Crankcase ventilation hose and rough idle

Tags: #problem

Location and common failure modes. The crankcase ventilation hose is located between the intake manifold and the engine block. Common issues: the hose isn't reinstalled correctly after service, or it cracks over time. Problems it causes: whistling sounds, breather-system malfunction, and — as the case below illustrates — rough idle and drivability symptoms serious enough to be mistaken for several other faults^12345678.

Real-world case — misdiagnosed for weeks after a thermostat/intake-gasket job. After a thermostat and intake-manifold-gasket replacement, one owner (Ксения) experienced: rough idle (RPM staying elevated), jerking on acceleration, a stiff brake pedal, and a non-functional handbrake in Park. A long diagnostic thread considered — and ruled out — throttle-sensor adjustment, the idle air control sensor, spark plugs not fully seated, missing electrical/ground connections, intake-manifold gasket misalignment, and a broken gasket seal leaking coolant into electrical circuits, before the crankcase ventilation hose was found disconnected — the actual root cause. Reconnecting it resolved every symptom^{9101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555615758592360616263}. Conflict/lesson: multiple plausible diagnoses were proposed and pursued (idle sensor, spark plugs, electrical connections, gasket seal leak) before the crankcase ventilation hose was identified as the actual cause — after any service that disturbs the intake manifold area, checking this hose first can save significant diagnostic time.

Throttle response failure — traced to vacuum-system leak, not just crankcase hose. A separate but related symptom: the gas pedal becomes unresponsive on all pedals (throttle and brake) simultaneously, causing fuel-injection control issues. Likely causes: vacuum-booster failure or a ruptured crankcase-ventilation hose (consistent with the hose failures already documented on this card). Solution: inspect all vacuum hoses on the intake tract for splits or damage, especially the crankcase-ventilation hose; this may require removing the intake manifold for a proper inspection. A FAW service visit is recommended for diagnosis if the issue isn't found by visual inspection⁶⁴⁶⁵⁶⁶.

Hissing noise only when hot — loose crankcase-ventilation tube. A separate report: a hissing/whistling sound near the throttle-body area appears only once the engine is warmed up. Cause: the crankcase-ventilation tube (routing blow-by gases) under the intake manifold works loose or partially detaches; it may also cause a slight RPM fluctuation. No dashboard error is logged. A full fix requires intake-manifold removal (labor-intensive — see the dedicated intake-manifold-removal card), but re-tightening the connection from underneath may be enough^676869707172.

Jerking at low throttle traced to a cracked coolant/intake tube near the thermostat. In a separate case, jerking in traffic when lightly touching the pedals was traced — during an unrelated thermostat replacement — to a cracked tube running from the thermostat to the intake manifold. Replacing the cracked tube resolved the jerking. Tip noted in the same thread: when refilling coolant, raise the hood and pour slowly to avoid trapping air pockets⁷³⁷⁴⁷⁵.

No PCV valve at all — the X80 ships with a dummy plug where a Ford Mondeo has a real valve. A detailed diagnostic thread establishes that the X80 has no functioning crankcase-ventilation (PCV) valve — the valve cover has only an empty housing with a dummy tube plugged in where the equivalent Ford Mondeo PCV valve would sit. This is identified as a likely contributor to rough idle/jerking around 1200-1500 RPM, compounded by the X80's electronic throttle pedal: ECU-to-throttle signal transmission has a perceptible delay (one owner reports that on light pedal input the throttle sometimes doesn't seem to open at all — driving with a very light foot can paradoxically feel smoother, "but it shouldn't have to work that way"), and under this theory the fuel/air mixture correction lags the crankcase-pressure swings enough to produce a jerk. Temporary workaround: partially constrict (pinch) the ventilation hose with a small clip, reducing flow by roughly 50% — one owner (Фаб) reported this measurably reduced bogging/hesitation on throttle tip-in and gearshifts, though some light throttle-off twitching remained; this is a single anecdotal report, not a confirmed fix. Permanent fix: source and install a genuine Ford Mondeo PCV valve (~1,000 RUB, RU market, June 2026, via Chinese marketplaces reachable over VPN; not listed on Ozon). Owners who installed the Mondeo valve and also cleaned the MAP sensor (see the MAP-sensor card) report reduced jerking and smoother AT shifting. A related design flaw noted in the same thread: the intake manifold sits low on this engine and accumulates sludge over time, and by ~140,000 km the EGR valve is typically fouled with oil sludge as a result (see the EGR card)^{7677787980818283848586878889909192}. Conflict: one member questioned whether the missing-PCV-valve theory actually holds and whether a muffler/exhaust modification might be the real fix instead — not resolved in this batch.

Rough running and elevated fuel consumption in hot weather — same root-cause cluster. Multiple owners separately report the engine running roughly ("like a diesel"), with increased fuel consumption and rough/jerky idle around 1200-1300 RPM, specifically when ambient temperature rises in summer — despite no stored error codes and normal-testing sensors. This is attributed to the same crankcase-ventilation design flaw, intake-manifold sludge buildup, and a MAP/MAF sensor needing cleaning, all already documented on this card^{9394958788969798}.

Intake-manifold air leak — smoke-test diagnosis. A separate but related case: no error codes were present, but an air leak was traced to the intake-manifold gaskets using a smoke generator (дымогенератор) — smoke visibly escaped around the manifold, confirming the leak point. A replacement silicone gasket kit (5 gaskets, for Mazda/Ford-compatible intake manifolds) costs roughly 500-1,000 RUB (RU market, June 2026) on Ozon; full disassembly of the intake manifold is required (see also the engine-gasket cross-compatibility card)^969799100.

Ford donor part number for the crankcase ventilation tube. Consistent with the "no real PCV valve" finding already on this card, one owner (Сергей Елисеев) gives a concrete Ford cross-reference part number for the crankcase-ventilation tube itself (патрубок картерных газов): 1440473 (also listed as FMVQ PTP024589). Symptom pattern matches the rest of this card: a hissing/whistling noise from the engine bay plus fluctuating idle speed points to this tube being clogged or cracked¹⁰¹.

Cold-start roaring/rocking noise — cracked intake bypass tube, inherited from Ford Focus 2 — from the second Telegram group. A separate community (FAW BESTURN X80 group) reports a distinct but related symptom: a loud roaring/rocking sound on cold start or rapid acceleration. The likely cause given: a cracked intake-manifold bypass tube or hose — described as a documented issue inherited from the Ford Focus 2 platform (the same lineage discussed elsewhere on this card for crankcase-ventilation/intake components). The tube/hose part itself is inexpensive, but replacement is reported as difficult, requiring removal of significant surrounding components. An alternative diagnosis offered in the same thread: the noise could instead originate from the timing chain/tensioner area at the front of the engine, more likely at higher mileage, rather than the intake tube — the two causes were not definitively distinguished in this exchange^{102103104105106}.

Media

Photo shows disconnected vacuum hose issue related to the problem

Related pages

• Idle stalling/shaking at stops — likely air-intake leak
• Low-speed jerking — multiple causes
• Manual transmission — 2nd-gear grinding
• Fuel pump — intermittent pressure drop
• Engine shuddering at 2.5k RPM and cruise

Sources

This page is compiled from 106 community messages. Every fact carries a numbered reference to its original author and message.

Last updated: 2026-10-29