I have a 2005 Ford Escape 2.3 L engine, there is a knocking type sound as soon as the engine is started and the speed of the "knocking" follows the RPM.
How can I be sure that it is in fact a rod knocking and not something else?
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When you start the engine and you hear the sound, does it sound better after the vehicle is warmed up?
A rod knock will only sound worse (louder) as the engine heats up. It will not go away as the engine gets warmer. If it does, it is probably something like an exhaust leak which closes itself as the engine manifolds get warm. The opposite could be true as well, where the noise comes on after it gets warmed up, but usually when it happens like this, it is pretty evident where an exhaust leak is at due to excessive soot buildup (black marking) at the leak on the exhaust manifold.
Has the noise stayed the same over time?
A rod knock will only get worse over time. If the noise is a light tap which has only stayed the same, it may be lifter/tappet/cam noise.
If there is an oil pressure gauge, did the pressure go up when you first heard the noise, then become much lower than it should be?
The cause of a rod knock is usually due to a spun rod bearing (could also just be a worn out bearing over time, but much less common). If the bearing spins inside its home in the large end of the connecting rod, oil pressure will first go up, due to less flow of oil where there normally would be. Then as the bearing quickly wears down, the oil flow will be much more due to there being a much larger clearance and an easy path for the oil to follow. The pressure going up at first may not always hold true, but you will see a large loss of oil pressure in a short period of time. How much? It depends on the engine and the amount of damage.
What were you doing at the time of the start of the noise?
Usually, a spun bearing happens because either you were doing something stupid with an old engine (revving an engine with 200k miles on it up to redline) or you ran it dry of oil. They can spontaneously spin the bearing, but that is not very common.
Rod knocks are very uncommon in engines where there is low mileage. Also, consider the maintenance history of the engine. If the oil was changed every 3k miles with a new filter, it's a lot less likely to happen. Newer engines are designed to last a long time and usually do.
While these are common items to consider with a rod knock, there is only one sure way to tell for sure and that is to tear the engine down. If you drive the engine long enough with a spun bearing, you can expect sooner or later to cause catastrophic damage to the engine with a rod being thrown through the side of the block, at which point you'll be in for much worse than a rebuild, but a replacement (plus, there'll be a huge mess to clean up). More than likely if you consider it is to be a rebuild, you should probably just look for a decent used motor to replace it. The cost will be much cheaper in the long run. You'll need to know the service history of an engine prior to purchase, but it will probably outlast the vehicle at that point.
Sometimes it is hard to tell the difference between a rod knock, wrist pin or piston slap in the early stages even when using a mechanics stethoscope. A main knock is a deeper sound that is more pronounced at the bottom of the block as apposed to the top when using a stethoscope.
Hard knocks that happen at startup and go away quickly are a sure sign it is an early failure and oil related to a rod or main knock, piston slap against the cylinder wall and wrist pin knocks will persist after warm up and get worse with engine rpm.
It takes a trained ear with a stethoscope to diagnose this problem with any accuracy.
Rod or main knocks will get more pronounced with increased miles on the engine, they will get much louder sooner than piston slap or wrist pins.
Pre-ignition or Detonation can mimic these knock sounds, and is a subject by itself.
Thank you for your interest in this question.
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