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Old 05-30-2013, 02:10 AM   #1
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Default Low Vacuum & Running Rich on Fresh Build

I never thought I'd actually make a thread about this, but here I am.

Freshly [re]built NB motor with forged rods/pistons, 0.5mm overbore, 9.0 CR. Running what is essentially a Flyin Miata FM2 kit (GT2560R turbo setup), with an [unfortunate] Hydra 2.7.

Symptoms:
-The car runs rough and idles rough
-Pulls -12 to -15inHg of vacuum at best (380 to 450 mmHg or 42 to 51 kPa absolute)
-Runs ~17% rich across the board (haven't gone into boost yet) on a reasonable base map, meaning a volumetric efficiency of like 35% at idle...

What I've checked/tried:
-6 vacuum gauges used; all yield same numbers; hooked to upper backside of manifold as well as upper center
-No vacuum leaks; MAP tuned and ISC is operating at a reasonable 30 to 35%
-Ignition timing is good
-Cam timing is verified stock (no VVT; Mazdaspeed motor)
-Wideband sensor properly calibrated
-EGR is connected but valve is sealing well and has been run disconnected
-Injectors cleaned/tested by RC, flowing dead on with excellent spray patterns
-Tried new coils & wires; no luck
-Cold compression test gave about 175psi on all 4.
-All plugs look evenly white after idling

There are a few more obvious tests to do, but the above pretty much rules out any issue I can think of.

I'm tearing my hair out on this one... Any advice or ideas would be much appreciated.

Thanks

Last edited by devgru38; 05-30-2013 at 02:52 AM.
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Old 05-30-2013, 09:39 AM   #2
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Is the engine broken in yet? Thats about how much vac I drew on first start but after 10 minutes on the road breaking it in it went down to the 30kpa and below range at idle.

Otherwise you have a vac leak.
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Old 05-30-2013, 10:54 AM   #3
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If your AFR is FUBAR, why would you expect decent vacuum?
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Old 05-30-2013, 12:10 PM   #4
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Originally Posted by Leafy View Post
Is the engine broken in yet? Thats about how much vac I drew on first start but after 10 minutes on the road breaking it in it went down to the 30kpa and below range at idle.

Otherwise you have a vac leak.
Yep, about 150 to 200 miles on it now. :/ How could I have a vacuum leak with idle speed control? Would the ISC have to bottom out? Not that I haven't checked everywhere w/ carb cleaner already...

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If your AFR is FUBAR, why would you expect decent vacuum?
AFR is FUBAR likely because of bad vacuum. I've tuned the entire table lean 17% just to see the result. It runs just about dead on stoich, and still pulls bad vacuum... Also, leaning the engine out 17% from what it should be isn't acceptable; something's clearly wrong. 35% VE is not correct.

With worse vacuum, the ECU assumes there's load on the engine and goes higher up the fuel map, injecting more fuel. I think if I can sort out the vacuum issue the fueling will sort it self out.
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Old 05-30-2013, 12:13 PM   #5
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After a year of searching for a vacuum leak, I found mine at the injector #4 against the block. Cleaning the mating surfaces fixed the leak and vacuum gauge immediately sat at stable idle around 19.
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Old 05-30-2013, 12:52 PM   #6
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After a year of searching for a vacuum leak, I found mine at the injector #4 against the block. Cleaning the mating surfaces fixed the leak and vacuum gauge immediately sat at stable idle around 19.
Was this on a map tuned car with idle speed control? Was the idle speed control not bottoming out?
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Old 05-31-2013, 10:13 PM   #7
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Hate to do this, but bump... Anyone have any suggestions?

Thanks again
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Old 06-01-2013, 01:04 AM   #8
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Are you positive the car is mechanically timed correctly.

If your intake/exhaust cams are both retarded by a belt tooth, the engine will pull less vacuum and run rich.

If your crank bolt was insufficiently torqued and the timing belt pulley has slipped, same result. In either case the ECU (stock or aftermarket, doesn't matter) will fire things at what it sees as the appropriate times, because the sensors say so, when in fact the engine is mechanically ahead of that point - thus everything is retarded.
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Old 06-01-2013, 01:49 AM   #9
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Originally Posted by Mobius View Post
Are you positive the car is mechanically timed correctly.

If your intake/exhaust cams are both retarded by a belt tooth, the engine will pull less vacuum and run rich.

If your crank bolt was insufficiently torqued and the timing belt pulley has slipped, same result. In either case the ECU (stock or aftermarket, doesn't matter) will fire things at what it sees as the appropriate times, because the sensors say so, when in fact the engine is mechanically ahead of that point - thus everything is retarded.
Damnit, I'm an idiot. First off, ^^^exactly what I was thinking, but I triple checked all timings... Just pulled the valve cover to manually check TDC vs crank pully and decided to shove a feeler in and check valve lash just for ***** and giggles... I can barely get a 1 thou in most of them... Should've checked this right off the bat, but figured that since the shims haven't changed, and the motor was just built, it couldn't be the issue...

So new question: my builder said he replaced all shims exactly as they were when he tore the motor apart; whether or not this is the case, they are all tight. Considering that the same shims were used, even if they were mis-ordered, how can this be? What can change valve lash across all valves without changing shims?

Also, idiot check on myself: would super tight valves be enough to fully explain these issues? Could that make enough of a difference to cause a 4 to 7 inHg decrease in vacuum?

Much thanks for the help!
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Old 06-01-2013, 06:44 AM   #10
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A valve job on the head will sink the valves and tighten them up. If they are that tight they may not be closing properly causing your issues... Burining an exhaust valve is possible..
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Old 06-01-2013, 07:16 AM   #11
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Originally Posted by Stealth97 View Post
A valve job on the head will sink the valves and tighten them up. If they are that tight they may not be closing properly causing your issues... Burining an exhaust valve is possible..
Makes perfect sense. Much thanks guys, this has been super informative. Guess I'll take it to a dealer to lash, since they'll have the shims.
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