| [GMCnet] Vacuum after FiTech installation [message #303894] |
Wed, 20 July 2016 20:05  |
Advanced Concept Ener
Messages: 112
Registered: December 2014
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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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| Re: [GMCnet] Vacuum after FiTech installation [message #303985 is a reply to message #303980] |
Fri, 22 July 2016 12:26  |
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wally
Messages: 643
Registered: August 2004
Location: Omaha Nebraska
Karma: 5
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Advanced Concept Ener wrote on Fri, 22 July 2016 11:45Vacuum 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
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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
Omaha NE
75 Glenbrook
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| Re: [GMCnet] Vacuum after FiTech installation [message #303988 is a reply to message #303894] |
Fri, 22 July 2016 12:51 |
Norisan
Messages: 18
Registered: April 2015
Location: Calgary
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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.
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| Re: [GMCnet] Vacuum after FiTech installation [message #304014 is a reply to message #303988] |
Fri, 22 July 2016 23:58 |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
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Think your old carb was in poor shape.
On Fri, Jul 22, 2016 at 10:51 AM, Norman Allinson wrote:
> 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.
>
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--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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| Re: [GMCnet] Vacuum after FiTech installation [message #304017 is a reply to message #303980] |
Sat, 23 July 2016 01:45 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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Advanced Concept Ener wrote on Fri, 22 July 2016 09:45Vacuum 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
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| Re: [GMCnet] Vacuum after FiTech installation [message #304396 is a reply to message #303894] |
Thu, 28 July 2016 00:38 |
Greenchevelless
Messages: 25
Registered: February 2013
Location: Tulsa, Okiehoma
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Junior Member |
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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'
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| Re: [GMCnet] Vacuum after FiTech installation [message #304483 is a reply to message #304396] |
Thu, 28 July 2016 23:39 |
Ed K
Messages: 14
Registered: February 2014
Location: Northern Illinois
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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
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