Troubleshooting Foaming and Odour in Coolants

What These Signs Mean and How to Fix Them

Coolants are essential for maintaining efficiency and precision in metalworking and machining operations. But if your coolant starts foaming like a bubble bath or smelling like rotten eggs, it’s not just unpleasant—it’s a sign of deeper issues that can damage tools, compromise quality, and put your workers at risk.

In this post, we’ll explain the most common causes of foaming and odour in coolants, what these issues mean, and how to fix (and prevent) them before they disrupt your operations.

Why Coolant Condition Matters

Coolants (especially water-soluble ones) are complex chemical formulations. When properly maintained, they:

• Control heat during machining

• Extend tool life

• Prevent rust and corrosion

• Wash away chips and debris

• Improve surface finish

When neglected or contaminated, they can:

• Break down and become ineffective

• Damage equipment and tools

• Pose health risks to operators

• Lead to costly downtime or rework

Foaming and odour are two of the earliest warning signs that something’s wrong with your coolant system.

Part 1: Understanding Coolant Foaming

What It Looks Like:

• Bubbles or froth on the surface of the sump

• Foam spilling out of machine tool reservoirs

• Inconsistent fluid delivery or low pump pressure

Why It Happens:

Foaming occurs when air gets trapped in the coolant, often due to:

• High-pressure pumps introducing air into the system

• Contaminants like oils or cleaners disrupting surface tension

• Incorrect coolant concentration (too rich or too lean)

• Agitation or turbulence in poorly designed sumps

• Mechanical issues like leaky fittings or cavitation

How to Fix Coolant Foaming:

✅ 1. Check the Concentration Use a refractometer to verify proper coolant mix.

• Too lean: Reduced lubricity and increased foam

• Too rich: Viscosity issues and poor flushing

✅ 2. Allow Coolant to “Rest” After Mixing Freshly mixed coolant can foam temporarily. Let it sit in a holding tank before use if possible.

✅ 3. Use Anti-Foam Additives Some manufacturers offer foam suppressants that are compatible with their coolants.

✅ 4. Inspect System Design Reduce turbulence by adjusting:

• Return line positions

• Flow speed from pumps

• Sump baffling to allow air escape

✅ 5. Eliminate Contaminants Skim tramp oil, avoid incompatible cleaners, and ensure hoses/fittings are secure to prevent air draw-in.

✅ 6. Replace Coolant if Degraded If foaming persists and fluid is old or heavily contaminated, a full drain and clean-out may be necessary.

Part 2: Understanding Coolant Odour

What It Smells Like:

• Rotten eggs (sulphur)

• Sour or musty smell

• Ammonia-like or chemical odour

Why It Happens:

Coolant odour is almost always caused by bacterial or fungal contamination. Microorganisms feed on coolant ingredients, creating gases and breaking down the fluid’s protective qualities.

Common Causes of Odour:

• Low coolant concentration (below 4–5%)

• Tramp oil accumulation (bacteria feed on it)

• Poor sump hygiene and chip build-up

• Warm, stagnant fluid—ideal for microbial growth

• Rare or inconsistent maintenance

How to Fix Coolant Odour:

✅ 1. Test the Concentration and pH

• Low concentration = more bacteria

• pH below 8.0 = potential for bacterial growth Adjust concentration and pH based on coolant manufacturer guidelines.

✅ 2. Skim Tramp Oil Daily Tramp oil provides food and cover for bacteria. Use a belt skimmer, coalescing filter, or weir to remove it.

✅ 3. Clean the Sump Thoroughly Sometimes odour means it’s time for a full system flush:

• Drain coolant

• Use a system cleaner or biocide flush

• Rinse thoroughly

• Refill with fresh, properly mixed coolant

✅ 4. Improve Circulation and Agitation Stagnant coolant = microbial growth. Keep coolant circulating during shifts and consider agitation pumps in large tanks.

✅ 5. Use Biocides or Preservatives (If Approved)Some coolant manufacturers allow supplemental biocide treatments. Always check compatibility to avoid chemical imbalance.

✅ 6. Increase Maintenance Frequency If coolant is left unmaintained for weeks, odour is inevitable. Set a routine for concentration checks, top-offs, and cleaning.

Prevention Tips: Keep Coolant Clean and Odour-Free

Best Practice	Benefit
Mix with clean, deionized water	Reduces mineral build-up and pH issues
Use a refractometer weekly	Keeps concentration in the optimal range
Skim oil and remove chips daily	Starves bacteria and prevents foaming
Clean sumps quarterly (at least)	Prevents sludge and biofilm build-up
Train operators on signs of degradation	Catches problems early before failures

✅ Tool Suggestion: Use a log sheet for each machine to track coolant condition, maintenance, and service history.

Final Thoughts: Foam and Odour Are Warning Signs, Not Just Nuisances

While foam and odour may seem like minor issues, they’re red flags that your coolant is degrading—or already ineffective. Addressing them promptly helps prevent tool damage, corrosion, safety concerns, and lost production.

Good coolant maintenance is about more than just fluid—it’s about keeping your entire operation running clean and efficiently.

Need help diagnosing a coolant issue or selecting the right fluid for your application? Our team can provide expert support, fluid testing, and supply recommendations tailored to your shop. Contact us today for help with coolants, maintenance tools, and troubleshooting guides.