Hard Start and Rough Idle on a 2010 TDI
A 2010 Volkswagen Jetta TDI with a 2.0L common-rail diesel has a hard start when cold and a rough idle. The engine cranks for 5-7 seconds before starting, and then idles with a noticeable shake. There is a P0087 (fuel rail pressure too low) DTC stored. The fuel filter was replaced 2,000 miles ago. A scan tool shows actual fuel rail pressure at 2,000 psi during cranking, while desired is 4,000 psi. At idle, actual rail pressure is 4,500 psi, desired is 5,000 psi. The in-tank lift pump delivers 5 psi, which is within spec. The high-pressure fuel pump (HPFP) is driven by the timing belt. A fuel return flow test from the rail shows 50 ml in 30 seconds, while spec is 20-40 ml. The injector return flow rates are all within 5% of each other. The fuel pressure regulator on the rail shows a resistance of 5 ohms, spec is 4-6 ohms.
1. What is the most likely cause of the low rail pressure and hard start?
Read the worked answer
The most likely cause is a worn or failing high-pressure fuel pump (HPFP) or a leaking fuel pressure regulator. The low rail pressure during cranking (2,000 psi vs. 4,000 psi desired) indicates the HPFP cannot build sufficient pressure. The return flow from the rail is higher than spec (50 ml vs. 20-40 ml), suggesting excessive fuel is bypassing, either through the pressure regulator or through internal leakage in the HPFP. The injector return rates are normal, so the injectors are likely not the cause. The fuel pressure regulator resistance is within spec, but it could still be sticking or leaking internally. To confirm, perform a leak-off test on the HPFP or command the regulator closed with a scan tool and observe rail pressure. If pressure builds, the regulator is suspect. If not, the HPFP is likely worn. Also, check the fuel volume from the lift pump to ensure it is adequate; 5 psi is within spec, but flow volume should also be checked.
2. What further tests would you perform to pinpoint the faulty component, and what safety precautions are necessary?
Read the worked answer
To pinpoint the faulty component, perform a high-pressure fuel pump leak-off test: disconnect the return line from the HPFP and crank the engine, measuring the return volume. If it exceeds spec, the pump is worn. Also, use a scan tool to command the fuel pressure regulator to full closed and observe if rail pressure rises; if it does, the regulator was leaking. Additionally, inspect the fuel for contamination (e.g., metal particles) which can indicate HPFP failure. Safety precautions: diesel fuel is flammable and under high pressure; wear safety glasses and gloves, avoid skin contact, and keep sparks and flames away. Relieve fuel pressure before opening the system. Use a suitable container for fuel and dispose of it properly. When cranking, ensure the engine is in a well-ventilated area.