Owner's Problem Description
The owner reports that their 1997 Mercedes Benz 280 engine is not firing and is burning oil. They have confirmed the timing chain is set correctly at top dead center (TDC) using the manual guidelines, but they suspect potential issues with the valve job due to inadequate tools used during installation. The engine cranks normally but emits smoke from the intake, suggesting oil burning, possibly from worn engine heads. They are considering testing the rebuilt engine head that was previously installed for $45.
Professional Technical Analysis
The 1997 Mercedes Benz 280, powered by a 2.8L inline-six engine, has reported issues with engine firing and oil consumption. With an estimated mileage of 150,000 miles, the symptoms include a lack of firing on cranking and visible smoke from the intake manifold, indicative of oil burning. The owner has attempted to set the timing correctly using the crankshaft '0T' mark and camshaft guides, which is critical for proper ignition timing. A misaligned timing chain could lead to severe performance issues. The smoke emission suggests that oil is entering the combustion chamber, often a result of worn valve seals, piston rings, or cylinder head gasket failures. Furthermore, the use of a rebuilt engine head raises concerns about its condition. This could be a contributing factor if the head was not machined correctly or if the valve job was improperly executed, leading to oil leaks into the intake. The PCM might not be receiving accurate feedback if the engine experiences misfires, which could also affect fuel delivery and ignition timing. Safety concerns arise with the emission of smoke, as it could indicate operational inefficiencies or further engine damage. Therefore, a thorough inspection of the valve job, along with testing oil pressure and conducting a compression test, is essential to determine the root cause of the oil burning and firing issues.
Possible Causes
Most common causes (ordered by frequency):
- Worn Valve Seals: Excessive wear on valve seals can allow oil to seep into the combustion chamber, particularly when the engine is cranked, causing smoke to emit from the intake. On the 2.8L engine, the valve seals are typically rated for 100,000 miles and can degrade with heat and mileage. If the seals have hardened or cracked, they cannot maintain a proper seal, leading to oil burning. - Visible smoke from the intake upon cranking, possible codes for misfires or rich conditions.
- Improper Valve Job: If the valve job was not executed correctly, it could lead to poor sealing of the combustion chamber, increasing the likelihood of oil entering the chamber. This is particularly relevant if the rebuilt head was not machined to the correct specifications, resulting in inadequate seating of the valves. - Smoke emission during cranking, potential compression loss in cylinder tests.
- Worn Piston Rings: The piston rings may be worn, allowing oil to bypass into the combustion chamber. This typically results in blue smoke and can also lead to lower compression in the affected cylinder. A compression test revealing less than 130 PSI could indicate this issue, especially in higher mileage engines. - Blue smoke from the tailpipe during acceleration, compression readings lower than expected.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial Compression Test: Perform a compression test on all cylinders to determine the health of the piston rings and valve seals. Expect values around 150 PSI for a healthy engine. If any cylinder shows significantly lower readings, this may indicate worn piston rings or valve sealing issues.
- STEP 2 - Visual Inspection of Valve Components: Remove the valve cover to inspect the condition of the valve components, including the seals and springs. Check for signs of wear, damage, or improper installation. This inspection can reveal whether the valve job was performed correctly.
- STEP 3 - Oil Consumption Test: Monitor oil consumption over a set period while running the engine. This will help determine the rate of oil burning and can confirm if the engine is consuming oil due to internal leaks.
- STEP 4 - Valve Seal Replacement: If worn valve seals are confirmed, replace them with OEM parts. Ensure the engine is cooled and depressurized before starting this procedure. Use the correct torque specifications when reassembling the components to avoid future leaks.
