Intake vacuum
Thread Starter
Intermediate
Joined: Nov 2004
Posts: 40
Likes: 0
Intake vacuum
I have a 1992 LT1 . It is coming up with H34 ( MAP sensor fault ) when the revs are raised to about 2500 rpm. All seems OK with the sensor but can anybody tell me what vacuum i should measure in the inlet chamber at idle and what should happen to the vacuum when the revs rise as my vacuum doesn't seem to alter above about 10 Hg which i think is why it is setting the code - shouldn't the vacuum rise with the revs and if so to what level .
Le Mans Master
Joined: Jan 2002
Posts: 9,036
Likes: 13
From: Edmonton AB
Originally Posted by uk vet
I have a 1992 LT1 . ... can anybody tell me what vacuum I should
measure in the inlet chamber at idle and what should happen to the
vacuum when the revs rise as my vacuum doesn't seem to alter above
about 10 Hg ... Shouldn't the vacuum rise with the revs and if so, to
what level .
measure in the inlet chamber at idle and what should happen to the
vacuum when the revs rise as my vacuum doesn't seem to alter above
about 10 Hg ... Shouldn't the vacuum rise with the revs and if so, to
what level .
at idle is representative of a stock, earlier Chev V8. (Aftermarket
camshafts can cause lower readings in the vicinity of 9-13" HG, even
less.)
If the throttle is opened rapidly, expect manifold vacuum to drop
substantially. If opened slowly, it might only drop a little or not at all.
Opening the throttle reduces the restriction between the cylinders
that are drawing through the intake and the atmosphere. So opening
decreases manifold vacuum rather than increasing it. (On some carb
circuits, opening the throttle opened a passage and may have caused
the vacuum reading to increase because of the placement of the
port in the venturi)
.
Thread Starter
Intermediate
Joined: Nov 2004
Posts: 40
Likes: 0
Sorry as i am in uk ( on nights at moment ) my posts are at strange times. The MAP and seal looks physically ok and when i put a vacuum guage to one of pass side outlets ( cruise ? ) it only reads about 10 hg .Yes i have a 93 manual. I now will look at how it falls with increase in revs . Thank you.
Pro
Joined: Aug 2004
Posts: 623
Likes: 1
Originally Posted by uk vet
Sorry as i am in uk ( on nights at moment ) my posts are at strange times. The MAP and seal looks physically ok and when i put a vacuum guage to one of pass side outlets ( cruise ? ) it only reads about 10 hg .Yes i have a 93 manual. I now will look at how it falls with increase in revs . Thank you.
Burning Brakes
Joined: Nov 2002
Posts: 1,097
Likes: 1
From: Abita Springs LA
I saw this the other day.....
Also this link will explain how to read a vacuum gauge.
http://www.secondchancegarage.com/public/186.cfm
Also this link will explain how to read a vacuum gauge.
http://www.secondchancegarage.com/public/186.cfm
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Melting Slicks
Joined: Nov 2002
Posts: 2,471
Likes: 2
From: 1993 Quasar Blue 'vert Scottsdale AZ
Mine peaks at 12" also (10" with the AC on). Mr. Crowley's excellent link suggests I have late valve timing. This vacuum problem began with the installation of an un-speced, out-of-the-box Hotcam. Anybody have any suggestions? ('93).
Pro
Joined: Aug 2004
Posts: 623
Likes: 1
Originally Posted by 65Z01
My gauge says low could be due to late ign timing but don't have a clue on how to set base timing on an LT1.
Have you checked for vacuum leaks??
Have you checked for vacuum leaks??
Le Mans Master
Joined: Jan 2002
Posts: 9,036
Likes: 13
From: Edmonton AB
Originally Posted by lefoy84
My L98 with Crane 114132 holds steady at 21 at idle.
210º/216º 0.440"/0.454" (w/ 1.5:1 rockers) CRANE #114132 POWERMAX 2040 (FORMERLY COMPUCAM 2040)
For comparison
207º/213º 0.415"/0.430" 117º LSA GM #10066049 L98 88-89
.
