Just tested my engine vacuum to try to determine the health of my engine. It always starts fine, no stalling (unless its been recently flashed), no dramatic loss of power moments... but then again i'm not so sure its running as strong as it once was.

anyway, my backyard results from a few minutes ago...

mileage: 45k
Idle vacuum: 16.9 - 17 inHG
idle speed: 830 rpm
water temp: 183 deg F
elevation: 400 ft
ambient pressure: 30.1 inHg

2000 rpm vacuum: 21 inHg

So is this rather mediocre or rather normal?

I haven't done any seafoam, Mazda cleaner, ect on the engine yet. I have been using LC FP+ fuel additive for amount 2 or 3 months now. I believe the spec in the service manual is 16.6 - 19.6 for the 6MT. Anyone know if the current engine replacement criteria is different?


Truthfully, my motivation to do this is twofold...
a) see if i'm missing a bunch of power
b) if my engine will be replaced under the latest MSP.... which means i need to take off my high flow cat and i'd rather not waste my time and money for gaskets on it

 

(subscribing)

 

You think that's bad? My idle vacuum is currently around 16.

 

i think i was going for if 17 is normal or bad... paging more smartz peoples

 

I've been told based on my vacuum and compression tests that my motor is tired and would appreciate a rebuild.

I'm going to do some datalogging next week and send the files to people smarter than me to try and rule out the possibility that something beside the motor is wrong.

 

That's a good question. We know 17 is better than 16 but not as good as 18. There are a few other factors that go into vacuum readings including ignition timing and clogged exhausts so if there's ever a concern one should do a compression check to support or dismiss one's fears.

Paul.

 

^^altitude too

last time I checked mine was only 16 in/hg, but this is at 5000 feet elevation

supposedly Mazda has some sort of altitude correlation with their vacuum test, but nobody seems to be able to get their hands on it

altitude would affect a compression test too, but possibly to a lesser degree

 

http://www.finishlineperformance.com...14-08-1924.pdf

look at page 9. hope that helps.

beers

 

Engine vacuum doesn't always correlate to compression, but it usually does follow to some extent.
15 in/Hg or lower with no load on the engine at sea level (give or take 1500 feet or so) is probably bad.
Anything from there up to 21 is "normal".
That said, my motor has gone from 61 kpa to 52 kpa over the past few months with very little change in compression.

Ignition timing doesn't have a significant effect on the idle vacuum on this motor. I don't know why.
I've advanced both leading and trailing - independently and together - by quite a bit (verified by a timing light) and there was no effect.

 

The old school rule of thumb I was told is you add 1 inHG for every 1,000 feet of elevation. I'd suppose a more accurate new school approach would be to check the weather channel and get the ambient barometric pressure and add in the difference between your location and sea level (29.9 inHG i believe)... however i've searched far and wide and can't get a confirmation on that.


Hmm... I seem to be feeling a bit better about my engine now. I might do an engine cleaning treatment and recheck the results. Its not like the test takes more then 3 minutes to perform and $15 for a gauge... i'm suprised more people haven't tried it actually since Mazda seems to think it correlates decently with engine health


...and thanks to the individuals i've gently towards this thread ....

 

sorry, absolutely useless

you take the Fluke reading using the special harness and input the data into an online spreadsheet and then it indicates whether the engine is good or requires displacement

somewhere in the spread sheet is the altitude correlation data, they never actualy let anyone see what it is, the system only provides the Tech with a go/no-go answer

and yes, you need to use standardized barometric data, we have a major airport nearby and that info is charted by the hour so it's easy to achieve

EDIT: Googling did reveal the 1 in/hg per 1000 ft elevation swag, I suppose that's good enough. I may have the car near sea level on Sunday and can take a reading if I do. Still can't locate a compression tester anywhere in the area except at the dealership that I refuse to ever even look at again

 

FWIW
I tested mine this morning
Idle temp at warm-up hot (no drive)
15-16
After a good beating, she read
14-15

2000 rpms jumps her to 21-22

Almost forgot, this is at 804 ft elevation.

 

Looks about the same as me. The problem is that small changes can effect it, so switching between the int-x and the accessport resulted with idle vacuum changing even though my engine was exactly the same as it was a few minutes prior.



I ran seafoam through my engine and it didn't seem to help with the vacuum readings - I noticed no change in vacuum or power delivery. A few weeks later Jeff tested my engine and it was within normal compression spec.


Unless your engine is extremely bad, I don't think the vacuum readings can be viewed into with much more accuracy than tea leaves.

 

And really who drinks tea nowdays anway?

 

Vacuum is a resultant of compression. In other words, you can't achieve strong numbers at low rpms (700 to 950) without good compression. From that standpoint, it's quite relevant (Taken with the other aformentioned factors) in determining engine health.

Seal.

 

Just finished w/ a seafoaming and the readings are identical.

 

Conversely, a strong engine can have a weak vacuum reading, as I mentioned above.

 

running seafoam through through the engine (flood 2 cans, soak, run out) coupled with concentrated doses in the fuel (1 can per 5 gal) and several autox events i.e. a lot of WOT (car isn't really driven on the street much if any) the vacuum readings have come up

 

i have just rebuilded my engine,having just 400 km on it and having over 8.4 bars on all seals and on both rotors,starting perfectly but the vacuum readings on idle are arround 15 inHg on idle and arround 14,5 with the air con on,but engine is working smooth.before the rebuild my readings were over 17 but the engine had 6.3 bars of compression.What do you think about this?P.S. i am watching the vacuum values with my greddy boost controller,elevation is 255 m and baro pressure 1005 Hpa and my car is turbocharged with the greddy kit,some updates on it and Cobb tuning.