Sudden change in vaccuum.
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Joined: May 2026
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Sudden change in vaccuum. Need some help diagnosing a cammed LS1 issue.
Freshly rebuilt LS1 (2000 Corvette engine, cammed, Holley Terminator X) used to idle around 60 kPa MAP when it was running right. Suddenly during one drive it started running like garbage and now idles around 78–80 kPa MAP (so significantly less vacuum).
Symptoms:
Things already checked:
This almost acts like a giant vacuum leak, but smoke tests are clean.
What would you guys check next? Valve spring/lifter issue? Cam timing jumped? Something else I’m overlooking?
Freshly rebuilt LS1 (2000 Corvette engine, cammed, Holley Terminator X) used to idle around 60 kPa MAP when it was running right. Suddenly during one drive it started running like garbage and now idles around 78–80 kPa MAP (so significantly less vacuum).
Symptoms:
- Rough/choppy idle
- Popping/backfiring
- Used to run fine at ~60 kPa
- Happened suddenly in one drive
Things already checked:
- Smoke tested multiple times through brake booster line — no vacuum leaks found
- MAP sensor reads ~103 kPa KOEO
- Switched between internal Terminator X MAP and GM MAP sensor — no change
- Spark confirmed on all plug wires with inductive timing light
- Pulled all 8 plugs and they all look fairly similar (none obviously dead/fuel soaked/oily)
This almost acts like a giant vacuum leak, but smoke tests are clean.
What would you guys check next? Valve spring/lifter issue? Cam timing jumped? Something else I’m overlooking?
Joined: Apr 2006
Posts: 3,924
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From: Grand Rapids, Michigan
Check your exhaust gaskets. A leak upstream could cause an issue. Valve springs could also be an issue. Use a stethoscope to listen around the valve cover area for noises. Pull the covers if you hear anything odd.
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