The oil cooler is clogged with dust, oily residue, or internal oil varnish. The cooler can't transfer heat, so the compressor runs hot.

What you'll see

Gradual temperature increase over weeks or months as the cooler gets progressively dirtier. The compressor runs fine after maintenance but slowly gets hotter until it trips. In dusty environments (workshops, construction, manufacturing), this can happen much faster. The oil cooler works like a vacuum cleaner -- it pulls in an enormous amount of cooling air, and all the dust comes with it.
Before you assume this is the problem

Check the thermostatic valve first -- a stuck thermostat can look exactly like a clogged cooler because in both cases, not enough heat is being removed from the oil. Feel the cooler hoses: if both are the same (warm) temperature, the problem is the thermostat, not the cooler. If the inlet hose is very hot and the outlet is noticeably cooler but the compressor still overheats, the cooler might be partially blocked. See: Oil Quality Issues (thermostatic valve).

Could also be:

See all causes of overheating →

How to diagnose

  1. Visual inspection -- external fouling

    Open the compressor enclosure and look at the oil cooler and aftercooler. You'll probably see it immediately -- a thick layer of dust, lint, or oily grime covering the cooling fins. In metalworking shops the dust is often mixed with oil mist, creating a sticky coating that compressed air alone won't remove. The aftercooler sits right next to the oil cooler and is usually just as dirty.
    Result: Visible dust/grime buildup = clean it. If it looks clean, check internal fouling.

  2. Check for internal oil varnish

    This one is harder to see. When a compressor has been running at high temperatures for a long time, the oil breaks down and leaves a brown/yellow deposit called 'oil varnish' on the inside of the cooler passages. It insulates the cooling surfaces and reduces heat transfer. You might notice the oil itself has a dark, almost sticky consistency. If the cooler has been cleaned externally but temps are still high (and the thermostat is OK), internal varnish is the likely culprit.
    Result: Internal varnish = very hard to remove. Usually requires cooler replacement.

  3. Check the oil separator pressure differential

    While you're at it: a clogged oil separator element creates a large pressure difference across the separator, which raises the internal pressure and discharge temperature. Check the differential pressure indicator if your machine has one. Maximum is typically 1 bar -- above that, replace the separator element. This isn't a cooler issue, but it causes the same symptom and is often overlooked.
    Result: Separator differential above 1 bar = replace separator element.

How to fix it

  1. Clean the cooler -- dry dust

    Blow compressed air through the cooler fins in the opposite direction of normal airflow. This pushes the dust out the way it came in. Do this from both sides if possible. For the aftercooler too -- they get equally dirty.

  2. Clean the cooler -- oily or sticky deposits

    If compressed air doesn't cut it (oily workshops, metalworking environments), you need to remove the cooler from the machine and wash it properly with soap and a high-pressure washer or steam cleaner. Blowing compressed air at oily deposits just moves the grime around -- you need to dissolve it and flush it out.

  3. Replace the cooler (internal varnish)

    If the cooler has internal oil varnish deposits, cleaning is usually not practical. The varnish coats the inside of the tubes and is extremely hard to remove. Some people try chemical flushing with caustic soda, but this risks damaging aluminum coolers. Most of the time, replacement is the best option. When you install the new cooler, also change the oil and fix whatever caused the overheating in the first place -- otherwise the varnish will come right back.

Common mistakes

Don't assume a cooler that looks clean on the outside IS clean. Internal fouling is invisible from the outside. Also: a cooler can have adequate temperature drop (inlet vs outlet) but still be insufficient. When flow is restricted through a partially blocked cooler, the temperature drop per unit of oil looks fine -- but the total heat removal (in kW) is too low because not enough oil is flowing through. Always consider flow rate, not just temperature difference.

Parts & tools

Compressed air for routine cleaning (blow reverse direction). High-pressure washer or steam cleaner for oily deposits. Replacement oil cooler if internally varnished. Replacement separator element if over 1 bar differential. Temperature gun for measuring inlet/outlet temperatures.

Review safety precautions before starting →

This issue can also cause

  • Oil Carry-Over Excessive oil carryover contaminating downstream air: usually a separator, oil level, or temperature problem.