ماء في خزان الوقود

Water in the fuel, from condensation, a leaking filler seal, or contaminated fuel taken on at a pump. On a diesel it is genuinely damaging rather than merely inconvenient, because the injection system depends on fuel for lubrication and water provides none — a slug of water through a high pressure pump can destroy it in one journey.

The symptoms are erratic and load-dependent. Water settles at the bottom of the tank and is picked up in slugs, so misfires, stumbles and pressure faults come and go with fuel level, cornering and gradient. A fault that worsens as the tank empties is the pattern, since the pickup sits where the water is.

Modern diesels carry a water-in-fuel sensor and a drain for exactly this, and a warning from it should be acted on rather than reset. Drain the filter bowl and look at what comes out — water separates visibly and the answer is immediate. Petrol engines tolerate small quantities but suffer corrosion and, in cold weather, ice in the lines; and a persistent problem in either fuel argues for a leaking filler seal rather than condensation.