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-   -   hmm high vaccum @ idle (https://www.miataturbo.net/general-miata-chat-9/hmm-high-vaccum-%40-idle-18887/)

trito 03-27-2008 07:12 PM

hmm high vaccum @ idle
So I was searching around old threads and I saw one about vacuum @ idle. I thought my vacuum was normal ( - 11-12psi) but I see that most people have - 18-22. My AFPR looks ok and my idle is around 1K. My fuel pressure idles @ 43 psi (I'm not using a AFPR) but I thought that might be because I'm using a 190lp HP pump. Injectors are new and so are the O-rings. So...

Where are good places to look for leaks?

What's the best method to find them?



lwindridge 03-27-2008 07:18 PM

- 11-12psi is normal (18-22 inch/mercury) - nothing to worry about I'd say!

Braineack 03-27-2008 07:35 PM

the higher the vacuum the better....the lower the vacuum you have problems...

-10 - -11psi = 18-20 ~hg.

trito 03-27-2008 07:43 PM

hmm ok I was wrong I guess. I thought my Prosport read everything in PSI but I guess if it's vacuum it reads inhg instead.

This is what I'm using.


so 10-11 inhg = vacuum leak?

Braineack 03-27-2008 07:50 PM

correct. :)

trito 03-27-2008 07:53 PM

so back to my orignal post...

Other than a smoke machine, what's a good method to find a leak?

patsmx5 03-27-2008 07:56 PM

I'm not converting units, but IF you do have a vacuum leak, I would first look to the last are you messed with. So, if you dropped in new injectors yesterday, I might start there. You can spray carburetor cleaner where you suspect there is a leak. If there is, engine speed will change, the engine will respond, making it obvious.

trito 03-27-2008 07:58 PM

koo thanks for the suggestion patsmx5!

chriscar 03-27-2008 08:37 PM

Pressure test!

My weapon of choice, made with nothing but Home Depot Racing's finest parts.


Alta_Racer 03-27-2008 08:47 PM

Low vacuum is not absolutely a leak.

What is your ignition timing doing and is the cam timing correct? Poor engine condition (ie compression) can lower vacuum as well. Poor A/F ratio can also make it take a dump.

To drop 6-7" of vacuum you wouldn't have to search real hard for a leak, it would be a very loud leak.



04 Miata 03-27-2008 08:59 PM

Did it drop after you installed the new injectors?? A bad seal at the injector could be the culprit. I use an unlit propane torch to search for leaks. Wanding it around the places a leak could occur will allow the fuel to pull in and raise the idle when you find the leak.

Braineack 03-27-2008 09:14 PM


Originally Posted by trito (Post 234623)
so back to my orignal post...

Other than a smoke machine, what's a good method to find a leak?

you dont have a leak. -10psi is great.

y8s 03-27-2008 09:55 PM

wait, that gauge only goes to 14 inHg. my car idles at like 22 or something. (bigger number = lower on the gauge)

jtothawhat 03-27-2008 09:59 PM

Could be your BOV spring too.

trito 03-27-2008 10:51 PM

unfortunately even if I had that I don't have anything to hook it up to :|

it's the same reading before the injectors.

Alta Racer:
I'll need to dig up my timing gun but last I check it was 10 at idle

it's 10 in/hg not psi, my mistake

Matt: yeah I realize now that it only goes to 14inHg(actually it goes to 15) but even so I'm getting 10-11 so there's something wrong there.

my bov is recirculating, would the spring still matter?

jtothawhat 03-28-2008 06:19 PM

Spring pressure does matter, the stiffer the spring the lower the number ect -20IN/HG with a 11 psi spring vs. -10IN/HG with a 6 psi spring. It allows air to escape at idle. I have a really soft spring in my Tial BOV and at idle I have -10INHG at idle, which most would say is really high, but it's all due to my BOV spring. I would check the spring tension on your Bypass Valve, and if you can tighten it an see if that corrects your problem.

trito 03-28-2008 06:36 PM

ok I can try that, my bov had 3 springs, I was using the middle tension one.

budget racer 03-28-2008 06:45 PM

ummm....can anybody explain this to me? i don't understand why there would be a difference in vacuum if the BOV is before the throttle plate.

Bryce 03-28-2008 06:58 PM


Originally Posted by budget racer (Post 235082)
ummm....can anybody explain this to me? i don't understand why there would be a difference in vacuum if the BOV is before the throttle plate.

I'm wondering the same thing. It wouldn't make a difference if your BOV is leaking at idle if your vacuum measuring point is further downstream IE: at the intake manifold.

Maybe jtothatwhatever is wrong..

Joe Perez 03-28-2008 07:30 PM

Uhm... yeah. You can remove the BOV entirely, leaving a large gaping hole in your intake plumbing, and manifold vacuum will be affected not one little bit. Trust me, mine fell off (well, it was blown off) a few weeks ago. :D

Manifold vacuum is all about the space between the exhaust valves and the throttle body, inclusive. If your rings or valves are shot, you'll have low vacuum. If you've got a leak somewhere on the throttle body or the intake manifold, you'll also have low vacuum (and high idle.)

If you do have any appreciable vacuum in the intake tract between the air inlet and the throttle body (where the BOV lives) then you really need to clean your air filter.

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