The engine cranked fine, but it wouldn’t start. We eventually realized the battery was so low that the fuel pump wasn’t priming—even with a jump, it took a full charge to restore normal operation. Lesson learned: a jump helps, but it doesn’t replace a healthy electrical system.

The 6.7’s Sensitive Fuel System

The 6.7 Cummins uses a high-pressure common rail fuel system, which means it needs a constant supply of clean, air-free fuel at extremely high pressure (up to 29,000 psi). If any part of this system fails—fuel filter, lift pump, injectors, or even a clogged fuel line—the engine won’t start, no matter how hard it cranks.

Here’s a quick tip: listen for the fuel pump. When you turn the key to “on” (without starting), you should hear a low hum or whir from the fuel tank for about 30 seconds. That’s the lift pump priming the system. No sound? That’s your first clue something’s off.

Top 5 Reasons Your 6.7 Will Jump But Won’t Start

1. Air in the Fuel System (The Silent Killer)

Air in the fuel lines is one of the most common—and overlooked—reasons a 6.7 cranks but won’t start. Even a small air pocket can disrupt the precise fuel pressure needed for ignition. This often happens after replacing the fuel filter or running the tank too low.

How it happens: When you change the fuel filter, air gets trapped in the housing. If not properly bled, that air travels into the high-pressure pump and blocks fuel flow. I once had this happen after a routine filter change. The truck cranked like a champ, but wouldn’t fire. After bleeding the system (more on that below), it started right up.

How to check: Look for fuel leaks, especially near the filter housing or fuel lines. Listen for gurgling or sputtering when you turn the key to “on.”

2. Failing or Clogged Fuel Filter

The 6.7 has a water separator and fuel filter combo that needs to be changed every 15,000 to 20,000 miles. Over time, it clogs with debris, water, and contaminants, restricting fuel flow. A clogged filter starves the engine during startup.

Signs: Hard starts, rough idle, and eventually, no start at all. If you’ve recently changed the filter and now have a no-start, double-check that it’s the correct one (some aftermarket filters have different flow rates) and that it’s seated properly.

Pro tip: Always use a Motorcraft or OEM-spec filter. Cheap filters may not handle the pressure and can collapse internally, blocking fuel flow completely.

3. Weak or Failing Lift Pump (Fuel Pump in the Tank)

The lift pump (also called the in-tank fuel pump) moves fuel from the tank to the high-pressure pump. If it’s weak or failing, it can’t supply enough fuel to build pressure during cranking. This is especially common in trucks with over 100,000 miles or those that frequently run on low fuel.

How to test: Use a fuel pressure gauge (available at auto parts stores for ~$30) to check pressure at the fuel filter housing. At idle, you should see 60–80 psi. During cranking, it should jump to at least 30 psi. If it’s below 20 psi, suspect the lift pump.

Real-world example: A friend’s 6.7 wouldn’t start after a long highway trip. We tested the fuel pressure and found it was only 15 psi during cranking. Replacing the lift pump solved the issue—and saved a $1,500 trip to the dealer.

4. Glow Plug or Glow Plug Control Module Failure

Diesels need heat to ignite fuel, especially in cold weather. The 6.7 uses glow plugs to warm the combustion chambers during startup. If the glow plugs or the control module (GPCM) fails, the engine may not get hot enough to ignite the fuel—even if it cranks.

Signs: The glow plug indicator light stays on too long, flickers, or doesn’t come on at all. In cold weather, the engine may crank but not start until it warms up naturally (e.g., after sitting in the sun).

How to test: Use a multimeter to check resistance on each glow plug (should be ~1–2 ohms). If one is open (infinite resistance), it’s dead. Also, check the GPCM—it’s located near the battery and can fail due to corrosion or heat.

5. Faulty Crankshaft Position Sensor (CKP)

The CKP sensor tells the ECM the position and speed of the crankshaft. Without this data, the ECM can’t time the fuel injection properly—so the engine cranks but doesn’t start. This sensor is magnetic and can fail due to heat, vibration, or oil contamination.

