Here's a breakdown of the likely culprits and how to approach diagnosing them:
1. Internal Engine Damage (Most Likely):
* Connecting Rod Bearing Failure: This is a serious issue. A knocking sound that's worse under load (in gear) is a classic sign. A connecting rod bearing in cylinder 3 is failing and causing the knock. This requires engine repair or replacement. Driving the truck with a connecting rod bearing issue will cause catastrophic engine damage.
* Camshaft Issues: While less likely to cause a knock *only* under load, a problem with the camshaft lobe for cylinder 3 (worn or damaged) could lead to poor performance and a change in sound under load.
* Piston Issues: A damaged piston or piston ring in cylinder 3 could also contribute to the knocking and poor performance.
2. Less Likely (but still possible) Causes:
* Valve Train Problem (Cylinder 3): A sticking or damaged valve in cylinder 3 could cause poor compression and a knock-like sound. A compression test (see below) will help diagnose this.
* Exhaust System Restriction: While less likely to cause a significant knock, a severely clogged exhaust system, particularly from cylinder 3's exhaust port, could affect performance.
Troubleshooting Steps:
1. Compression Test: This is crucial. A compression test will measure the pressure in each cylinder. Significantly lower compression in cylinder 3 confirms internal engine damage. You'll need a compression tester to perform this.
2. Leak-Down Test (More Advanced): This test identifies where the compression is leaking – past the rings, valves, or head gasket. It provides more specific information than a compression test.
3. Visual Inspection: If possible, inspect the engine for any obvious damage or leaks. Look around the cylinder 3 area for anything unusual.
4. Listen Carefully: Try to pinpoint the location of the knock using a long screwdriver or mechanics stethoscope. This can help determine if the noise is coming from the connecting rod, piston, or elsewhere.
Important Considerations:
* Do Not Continue Driving: Driving the truck with a potential connecting rod bearing failure is extremely risky. You risk total engine seizure and extensive damage.
* Professional Mechanic: Given the symptoms, it's highly recommended to take your truck to a qualified mechanic for diagnosis and repair. They have the tools and expertise to diagnose the problem accurately and safely.
The improved running with cylinder 3 disconnected only strengthens the case for a serious mechanical issue within that cylinder. Ignoring this could lead to a much more expensive repair later. Get it checked out as soon as possible.
