[GMCnet] Vacuum after FiTech installation

[Advanced Concept Energy Solutions]

After installing FiTech FI system on my 455 the intake manifold vacuum has dropped from 22" at idle to 17.7". I have looked all over for leaks but found none. Idles smooth, runs great, mileage seems better than with carb but haven't driven far enough to verify. Vacuum at all loads seems lower by at least 3". Have stock intake manifold. What vacuum are others getting with stock manifold? This is new engine and trans, 3:46 gears, all new vacuum hoses, new brake booster, new electric vacuum pump on brake booster. Tried squeezing off vacuum lines one at a time to see if it affects manifold vacuum but nothing. Torqued down on TB again ( with all those spacer gaskets) but no difference. I did find a bad booster check valve but replaced it and picked up 1" to get to 17.7".

Jon Darcy ACES
North Jersey 76 stretch, flares, 4 bag, Alcoa's, bunkhouse,MAC Dash,Manny Trans, FiTech FI,Honda 6010

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[Espen Heitmann]

Is your vacuum port to the gauge at the same place ?
--
1973 26'-3 # 1460 Parrot green Seqouia Known as the Big Mean Green Machine,
And just sold my 1973 26'-2 # 581 White Canyon lands under the name Dobbelt trøbbel
in Norway

[Espen Heitmann]

And did you adjust the ignition timing ?

[Jim Kanomata]

They need to know manifold vacuum from throttle vac.

--
Jim Kanomata
Applied/GMC, Fremont,CA
ji...@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502

[Advanced Concept Energy Solutions]

Vacuum on the dash vacuum gauge reads from a fitting on the front of the manifold also now shared with the trans. The gauge reads about 17.5 at 750 rpm idle with no AC in park. The electronic readout LCD plugged into the FiTech TB reads 17.7 " on it's dashboard display. Not sure where on the throttle body it reads this vacuum but it must be below the plates. I am not really sure that this is a problem since the engine runs well. I am just mentally tuned to higher vacuum means better mileage and I drive using the vacuum gauge. I had it placed in the center of the new Mac dash. Just wondering if others are seeing this same vacuum. Should I be adjusting the idle screw to close the plates even though the other numbers are where they should be? One odd thing with the FiTech was that one of the small vacuum ports didn't have vacuum on it and evidentially wasn't drilled out. Maybe this winter I will pull it off and look at this but for now it runs.
Thank you everyone for your feedback on this and also my question on steering play. ( another winter project)

Jon Darcy ACES
North Jersey 76 stretch, flares, 4 bag, Alcoa's, bunkhouse,MAC Dash,Manny Trans, FiTech FI,Honda 6010

[Wally Anderson]

Advanced Concept Ener wrote on Fri, 22 July 2016 11:45

If you are running closed loop fuel at idle that might explain less vacuum at idle. Our Qjet ran richer than 14.7 airfuel ratio which is probably what
fitech trims to. Ours is happier around 13.5 AFR at idle.
--
Wally Anderson
1975 Glenbrook
Megasquirt 455 port injection science project
Omaha Nebraska
Bob Stone hydroBOOOOST
Greater Midwest Classics
GMCES
http://wallyandsue.blogspot.com/

[Norman Allinson]

My Vacuum dropped by about 2" at idle, in park after installing the Fitech, however my gas mileage is definitely improved, the plugs look good, and
it's running so much better that I'm not concerned. I'm guessing that my carb was in need of attention.
--
Norm.

[Jim Kanomata]

Think your old carb was in poor shape.

--
Jim Kanomata
Applied/GMC, Fremont,CA
ji...@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502

[George Beckman]

Advanced Concept Ener wrote on Fri, 22 July 2016 09:45

> Vacuum on the dash vacuum gauge reads from a fitting on the front of the manifold also now shared with the trans. The gauge reads about 17.5 at
> 750 rpm idle with no AC in park.
>

> Should I be adjusting the idle screw to close the plates even though the other numbers are where they should be?
>

> Jon Darcy ACES
> North Jersey 76 stretch, flares, 4 bag, Alcoa's, bunkhouse,MAC Dash,Manny Trans, FiTech FI,Honda 6010

Jon,

Don't adjust the plates unless something in the FiTech manual says to do so. With EFI, the idle speed is not changed by the throttle plates. Opening
or closing them will not change the idle speed.

That being said, it could be that FiTech sets up the unit with the plates further open at idle that the Q-Jet plates. FiTech may want more air moving
past the injectors at idle. More air will keep fuel moving on the intake runners during warm up. Our intakes are not the best when it comes to the
runners and can easily "store" droplets of gas when cold.
--
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George

[Dan]

Im running the fitech on my 454 chevelle. Ive got a huge cam and before with my holley carb I barely had brakes. Now with the fitech my vacuum has
dropped so much that I cant drive the car until i get a vacuum pump or convert to something else for the brakes.
--
1976 Palm Beach 26'

[Ed Kline]

On the Velocity TV program Phatom Works they had a similar problem on a 1966 Pontiac GTO with an aggressive cam and determined wasn't getting enough
vacuum and added a Hydro-boost system which resolved the brake problem. There is a Hydro-boost kit for GMC motorhome (Applied GMC carries).
--
76 Palm Beach
74 Corvette
08 Jeep Liberty
02 Honda Silverwing