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HELP- officially stumped

Discussion in 'Chevy Trucks' started by LUCKY 7, Dec 8, 2010.

  1. LUCKY 7

    LUCKY 7 Senior Member
    Messages: 107

    I hope someone can help me with this one. I have an '88 1 ton with the 7.4L & turbo 400 trans. I am having idle troubles. When using the truck, the idle gets high- 1000-2000 rpm- bad when shifting! First, I replaced the idle control valve, no change. No vacuum leaks. Then I realized what was triggering the problem- when I sit in gear holding the truck still, the idle starts to climb higher & higher. When I put it in park or neutral, the idle stays up there- won't come down until you shut it off. Start it right back up again- low idle. A friends scanner said maybe torque converter but the symptoms don't seem quite right.I've eliminated as many possibilities as I can. Anybody know? Thanks in advance!!
     
  2. NICHOLS LANDSCA

    NICHOLS LANDSCA PlowSite Veteran
    Messages: 4,316

    Carb or EFI? 88' could be either
     
  3. LUCKY 7

    LUCKY 7 Senior Member
    Messages: 107

    Sorry- EFI (throttle body)
     
  4. 2COR517

    2COR517 PlowSite Fanatic
    Messages: 7,115

    Can you watch the IAC position on the scanner while driving?
     
  5. LUCKY 7

    LUCKY 7 Senior Member
    Messages: 107

    I haven't tried to leave a shop with a scanner yet so I don't know. I don't even have to move the truck to make this problem show up- just put it in gear, hold the brake, and here it comes. When my friend was checking it with his scanner he said it gave no codes. I seem to remember the IAC was reading way high. We scrolled through the symptoms and it suggested that the problem was the torque converter but if that was the case I think it should drop down the idle when you put it back in park- this one stays on high idle.
     
  6. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    It isn't a torque converter issue I can promise you that. More like a faulty CTS, MAP, TPS or vacuum leak under the throttle body. Scanner info will rule out the CTS, MAP and TPS without throwing parts at it. A fresh gasket under the TB and adapter plate are almost a requirement as PM every few years on the old TBI "adapted" 454's.
     
  7. LUCKY 7

    LUCKY 7 Senior Member
    Messages: 107

    We replaced both gaskets for the throttle body. He did something with the MAP too- I think he bypassed it to see if it changed. He did that with every vacuum & sensor we could find. Keep throwing ideas out there guys- I need 'em. Thanks again for the help.
     
  8. jb1390

    jb1390 Senior Member
    Messages: 710

    If you force the truck into open loop using the scanner-does it make any difference?
     
  9. LUCKY 7

    LUCKY 7 Senior Member
    Messages: 107

    Not sure I understand what you're asking. I'm no mechanic by any means. Someone else has been running the scanner. Another guy is telling me that it's the ECM. Could the signs of a bad ECM be triggered by holding the brake pedal in gear? Also, if it is the ECM, what other kinds of changes can I expect to see? Thanks
     
  10. jb1390

    jb1390 Senior Member
    Messages: 710

    I know I had a similar problem on my 1993. Idled around 1200 rpm-sometimes would lope real low. I never really figured out what caused it-it was a very intermittent thing, so it wouldn't always act up when on a scanner.

    Using a decent scanner, you can force the truck into open-loop, which primarily ignores the reading form the oxygen sensor.

    The other thing you could try-is OBD1 will largely ignore sensors that are unplugged. If you unplug the o2 sensor-it will throw a code but it will run anyway. If you can unplug a problem sensor, and it runs fine, you'd know what to replace.

    With a scanner, it is easier to just look at the values.....

    I know one mine I tried changing the ECM, with no improvement. Doesn't mean that it won't work for you though.

    What is the vacuum reading at idle? Is it steady?

    Plugged EGR could also cause what you are experiencing. I know that was a very common problem on the TBI 350's, not sure on the 7.4.
     
  11. mmaddox

    mmaddox Member
    Messages: 57

    Vacuum leak, TPS, A/C idle/fast idle solenoid. ECM will not increase idle speed. Butterflies closing completely?
     
  12. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Does it have hydro-boost?
     
  13. LUCKY 7

    LUCKY 7 Senior Member
    Messages: 107

    No- not mod. brakes. ECM was good, MAP was good, no intake leak- inside or out, IAC good, disconnected A/C- no change. I know there's other things we have checked just can't think of all of them right now.
     
  14. LUCKY 7

    LUCKY 7 Senior Member
    Messages: 107

    Also, the IAC numbers peg out at 146 steady when running and won't come down. I now have the IAC disconnected so that if I have to plow at least the idle is low so it's easier on the tranny. We have checked for vacuum problems everywhere we can. At this point everyone involved feels like we're looking for something REALLY odd because all of the usual problems have been looked at, but I am guessing B&B will figure it out before the guys that are actually looking at it!:laughing:
     
  15. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    If it's pegging the IAC counts in gear then it has a vacuum leak with your foot on the brake. Remove and plug the port directly in the intake for the brake booster supply line and retest.

    See where I was going when I asked about hydro-boost or not?
     
  16. LUCKY 7

    LUCKY 7 Senior Member
    Messages: 107

    We plugged it- no leak found.
     
  17. LUCKY 7

    LUCKY 7 Senior Member
    Messages: 107

    torque converter it is!!!:eek: