Low vacuum - possible causes? (LT1)
Thread Starter
Safety Car
Joined: Jul 2001
Posts: 4,871
Likes: 15
From: Detroit's West Side MI
Low vacuum - possible causes? (LT1)
I posted a similar thread yesterday, but due to only a few responses (Readeasy - as always happy to help 
) I thought I should expand the topic.
The basic issue is a lack of vacuum (-5 in/hg idle, at higher RPM around 7-9 in/hg). I'm also throwing a code 34, which is low vacuum. (actually high vacuum, since it's measured negative)
I've had severe spark knock so I've been chasing a timing problem. Replaced timing set and optispark (upgrade to '95 version) and had no change.
Car has no EGR, LT headers, and other general specs listed below in sig. Based on yesterday's thread possible causes are:
Leak between intake and heads
bad cam grind
slight leak at EGR valve
Other things that were checked and OK:
compression in all cylinders
valves (and valves not seating)
MAP sensor
Old thread here: http://forums.corvetteforum.com/show...88&forum_id=48
) I thought I should expand the topic.The basic issue is a lack of vacuum (-5 in/hg idle, at higher RPM around 7-9 in/hg). I'm also throwing a code 34, which is low vacuum. (actually high vacuum, since it's measured negative)
I've had severe spark knock so I've been chasing a timing problem. Replaced timing set and optispark (upgrade to '95 version) and had no change.
Car has no EGR, LT headers, and other general specs listed below in sig. Based on yesterday's thread possible causes are:
Leak between intake and heads
bad cam grind
slight leak at EGR valve
Other things that were checked and OK:
compression in all cylinders
valves (and valves not seating)
MAP sensor
Old thread here: http://forums.corvetteforum.com/show...88&forum_id=48
Thread Starter
Safety Car
Joined: Jul 2001
Posts: 4,871
Likes: 15
From: Detroit's West Side MI
Originally Posted by bogus
chase the various vacuum hoses... I would not be surprised if some were bad.
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,009
From: Texas
Originally Posted by Redeasysport
Brake booster hose is bigger.PVC hose too.
In addition, the brake hose connects to a check valve in the brake booster. If the check valve is bad, it's a Huge leak.If the injectors aren't properly seated in the intake, there can be a vacuum leak past the injector tips.
Make sure your rockers aren't set too tight. Might have some valves staying open just a little.
What were your compression test results? Were the readings consistent across the cylinders?
Did you do a leak down test?
Btw, spark knock can occur if the knock sensors are over torqued. Factory spec is ~11 lb/ft, max of ~15 lb/ft
Melting Slicks
Joined: Nov 2002
Posts: 2,471
Likes: 2
From: 1993 Quasar Blue 'vert Scottsdale AZ
Originally Posted by Redeasysport
Brake booster hose is bigger.PVC hose too.
Thread Starter
Safety Car
Joined: Jul 2001
Posts: 4,871
Likes: 15
From: Detroit's West Side MI
Thanks for the suggestions!
Compression is good on all cylinders, so head gasket is ok (whew). Valves are ok, we checked all the rockers again to make sure none were too tight. But again, compression is fine, so they're all seating.
No cats on this car.
I don't have the clips on the injectors, so that's a good place to start, they could be riding up. I'll also check the brake booster - what's the best way to do this?
I think it's going to be the intake, though. Hopefully I damaged or misaligned the gasket. That's an easy fix...
I also just replaced the TB. But it was running poorly long before that.
Compression is good on all cylinders, so head gasket is ok (whew). Valves are ok, we checked all the rockers again to make sure none were too tight. But again, compression is fine, so they're all seating.
No cats on this car.
I don't have the clips on the injectors, so that's a good place to start, they could be riding up. I'll also check the brake booster - what's the best way to do this?
I think it's going to be the intake, though. Hopefully I damaged or misaligned the gasket. That's an easy fix...
I also just replaced the TB. But it was running poorly long before that.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Thread Starter
Safety Car
Joined: Jul 2001
Posts: 4,871
Likes: 15
From: Detroit's West Side MI
Oh, and I did not do a leakdown test, but I don't know that it will help. I know the car isn't holding pressure, if it weren't so loud I would think I could probably hear the leak hissing.
I plan to take some carb cleaner and go over everything. Hopefully that will expose the leak. Maybe I should use ether...with that much of a hole I could turn my car into a bomb.
Next question - could this much of a vacuum leak cause spark knock? Would the computer overcompnesate and advance timing?
In my head, if there is low vacuum the computer would see that as WOT, and dump more fuel, not lean it out. Am I thinking about this the wrong way?
I plan to take some carb cleaner and go over everything. Hopefully that will expose the leak. Maybe I should use ether...with that much of a hole I could turn my car into a bomb.
Next question - could this much of a vacuum leak cause spark knock? Would the computer overcompnesate and advance timing?
In my head, if there is low vacuum the computer would see that as WOT, and dump more fuel, not lean it out. Am I thinking about this the wrong way?
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
It may be putting more fuel than necessary because it is sucking so much air trying to keep the A/F even.The ECM will do strange things when it is trying to compensate for a bad running engine.I would solve the Vacuum issue before trying to figure out anything else.It may run fine when that is fixed.
Carb cleaner will not help much on the leak either propane or starting fluid will work better.
I don't see any reason you couldn't clamp off the vacuum lines to start it and measure the vacuum.
Carb cleaner will not help much on the leak either propane or starting fluid will work better.
I don't see any reason you couldn't clamp off the vacuum lines to start it and measure the vacuum.
Last edited by Redeasysport; Mar 17, 2006 at 12:38 PM.
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,009
From: Texas
Originally Posted by ScaryFast
I'll also check the brake booster - what's the best way to do this?
Melting Slicks
Joined: Aug 2003
Posts: 3,395
Likes: 393
From: Lakeville MI
I didn't see any mention of ignition timing.
Late or retarded ignition timing will require a large throttle opening or high IAC setting at idle. This can result in low vacuum readings.
Mixture, leaks, improper cam timing, over adjusted exhaust valves are some other potential causes.
Late or retarded ignition timing will require a large throttle opening or high IAC setting at idle. This can result in low vacuum readings.
Mixture, leaks, improper cam timing, over adjusted exhaust valves are some other potential causes.
