Engine runs hot because there's not enough coolant circulating or not enough compressor oil absorbing heat. Two separate systems that both cause overheating when low. Check both.

What you'll see

Engine temperature gauge reads higher than normal and keeps climbing. If the coolant level is critically low, you may see steam from the overflow. For low compressor oil, the element outlet temperature rises -- the compressor safety switch trips before the engine overheat switch. Both can happen simultaneously on a neglected machine. If the engine oil pressure drops too low, you'll get a separate low-oil-pressure shutdown rather than an overheating shutdown.
Before you assume this is the problem

If the coolers are visibly dirty, clean them first -- that's more common than low fluid levels. See: Clogged Radiator or Cooler. If the engine overheats immediately on startup from cold, the issue is more likely a seized component or blocked oil flow.

See all causes of overheating / high temperature shutdown →

How to diagnose

  1. Check the coolant level

    Check the coolant expansion tank (header tank) -- the level should be between the MIN and MAX marks. If the engine has cooled down, you can also check the radiator cap carefully. If the coolant is low, there's a leak somewhere -- coolant doesn't evaporate out of a sealed system. Look for puddles under the machine, wet spots on hoses, weeping from the water pump seal.
    Result: Level between marks = OK. Low = top up and find the leak.

  2. Check compressor oil level

    Look at the oil sight glass on the separator vessel. Check it when the compressor is running at operating temperature and loaded -- the oil level drops when the machine is running because oil is distributed throughout the system. The level should be in the middle of the sight glass during operation. If you can't see oil, the level is critically low.
    Result: Oil visible in middle of sight glass during operation = OK. No oil visible = critically low.

  3. Check coolant quality

    Coolant should be clean and the correct color for the type used (green, pink, or orange depending on the manufacturer's specification). Rusty brown coolant has lost its corrosion protection and may have caused internal deposits that reduce cooling efficiency. The pH should be 7.5-9.5 -- below 7 and it becomes acidic and damages the engine.
    Result: Clean, correct color, no rust particles = OK. Brown, rusty, or acidic = flush and replace.

How to fix it

  1. Top up coolant

    Use the correct coolant mix -- typically 50% glycol antifreeze and 50% clean water. Don't use plain water as it provides no corrosion protection and freezes in cold weather. Don't mix coolant types (e.g., don't add green to a pink coolant system). If the system was very low, you'll need to bleed air from the cooling system after topping up.

  2. Top up compressor oil

    Add the correct compressor oil through the filler cap on the separator vessel. Use only screw compressor oil -- not engine oil, hydraulic oil, or general-purpose oil. Screw compressor oil has anti-foaming additives that are critical for proper oil separation. Don't overfill -- too much oil causes oil carry-over in the compressed air.

  3. Find and fix leaks

    If coolant or oil levels keep dropping, find the leak. For coolant: inspect all hoses, the water pump seal, radiator, heater core connections, and cylinder head gasket (white smoke = coolant leaking into combustion). For compressor oil: check hose connections, the oil filler cap seal, the oil filter housing, and the separator vessel drain.

Common mistakes

Don't run a compressor with the oil level above the maximum mark on the sight glass. Overfilling causes oil carry-over -- the excess oil gets blown out with the compressed air. It seems like 'more is better' but it's actually worse. For coolant: don't use pure water, and don't just keep topping up without finding the leak. A small coolant leak can become catastrophic engine damage if the head gasket fails.

Parts & tools

Correct coolant/antifreeze. Compressor oil (match the existing type). Funnel. Coolant pH test strips. Replacement hoses and clamps if leaking.

Review safety precautions before starting →

Safety

Never open the radiator cap or coolant expansion tank on a hot engine. The system is pressurized (typically 0.5-1 bar). Steam and boiling coolant will erupt from the opening. Wait at least 30 minutes after shutdown. Check compressor oil level through the sight glass -- no need to open anything.

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