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Automobile Repair/Before you buy

< Automobile Repair

Tests to conduct before purchasing a second-hand vehicle
The sump compression of an engine must be negative. Pull out the dipstick while engine is idling, but don't engage the accelerator or throttle. If there's any wind, gas or oil escaping the dipstick holder then the piston rings are worn. This condition is known as "blow-by".

White smoke from the exhaust indicates engine oil remaining on the engine cylinder walls that the piston rings haven't scraped off, i.e. loss of oil control. Black smoke from the exhaust indicates dirty fuel injectors on a diesel engine. Dirty fuel injectors aren't a serious problem. The rings gets progressively worn down. A slight escape of wind from the dipstick is the first stage of ring failure. It will get worse until gasses and oil start escaping the dipstick holder. Reduced compression or blow-by on the rings has nothing to do with the condition of a diesel pump.

Reduced engine compression/white smoke are the result of the piston rings not sealing tight with the bore. Blow-by on the rings results in less power transferred to the crankshaft. The compression gases that escape past the rings are forced out via the sump. This results in the sump leaking oil.

First conduct a 'dry test' and then a 'wet test'. The dry/wet readings must be very close to each other. To do a dry test for petrol engines, unplug the spark plugs and on a diesel engine, remove the glowplugs. Place compression tester inside glowplug/sparkplug hole and crank engine a maximum of 10 times, during which needle indicates maximum compression. Note this maximum compression reading and write it down. Repeat test for remaining cylinders. All four readings must be within 10% of the vehicle official specs and very close to each other. Should a reading be out of spec, insert 10mL engine oil into the glowplug/sparkplug hole and repeat compression test. The oil seals any faulty rings that may be skewing the test results. Should the reading differ from the dry reading, it indicates a failing ring. Above sea level the compression readings on all four pistons must be within 20% of the specs.

Always have a mechanic perform a compression test on any used vehicle before purchase. Don't use the mechanic next door to the car sales location. You want ensure an unbiased opinion.

If there's no air, gases or oil escaping the dipstick holder, but the compression readings are not correct then the cylinder head is cracked or there is a blown head gasket. Look for a white emulsification on the dipstick. It would indicate a cracked cylinder head, leaking engine coolant on the piston. Check the dipstick end for gear oil or engine oil. Unscrupulous sellers add SAE90 (thick) gear oil to mask faulty ring symptoms.

Prevent engine overheating:
Replace water pump, thermostat and radiator hoses. Have radiator cleaned at a radiator workshop. Make certain that the radiator capacity is big enough for the engine. Diesel uses 3core, petrol 2core radiators for LDV. Remove radiator cap and check for oil. Oil in the water means a cracked cylinder head. This is major structural damage to an engine. Machinist shops test head integrity via a pressure test. There must be engine coolant in the radiator. Clear water in radiator indicates poor maintenance of engine.

Repairing the internal combustion engine and condition inspection
1) Forged pistons need 80% more bore/piston clearance than cast iron pistons. Cast iron pistons expand less.
2) Custom, extremely hard, cast iron pistons can be made by a machinist so that bore/piston clearance can be kept very low.
3) The skirt is the bottom and crown the top part of a piston.
4) Clearance measurement is done at the skirt of the piston.
5) Mitutoyo bore gauge measures the inside diameter of an engine bore. It resolution is 0.0001 inch.
6) Ribbon gauges are thin long gauges that stick in between piston and bore to measure the clearance. Determine with a scale the pound/inch force needed to pull out the ribbon gauge. It must be a certain force. A 2.4D Toyota engine shall have 0.007 inch clearance between piston/bore. Use 0.002/0.004 inch ribbon gauges. If they are binded when pulled out, hone bore more. The piston is placed inside the bore without rings to measure the clearance.
7) Generally for each inch of bore diameter, the piston/bore clearance must be 0.0005 inch of bore.
8) If the piston skirt/bore clearance is to small, the piston will expand and seize the engine when hot.
9) Smear main bearings, big-ends,crankshaft with assembly grease before torquing bolts.
10) Ring gap-size is measured with a feeler gauge. It must be a certain minimum size to prevent the ends clamping together as the engine heats up during operation. The ring gap doesn't determine if the bore/piston clearance is correct. Insert an inverted piston at the top of the block to make the ring absolutely level, remove the piston and measure the gap. Rings in the first stage of failure will allow just air and/or a small amount of white gas with it. It sometimes is possible to only fit new rings and not new sleeves with first stage ring failure. The next size rings are fitted to the piston. But the ring gap must be manually resized via a file. Clamp a file onto a work bench and file of the ends of the oversized ring. Fit the ring in the piston and measure for ring gap clearance via a feeler gage. Repeat process until all the rings gap-size is within specs.
11) A toolshop bores the bore/sleeves to the exact size of the piston skirt. After this the bores are honed to the specified clearance via a honing machine. Insert the piston without rings to test for clearance as described.

Suspension check on light-duty delivery vehicle
The front suspension consists of:
1) Lower control arm, bushing and connecting bolt x2
2) Upper control arm x2
3) Torsion bar x2
4) Upper and lower balljoints x4
5) Tie-rod ends x2
6) Wheel bearings x2
7) Draglink x1
Load 600 kg (~1300 lbs) on the bakkie (pick-up truck). Drive at 60 km/h (~40mph) and slam on the breaks hard. If the bakkie yanks to one side, then the lower-control-arm-bushing and bolt are worn out. The lower-control-arm are fastened to the chassis via the bushing and bolt. The torsion bar mounts on the connecting pin. To check the tie-rod ends and wheel bearing, jack the front up and manipulate the wheel - there should be no play. Wheel alignment can't be done unless the entire front suspension is in working order. Check for play on the steering wheel - there should be none. If there is, a nut on the steering box is turned to increase the tension. But before this is done, you must be certain that the entire suspension is order, in particular the draglink and tie-rod ends.

Unleaded fuel won't damage your engine.