TBI running on a mechanical pump

Discussion in 'The Garage' started by Chevy305, Nov 8, 2019 at 2:46 AM.

  1. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

    Joined:
    Jan 23, 2006
    Posts:
    12,082
    Likes Received:
    1,270
    Location:
    New Hampshire
    Build(s):
    3
    Anyone know if a TBI system can run on a mechanical fuel pump? I'm in the middle of doing a TBI swap to my '78 K10 and was looking to see if I could test fire it using the existing fuel system.
     
  2. Kain

    Kain 1/2 ton status Premium Member

    Joined:
    Mar 16, 2011
    Posts:
    3,596
    Likes Received:
    620
    Location:
    Lubbock Texas
    Build(s):
    1
    no not enough pressure
     
  3. Bent77

    Bent77 lil meanie Staff Member Moderator GMOTM Winner

    Joined:
    Dec 3, 2002
    Posts:
    26,600
    Likes Received:
    32,873
    Location:
    NoCo
    Build(s):
    1
    I don't know a mechanical pump that will do it, being you need 10-14 psi ideally
     
  4. paratrooper307

    paratrooper307 1/2 ton status Premium Member

    Joined:
    Sep 26, 2010
    Posts:
    1,068
    Likes Received:
    92
    Location:
    North Carolina
    Nope. Carbs need lots of flow but low pressure. Even performance mechanical pumps are likely to be below 10 psi.
     
    Kain likes this.
  5. Smokinthehippies

    Smokinthehippies 3/4 ton status Premium Member GMOTM Winner

    Joined:
    May 6, 2009
    Posts:
    8,207
    Likes Received:
    2,606
    Location:
    Millard, NE
    Build(s):
    2
    I know it wouldn’t be enough to “run” as it’s like half the needed psi, but wouldn’t the amount of fuel available be enough to fire and fuel for a couple of seconds? mall he’s trying to do is test fire and make sure it’s correct before swapping fuel system over. I would think you could accomplish that on brake clean honestly so the tbi should be Able at least to fire off.
     
  6. paratrooper307

    paratrooper307 1/2 ton status Premium Member

    Joined:
    Sep 26, 2010
    Posts:
    1,068
    Likes Received:
    92
    Location:
    North Carolina
    He could try it, but injectors need pressure to work function and atomize the fuel. It won't hurt anything, but with most mechanical fuel pumps around 6-8 psi, the system won't work properly, and I'm guessing the goal of the test fire is to make sure the TBI system works properly, not just get the engine to fire.
     
  7. Blue85

    Blue85 Troll Premium Member

    Joined:
    Jul 26, 2000
    Posts:
    11,000
    Likes Received:
    2,094
    Location:
    Grand Rapids area
    Build(s):
    1
    Most mechanical pumps are rated for like 5psi and TBI wants 12+. If you deadhead a mechanical pump it might generate enough pressure, but would it be steady pressure or pulsating? Driven from the cam, the pump will pulse once per every 2 engine revolutions and the TBI injectors aren't timed to the engine, so the erratic nature should spread evenly across all cylinders on average. I don't think it could run enough to drive right, but it could answer the question of having all the wiring correct.

    I don't know what line adapters you would use to connect the pump output to the fuel rail, but it might not be much more time/money to pick up a cheap inline electric fuel pump. Or find a friend with a TBI truck, park it next to your project and run some rubber fuel line across...
     
    campfire and 79rustyk10 like this.
  8. diesel4me

    diesel4me 1 ton status Premium Member

    Joined:
    Jul 24, 2003
    Posts:
    25,074
    Likes Received:
    6,586
    Location:
    Massachussetts
    There are mechanical fuel pumps that put out 14-16 psi,but they cost $220 and probably wont provide a stedy fuel flow that would operate a TBI system properly,the pump I looked at online made no mention of it being able to be used with TBI..

    A cheap "test pump" could be rigged up using an external electric pump off a car that has enough pressure & volume,from a salvage yard..some cars like Mercedes and Volvo's with Bosch fuel injection had pressures as high as 80 psi,you'd need a regulator to choke it down to TBI standards..
     
  9. Bent77

    Bent77 lil meanie Staff Member Moderator GMOTM Winner

    Joined:
    Dec 3, 2002
    Posts:
    26,600
    Likes Received:
    32,873
    Location:
    NoCo
    Build(s):
    1
    Anyone mention there would not be a priming cycle either?
     
    1-ton likes this.
  10. jekquistk5

    jekquistk5 Weld nekid Premium Member GMOTM Winner

    Joined:
    Jan 18, 2004
    Posts:
    13,248
    Likes Received:
    4,807
    Location:
    Pinckney, MI
    Build(s):
    3
    If i was too the point in my swap I would send you my tbi pump as I can't use it anymore. But tbi pumps and senders are cheap, hell just go with a vortec pump now so when you ls swap it its done. Rob has been running the vortec pump and sendor on his tbi for 2 years no issues. You will need to make sure the return line is functioning properly.
     
