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TBI Help Needed

Discussion in 'The Garage' started by the cowboy, May 8, 2004.

  1. the cowboy

    the cowboy Registered Member

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    All right guys, I am at my wits end.
    I have an ’89 ¾ ton suburban, 350TBI, th400, np241, 4” lift. There is ~250k miles on the truck, 15k on a GM crate motor I put in a little over a year ago. It is all stock, except the exhaust (dual). Things ran great after the new motor swap, no complaints at all for well over a year.

    I was on my way home for spring break and it started running really bad at idle, but OK at speed (>2000 rpm). I determined that it was running rich. Toward the end of the 500 mile trip it finally threw a code; EGR, MAP and 02 sensor. I have replaced all 3, along with the EGR solenoid, the Idle Mixture sensor, Electronic Spark Control module, new plugs, plug wires, and a rebuild of the throttle body. I checked compression, it is 125 psi all around. It continues to run extremely rich at idle, so much so that to start it hot I have to hold the throttle open like you would with a flooded carb.

    About the only thing left to replace is the Throttle Position Sensor, but I am kinda tired of throwing new $40 parts at this thing. The problem continues to get progressively worse. Does anyone have any suggestions? /forums/images/graemlins/1zhelp.gif

    Thanks a ton in advance for any replies. This forum looks to be a great resource…
     
  2. heavy4x4

    heavy4x4 1/2 ton status

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    After putting those new parts on, is the Service Engine Soon light still on? If so, disconnect the battery for ~5 minutes to "clear" the computer.
     
  3. the cowboy

    the cowboy Registered Member

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    I reset the computer after each of the new parts, and now the SES light won't come on at all. That suprises me with the volume of black smoke; I would expect the O2 sensor to cry foul. The paperclip trick now says "code 12" and that is all. I have spent time trying to trip a code, with no luck.

    I have heard that the computer rarely goes bad on these trucks. Does that sound right?
    Does this sound like something a faulty Throttle position sensor would do?
     
  4. Thunder

    Thunder 3/4 ton status

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    Does it run rich after it is warmed up? If so it may be the coolant temp sensor located by the thermostat housing. It may be telling the ECM your engine is cold. So the ECM is keeping the mixtire rich.
    A scanner would really help to diagnose problems like this.
     
  5. shaun

    shaun 1/2 ton status

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    wellthe TPS and the MAP sensor worktogether to control the amount of fueldelivered. TPS works off voltage (I think .54 but am not sure right now) and the MAP runs of pressure in the intake.If either of these are off the other will compensate for it and this can throw off your fuel delivery. You should also check your fuelfilter because it could be clogged causing itto work only when it has moreof a draw (higher idle). Atleast this is how I understand this stuff.I am new to TBI so this is just my.02


    Shaun
    hope it helped /forums/images/graemlins/ears.gif
     
  6. HarryH3

    HarryH3 1 ton status Author

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    Stop throwing parts at it! /forums/images/graemlins/yikes.gif You have spent enough on parts to have paid for a decent scanner. /forums/images/graemlins/frown.gif I picked up the Auto X-Ray EZ-Link at Checker Auto Parts for $150. It lets you look at all of the engine data to get a MUCH better idea of what's going on. /forums/images/graemlins/thumb.gif

    If you have a laptop computer you can build your own cable for around $10 and use the free WinALDL software. A search on Google for WinALDL will point you to a download site. /forums/images/graemlins/thumb.gif

    Just because the ECM sets a certain flag, it doesn't neccessarily mean that it has detected the true problem. For example, a weak coil can cause the O2 Rich flag to set. It doesn't mean that the O2 sensor is bad, it just means that the O2 sensor is seeing a rich condition when it shouldn't, because the weak spark isn't igniting the fuel properly. Replacing the O2 sensor for this problem won't do anything but lighten your wallet. /forums/images/graemlins/frown.gif
     
  7. 4xcrazy

    4xcrazy 3/4 ton status

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    Computers can and do go bad on them, a buddy of mine's computer fail on him, would not produce any codes, wouldn't produce a code 12 for that matter though. Problem with buying a scanner, is it won't tell you if the fuel pump is going bad though, thats kinda what mine did when i finally replaced the pump and all was good.
     
  8. unclematty

    unclematty 1/2 ton status

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    should've checked fuel pressure before throwing parts at it. expensive lesson /forums/images/graemlins/thinking.gif /forums/images/graemlins/hack.gif /forums/images/graemlins/weld.gif /forums/images/graemlins/grind.gif
     

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