A failed head gasket allows compressed air to leak from the cylinder, causing the pump to work harder and run hotter. On two-stage compressors, a gasket failure between stages can cause extreme first-stage overheating as high-pressure air bleeds backward.
What you'll see
If the overheating is equal across all cylinders and the compressor is in a confined space, ventilation may be the primary issue. If one specific cylinder is overheating, check the valves first -- they're a more common failure than head gaskets. See: Poor Ventilation, Leaking Valves.
How to diagnose
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Listen for hissing at the head joint
With the compressor running, listen carefully (but keep your distance) around the cylinder head gasket joint. A blown external gasket produces a hissing or whistling sound. Spray soapy water around the joint while running -- bubbles pinpoint the leak.
Result: No hissing or bubbles = gasket sealing. Hissing and bubbles = blown gasket. -
Check if this followed a recent head removal
If the head was recently removed (for valve work, inspection, etc.) and the compressor immediately started overheating, the gasket installation is suspect. The gasket may have been reused, installed wrong, or the head bolts weren't torqued properly.
Result: Recent head work + new overheating = gasket installation issue.
How to fix it
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Replace the head gasket
Remove the head, discard the old gasket (never reuse), clean both mating surfaces completely, check both surfaces for flatness with a straight edge, install a new gasket, and torque the head bolts in a cross pattern to the correct spec. Re-torque after one hour of running.
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Check head flatness
A warped head won't seal even with a new gasket. Place a straight edge across the mating surface and look for gaps. If you see daylight, the head needs lapping on a flat surface with emery paper, or replacement if badly warped. Warping can be caused by the overheating itself, creating a chicken-and-egg problem.
Reusing old gaskets is the number one mistake. A compressed gasket won't seal a second time. Also: don't tighten head bolts in a circular pattern -- use a cross (star) pattern to distribute clamping force evenly. Uneven clamping causes the gasket to seal on one side and leak on the other. And check the torque spec -- overtightening cracks cast iron heads, undertightening allows the gasket to blow out.
New head gasket. Gasket scraper. Straight edge. Torque wrench with correct socket. Fine emery paper on flat surface (for lapping if needed).
Hot compressed air escaping from a blown gasket can burn. Keep hands clear of the head area. Let it cool before working on it.
This issue can also cause
- Not Building Pressure / Air Blowing Out Inlet Compressor runs but pressure builds slowly or not at all: often air blows back out through the inlet filter.