    Chevy305 likes this.
  11. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

    Joined:
    Jan 23, 2006
    Posts:
    12,082
    Likes Received:
    1,270
    Location:
    New Hampshire
    Build(s):
    3
    Thanks for the replies everyone .
    I am really just looking to test fire the engine to make sure the bulk of the wiring works before I tie it all up nice and neat. But I won't waste my time hooking up the line and go straight to a TBI pump.

    I already have a TBI pump/sender to drop in. But damn I wish I thought about using a vortec pump. Oh well, at least I thought ahead when I replaced the gas tank 2 years ago to buy the baffled '87 TBI tank.
     
    Last edited: Nov 8, 2019 at 11:33 AM
  12. 6.2Blazer

    6.2Blazer 1/2 ton status

    Joined:
    Feb 24, 2000
    Posts:
    4,863
    Likes Received:
    140
    Location:
    Ohio
    FYI, spraying starting fluid or brake cleaner down the throttle body is not the same situation as you are completely bypassing the fuel injectors. The issue is that the fuel pressure being too low will not allow the fuel injectors to open up (pop off) and flow any fuel.
     
  13. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    28,889
    Likes Received:
    871
    Location:
    Roy WA
    Simple test that it SHOULD start is to put a test light across the injector leads. If it blinks while cranking, the ECM is seeing the distributor rotate, and firing the injectors because of that, which indicates the electrical side of the ignition circuit is working properly, and the injectors are getting power and being fired by the ECM. Fuel then would be the only unknown, but that's 12V to an electric pump, that works or it doesn't.
     
    Chevy305 likes this.
  14. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

    Joined:
    Jun 13, 2003
    Posts:
    32,269
    Likes Received:
    5,528
    Location:
    Jersey Shore
    Build(s):
    3
    didn't read anything, but... you'd be surprised at what a lot of stock TBI rigs are running around with for pressure... :whistle: my k5 and Mutt both had about 7, 8 lbs to em and ran "ok" before going TPI pump and reg...... obviously it was a noticeable gain going to 13.5... :saweet:

    that said, i wouldn't even bother trying...
     
    Last edited: Nov 8, 2019 at 4:07 PM
    Chevy305 likes this.
  15. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

    Joined:
    Jan 23, 2006
    Posts:
    12,082
    Likes Received:
    1,270
    Location:
    New Hampshire
    Build(s):
    3
    That's a good test to keep in mind. I'm just going to install the fuel pump and be done with it. If I still have problems then I'll try this test.
    Especially since I'm just going to get it running on fuel only at first and leave my HEI in place.
     
  16. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

    Joined:
    Jan 23, 2006
    Posts:
    12,082
    Likes Received:
    1,270
    Location:
    New Hampshire
    Build(s):
    3
    Hey while I have everyone's TBI attention, where exactly does the oil pressure switch go? I take it that it goes into the oil pressure port above the oil filter? Does it thread straight in or do I need to install a 90° fitting to point the connector up wards? To bypass it I just jumper it correct?
     
  17. sweetk30

    sweetk30 selective wrench turner guy Premium Member

    Joined:
    Nov 1, 2004
    Posts:
    32,902
    Likes Received:
    8,114
    Location:
    horseheads , ny 14845
    Build(s):
    4
    above filter is dash gauge .needs the gm 90* brass fitting . gets a metal cage over it also .

    safety sensor is top of block back of intake .
     
  18. 6872xtc

    6872xtc make mine modified Premium Member GMOTM Winner

    Joined:
    Sep 16, 2008
    Posts:
    5,622
    Likes Received:
    5,026
    Location:
    Palisade,Colorado
    Build(s):
    1
    While I have only worked on less than half of a dozen TBI trucks, and all of them small block trucks, my '90 Jimmy and my '95 K2500 have the oil pressure switch in the block at the back of the intake/ lifter valley. Nothing above the oil filter. An '87 K10 that I parted out was the same. I don't know what would make them different. All of them have the fuel pump switch and the sender for the gauge or light all in one.
    The fuel pump control circuit is 2 terminals in the switch, one terminal is for the gauge or light.
     
  19. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

    Joined:
    Jan 23, 2006
    Posts:
    12,082
    Likes Received:
    1,270
    Location:
    New Hampshire
    Build(s):
    3
    That's weird, the switch I bought has a 1/4" NPT threads while the fitting at the back of the valley is 1/8" so I figured it would go to the other oil pressure port at the filter. The pigtail for the switch seems like it could reach either location.

    EDIT:
    Found this older thread detailing how the oil pressure switch works. Good read, definitely just learned something.
    https://ck5.com/forums/threads/do-i-really-need-the-fuel-pump-oil-pressure-switch.227486/
     
    Last edited: Nov 8, 2019 at 8:28 PM
  20. 6872xtc

    6872xtc make mine modified Premium Member GMOTM Winner

    Joined:
    Sep 16, 2008
    Posts:
    5,622
    Likes Received:
    5,026
    Location:
    Palisade,Colorado
    Build(s):
    1
    I don't believe that it will be a problem to put it down lower. I believe that some big block trucks had a switch there.

    I know that I have found a step up 90* fitting in some trucks at the back of the intake. It even has an extension to raise it up .
     

Share This Page