Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

TBI Vacuum line question...MAP Sensor

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Hoover87, Jul 4, 2002.

  1. Hoover87

    Hoover87 Registered Member

    Joined:
    Nov 7, 2001
    Posts:
    43
    Likes Received:
    0
    I'm hoping to start up the 350HO with the TBI tomorrow, but have one more little item I hope to solve/comfirm before I turn the key.

    Which vacuum source does the MAP Sensor connect to? I missed it on the pics I took and don't have a diagram. Basically which vacuum port does it connect to?

    Oh ya, one more item I can't figure out. On the original 305 there was a vacuum fitting behind the TBI on one of the manifold runners....what is that supposed to be connected to?

    Original stuff is an '87 305 TBI truck...any info would be great!!

    Thanks,

    Mike
     
  2. HarryH3

    HarryH3 1 ton status Author

    Joined:
    May 31, 2000
    Posts:
    10,384
    Likes Received:
    2
    Location:
    Georgetown, TX
    Looks like the MAP sensor on my '90 Sub is connected to a fitting on the back of the throttle body, just above the fuel line connection.

    The fitting on top of the manifold connects to the power brake booster. Did yours have other stuff connected there?
     
  3. Hoover87

    Hoover87 Registered Member

    Joined:
    Nov 7, 2001
    Posts:
    43
    Likes Received:
    0
    Ya, I think something else was connected to the fitting on the manifold. The fitting I'm refering to is on the manifold behind the throttle body. Only one hose port.

    The power brake booster on this truck was run to a metal pipe that screwed into the manifold in front of the throttle body.
     
  4. Thunder

    Thunder 3/4 ton status

    Joined:
    Jul 20, 2000
    Posts:
    8,946
    Likes Received:
    1
    Location:
    Northeast Nevada
    On my 89 with TBI. Looking at the TB from the front.
    There is one large connection in the middle bottom of the TB for the PVC hose.
    Under this is the port for the fuel evaporation canister.
    To the right (passenger side) there is a small port where the EGR solonoid vacume line hookes to.
    On the left side on the manifold is the steel line that goes to the brake booster.

    In the rear of the TB.
    On the TB there is a port for the MAP sensor vacume line.
    On the manifold is a port whis has a line connected to it that goes to the Vacume anplifier(big plastic globe on the drivers side firewall). and tees off to the Cruise controll and heater vent accuators.
     
  5. Blazer_Boy

    Blazer_Boy 1/2 ton status

    Joined:
    Dec 16, 2000
    Posts:
    1,669
    Likes Received:
    0
    Location:
    Sioux City, IA, USA
    Is that the GM 350 H.O.? I didn't know they were computer compatible. Of course I had people telling me that a bigger cam would rob my vacuum and cause trouble. I put on the vac gauge and I have 22" with my 4x4 xtreme cam. Kinda nice not having to mess with some calibrations when I'm pulling stock vacuum.
     
  6. Hoover87

    Hoover87 Registered Member

    Joined:
    Nov 7, 2001
    Posts:
    43
    Likes Received:
    0
    Thunder...thanks a million! That is exactly the info that I need.

    Blazer Boy....yes, it is the GM 350 H.O. 330 horse crate. I'll find out today how compatable or uncompatable the thing is with the computer. 50% of the people I talked to said it couldn't be done, 25% said it would work but will take some tweaking, the last 25% said it worked really really well. I figured what the heck...why not take a crack at it. I think its just one of those things that has to be done right to make it work....an all or nothin' project. We'll see.

    Thanks again to all who replied.

    Mike
     
  7. Thunder

    Thunder 3/4 ton status

    Joined:
    Jul 20, 2000
    Posts:
    8,946
    Likes Received:
    1
    Location:
    Northeast Nevada
    The only problems with the 350 HO engine and computers is the cams the exahust duration is around 220-222 degrees It may cause a loss of vacume affecting idle and off idle performance. The stock computer likes cams with no more than 214 degrees duration.
    Also the Stock fuel settings may not be enough to feed the engine to bring out all the power it is capable of. This can easily be fixed with a new chip burned for the cams profile. Or beings it is not much over 220 degrees you can get an adjustable MAP sensor from turbocity that you can probabally dial it in with. I also recomend you get an adjustable fuel pressure regulator. It really helps with performance engines.
    Here's the link to their TBI page:turbocity/TBI

    Also if you find you need more fuel pressure than the stock regulator provides GM makes a regulator for marine applications P/N 17113186 It is a direct bolt-on jobbie, and will give you about 18-20psig fuel pressure. Additionally, this regulator has vacuum compensation capability. If you hook the included vac port to manifold vacuum, then you'll get a part throttle fuel pressure decay of roughly 1psig per every 2"Hg manifold vacuum. These allow you to get more fuel under hard acceleration then cut the pressure back when cruising. I have heard these work very well on modified and HO 350s.

    I'm sure with a little tweaking you will end up with a very good running TBI fed engine that will get decent gas milage too.
     
  8. Hoover87

    Hoover87 Registered Member

    Joined:
    Nov 7, 2001
    Posts:
    43
    Likes Received:
    0
    Well, she started up yesterday with the first turn of the key. We ran it for the first 20-30 minutes of break in with the stock chip in it cause I forgot to swap in the new one from Faschip, but it seemed to run okay. No big deal, just sitting there with the engine rev'ed. Anyway, after we set the timming the stock 305 chip chip would idle the engine just fine but coughed and choked when rev'ed. I then swapped in the custom chip and it didn't want to idle well but would rev better. We swapped a few vacuum hoses, hooked up the PCV valve and it seemed to run a little better, but still needs some tweaking. I'm going to fiddle with it some more today. I think I will order an adjustable MAP sensor and give that a try if I can't get anywhere with it today. The big thing is that she is in and running, now she just needs a little tweak...I hope.

    I haven't tried to drive it yet because I just discovered that my clutch slave cylinder is shot. That showed get here next week as I am living overseas at the moment. Plenty of time for some more tweaking.

    So far this is waht I have into the project:

    '87 Chevy shortbox 4x4 305 TBI truck, 4-speed manual
    GM Crate 350 H.O.
    GM Vortec to TBI manifold
    Brand new stock 454 TBI, purchased at TurboCity
    Intake Manifold bored to match 454 throttle body
    Custom Chip from FastChip
    Misc. stuff, plugs, wires, filters, blah, blah, blah....

    Thanks again for the input and ideas.

    Mike
     

Share This Page