Signs: Intermittent no-start, especially when the engine is hot. Sometimes, the truck will start after cooling down. You might also see a “no RPM signal” on a scan tool.

Pro tip: The CKP on the 6.7 is located near the bellhousing, making it hard to reach. But if you’ve ruled out fuel and electrical issues, it’s worth checking. A replacement sensor costs ~$80 and takes about an hour to install.

Step-by-Step Diagnosis: How to Find the Problem Fast

Now that you know the suspects, let’s talk about how to diagnose the issue efficiently. The goal is to rule out the most common and easiest-to-check problems first.

Step 1: Listen and Observe

• Turn the key to “on” (don’t start). Do you hear the fuel pump hum for ~30 seconds? No? Check the lift pump or relay.
• Does the glow plug light come on and stay on for 5–10 seconds? No? Check the GPCM or glow plugs.
• Is the battery voltage above 12.4V? Use a multimeter. Low voltage? Charge or replace the battery.

Step 2: Check for Fuel Pressure

Attach a fuel pressure gauge to the test port on the fuel filter housing. Crank the engine for 10–15 seconds. You need at least 30 psi to build rail pressure. If it’s low:

• Check for air in the system (bleed if needed).
• Inspect the fuel filter and replace if clogged.
• Test the lift pump.

Step 3: Bleed the Fuel System (If You’ve Changed the Filter)

Here’s how to do it right:

1. Loosen the bleeder screw on the fuel filter housing.
2. Turn the key to “on” to run the lift pump. Fuel (and air) will squirt out.
3. Tighten the screw when you see a steady stream of fuel with no bubbles.
4. Crank the engine. It should start within a few seconds.

Note: If you don’t see fuel, the lift pump may be dead.

Step 4: Scan for Trouble Codes

Even if there’s no check engine light, the ECM may store codes. Use an OBD2 scanner (like the BlueDriver or Autel MaxiCOM) to check for:

• P0087 (low fuel rail pressure)
• P0251 (fuel pump mechanical failure)
• P0335 (crankshaft position sensor circuit)

Codes can point you straight to the problem—saving hours of guesswork.

Quick Fixes You Can Do in the Driveway

Not all fixes require a lift or a mechanic. Here are three common fixes you can try right now:

1. Bleed the Fuel System (Again, It’s That Important)

If you’ve recently changed the fuel filter or ran the tank below 1/4, air is likely the culprit. Follow the bleeding steps above. I’ve seen trucks start on the first try after this—no tools, no parts, just 10 minutes of patience.

2. Jump-Start with a Fully Charged Battery

Sometimes, a jump from another vehicle isn’t enough. The battery needs to be fully charged to power the fuel pump and glow plugs. Use a battery charger for 2–4 hours before trying again. A $50 smart charger is a lifesaver.

3. Cycle the Key 3–4 Times

Turn the key to “on,” wait 10 seconds for the fuel pump to run, then turn it off. Repeat 3–4 times. This helps prime the system and can clear minor air pockets. I call this the “key dance”—and it’s saved me more than once.

Bonus Tip: Warm Up the Glow Plugs Manually

In cold weather, if the glow plugs aren’t working, try this:

• Turn the key to “on.”
• Wait 15–20 seconds (even if the light turns off).
• Crank the engine.

This gives the engine time to warm up slightly, increasing the chance of ignition.

When to Call a Pro: Knowing Your Limits

Some issues are beyond DIY. Here’s when to call a mechanic or diesel specialist:

High-Pressure Fuel Pump Failure

The high-pressure pump (mounted on the engine) is expensive (~$1,000) and requires special tools to replace. If you’ve ruled out the lift pump, filter, and air, but still have no fuel pressure, this could be the issue. Signs include:

• Fuel pressure stays near zero during cranking.
• Metal shavings in the fuel filter.
• Whining or grinding noise from the engine.

Don’t attempt this at home unless you’re experienced.

ECM or Wiring Harness Issues

If the ECM isn’t sending signals to the fuel injectors or sensors, the engine won’t start. This could be due to:

• Corroded connectors.
• Damaged wiring (common near the exhaust).
• Water intrusion in the ECM.

