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Help me figure whats wrong with my TBI 355

Discussion in 'The Garage' started by sled_dog, Feb 25, 2005.

  1. sled_dog

    sled_dog 1 ton status

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    Excuse the length but need to give some background.

    Alright heres the story, I put my 355 into my Suburban in place of my old tired 350. Now the motor has an RV camshaft, something like 204 208 or 210 duration. Really mild, low lift. 76cc heads with flat top pistons, so not lots of compression(8.5 or so). I'm running a throttle body I modified("ported" a little) and a 1" open TB spacer. I have headers that dump into 2.5" pipes, straight back to flowmaster 40s then they 90* out the sides before the tires. I put a new fuel filter in when I replaced the motor.

    Alright now, here is the issues. My TBI motor runs like it is carburated. When its cold, if I jump in and go it doesn't like me very much. It will go but I have to rev the piss out of it before I shift. If I lug the motor at all it spits and sputters back at me. The timing is spot on, I checked that a bunch of times before I left for school. Now when its warm, and I shift it will pop between gears. A very slight backfire. At speed on the highway, say cruising at 65, it will miss once in a while. Barely noticeable, except on a 5 hour drive across PA(just cause of the time to listen for it). When I first got the truck running the fuel pump made a nasty bit of noise. Can't even describe it, each time it turned on to prime it would just sound bad. I drove it a little and that stopped. Now all I get is the normal fuel pump noise. I am getting crap for gas mileage. Maybe like 8-10 around town and like 8 on the highway at best. Thats not even beating on it(much). On the highway I cruise at 65 which with the 245/75s, 3.42s, and 465 thats about 2500 RPMs. I think I should be doing better on gas, even if I get throttle happy sometimes.

    What do you guys think? Could an O2 sensor cause all this? The sensor is new but it still a single wire, and it is installed in the collector(actually in the collector) at the end of my long tube header. Think a heated would fix this? Seems to me like its running rich. Inside of exhaust pipes are a little black, smells of gas(no cats mind you) and crappy mileage. And as far as I know when a motor runs rich it will actually fool you and make you think its running lean(popping and missing at RPM). But I have no idea why it would run rich if I've only done things to improve airflow and not fuel. Any opinions? Computer tune is stock. I have yet to pull a plug and check color.
     
  2. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    have you checked to see if its throwing any codes?
     
  3. fortcollinsram

    fortcollinsram 1/2 ton status

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    I would start off by scanning it and seeing what kind values you are getting. Keep a special close eye on the MAP sensor and the O2 and the BLM and INT number. Do you have all the necessary stuff hooked up for TBI such as VSS, ESC, etc...???

    I would start off though by looking at what is happening at WOT with the O2 readings (even though the ECM doesn't reference the O2 at WOT, you can get an idea if you are way off in terms of fuel delivery.

    Chris
     
  4. sled_dog

    sled_dog 1 ton status

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    how do I scan to get the readings? I've heard of it but no clue how. I know how to check codes, but its not throwing any. Everything is hooked up.
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    reading up on winALDL will get you started on how to actuall scan the vehicle.

    O2 sensor is not referenced when the vehicle is cold. Sounds like something engine temp related, anytime I suspect something like that, I recommend checking what the CTS is telling the ECm the temp is.

    From what others post though, CTS problems manifest themselves in various hard start conditions, it doesn't sound like that is an issue for you?
     
  6. sled_dog

    sled_dog 1 ton status

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    nope no hard start. When it is really really cold(like 0*) it has to crank a few times but thats all.
     
  7. jarheadk5

    jarheadk5 1/2 ton status

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    Also sounds like you could be getting false air. Check all your vacuum hoses, not only for connection, but condition as well. I seem to recall someone else here recently having TBI driveability issues, turns out the vacuum hose to the MAP was dryrotted all to hell.
    Also check your TBI base gaskets (at the TB and the spacer). I once had a similar issue, but not as bad, and I found a small vac. leak at the base of the TB. New gasket, problem gone. And the whole time mine was doing it, it never threw a code...
     
  8. Corey 78K5

    Corey 78K5 1 ton status

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    I would say it's Your computer!!!!! It probably cant figure out what is up with all the changes in the engine since it was programed for a stock 350:whistle: You will probably need to have Your computer reburned to accept all the changes.
     

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