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Old 12-12-2011, 07:45 PM   #1
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Default really high Vacuum

I got a p0171 code at Road Atlanta about 2 weekends ago. I had checked my vacuum the week before Road Atlanta and read 21.5 in/Hg, so I figured I was unlikely to be a vacuum leak. I went ahead (anyway) and retightened down all my hose clamps and any fittings going to my catch-can/intake manifold. Checked the vacuum when I got back and it read 25 in/Hg.

I suspect I was probably leaking a bit some where and tightening everything down fixed it up, but the real question is why is my vacuum so fuggin high?

Most threads I see say about 20-21 in/Hg is average for the miata and 18 in/Hg or higher is factory spec. Any problems that lead to high vacuum, or should I enjoy it while it lasts? motor has 67k miles, Naturally aspirated, msm cam, and my intake runs down through the undertray into the front bumper.
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Old 12-12-2011, 08:04 PM   #2
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wouldn't that be a good thing?
the higher the vacuum, the lower the manifold pressure, the less fuel you're injecting.

I idle at about 35kPa (megasquirt) which is like, 20 inHG? Assuming 0kPa = 30inHG and 100kPa = 0inHG.

So 25inHG is like 15-20kPa? That's boss, I can only see that when coasting/engine braking.
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Old 12-12-2011, 09:37 PM   #3
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Your math had me going there for a second, but I take it you are subtracting atm. out?
100 kPa=29.9 in/Hg (so you are comparing absolute in/Hg with gauge Kpa?)
I don't understand how this whole process works, so if anyone can explain how you got less manifold pressure = less fuel, then please go ahead! Sorry reading higher vacuum/less fuel has me concerned that it might be the cause for the lean condition the ECU is giving me. But that sounds really backwards.

I just want to make sure there isn't something I did wrong to get such high vacuum idle. I will be hooking up an MTX-L to my car soon, just to start getting ready for the megasquirting in the next month or so.
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Old 12-12-2011, 11:52 PM   #4
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high vacuum is most likely a valve that isnt sealing correctly. its allowing air to be pulled into the chamber during the downstroke after combustion. Which would throw the p0171 lean code. just a thought.
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Old 12-12-2011, 11:57 PM   #5
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one at a time bring each cyl to TDC then do a leakdown test. if it passes. start looking towards injector pulses. see if one isnt firing correctly.
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Old 12-13-2011, 12:21 AM   #6
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Thanks for the suggestion Jeff. I have been planning to do a leakdown test for a while now. I do have a valve guide/seal oil leak, so I think that further points towards a valve issue.

I will post more about the leakdown results in the next two weeks.
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Old 12-13-2011, 01:05 AM   #7
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on my 1.8 2003 factory motor i had a nice n steady 25 mmhg of vacuum at factory hot idle.
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Old 12-13-2011, 01:09 AM   #8
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here's how pressure works:

At sea level, everything on earth is under 1 atmosphere. 1 atmosphere is about 100kpa absolute pressure.
100kpa of absolute pressure = 0kpa of relative pressure = 0inHG of vacuum = 0psi of boost.
0kpa of absolute pressure = -100kpa relative pressure = 100% vacuum = about 30inHG = not boost anymore.
200kpa of absolute pressure = 100kpa relative pressure = not vacuum anymore = about 15psi of boost
300kpa of absolute pressure = 200kpa relative pressure = not vacuum = 30 psi of boost

this gauge is in inHG vacuum, psi boost and relative pressure in kPa

So when your engine is reading 25inHG that means it's actually sealing better than an engine at 20inHG. Basically what I'm saying is that 25 inHG vacuum is pretty damn awesome and is a sign of a healthy engine, IMO.
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Old 12-13-2011, 02:27 AM   #9
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uh yeah i meant inHG not mmHG

BTW

Timing and piston compression ratio will also affect vac pulled. So will RPM.
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Old 12-13-2011, 05:33 PM   #10
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Gotcha, thanks for the suggestions guys. Another thing to mention, which I found to be odd, is my vacuum gauge doesn't do the vibrating like normal anymore. Which I think follows what Jeff said.

But either way, a leakdown is called for here.

Fae, it makes sense yours would run higher vacuum as you have 10:1 compression, but my compression hasn't changed at all. So why did it go from 21.5 to 25?

I really just need to get my daily so I can get into the engine and figure **** out. any other suggestions?
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Old 12-13-2011, 05:35 PM   #11
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Quote:
Originally Posted by Track View Post
any other suggestions?
Moar Boost!!
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Old 12-13-2011, 08:48 PM   #12
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Quote:
Originally Posted by Track View Post
Fae, it makes sense yours would run higher vacuum as you have 10:1 compression, but my compression hasn't changed at all. So why did it go from 21.5 to 25?

maybe you had a vacuum leak that fixed itself?
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Old 12-13-2011, 08:57 PM   #13
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Quote:
Originally Posted by illeniummx5 View Post
high vacuum is most likely a valve that isnt sealing correctly. its allowing air to be pulled into the chamber during the downstroke after combustion. Which would throw the p0171 lean code. just a thought.
Not this.
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Old 02-24-2014, 11:41 PM   #14
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bringing this from the dead a bit. Don't know why/if I have high vacuum, but I do have a good bit of leaking overall and an oil leak in piston 2. I suspect a valve seal...but could be something else.

The lean condition is due to mods though, over the past few years, as I have driven on street the lean CEL never came on, but it does on track where i am mostly in open loop.

MSM cam + I/H/E. With that said, I am still in safe AFRs for N/A car.
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