Why Your Mercedes W204 C200 CDI Idle and RPM Are Jumping - And What to Do About It

If you’re driving a 2008 Mercedes-Benz W204 C200 CDI with the OM646 manual transmission, you might have noticed the idle revs bouncing between 750 and 850 RPM, sometimes fluctuating by as much as 50 RPM. After about 20 minutes of driving, things settle down, unless you turn on the A/C, which also stabilizes the idle.

But the problem doesn’t stop there: while cruising at 1,500 to 2,000 RPM, the engine revs can jump by 50 RPM, even with the A/C on. On top of that, you’re getting an occasional P0087 fuel rail pressure low code, which triggers a limp mode that limits RPM to 3,000 and disables ESP.

Restarting the car or clearing the code brings everything back to normal, but it happens one to three times a month, despite driving 60 kilometers daily.

You’ve already done some smart troubleshooting. You replaced a cracked hot air intercooler hose, a completely torn PCV membrane, a 12-year-old fuel filter, and a very dirty air filter, all good moves, especially since the car wasn’t driven much before you got it. The fuel rail pressure readings at idle are bouncing between 3,200 and 3,500 PSI, which is a red flag.

That pressure instability, combined with the P0087 code, points to a fuel delivery issue, likely a failing high-pressure fuel pump, a clogged fuel injector, or a faulty fuel pressure regulator. The fact that engaging the clutch at idle raises and stabilizes the RPM to 950 suggests the engine is compensating for a lean condition or a vacuum leak.

The good news: your new fuel filter and air filter rule out those common culprits. But the cracked intercooler hose and torn PCV membrane could have let unmetered air into the system, messing with fuel trims and pressure over time. Even if those parts are replaced, the damage may have already stressed the high-pressure pump or injectors.

At 189,500 kilometers, a fuel system cleaning or injector balance test at a shop like Boca Auto Care could pinpoint the exact cause. In the meantime, check for any remaining vacuum leaks, especially around the intake manifold and turbo hoses, and consider having the fuel rail pressure sensor tested.

If the problem persists, a high-pressure pump replacement or injector refurbishment may be the final fix to banish that limp mode for good.