Low Compression and Blow-By on a Common-Rail Diesel
A 1.5-litre four-cylinder common-rail diesel in a light commercial vehicle has run 1,20,000 km. From cold it needs 5–6 seconds of cranking, then emits blue-white smoke for about 30 seconds. Power is down on gradients, oil consumption is 0.8 litre per 1,000 km, and there is visible blow-by at the oil filler cap. A dry compression test gives 26, 25, 18 and 25 bar against a specification of 28–32 bar, minimum 22 bar. A wet test with 5 ml of oil raises cylinder 3 from 18 to 24 bar; the other cylinders change by about 1 bar. A cylinder leakage test on cylinder 3 shows air escaping from the oil filler neck. Glow plugs and injector corrections are normal. The workshop manager suggests replacing only the head gasket and valve stem seals.
1. Interpret the compression and leakage results. Do they support the manager's proposal?
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The wet test is the key discriminator. Adding oil temporarily seals the rings, so a rise of about 6 bar on cylinder 3 points to piston-ring or bore sealing loss. A valve or head-gasket leak would not normally improve that much with oil. The leakage test confirms the path: air escaping from the oil filler neck means combustion pressure is reaching the crankcase past the rings and piston, not past the valves or into the coolant.
The manager's proposal does not fit the evidence. A failed head gasket would usually show coolant loss, bubbles in the expansion tank, cross-cylinder leakage, or white steam, not isolated low compression with crankcase leakage. Worn valve-stem seals can cause smoke after idling, but they do not explain 18 bar compression or 0.8 litre/1,000 km oil use with blow-by.
The correct next step is to strip cylinder 3 and measure the bore, piston, and rings before deciding between honing and reboring.
2. What measurements would you take before ordering parts, and how would you decide between a hone-and-ring repair and a rebore?
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Measure the bore at several depths and in two axes with a calibrated bore gauge: about 10 mm below the deck, mid-stroke, and near the bottom. Calculate taper and ovality against the manual limits. Check piston-to-bore clearance, ring end gap in the worn and unworn part of the bore, ring groove side clearance, and piston skirt wear. Inspect the bore for scoring, scuffing, and a wear ridge at the top.
Use the manual limits as the decision rule. If taper or ovality is beyond the maximum, or the bore is already at the wear limit, a hone cannot restore a true cylinder; rebore to the next oversize and fit matching pistons. If the bore is within limits and only the rings are worn, a controlled hone with the correct cross-hatch and new rings is acceptable. In either case, check the turbocharger shaft play and intercooler for oil, since oil consumption may have more than one source, and verify crankcase ventilation is not restricted.
After repair, run in correctly and recheck compression, blow-by, and oil consumption.
3. Explain why the cold-start symptom and blue-white smoke fit this diagnosis, and state one risk of delaying the repair.
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Low compression on one cylinder lowers the end-of-compression temperature and pressure, so cold starting is slow and that cylinder misfires briefly; unburnt diesel appears as blue-white smoke until heat builds up. The same ring leakage pressurises the crankcase and pushes oil mist through the breather, increasing oil consumption. Delaying repair allows diesel to dilute the oil and combustion soot to contaminate it, while the worn bore continues to taper. That raises the risk of bearing wear and, in a diesel, of uncontrolled running on its own oil mist. It also damages the turbocharger and exhaust after-treatment. The repair should therefore be based on measured bore condition, not on the cheapest gasket job.