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Old 10-06-2009, 03:13 PM   #1 (permalink)
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Vacuum gauge reading low possibly? Help

I installed a vacuum gauge in my car last night. I didn't use the tube that came with it because it did not seem very durable. I bought a tube from Napa and used that.

Here's where i have it connected on the vacuum line:





This is the gauge:



Today i was able to see how it was acting and i think it maybe reading a little low from the research that i've done

At idle it is at 17-18, which i think is normal. When i was accelerating slowly from a stop, it would usually be at 5. Once i was up to speed cruising around town at speeds between 30-45 MPH on level roads, it would be 15-18. On the highway going 55-60 MPH, on level ground it was at about 12-15, going up a hill it would drop to 5-10, and going down a hill it would be at 15-20. When i had my foot off the gas coasting to a stop, it would be at 20-23, usually with the needle shaking at whatever number it was at. If my car was in fuel cutoff mode, slowing down or coming to a stop (wideband read ---- and SG2 read 9999), it would be at a steady 23.

Never the whole trip, to and from my destination, did it ever go above 23 .

Are these reading usual or are they low? If they are low, what should i do?

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Old 10-06-2009, 03:41 PM   #2 (permalink)
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I think the readings seem slightly low.

My Echo reads about 21 at idle and it reads slightly higher under similar circumstances.
It's a manual and has a very low idle speed, it will be higher for an auto in gear, or any car with a higher idle speed.

The fluctuating gauge needle is an indicator of one of two possible problems.

It could be an engine problem, but I would not assume that to be correct yet.

Its more likely that you have tapped the vacuum near one of the individual runners in the intake manifold. I can't really tell from the photos.

You need to tap the vacuum from the intake plenum, where all the manifold runners meet.

My preference is the hose to the brake booster, which should connect to a steady vacuum source. If you try that and it still fluctuates it might indicate there is a problem with the engine, induction, or ignition systems.

Idle vacuum is also a function of idle speed, and engine compression. My Echo is 10.5 to one so it reads fairly high idle vacuum.

Also understand that your altitude makes an difference as well as atmospheric pressure.
Higher altitudes and lower atmospheric pressures will reduce idle vacuum readings.

Otherwise your ranges seem to closely match mine, just slightly lower.

My 76 Z car would do about 18 at idle, but it's compression ratio was 8.5 to 1.

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Mech
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Old 10-06-2009, 05:43 PM   #3 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
...You need to tap the vacuum from the intake plenum, where all the manifold runners meet.

My preference is the hose to the brake booster, which should connect to a steady vacuum source. If you try that and it still fluctuates it might indicate there is a problem with the engine, induction, or ignition systems.

Idle vacuum is also a function of idle speed, and engine compression. My Echo is 10.5 to one so it reads fairly high idle vacuum...
The compression ratio for my car is 10.1:1 with the idle at 650-675 RPM. If the vacuum readings i'm getting now stay this way, i'll try connecting it to the brake booster like u had said.

This is where i have it connected to currently (sry the arrows are kinda small):



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Old 10-06-2009, 07:08 PM   #4 (permalink)
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I have one on a 93 Chevy Van and a 2000 Ford Mercury with the 4.6 motor and your readings sound normal to me.

And the bouching pointer may just be the egr system, I have seen the very same thing on my Ford once it warms up and the egr starts to cut in...

I am a old time machinc and had a panic reaction to seeing the pointer bounch around at cruse untill I read it is more or less normal on some new cars...

A reading ot 10 to 12 at 70 MPH is what I see and I am getting 29 MPG at that....

Rich
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Old 10-06-2009, 07:24 PM   #5 (permalink)
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if you are "Teed" into the Purge line , when the purge valve is duty cycled by the ECM / PCM
the vacuum gauge will pulse with the duty cycle .

17 to 18" at hot idle is too low at sea level
but more important is that at hot idle there should be ZERO flutter in the needle

the brake booster vacuum line has a check valve in it ,
if you "T" into the line after the check valve the vacuum value you measure will be incorrect.
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Old 10-07-2009, 04:34 PM   #6 (permalink)
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I noticed today that whenever i was stopped with the car in drive and had my foot on the brake, the needle was between 17 and 18 and also was shaking a little. When i put the car in park and released the brake pedel, the needle went up to 20 and was steady.
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Old 10-07-2009, 04:38 PM   #7 (permalink)
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That is what I get as well, in gear 17/18 out of gear 19/20.

I see the bounchy pointer at cruse....right where the EGR should be working...

Not totatly happy with a shaky pointer at idle in gear but is it a 4 cylinder...?

My Toyota shakes the whole car at idle in gear....

Rich
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Old 10-07-2009, 05:17 PM   #8 (permalink)
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Quote:
Originally Posted by racprops View Post
Not totatly happy with a shaky pointer at idle in gear but is it a 4 cylinder...?

Rich
Yes sir, 4 cylinder 1.8L engine.
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Old 10-07-2009, 06:18 PM   #9 (permalink)
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vacuum values

operating temperature
UN boosted engines .
typical "normal" at sea level vacuum values are ;

21 to 21" at hot idle ,no load, the needle must be steady smooth , flutter is NOT allowed

no load, at 2k rpm 2" more than at idle what ever the idle value is
UNless the EGR is opening like on some subaru

WOT snap values should be 0" when the throttle is opened wide and then as the engine decelerates about 24 to 25" , no load .

WOT snap means ;
as quickly as you are able ...
snap the throttle to MAX opening and the millisecond it is open , let the return spring snap it closed , DO NOT DO this in gear .

flutter at hot idle with no load =
valve train problem

no load low vacuum at hot idle almost always =
incorrect camshaft timing

no load , low vacuum at 2k rpm hot usually means restricted exhaust
...............................
these are typical values ;
there are exceptions ;
they do not apply to some engines , like 2.0 9A VW dohc engines
values will be lower at high altitude

Last edited by mwebb; 10-07-2009 at 06:22 PM.. Reason: left out the "no load"
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Old 10-07-2009, 06:51 PM   #10 (permalink)
Cycling more to drive les
 
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MWebb; in case there is an issue with incorrect cam timing, like if there's timing advance or retard, how would that effect fuel economy?

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