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Help a TPI newbie with low RPM stumble and a constantly running fuel pump......

Discussion in 'The Injection Section' started by CobraEatr, May 22, 2008.

  1. CobraEatr

    CobraEatr Registered Member

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    Hello everyone. I'm glad to finally be a full member on this board.

    I just recently bought a 1984 K5 Blazer from a guy who performed a carb to TPI swap. The engine is a rebuilt 350 from an ’89 Blazer and he used the complete TPI system and computer from an ’85 IROC.

    The truck starts perfectly each time it’s cranked. It idles nicely and for the most part it drives well. The only problem is at low RPMs in the range of 1mph to around 15mph. The engine seems to ‘bog-down’ in this range and it is only corrected if I give it a fair amount of gas. If I lightly feather the gas pedal then it surges and hesitates. Once I get above 15-20 mph it’s smooth sailing. It does this each and every time that I get back into this range. If I stomp on it from the start then it speeds up well with no hesitation. I CAN drive it around but I prefer not to until this gets fixed.

    Also, the fuel pump runs constantly when I turn the key to the accessory ON position (but before I actually crank the engine). In my other car (a ’93 Z28) my fuel pump will pressurize and then stop, but this isn’t the case with the Blazer. I’m not sure if this is the root of my low RPM stumble or not but I thought it was worth mentioning. All the fuel accessories are aftermarket. He was using an inline fuel pump that is located in the engine bay.

    One extra item to note is that all emissions have been removed from this motor. Most things I’ve read on this type of problem seem to be solved with an EGR fix, but that won’t work in my case.

    I’ve cleaned my IAC and I replaced my TPS. I set the TPS at 0.54V. The problem is still there though.

    I've spent almost one full month searching for answers to this over on thirdgen.org but I've never found anything that helped. Therefore I decided that it was time to pony-up and become a paying member on this board. Any info is greatly appreciated.

    Thanks!
     
  2. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    The first thing you need to fix is the fuel pump issue. The pump should only cycle for 2 seconds in the on position then shut off until you've started the vehicle. The second thing that you need to address is the fuel pump location. An electric pump is designed to push fuel not suck fuel. The pump needs to be mounted as close as possible to the fuel tank and also as close to the bottom as feasibly possible. Is there an inline fuel filter somewhere in the system? Remember that it must be a high pressure filter and not a low pressure like for a carb or TBI application. TPI runs in the 45 psi range. I would also check your fuel pressure and go from there. Some or all of these items may be your problem.
     
  3. CobraEatr

    CobraEatr Registered Member

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    Thank you, I'll try all of those things. I need to take a picture and post it of what the fuel system looks like. In the engine bay, on the driver's side and close to the front of the grille there is an electric fuel pump, an inline fuel filter, and a large canister of some type that I have no idea as to what it is. All of these things are connected with nice braided hoses. The fuel pump seems to sit pretty low but I too thought it was weird that it was located in the engine bay.

    I'll check the fuel pressure this weekend and post it back on here. I guess I can borrow a FP guage from an auto parts store to check it. In the meantime, does anyone know what is causing the fuel pump to constantly run? Would replacing my fuel pump relay or my oil pressure sensors fix this problem?
     
  4. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    Someone probably wired the fuel pump incorrectly and wired it to a key on hot lead. The fuel pump hould be ran trough the ECM so that when there's a crank signal then the fuel pump will run. You have any idea if this is some sort of aftermarket harness or did the PO wire this thing himself? The oil pressure sensor allows the fuel pump to continue working if the fuel pump relay fails. Most people get it confused and think it shuts the fuel pump off if there is low oil pressure which is incorrect.
     

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