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Old 11-15-2010 | 12:42 PM
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Vacuum reading

Hey I jus installed a vacuum gauge on my car and was wondering what all us Non-turbo guys are pullin on the average! I'm between 15 and 16.5 at idle and when I let out throttle in gear about 23-24?
Old 11-15-2010 | 02:14 PM
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Even for turbo guys we get the same thing.... Sounds bang on to mine
Old 11-15-2010 | 02:16 PM
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yep - mine too (also turbo).
Old 11-15-2010 | 02:54 PM
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How much could pre-mix or a de-carbon cleaning change those numbers?
Old 11-15-2010 | 03:30 PM
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My 07 8 gt auto idles smooth at about 750 rpm, 4 or 5 maf, and 16 constant vacuum at 2900ft elevation.
Old 11-15-2010 | 03:38 PM
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Originally Posted by Old Rotor
How much could pre-mix or a de-carbon cleaning change those numbers?
Unless you have some sort of seal failure I dont think it should change
Old 11-15-2010 | 03:46 PM
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I am usually closer to 18 warmed up for me. 21-22 cold (higher idle) . Same with or without the supercharger. 24ish in gear off throttle.
Old 11-15-2010 | 07:42 PM
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Originally Posted by Charles R. Hill
The factory spec is 17-20", idling at sea level, fully warmed-up. Most with above 50K miles will dip into the 16s. When they get into the 15s I start to look for issues or plan a rebuild for the following year.

The factory spec, in grams/second of airflow, is 5-6.

It isn't that uncommon for vacuum gauges to be off by 1".
true dat
Old 11-15-2010 | 08:24 PM
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Originally Posted by Highway8
I am usually closer to 18 warmed up for me. 21-22 cold (higher idle) . Same with or without the supercharger. 24ish in gear off throttle.
So if you live near Fairfield, Ca., you could be under 100ft elevation which would account for your good 18 inches of vaccum.

You generally lose 1 inch for every increase of 1000 ft of elevation assuming same temp, etc.

So mine should read 18 to 19 inches in Fairfield, Ca.
Old 11-15-2010 | 09:02 PM
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So I don't know much about these things....what's the simplest way to test the idle vacuum reading? (Aside from paying mazda $100 to do it...) Assuming you hook up some tool to the accessory/vaccum nipple thing?
I know my compression numbers are borderline, and that mazda now uses vacuum readings as a measure of engine health, so i'm very curious....
Old 11-15-2010 | 09:13 PM
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I see -9 PSI cold and a little over -7 PSI hot.
That translates roughly to 18" & 14", respectively.

I am at 1300 feet.
Old 11-16-2010 | 09:35 AM
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Originally Posted by Charles R. Hill
The easiest is to use the nipple right behind the throttle body.
Thanks for that tip Charles!
Old 11-16-2010 | 10:07 AM
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How much does a vacuum reading equate to determining low compression?
Old 11-16-2010 | 10:33 AM
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There is a correlation, but it is kinda fuzzy as the range of vacuum readings for both a "good" and "bad" motor overlap quite a bit.
In my experience, a warm 90 PSI motor will make about the same vacuum as a warm 115 PSI motor.
Old 11-16-2010 | 10:08 PM
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Accessory load can decrease vacuum. Make sure you turn everything off.
Old 03-27-2011 | 05:05 PM
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Okay, so I tested mine. I have about 26,000 on my Mazda reman. This picture is what I was seeing hot at idle.

http://www.flickr.com/photos/9krpmrx8/5565947762/

I was a little worried at the somewhat low reading for such a low mileage engine. But then I tested Hoss-05's car (05' M/T 10k on reman) and sillwhitegirls car (07' M/T 43k original engine) and basically their readings were the same.

Thoughts? I did a zoom zoom cleaning right now and I will test after I get back from a drive.
Old 03-27-2011 | 06:39 PM
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Okay I tested mine with a different vacuum tester I had and it was still at a tad bit past 15 at hot idle after the zoom zoom cleaning. So both of my vacuum testers seem to be inline with each other. I also tested grungepups 07' M/T with 50k on original engine and he was steady at about 14
Old 03-27-2011 | 07:30 PM
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Hot idle on a healthy motor can be anywhere from 13" to 20", depending on altitude and other things.
Old 03-27-2011 | 08:55 PM
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Originally Posted by MazdaManiac
Hot idle on a healthy motor can be anywhere from 13" to 20", depending on altitude and other things.

Oh ok, I thought factory spec was 17-20. I have a secondary air leak (blip of powerloss) somewhere that only shows at 6,000-7,000RPM. You can see it on the data logs and I am trying to diagnose and I was worried my motor might not be up to snuff. The issue does not happen all the time, but about half the time I take her to redline. But my MAF readings at redline (195 or so) seem lower than others are.
Old 03-28-2011 | 04:59 AM
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Originally Posted by 9krpmrx8
Okay I tested mine with a different vacuum tester I had and it was still at a tad bit past 15 at hot idle after the zoom zoom cleaning. So both of my vacuum testers seem to be inline with each other. I also tested grungepups 07' M/T with 50k on original engine and he was steady at about 14
Last time I was in SA, I tested 18 inches vacuum hot at idle on my VW vacuum guage from my 1980 Scirocco.

When, I come over Thursday after your workday, we can test with both guages if you want.

We can even put those new plugs in mine, and see if that improves it some more if you like testing stuff.
Old 03-28-2011 | 10:28 AM
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Originally Posted by Charles R. Hill
Factory spec be damned; most healthy Renesis engines will produce about 15-17 in./hg. at idle but equally important is that the vacuum gauge needle is steady and not bouncing around.
Originally Posted by Charles R. Hill
Might this be a bad APV motor, perhaps not opening all the way?

Mine was sitting steady however grungepup had the lowest reading and it was moving a bit back and forth. He has kind of suspected his engine was on the outs but his car runs really strong so who knows.

I will have to look into testing the APV motor, it does have 122,000 on it. On a side note though, I cleaned my ESS after doing the zoom zoom cleaning and the problem has not came up since but we will see after a couple of days of driving if it comes back.
Old 03-28-2011 | 01:17 PM
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trying to analyze engine sealing outside of gross variations is a waste of time & effort, which makes it the perfect forum topic
Old 03-28-2011 | 01:21 PM
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Originally Posted by TeamRX8
trying to analyze engine sealing outside of gross variations is a waste of time & effort, which makes it the perfect forum topic

Yeah, well hopefully I will have a compression tester soon enough and I can test it that way as well.
Old 05-12-2011 | 04:13 PM
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Sub, My car seems to be acting the same way...
Old 05-12-2011 | 04:23 PM
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Originally Posted by WTBRotary!
Sub, My car seems to be acting the same way...

Replacing my plugs fixed it. I will get some vacuum readings tonight to see if that changed.


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