4.3 vortec - cold start issue

Discussion in 'Chevy Truck Forum' started by GregoryJ, Dec 13, 2011.

  1. GregoryJ

    GregoryJ New Member

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    I have a 95 GMC sonoma 4.3 vortec with 191k miles. Having problems w/ cold start. Troubleshooted down to fuel delivery, at least I think so. Here's my fuel pressure readings. Static: 58psi, but it drops like a rock. 48psi in 30 seconds, 42psi in a minute. Running: 54psi. Dead Head: 68psi. Changed fuel pump, but no major improvement in readings, and no improvement on cold start. Warm start is no issue at all. Recommendations?
     
  2. sac36582

    sac36582 Senior Member

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    There have been lots of posts regarding this issue; search for the post titled "hesitation problem when cold" and look through all that I've done to my truck in order to get an idea of what to test (versus just replacing parts) and how. My symptoms are 1) rich smelling exhaust that has already taken out the old cat and is busily working on taking out the brand new one that I just installed (its already starting to rattle like the old one,) 2) heavy deposits under the distributor cap, probably due to the spark jumping a huge gap because of the 3) massively advance ignition timing, 4) excessively high idle speeds and 5) a hesitation at part or no-load throttle positions at road speed.

    It sounds like you may be having problems with your fuel regulator; does the pressure drop you mentioned take place with the engine running at idle, driving down the road or when you shut it off? If it won't hold pressure, you probably have leaky poppet injectors or a bad regulator. Unfortunately, you wont be able to actually see the problem without disassembling the intake.

    This might not be a bad idea if you plan to keep the vehicle for a while, since the intake manifold gaskets at the head are known to rot out after this many years/miles and would give you the incentive to do some preventative maintenance. I'd also look for water/steam coming from the exhaust pipe well after start-up when the engine/exhaust system is at regular operating temp. If you still have steam, it could mean that you have a water leak that is going into the lifter valley and then being evacuated by the PCV system or into a cylinder. Either way, you will see where the water is washing the intake port or block. Same with a leaking fuel regulator, there will be a washed spot under the leak. You won't be able to determine if the poppets are leaking, since the fuel will wash the intake ports post injector anyway.

    Start with testing/cleaning the MAF (using the correct cleaner,) the knock sensor and the FI system thermostat. The knock sensor is a piezoelectric device so you have to measure it in the A/C milli-volt range or check the timing for a response with a light. Clean the terminals under the cap with some emery cloth and see if that doesn't make it start better on the initial fire-up of the day. There are descriptions of test procedures in the above mentioned thread. Also, check your engine timing and if its way off towards advanced, you probably have a worn cam. I'm taking my truck down today to measure cam lift and if that's the answer to my otherwise unsolvable (truck) problems, I'll post the results.

    -MED
     
  3. GregoryJ

    GregoryJ New Member

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    I reviewed your post "hesitation problem when cold". Bad CAT. Nice to get it resolved !

    I'm with you with thoughts about my pressure regulator and injectors. My pressure drops during the Static test, engine not running, from 58psi very rapidly. Shop emissions manual fuel diagnosis chart A-6 is telling me to check for a partially disconnected fuel pulse dampener ( pulsator ) or a faulty in-tank fuel pump. Changed the pump like you to a new Bosch unit already. No improvement. I've cleaned the MAF sensor and throttle body intake. Replaced the IAC too.

    DOES ANYONE KNOW WHAT/ WHERE THE FUEL PULSE DAMPENER ( PULSATOR ) IS?
     
  4. sac36582

    sac36582 Senior Member

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    The fuel pulse dampener goes on the output side of the pump between the pump and the fuel supply tube that goes up to the top of the tank in the sender housing. It looks like a two-bulb rubber thingy on one side that has a metal backing plate. The new fuel pump would have come with a short piece of rubber tube to eliminate this device. Some people say to keep it, some say to replace it with a 10-12" long piece of rubber tube made into a loop (this does much the same thing as the original, but you don't have to worry about it going bad) and some people say just get rid of it by putting in the short piece of hose.

    I think that with your new Bosch turban style pump, you don't need it. Do you remember putting it back on when you changed the pump or did you connect the output of the pump to the tank outlet line with the short piece of hose?
    -MED
     
  5. chevychase

    chevychase Semi-Admin
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    Changed pump .. has to be either fuel damper / check valve or fuel pressure regulator ... or leaky injectors... or hole in line in tank.
     
  6. GregoryJ

    GregoryJ New Member

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    Guys, thanks for all the help. Landed up taking the top of the upper intake off and replaced the "spider" injector/ regulator assembly. The regulator was dripping fuel when I fired up the pump and pressurized the system. Starts immediately now when cold. I now know how critical fuel pressure is to cold start.

    I did not have a pulsator on my fuel pump. I replaced the fuel pump with a Bosch. I think I had a replacement Bosch unit in there already.
     
  7. dirtrider73068

    dirtrider73068 Senior Moderator
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    More than likely the fuel pressure regulator was bad leaking fuel thus loosing pressure.
     

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