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Home >> Pontiac >> 1978 >> Sunbird Base, 2.5L Eng VIN 1, Automatic Trans >> Repair and Diagnosis >> Engine Performance >> Engine Control Systems >> Electronic Fuel Control >> Testing & Diagnosis >> Vacuum System Checks
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Vacuum System Checks

NOTE: Increased altitude will change vacuum readings. For example, at 4, 000 feet, vacuum reading will be approximately 1 1/2" lower for the same reading at sea level. Note and make necessary allowances.
  1. Check vacuum hose diagram in this section or on emission control decal under hood and note proper routing.
  2. Disconnect vacuum hose from vacuum storage tank (to vacuum modulator) and "tee" in a vacuum gauge to this line.
  3. Start engine and run for one minute then turn off. Observe gauge. It should read minimum of 13-15 in. Hg. After 5 minutes, vacuum level should still be around 10 in. Hg or more. If not, vacuum leaks are present. If hose leaks are not found, replace vacuum supply reservoir and repeat test.
  4. Check carburetor diaphragm(s) by applying vacuum to control port and watching for bleed-down. If vacuum will not hold, replace diaphragm(s).
  5. Disconnect vacuum gauge and reconnect all vacuum lines.
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