By design the PCV (positive crankcase ventilation) valve regulates the buildup of excess gases, vapors and corrosive substances that escape from the combustion chamber. 

A failing PCV valve will cause these harmful gases to accumulate inside the engine crankcase causing pressure buildup that can affect the quality and longevity of engine oil and lead to sludge buildup within the internal system, which could ultimately cause major engine failure. A failing PCV valve will also affect chemical concentrations within exhaust gases, In-turn leading to increased smog emissions.
Dirty PCV from the Mazda CX7 before being cleaned.

The importance of understanding of the PCV/Valve system operation in automobile engine diagnosis can never be over emphasized, reasons being that a malfunctioning PCV system or Valve can disrupt the performance of the engine and lead to contamination and damage of other internal engine components.

A PCV system/Valve clogged or stuck in close position causing pressure build-up will Increase internal engine pressure failure of one or more oil seals or gaskets, causing engine oil leaks, moisture and sludge build-up inside the engine, engine surges and possibly black smoke.
While a stuck open, disconnected or ruptured PCV system or Valve causing vacuum will cause engine misfires at idle, lean air-fuel mixture, presence of engine oil in PCV valve or hose, increased oil consumption, hard engine start and rough engine idling.

A stuck open PCV valve can trigger the Malfunction Indicator Light (MIL) due to an increase in air flow and cause the ECU/PCM to point at either the Mass Air Flow meter or Oxygen sensor malfunction instead, and make the diagnosis harder than it should be to detect the real source of the problem. It’s only a detailed diagnosis and interpretation, pin-point tests as specified by the manufacturer that can help narrow down the source of the problem.

The same PCV from the Mazda CX7 after being cleaned.

CASE STUDY: We will be looking at two scenarios that will further explain and differentiate the effects of the electrically controlled heated PCV system/Valve and the vacuum controlled non heated PCV system/Valve.

The first case study is that of a 2008 Ford Edge, V6 3.5L engine with 178,230 mileage, equivalent to 285,168km. The client came to our workshop and complained of his vehicle not accelerating (throttle pedal not functioning while the engine is still running) after some minutes drive, that is, the vehicle will just stop moving while the engine is running, no response from the throttle pedal depression and the MIL was ON. This vehicle has an electrically controlled heated PVC system/Valve, which implies that the PCM/ECU has a hand in the functioning and monitoring of this particular PCV system/Valve. 

Further investigations revealed that the engine of the vehicle had being changed most probably because of the high mileage, which was later confirmed by the owner, that the engine was changed in America before shipping to Nigeria.
Ford Edge PCV system
On diagnosing the vehicle, fault codes P2195 and P2197 were extracted with P2195 as the freeze frame code, meaning Bank 1 Lack of HO2S, sensor switch open/close and Bank 2 Lack of HO2S, sensor switch open/close respectively, resulting to P0171 and P0174 meaning, Bank 1 running Lean and Bank 2 running Lean respectively.

Air FLow.

While we were carrying out the pin-point test, we discovered the PCV system/Valve was not giving out return signal to the PCM/ECU and bearing in mind that the freeze frame fault code P2195 is related to the Bank 1 PCV system/Valve, we went straight to the PCV connector, on locating it we discovered that it was not connected to the PCV system/Valve, we connected it and that was the fix. This was an easy fix with the client not buying any part and no time wasting.

The second case study is that of a 2007 Mazda CX-7, L4 engine with turbo 2.3L with 139,519 mileage, equivalent to 223,220km. The client complained that the vehicle stalls/hesitates at start up in the morning, engine shuts down after running for awhile, vibration and rattling sound from the engine and visible blue/white exhaust smoke after prolong idling. This vehicle is designed with a vacuum controlled PCV system/Valve, which implies that functionality and monitoring is controlled by vacuum (air flow). Further investigation revealed that this is a RECALL issue (factory fault) and probably the first owner had a poor maintenance culture or the service information given by manufacturer was wrong (that is service every 10,000 miles).

On diagnosing the vehicle, fault codes P0300, P0301 were extracted, with P0301 as the freeze frame code, meaning multiple misfire and misfire on cylinder 1. Going by the repair pattern provided by the manufacturer’s TSB (Technical Service Bulletin) we would need to change the ventilation hose, cylinder head over, engine hanger, Air pipe, Vacuum hose, Plugs and all components included in the ventilation set. Due to non-availability of the parts, we opted to flush the engine of SLUGDE, clean the air inflow ways and the PCV valve.

From the case studies above we would deduce that the electrically controlled heated PCV system/Valve is more intelligent and gives direct solution, while for the vacuum controlled non-heated PCV system/Valve can pick on any of the emission system components as the main cause of the problem. This will further emphasize the need to understand how the system works, it’s effect and specific symptoms before disassembling any part.

The diagrams below are snap shots from Alldata diagnostic/repair software.


Knowledge is Power.

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