The Vaccum reading tells the health of the motor correct?
06-05-02, 02:38 AM
#1
Full Member
Thread Starter
Join Date: May 2002
Location: Cloverdale Bc
Posts: 104
Likes: 0
Received 0 Likes
on
0 Posts
The Vaccum reading tells the health of the motor correct?
Ok, I was wondering weather your supposed to take the vacuum reading at operating temp in gear or in neutral? mine reads just a tad off of 20hg. But when I first started it up it sits at like 13 and then comes down to 20 when engine is at operating temp. When the car is moving its at 20hg before I get on the boost and the throttle. So when are you supposed to really check it, in neutral, motion or in gear or what?
06-05-02, 03:06 AM
#4
Meesto Spakaro
Join Date: May 2001
Location: Redondo Beach, CA
Posts: 489
Likes: 0
Received 0 Likes
on
0 Posts
idleing in gear for a long time just ruins your clutch 
idleing in neutral
16-18hg @ 750 is normal. Of course, if it's lower it can mean lots of things...boost leak, bad coils, bad plugs...etc etc. If you're losing compression you can tell...the car will start idleing erratically, and will shake.
I get about 14hg @ 750. Do I have bad compression? Nope... if you have any porting done, the vacuum tends to be lower. What is your idle at? If you have a steady idle at 750, then 20hg is excellent.
idleing in neutral
16-18hg @ 750 is normal. Of course, if it's lower it can mean lots of things...boost leak, bad coils, bad plugs...etc etc. If you're losing compression you can tell...the car will start idleing erratically, and will shake.
I get about 14hg @ 750. Do I have bad compression? Nope... if you have any porting done, the vacuum tends to be lower. What is your idle at? If you have a steady idle at 750, then 20hg is excellent.
06-05-02, 03:56 AM
#6
Full Member
Join Date: Sep 2001
Location: South FLorida
Posts: 55
Likes: 0
Received 0 Likes
on
0 Posts
What if you have 10hg at idle at 750?? It happened to me suddenly, before that i was idling at about 1500 and 18hg. My motor is street ported by the way. Also I noticed that now I dont get any boost at all until 3000rpm. Anyone have any clues about whats wrong with me car?
06-05-02, 06:27 AM
#7
Passenger
Posts: n/a
well from my standpoint blowing the snot out of a motor (front rotor comp test. 56/32/30) that is wrecking a motor running 18psi by accident dam seq. turbo set up that is half the reason I went non-seq no spike...anyhow if you have fucked a motor up it will not hold idle after you rev it past 3000 rpm in (n) it will cut out. and if you get it to idle by itself the vacuum will jump from 5-10hg back and forth that aint no vacuum leak that is a dead motor...
Trending Topics
06-05-02, 08:27 AM
#8
Rotary Freak
Join Date: Dec 2001
Location: Hattiesburg, MS
Posts: 1,668
Likes: 0
Received 0 Likes
on
0 Posts
My vacuum at idle (~750 RPM) is 10 in. Its the original engine with 89K. I haven't been able to find any vacuum leaks, and have done the vac hose job. The car seems to run fine, though.
06-06-02, 03:44 AM
#13
Rotary Enthusiast
Join Date: Feb 2001
Location: Huntington Beach, CA, USA
Posts: 1,025
Likes: 0
Received 4 Likes
on
4 Posts
10"Hg is pretty low even for a streetport. However remember vacuum goes up as rpms go up until the turbos start to spool and cancel it out, so if your idle was at 1500 and now its at 750 then something happened to change your idle, your vacuum was probably the same all along. FWIW the proper idle speed is 850 NOT 750. I think the misconception about 750 comes from the shop manual specifying 750rpm as the idle speed in diagnosis mode. This is only the proper idle when the TEN terminal is grounded! Although I don't like to compare vacuum readings across different cars because they tend to vary a lot based on idle rpm and guage, 10" is low enough to indicate a problem on a stock port engine. Even on a street port that sounds low, but then again, there are mild street ports and there are big street ports. If the car is running well with a smooth idle you could simply have a leak going into the gauge itself. Best bet is to get a compression test, if that checks out, and you're sure you don't have any vacuum leaks, then it could be normal. Only you can tell if the car is driving right or not.
Last edited by Nathan Kwok; 06-06-02 at 03:49 AM.
06-06-02, 05:55 AM
#14
A CHANGE in vacuum at idle is an indicator of engine condition, but there are lots of variables (gauge accuracy, idle speed, altitude, porting, the weird crap the car does sometimes that makes the vacuum drop at idle, etc.) so it isn't that useful comparing one car to another. It is somewhat useful to give ranges and stuff, but don't think you necessarily have a blown motor if you get 14 and someone says 15 is the lower limit for a healthy engine.
-Max
-Max
06-06-02, 05:17 PM
#16
Full Member
Join Date: May 2002
Location: Denver
Posts: 158
Likes: 0
Received 0 Likes
on
0 Posts
I don't think you can tell of motor health by vaccum. When I was in San Diego CA my reading was 19-20 and now that I'm in Denver CO, my reading is 14-15. I think altitude plays a big part.
06-08-02, 10:27 AM
#20
Senior Member
Join Date: Mar 2001
Location: Tulsa, OK
Posts: 514
Likes: 0
Received 0 Likes
on
0 Posts
Hey Gordon, so it doesn't matter what rpm your at during idle? The one wierd thing about my car is that it likes to idle rich, but it makes the vacume go down to about 10 or 11. If I lean it out at idle it will kick it up to 15, but when starting from a stop makes the car hesitate due to lack of fuel for a moment untill it gets out of that rpm range.
Thread
Thread Starter
Forum
Replies
Last Post
