* Low oil pressure: 15 psi under load is critically low for a Chevy 350. This suggests insufficient oil lubrication, leading to metal-to-metal contact and knocking. The likely culprits are:
* Worn bearings: This is the most probable cause. The main and/or connecting rod bearings are worn, allowing excessive clearance and oil leakage.
* Worn oil pump: The pump may not be delivering enough oil. This could be due to wear within the pump itself or issues with the drive system.
* Clogged oil filter or pickup: A clogged filter or pickup restricts oil flow.
* Low oil level: Always check your oil level before driving. Low oil is an obvious cause of low oil pressure.
* Knock on cold start: This could be exacerbated by the low oil pressure. The lack of lubrication during cold startup, when there's increased friction, could cause the knock. However, it could also be due to other reasons, including:
* Connecting rod knock (related to low oil pressure): This is a serious issue that gets worse as the engine warms up.
* Piston slap: This is less serious than a rod knock and is often more pronounced when the engine is cold. It is a sound that comes from the piston slapping the cylinder wall. This can sometimes be caused by worn piston rings.
* Valve train noise: While less likely, noisy lifters or rocker arms can cause a knocking sound on cold starts, but this wouldn't usually be connected to low oil pressure.
Why a bad sensor is unlikely: While a faulty oil pressure sensor could give you a *false* reading, it wouldn't *cause* the low oil pressure. A faulty sensor would show a low reading, even if the pressure was actually okay. In your case, the low pressure is real and dangerous.
What you should do:
1. Do NOT drive the vehicle: Continuing to drive with low oil pressure will almost certainly result in a seized engine – a very costly repair.
2. Check the oil level: Make absolutely sure the oil level is correct.
3. Have the oil pressure checked with a mechanical gauge: Don't rely on the gauge on your dashboard. A mechanic can connect a mechanical gauge to get an accurate reading.
4. Take it to a mechanic immediately: They will need to diagnose the problem properly and determine the extent of the damage. The low oil pressure and knocking indicate serious internal engine problems. Repair costs could be substantial depending on the extent of damage.
Ignoring this issue will almost certainly lead to major engine failure. Get it checked as soon as possible.
