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TBI running like crap

Discussion in 'The Garage' started by 1979jimmy350, May 19, 2005.

  1. 1979jimmy350

    1979jimmy350 1/2 ton status

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    this is on a 89 TBI engine. The engine idles fine but when the throttle is more than one third down it starts to pop and if you floor it it will kill the engine some times. If the transfer case is in low range it will not do it as bad or some times not at all. It is throwing a code 34. I have connected a vacum gague and it gets around 20 inches of hg at idle and around 15 under part throttle. The needle stays stedy and is not bouncing around.
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    Sorry, no links to a troubleshooting chart.

    Heres a post on that code though code 34 issues

    I wouldn't just replace things. As you can see it can be at least 5 things (to some that wouldn't be a "lot" lol) so that can get expensive.

    Autozone will scan the vehicle for free, (have one nearby?) IF thats a data scanner and not just code reader, you should be able to see what the MAP is reading "normally"...engine vacuum reading will correlate to the MAP value in the scanner data, if it doesn't read correctly at idle, then the MAP is bad. Thats why the GM troubleshooting flow charts are so helpful in most cases.

    TPS is easy to check with a scanner as well, making sure reading is progressive and consistent.
     
  3. 1979jimmy350

    1979jimmy350 1/2 ton status

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    I had a good MAP laying around i swaped it out and it did not help. I am able to plug my lap top in to the computer and get data. What is the normal reading at idle for the map. Also when it was poping i did not see a radical change in the map readings
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    From what I saw over at thirdgen.org, 40-50KPA at idle is supposedly normal. TPS should be around .5V at idle.
     
  5. shane74

    shane74 1/2 ton status

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    Check the EGR. I just went through similar issues w/ my '87 TBI burb. Very rarely is it any of the other sensors and crap (ask me how I know :crazy: ) If it is the EGR, buy the OEM one. I replaced 2 aftermarket ones within a year of each other. The second one went bad again just a couple weeks ago. I replaced it with an OEM one. They have a 1 year warranty, where as most after market ones only have a 3 - 6 month warranty. Both times they went out, they were just outside the warranty. I spent more on to "cheap" aftermarket ones than if I had just dropped the $$ for an OEM one. The first OEM one lasted 18 years. I sure the second should last just as long.

    Oh yeah, every time my EGR went bad, the computer NEVER stored a code for it. :mad:
     
  6. 1979jimmy350

    1979jimmy350 1/2 ton status

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    was your egr sticking open or not opening up?
     
  7. shane74

    shane74 1/2 ton status

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    The shaft inside it broke (both times) and it would sometimes stick open, or closed. That's what made it so hard to diagnose inthe driveway. This last time it was stuck open so that it couldn't build much vaccum. The engine was popping and stumbling almost like the cam was bad.

    Oh yeah, code 33 is MAP sensor too low and code 34 is MAP sensor too high. I'll bet you $100 that your MAP sensor is fine. I got those exact same codes and all three times it was the EGR. If you keep trying to drive it, you'll start getting O2 sensor, TPS, ESC, running too rich, and running too lean faults too. Again, ask me how I know :whistle: :doah:
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

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    Dunno how the '747 handles EGR, but the '165 (that I run) has an EGR temp diagnostic switch...the ECM "looks" for the switch to activate when EGR is introduced since the exhaust gas is hot.

    I never really looked into it, although I'm relatively certain a scanner (or setup like the poster has) should show if EGR is commanded or not by the ECM.

    EGR is a weird duck, in these older applications it seems that the only time a code comes up is if the ECM doesn't see EGR when it expects to. If EGR is being introduced when it's not commanded (such as stuck open valve) you'll see other codes start popping up as the mix gets screwed up, etc.
     
  9. shane74

    shane74 1/2 ton status

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    You would think wouldn't you? Apparently, the GM engineers didn't. No the EGR is not commanded by the ECM. It is strictly mechanical.

    Yes it is a VERY weird duck. It will do all sorts of wacky things when it goes bad. Since it's not controlled by the ECM, running a standard code scan or watching "live" diagnostics won't tell you much. It can make you chase your tail trying to findthe source of the problem. When it does go bad, you will all sorts of codes for the MAP sensor, ESC sensor, TPS sensor, Knock sensor, O2 sensor, TCS (timing control) and there are probably others I am forgetting about. Hell it'll make your tranny do wierd crap too. It another one of those things you'd like to b*tch slap a GM engineer over. They made everything else ECM controlled, why not the friggin' EGR?? At least that way, when it went bad, you'd get a code for it. Anyway, my bet is still on the EGR, if it's stuck either way (open or closed) the rig will run like crap.

    The first time it went bad it did EXACTLY as was mention in the first post. The second time it ran fine above 1500 rpm. The third time, it was intermittent. It was sticking both open and closed. Sucked big time.
     
  10. dyeager535

    dyeager535 1 ton status Premium Member

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    So the TBI setups (on trucks at least) don't use an EGR solenoid controlled by the ECM at all??

    I guess I shouldn't be surprised, cheap bastards GM lol.
     
  11. shane74

    shane74 1/2 ton status

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    Mine doesn't. It's all vaccum related.
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    Looked at a wiring diagram over on chevythunder for the TBI stuff.

    According to that diagram, EGR solenoid was 305 only. :confused:

    In any case, I don't own one, so I really don't know. Only thing the solenoid does on mine is allow vacuum through it when the ECM commands EGR. Vac line from plenum to solenoid, solenoid to EGR valve.
     
  13. shane74

    shane74 1/2 ton status

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    On the 5.7's, the EGR is connected to the MAP sensor w/ a vaccum line. The EGR draws vaccum from the intake manifold. That is why the ECM will throw the MAP codes when it's really the EGR. Also, that is why you'll get codes 33 and 34 (MAP too low/high). Since the EGR is mechanical, the ECM doesn't "know" to look for it. When the EGR goes bad, the MAP sensor goes all wacky and the ECM starts trying to compensate with fuel delivery and timing. A big indicator that the EGR is going bad is a surging idle and poor fuel mileage. Those are the 2 biggest tell tale signs that it is about to go out. I had to learn all this the hard way. If it isn't the EGR I will be really suprised.
     
  14. 4xcrazy

    4xcrazy 3/4 ton status

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    Check your fuel pressure, mine did this same thing a few years back, threw one code, replaced the part it was complaining about, then would throw another,different code,,finally found out it was just weak fuel pressure, running way too lean, mine did the exact thing with the dying out when you get on it sometimes, heck i drove mine back from San Diego in 3rd gear because it didn't have enough power, or whatever to even get it through the gears.

    Yes i realize what everyone is saying to check, because of the code you get, but i have found out on these things, that one thing that may start going bad, will give a false reading to something else, basically throwing false codes, which is what happened in my case.
     
  15. 1979jimmy350

    1979jimmy350 1/2 ton status

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    i am starting to lean twords the fuel pump because we had to tap on the fuel tank a few weeks ago to get the pump to work. But the condition seems to be getting better now it will only pop at one third throttle then will stop when it is at full throttle but sounds like it is only running on half the cylinders but is not shaking like crazy. At least the TBI is only a temp fix
     

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