Your shop hits you with that smell on Monday morning — sour, sulfurous, unmistakable. Here's what's actually happening in the sump, how to confirm it in ten minutes, and the fix that keeps it from coming back.

What's actually happening

Water-soluble coolant is a fine home for bacteria, and machines feed them: way oil, hydraulic oil, and gearbox oil leak into the sump and float on top as tramp oil. That floating layer does two things — it feeds bacteria, and it seals the surface so oxygen can't get in. In the oxygen-starved sump underneath, odor-producing bacteria (including the sulfate-reducers behind the rotten-egg smell) multiply rapidly, especially over idle weekends.

Confirm it in 10 minutes

• Look: a rainbow sheen or visible oil layer on the sump means tramp oil. Any visible layer is enough to seal the surface.
• Smell the pattern: worst after idle time, better after running — classic oxygen starvation.
• Test: a dip slide (paddle test) gives you a bacteria count overnight. Readings in the 10⁵–10⁶ CFU/mL range are the common action level.
• Check concentration with a refractometer — lean coolant loses its bio-resistance. Trust the refractometer, not the mixer dial.

The fix

1. Remove the tramp oil — continuously. A skimmer running around the clock (especially over nights and weekends) keeps the surface open and takes away the bacteria's food and cover. This is the root-cause step.

2. Restore concentration. Verify with a refractometer and correct to your fluid supplier's spec.

3. If the sump has already turned: clean and recharge it, following your coolant supplier's cleanout procedure — then let the skimmer keep the fresh fill fresh. Odor-control treatment can help maintain a healthy sump; use products per their label.

What NOT to do

• Don't just add fragrance or more biocide and walk away. If the oil layer stays, the problem returns — and follow product labels on any biocide use.
• Don't dump the sump without fixing the cause. New coolant in a dirty, oil-fed sump turns again in weeks.

Common questions

Why does my coolant smell like rotten eggs on Monday morning?
Over an idle weekend, a tramp-oil layer seals the sump surface and starves the coolant of oxygen. Sulfate-reducing bacteria thrive in the oxygen-poor sludge underneath and produce hydrogen sulfide - the rotten-egg smell. Running the machine stirs it up and releases the odor.

How many bacteria are too many in machine coolant?
Dip-slide readings in the 100,000 to 1,000,000 CFU/mL range (10^5-10^6) are the common industry action level. At that point, address the causes - tramp oil, weak concentration - rather than only masking the smell.

Will an oil skimmer alone fix rancid coolant?
It fixes the leading root cause. Removing the tramp-oil layer restores oxygen to the sump and takes away the bacteria's food and cover. Pair it with correct concentration (verified by refractometer) and, if needed, odor-control treatment. If the sump has already turned, clean and recharge it first - then the skimmer keeps the new fill fresh.

Related problems: Oil floating on coolant · Coolant doesn't last · Oil mist in the shop

Written by the coolant-management team at Zebra Skimmers Corp, Solon, Ohio — 30 years of keeping machine-shop coolant clean. Last updated August 2026. This guide is general equipment and fluid-maintenance information, not professional safety, medical, or regulatory advice.