Our me­chan­i­cal en­gi­neer in­ter­prets (or de­bunks) what these mo­torists were told by their me­chan­ics.

Torque (Singapore) - - CONTENTS -

A me­chanic told me that my car had a “head gas­ket leak”. He showed me some ev­i­dence of oil seep­age and rec­om­mended that I have it fixed. He said that it would get worse, even if a re­pair was not ur­gent. Ex­actly how se­ri­ous is this? The head gas­ket is the seal be­tween an engine’s cylin­der block and cylin­der head.

The pis­ton moves in the cylin­der block and the head is where the valves are lo­cated and where com­bus­tion is ini­ti­ated. As such, the head ex­pe­ri­ences very high pres­sures and is sub­jected to much of the heat gen­er­ated dur­ing com­bus­tion. The head gas­ket also iso­lates the oil and coolant chan­nels be­tween the block and the cylin­der head. So the im­por­tance of the head gas­ket is quite ob­vi­ous.

A small leak at the gas­ket – the joint be­tween the head and the block – is not se­ri­ous enough to cause com­pli­ca­tions. How­ever, no gas­ket leak will ever “self-heal”. It will only get worse over time. If al­lowed to per­sist, the gas­ket will likely rup­ture, caus­ing oil con­tam­i­na­tion of the coolant and vice-versa.

This is a con­di­tion that must be avoided to pre­vent the in­evitable dam­age caused by coolant mix­ing with engine oil. This re­sults in the de­te­ri­o­ra­tion of engine lu­bri­ca­tion and, ul­ti­mately, fail­ure of var­i­ous mov­ing com­po­nents, which will mean a full engine over­haul and a hefty re­pair bill.

For a frac­tion of the cost, a gas­ket re­place­ment is a worth­while ex­er­cise. Dur­ing my car’s last rou­tine ser­vic­ing, my me­chanic rec­om­mended that I do an engine flush to clear car­bon build-up. Is this use­ful or just a way for the work­shop to earn more money? What can I do to pre­vent car­bon build-up in the first place? In an engine flush, a liq­uid is poured into the oil-filler on the valve cover to mix with the engine oil. The engine is then run for a few min­utes and the mix­ture is drained – along with sludge and con­tam­i­nants. The engine is then re­plen­ished with fresh oil. Be­tween gaps in the mov­ing parts of an engine, there is a


thin film of oil that is crit­i­cal to pre­vent­ing wear. An engine flush could strip off this thin film. An engine that is reg­u­larly ser­viced, filled with goodqual­ity lu­bri­cant and fit­ted with a rec­om­mended oil fil­ter will not re­quire flush­ing – even af­ter 300,000 kilo­me­tres. If there is a ma­jor engine prob­lem, such as worn main­bear­ings or leak­ing pis­ton rings, then these parts should be re­placed. Dur­ing re­place­ment, the engine will be cleaned. And dur­ing re-as­sem­bly, parts are al­ways coated with oil to ini­ti­ate the thin film of lu­bri­cant men­tioned ear­lier. Even in ex­treme sit­u­a­tions where there is con­tam­i­nant build-up in the engine (such as ra­di­a­tor coolant get­ting into the engine), it is better to per­form a short-in­ter­val oil change (per­haps within the first 1000 kilo­me­tres) af­ter the re­pair work is com­pleted to flush out rem­nants of wa­ter. There is no need for an engine flush. In any case, engine oils con­tain a num­ber of ad­di­tives, in­clud­ing de­ter­gents. Re­cently, my car lost power while ac­cel­er­at­ing. It also emit­ted white and blue smoke. My me­chanic told me that I had to re­place the “PCV valve”. What does this part do and is the prob­lem se­ri­ous? A small amount of com­busted gases will in­evitably es­cape past the pis­ton rings into the crank­case where the engine’s oil is “stored”.

If al­lowed to build up, the gases will cre­ate a high pres­sure in the crank­case and cause de­te­ri­o­ra­tion of the con­tent of the oil sump in the crank­case. Even­tu­ally, sludge will form and the engine oil will lose its lu­bri­cat­ing prop­er­ties.

In the early days, the blowby gases were vented out of the crank­case and into the at­mos­phere. Elim­i­nat­ing this air pol­lu­tant was one of the first emis­sions-con­trol pro­cesses. These gases are now chan­nelled back into the com­bus­tion cham­ber via the in­take man­i­fold, while the crank­case is pro­vided with fre­shair ven­ti­la­tion via the air-fil­ter.

All cars have what is called the pos­i­tive crank­case ven­ti­la­tion (PCV) valve. This sim­ple but ef­fec­tive com­po­nent lies in the plumb­ing be­tween the crank­case and in­take man­i­fold, and fa­cil­i­tates the re­cy­cling of the blow-by.

It is de­signed to open at a spe­cific pres­sure dif­fer­en­tial be­tween the in­take man­i­fold and crank­case. This is so that suf­fi­cient ven­ti­la­tion is pro­vided while pre­vent­ing a con­di­tion of vac­uum in the crank­case that would cause ex­ces­sive oil to be drawn by the in­take man­i­fold.

A PCV valve that is stuck “open” will lead to oil build-up in the in­take man­i­fold, lead­ing to a plume of bluish white smoke when ac­cel­er­at­ing. On the other hand, a PCV valve that is jammed “shut” will cause pres­sure build-up to a point where air-oil fumes from the crank­case will be di­verted to the air fil­ter. This also re­sults in bluish white smoke when ac­cel­er­at­ing.

The con­di­tion is not se­ri­ous, nor is it ex­pen­sive to fix. But it should be fixed. Pro­longed run­ning of the car with a faulty PCV valve will lead to for­ma­tion of sludge in the engine oil or con­tam­i­na­tion of the air fil­ter, in­take man­i­fold as well as other air and vac­uum lines.


Ques­tion­able rec­om­men­da­tions by me­chan­ics could throw a span­ner in the re­pair works.

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