Cummins/Onan — Checking Exhaust Back Pressure
Testing & diagnosis · Exhaust piping
From a Cummins/Onan service manual covering: ST-50, T-18A, VT-50. Check it applies to your exact model before use.
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Before you start
- Line to manometer must be the specified size and length to minimize reading variation
Procedure
- Select a measurement point close to turbocharger outlet flange with uniform flow, one pipe diameter from flow direction changes.
- Weld a 1/8 inch [3.2 mm] pipe coupling to exhaust tubing and drill through.
- Remove burrs and mount a 90 degree fitting to the coupling.
- Use 3 ft [0.9 m] of 1/8 inch [3.2 mm] I.D. copper tubing plus 10 ft [3 m] of 3/16 inch [4.8 mm] I.D. soft rubber hose to manometer.
- Compare reading against 3 inches [76.2 mm] of mercury maximum.
Tools required
- mercury or water filled manometer Amazon
- 1/8 inch pipe coupling Amazon
- 90 degree fitting Amazon
- copper tubing Amazon
- rubber hose Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Exhaust back pressure maximum 3 inches [76.2 mm] of mercury
Source: Cummins/Onan service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.