Diagnosing Air Intrusion and Cylinder Misfire on the Caterpillar 226

Air intrusion in the fuel system of compact loaders such as the Caterpillar 226 can cause sudden misfires, rough running, and cylinder‑specific skipping. When a machine runs smoothly for a few seconds and then begins to stumble, the root cause is often hidden somewhere in the suction side of the fuel system. This article explains how air enters the fuel supply, why symptoms may shift between cylinders, and how experienced mechanics diagnose such problems. Real‑world stories, terminology notes, and practical troubleshooting insights help illustrate the complexity of fuel‑system air leaks.

Initial Symptoms and the Onset of the Problem

The operator of a Caterpillar 226 skid steer noticed that the machine would start cleanly, run smoothly for a few seconds, and then begin skipping on cylinders No. 2 and No. 4. The fuel system quickly filled with air, causing rough operation. Overnight, the air would settle out, allowing the machine to start normally again—only for the symptoms to return moments later.

The issue began after the machine was accidentally tipped over. Once uprighted and restarted the next day, the engine initially skipped on cylinder No. 1. After resealing components and servicing the injection pump, the misfire shifted to cylinders No. 2 and No. 4. This shifting pattern is a classic sign of air intrusion rather than mechanical failure.

Terminology Notes

• Suction Side: The low‑pressure portion of the fuel system between the tank and the transfer pump; the most common location for air leaks.
• Transfer Pump: A low‑pressure pump that supplies fuel to the injection pump.
• Injection Pump Reseal: A service procedure where internal seals are replaced to prevent air or fuel leakage.
• Tank Vent: A small valve or passage that allows air to enter the tank as fuel is consumed; if blocked, it can cause vacuum conditions.

Why Air Intrusion Causes Cylinder‑Specific Misfires

Air entering the fuel system disrupts the steady flow of diesel to the injectors. Because the injection pump distributes fuel sequentially, air pockets may reach certain cylinders first. This can cause:

• Misfire on one cylinder
• Then misfire on another
• Then temporary smooth running
• Then renewed skipping

This shifting pattern is a strong indicator that the problem is not mechanical (such as bent rods or valve issues) but rather fuel‑system aeration.

Troubleshooting Steps Already Attempted

The operator performed an extensive list of repairs and checks, including:

• Resealing all suction‑side components
• Swapping injectors
• Replacing the transfer pump
• Installing a new head gasket
• Checking push rods
• Performing a valve leak‑down test
• Resealing the injection pump

Despite all this work, the problem persisted. This highlights how elusive air leaks can be, especially after a rollover event.

A Simple but Overlooked Cause: The Tank Vent

A seasoned mechanic suggested a simple test: Run the machine with the fuel cap removed.

If the tank vent becomes plugged—especially after a rollover—fuel cannot flow freely. As the transfer pump draws fuel, it creates a vacuum in the tank. Eventually, the pump begins pulling air through the smallest leak in the suction line, even if all fittings appear tight.

A blocked tank vent can cause:

• Air bubbles in the fuel
• Sudden misfires
• Fuel starvation
• Erratic cylinder skipping

This is one of the most common and most overlooked causes of air intrusion.

A Real‑World Story: The Loader That “Fixed Itself” When the Cap Was Removed

A contractor once spent days chasing an air leak on a compact loader. He replaced hoses, clamps, filters, and even the transfer pump. Nothing worked. Out of frustration, he removed the fuel cap—and the machine ran perfectly. The tank vent had been blocked by a small piece of debris. This simple fix saved him from replacing the injection pump unnecessarily.

Other Possible Causes After a Rollover

When a machine tips over, several things can happen inside the fuel system:

• Sediment in the tank may shift and block the pickup
• Hoses may stretch or crack
• Seals may unseat
• The tank vent may clog with debris
• Air may enter through fittings that were previously tight

Even if the machine appears undamaged, the internal fuel pathways may have been disturbed.

Why Air Settles Overnight

Air naturally rises to the top of the fuel system when the machine is off. Overnight, the bubbles migrate upward, allowing the system to temporarily regain normal fuel flow. Once the engine starts and suction increases, the leak reintroduces air, causing the misfire to return.

This pattern—smooth start, rapid deterioration—is a diagnostic hallmark of suction‑side air leaks.

A Case Study: The Shifting Misfire

The operator’s experience—initial misfire on cylinder No. 1, later shifting to No. 2 and No. 4—matches classic air‑intrusion behavior. When the injection pump was resealed, the internal distribution of fuel changed slightly, altering which cylinders received air pockets first. This explains why the misfire “moved” even though the underlying issue remained unchanged.

Conclusion

Air intrusion in the fuel system of a Caterpillar 226 can be extremely difficult to diagnose, especially after a rollover. Even after extensive repairs, a simple issue such as a blocked tank vent or debris in the pickup tube can cause persistent aeration and cylinder misfires. The shifting misfire pattern, overnight settling of air, and rapid return of symptoms all point toward a suction‑side air leak rather than mechanical failure. Real‑world experience shows that sometimes the simplest checks—like removing the fuel cap—can reveal the true cause of a complex problem.