Case­book

Garage­man saves a me­chanic from trou­ble with a Free State tune-up and chats car­bon

Car (South Africa) - - CREDITS - BY: Garage­man

CAR­BON BLOWOUT

Drift­ing has be­come a mo­tor­sport in many coun­tries, de­spite the fact the cars in the pin­na­cle of rac­ing, F1, do not drift spec­tac­u­larly, if at all. To see a proper drift, you have to nd a video of Fan­gio or As­cari han­dling a Maserati or a Fer­rari: the four wheels are par­al­lel as they point to­wards the side of the road while the car ne­go­ti­ates a cor­ner in a glo­ri­ous slide con­trolled mainly by the throt­tle.

“Side­ways” Syd­ney, one of our town’s more colour­ful me­chan­ics, de­serves his nick­name. He even has a court case be­hind him to prove it. Some years ago, he crashed a cus­tomer’s car by try­ing to drift it the way they do in the movies. The traf c con­sta­ble tes­ti­fy­ing in court said: “I saw the ac­cused ne­go­ti­at­ing a cor­ner on the high­way with the car at a weird an­gle. The mo­ment he spotted the po­lice car, he got such a fright he slowed down. This caused the car to run into the bushes next to the road.” Side­ways had to pay a hefty ne and the work­shop em­ploy­ing him had to re­pair the car but his boss kept him on; good me­chan­ics are scarce.

Many me­chan­ics, es­pe­cially the back­yard fra­ter­nity, like to rev an en­gine up to the red­line in neu­tral in or­der to “clear out the cob­webs”. This can be harm­ful be­cause the pis­tons rely on the air com­pressed dur­ing the up­stroke to act as a damper for the erce de­cel­er­a­tion they face in com­ing to a stop at top-dead-cen­tre. There is very lit­tle air to com­press while the gear­box is in neu­tral; the en­gine is not un­der a load.

Some me­chan­ics, my­self in­cluded, be­lieve a good run at high revs, known as a “Free State tune-up”, helps to clean out some of the de­posits. It has to be done on the road and es­pe­cially up­hill so the en­gine has to work hard. Side­ways’ boss doesn’t be­lieve in this the­ory and has banned him from do­ing it. In fact, poor old Side­ways has been asked to drive cus­tomers’ cars se­dately in or­der to keep his job.

Two weeks ago, Side­ways was caught “in­dulging in harm­less fun” and he asked me to have a heartto-heart with his boss. I pre­pared for my case by con­sult­ing some SAE au­to­mo­tive-en­gi­neer­ing pa­pers writ­ten by en­gi­neers who have in­ves­ti­gated the ef­fect of high-speed runs on en­gine clean-ups.

It turns out mod­ern EGR and PCV sys­tems re­cy­cle oil mist through the in­take man­i­fold and this even­tu­ally leads to a car­bon build-up. These de­posits start to form and bond with a metal sur­face when pis­ton tem­per­a­tures are in the range of 195 to 290 de­grees Cel­sius, which is nor­mal for ev­ery­day driv­ing. Above 290, it’s too hot for the mol­e­cules to bond with the metal or each other. At about 325, it’s hot enough for the de­posits to break down and sep­a­rate from the metal.

I vis­ited Side­ways’ boss and showed him the proof that an oc­ca­sional bit of hard work is good for an en­gine. He ac­cepted the sci­enti c ev­i­dence re­luc­tantly and ad­mit­ted he was most likely too hard on Side­ways. He just didn’t want an­other mishap in­volv­ing a cus­tomer’s car.

CAR­BON BUILD-UP

Peter Wat­son was such a cute lit­tle boy his fam­ily started to call him “Pixie”. Dur­ing his high school days, he de­vel­oped a keen in­ter­est in photography and his name grad­u­ally changed to Pixel. These days, he is known as Me­gapixel for a rea­son which can well be imag­ined.

Me­gapixel drives a 2015 Peu­geot 308 1,2T. Soon af­ter the 60 000 km ser­vice plan ex­pired, the car be­gan to lose its sparkle. Idling was no longer smooth and the ac­cel­er­a­tion be­came more leisurely. He brought the car to us and I gave the job card to Au­gust.

It was one of the rst GDI (gaso­line di­rect-in­jec­tion) en­gines that we ser­viced be­cause most of them are still un­der war­ranty. We were all aware some tend to car­bon-up the area be­hind the in­take valve, so the rst thing Au­gust did was re­move one of the spark plugs and ex­am­ine the com­bus­tion cham­ber with a scope. The in­take valve area was a lot dirt­ier than the ex­haust valve space. Au­gust then re­moved the in­take man­i­fold and con rmed there was a lot of car­bon in the ports.

The gen­eral opin­ion in the mo­tor trade is this build-up is so hard, the only solution is to re­move the cylin­der head and send it to a spe­cial­ist shop to clean it by us­ing ul­tra­sound and a glass-bead spray. I had to get Me­gapixel’s per­mis­sion to do that and the poor man had no choice but to agree.

Ap­par­ently, on GDI en­gines, the area be­hind the in­take valve of­ten reaches that 195 to 290 de­grees Cel­sius tem­per­a­ture which pro­motes the for­ma­tion of car­bon. This does not hap­pen on in­di­rect-in­jec­tion (into the man­i­fold) en­gines be­cause the fuel in the airstream has a cool­ing ef­fect so the tem­per­a­ture stays be­low 195 de­grees Cel­sius.

The pre­vi­ously men­tioned SAE ar­ti­cle does also state a Free State tune-up will not clean up the car­bon in a GDI en­gine.

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