Vacuum Gauge Blues
Thread Starter
Melting Slicks
Joined: Apr 2001
Posts: 2,030
Likes: 363
From: Lubbock Texas
Vacuum Gauge Blues
Those of you that followed my thread "Need Engine Diagnostic Guru Help" know that I had 3 issues, one of which was low vacuum at idle. I was only getting 9-10" of vacuum at idle (700 rpms). The cam wasn't radical enough to produce such low readings so everything was pointing to a vacuum leak which I never found despite repeated efforts.
The one thing I never bothered to question was my brand new Actron vacuum gauge. Finally checked its reading with my new Mityvac hand pump which showed the gauge was 4-5" too low! Took it back to Sears and exchanged it for another one. The new one reads the same as the Mityvac gauge and I was able to confirm that vacuum at idle is actually in the 14 to 15" range. Still a bit on the low side but not anywhere near the problem I thought it was. Da-damn!
DC
The one thing I never bothered to question was my brand new Actron vacuum gauge. Finally checked its reading with my new Mityvac hand pump which showed the gauge was 4-5" too low! Took it back to Sears and exchanged it for another one. The new one reads the same as the Mityvac gauge and I was able to confirm that vacuum at idle is actually in the 14 to 15" range. Still a bit on the low side but not anywhere near the problem I thought it was. Da-damn!
DC
Melting Slicks
Joined: Mar 2007
Posts: 2,100
Likes: 12
From: Frederick Maryland
Make sure you check the engine vacuum when everything is set to normal specs and the engine warmed to operating temperature. Don't know what year you have, but if it has points, make sure the dwell is properly set, timing is properly set, carb properly adjusted, etc.
14"-15" of vacuum may well be normal for your Corvette engine, depending on which one (L48 vs. L82, etc.). Even mild Vette engines have a bit more cam than a standard run of the mill small block in most Chevies of the period. That alone can make for a somewhat lower vacuum reading.
If you do have an issue, is the vacuum gauge holding steady or is it erratic? That can tell you which way to focus your efforts.
A vacuum gauge is one of the simplest and most useful diagnostic tools ever.
14"-15" of vacuum may well be normal for your Corvette engine, depending on which one (L48 vs. L82, etc.). Even mild Vette engines have a bit more cam than a standard run of the mill small block in most Chevies of the period. That alone can make for a somewhat lower vacuum reading.
If you do have an issue, is the vacuum gauge holding steady or is it erratic? That can tell you which way to focus your efforts.
A vacuum gauge is one of the simplest and most useful diagnostic tools ever.
Thread Starter
Melting Slicks
Joined: Apr 2001
Posts: 2,030
Likes: 363
From: Lubbock Texas
Block: 350 ci 4-bolt
Heads: Camel Hump Heads – 2.02 intake valves,
Exhaust valves and cc not known.
Hardened valve seats with 3 angle valve grind.
Cam: Crane 967601 BluePrint Hydraulic lifter camshaft
RPM range 2000 – 5600
Duration @ 0.050” lift: 222/222
Valve lift: 0.447”/0.447”
Replaces GM 350 HP 3863151
Intake: Holley Contender
Carb: Holley 650 Double Pumper Model 4777
Choke: Electric
Crank: Forged and balanced, believed to be standard stroke
Timing Chain: Edelbrock Performer
Pistons: TRW L2490F-30 Forged Pistons
Exhaust: Sanderson Cast Headers model QP1000 into 3” exhaust w/ turbo mufflers
Ignition: GM HEI
Fuel Pump: Standard Mechanical
Transmission: TH400
The vacuum is steady and yes the gauge is helpful. Even the original gauge that read low, put me onto possibly worn rings. When I blip the throttle the vacuum drops completely to zero and then rebounds to about 20” before returning steady to normal. Apparently it is not supposed to drop completely to zero and the info I found suggests this is due to poor rings. Afterward, I confirmed that I have low compression in #1, #5, and #8.
I'm just frustrated I spent so much time chasing a low vacuum problem due to a faulty gauge.
DC