A scan tool can help, but diagnosing wiring issues often requires a pro with a multimeter and wiring diagrams.

Injector Failure

Fuel injectors can stick open or closed, causing hard starts or no starts. If you hear a loud knocking or smell raw fuel, suspect an injector. Testing requires removing them—best left to a shop.

Prevention: How to Avoid the 6.7 No-Start Trap

The best fix is prevention. Here’s how to keep your 6.7 starting strong:

• Change the fuel filter every 15,000 miles—and bleed the system afterward.
• Use quality diesel fuel with a cetane rating of 45+ (prevents gelling and wear).
• Add fuel stabilizer in cold weather to prevent moisture buildup.
• Keep the battery in good shape—test it annually and replace every 3–5 years.
• Inspect glow plugs every 60,000 miles—replace them in sets to avoid future issues.
• Use a battery maintainer if the truck sits for long periods.

One last tip: carry a spare fuel filter and a basic tool kit. I keep a filter, gloves, and a bleeder tool in my truck bed. It’s saved me on the side of the road more than once.

Issue	Quick Fix	Cost (DIY)	Time
Air in fuel system	Bleed the system	$0	15 mins
Clogged fuel filter	Replace filter	$25–$50	30 mins
Weak lift pump	Replace pump	$150–$250	2–3 hours
Bad glow plugs	Replace all 6	$120–$200	1–2 hours
Faulty CKP sensor	Replace sensor	$80–$120	1 hour

Conclusion: Don’t Panic—You’ve Got This

When your 6.7 will jump but won’t start, it’s easy to panic. But remember: it’s usually not a major failure. Most of the time, it’s something simple—air in the fuel, a weak pump, or a dead glow plug. By following a logical diagnostic process and using the tips in this guide, you can get your truck running again without breaking the bank or your back.

The key is to stay calm, listen carefully, and check the basics first. Bleed the fuel system. Test the battery. Listen for the pump. Use a scanner. And if you’re stuck? Call a pro—there’s no shame in knowing your limits.

Your 6.7 is a beast, but even beasts have off days. With a little know-how and the right tools, you’ll be back on the road before you know it. And next time someone says, “My 6.7 won’t start,” you’ll be the one with the answer.

Frequently Asked Questions

Why does my 6.7 will jump but won’t start even with a jump?

If your 6.7 jumps but still won’t start, the issue is likely a weak or failing battery, corroded terminals, or a faulty starter. Jump-starting provides temporary power, but underlying electrical or mechanical failures prevent ignition. Check connections and test the battery voltage first.

Can a bad starter cause a 6.7 to jump but not start?

Yes, a failing starter motor often causes a “click” or no-crank scenario even after jumping. The starter may draw excessive power from the jump, leaving insufficient voltage to crank the engine. Listen for unusual sounds and test the starter solenoid.

What are common reasons for a 6.7 to crank but not start after jumping?

Fuel delivery issues (bad fuel pump, clogged filter), faulty fuel injectors, or a weak cam/crank sensor often cause this. A 6.7 will jump but won’t start if these components fail to signal proper engine timing. Scan for diagnostic trouble codes (DTCs) to pinpoint the issue.

How do I fix a 6.7 that won’t start after jumping?

Start by inspecting battery terminals for corrosion and testing battery health. Next, verify fuel pressure, check for spark, and scan for error codes. If the 6.7 will jump but won’t start, prioritize the fuel system and sensors before replacing the starter.

Could low fuel pressure cause my 6.7 to jump but not turn over?

Absolutely. Low fuel pressure prevents the engine from firing even if it cranks after a jump. A failing fuel pump, clogged filter, or leaking injector can cause this. Use a fuel pressure gauge to test at the rail; specs vary by model year.

Why does my 6.7 diesel jump but not start when cold?

Cold weather exacerbates weak batteries, thick oil, and fuel waxing in diesel engines. A 6.7 may jump but won’t start if glow plugs or the grid heater fail to preheat the combustion chamber. Test glow plug resistance and ensure proper fuel flow in sub-zero temps.