Water, fuel dilution and soot in oil: how to detect them
Water, fuel and soot all damage the oil film or the oil itself. Each has quick tests and traps. Know the form the water takes before you compare two methods.
Water: dissolved, emulsified or free
Water in oil can be dissolved in it, finely mixed as an emulsion, or free and separated. The form depends on the oil and its temperature and on how the sample was handled. Dissolved water is a measure of the oil's humidity. Free water and emulsion are more damaging: they thin the film, corrode surfaces and attack the additives. A hot, mixed sample reads differently from a cold one that has settled.
| Method | Strengths | Watch for |
|---|---|---|
| Crackle test (hot plate) | Quick field screen for free water | Gives yes or no, not a number |
| Infrared | Gives a quantity of dissolved water, and shows scattering that points to emulsion | Compare with titration only when the water is mostly dissolved and the sample is well mixed |
| Karl Fischer titration | Accurate reference for total water | Lab method with reagents; sample must be homogeneous |
Infrared and water
Water absorbs infrared light at a known band for the oxygen-hydrogen bond, near 3400 wavenumbers in most mineral oils (the range is about 3200 to 3800). The deeper the absorption, the more dissolved water. Scattering across the spectrum points to emulsified or free water. Mix the sample well before testing.
Fuel dilution in engine oil
Fuel in the sump thins the oil and reduces the film. It comes from leaking injectors, long idling or poor combustion. Viscosity and flash point both change with fuel, but neither gives fuel directly. A headspace test measures fuel vapour above a heated sample and gives a direct percentage in minutes without chemicals. Gas chromatography is the lab reference and can separate the fuel from other light materials.
| Method | Strengths | Limits |
|---|---|---|
| Viscosity drop | Already measured | Other causes of thinning give the same result |
| Flash point | Direct physical sign of fuel | Needs heating a fuel sample, so a fire hazard; needs a correlation to give a percentage |
| Headspace vapour sensor | Direct, fast, no chemicals | Needs the same procedure and temperature each time |
| Gas chromatography | Specific and accurate | Costly, lab only |
Soot and glycol
- Soot comes from combustion and thickens the oil. It is measured by infrared or by a soot meter, and a high level usually points to an air-fuel or injection problem.
- Glycol means coolant is leaking into the oil. Infrared can detect it, and sodium, potassium and boron in the spectrographic results back it up.
- Treat any glycol as a stop-and-investigate signal, because it attacks bearings and additives.
What to do first
- Check the sample first: was it hot, mixed and taken from the right place?
- Repeat the test on a second sample before a major decision.
- Find the entry point: breather, seal, cooler, injector, or condensation from stop-start cycles.
- After the repair, take a second sample to confirm the level has fallen.