Another 95 Yukon question - bog off idle

Discussion in 'The Choo-Choo shop' started by Smartin, Nov 14, 2011.

  1. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    I'm starting to run out of things to clean/replace on this thing, short of ripping the engine out.

    Problem - at low RPM's (800-1300) I have a stumbling issue. It is most noticeable when I take off from a stop at light throttle. It will nose over, then come out of it quickly. If I push through it with a heavier throttle, then the problem is not there...only if I am in that RPM range it will stumble. And it's a steady stumble, kind of cyclical. If I sit there and brake torque it up to those RPM's, the truck will come in and out of this stumble...like I am pushing the gas pedal down and back up again...hard to explain.

    Plugs, wires, cap, rotor, injectors, throttle body cleaned, MAP sensor replaced.

    What am I missing? I'm tired of throwing money at it.
     
  2. Skyhawk

    Skyhawk Well-Known Member

    Sounds like throttle position sensor is dropping out.
     
  3. skierkaj

    skierkaj Day 2 Street Screamer

    Fuel Pressure Regulator!
     
  4. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    Thanks guys...I've thought about both of those parts, but no one I've talked to around here has mentioned them. They are both relatively cheap, so I think I'll pick them up on the way home tomorrow.
     
  5. 462 Chevelle

    462 Chevelle 462 chevelle

    Sounds like vacuum leak to me
     
  6. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Throw a fuel pressure guage on it and see if its bleeding pressure off, the plastic spider under the intake may have developed a crack.
     
  7. Daves69

    Daves69 Too many cars too work on

    Adam had a similar problem with my old 94 truck turned out to be the distributor pick up coil O'reilly p/n D1987


    [​IMG]
     
  8. 462 Chevelle

    462 Chevelle 462 chevelle

    Never seen a pick up coil like that on a chevy. usually either ignition module or little black box looking thing. 95 is throttle body i believe. I think the spider injection started in 97
     
  9. gsgns4me

    gsgns4me Well-Known Member

    My $.02

    If you have the latest, greatest, platinum, cubic zirconium, mucho deniro spark plugs installed.....replace them with the recommended AC Delco plug.

    Pick up coil and/or worn out distributor is my 2nd guess.
     
  10. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    Fuel pressure regulator and throttle position sensor were not the problem.

    Pickup coil is not installed yet, as it requires removal of the distributor. I'm too lazy to tackle that in the dark cold driveway. Maybe I'll get to it on Sunday.

    I don't remember which plugs I bought...I don't usually go for the glitzy ones anyway.
     
  11. 462 Chevelle

    462 Chevelle 462 chevelle

    you dont have to pull the distributor to change the pickup coil?? if i remember correcty on your year model its either 2 plugs and 2 7mm screws of its 2 torx 1 plug and you have to get the dist in the proper position to get it out. coudl you confirm if this is a spider injection motor or is it a throttle body? if its a spider injection.. put fuel pressure guage on it. record reading. use snapon scan tool to engage injectors 1 by 1 and see what kind of pressure drop you get on each injector. ive found 5 to 8 is the usual drop. if its throttle body.. still sounds like a vacuum leak.... good luck. :beers2:
     
  12. gsgns4me

    gsgns4me Well-Known Member

    The distributor shaft has to be removed from the distributor to replace the pick-up coil. Also called a "pole-peice" by some.
     
  13. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    It's TBI, no spider injection. TB has been removed and cleaned. There is no other indication that it would be a vacuum leak. I think it would be running poorly in more than just the spot it is currently having problems if there was a vacuum leak.
     
  14. 462 Chevelle

    462 Chevelle 462 chevelle

    My experience with vacuum leaks is that they give low rpm trouble but other wise you don't notice. at the track i had my cap come off of a hose the size of my thumb and it wouldn't run worth a Crap till about 1500 rpms and i still ran 12.8s. it sounded like it had a huge cam.
     
  15. GSXMEN

    GSXMEN Got Jesus?

    Adam - When you pull the dist., also check the shaft/bushing play. Both top and bottom. My brother's 92 C1500 went on him.....ended up needing the whole dist.

    His current 92 full size Blazer has a 350 and is doing pretty much the exact same thing. Pulls strong when you're on it, but it does have that nose over right off idle.

    If you check the fuel pressure with a guage, look for 13.5 lbs.
     
  16. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    And the winner is.....


    No one!


    I replaced the whole distributor, and no change. It ended up being almost as cheap to replace the whole thing as it would to replace just the electronic parts. I didn't want to take a chance thinking it might be too much play in the shaft bushing. I had that problem in the Centurion and it plagued me until I finally found the problem.

    I also replaced the idle air control valve. No difference.

    WTF is left? EGR valve?
     
  17. Doo Wop

    Doo Wop Where were you in '62?

    Spray a bit of WD-40 around known vacum areas. If idle picks up a bit....fix whatever is vacum related there.
     
  18. gsgns4me

    gsgns4me Well-Known Member

    You don't have Tornado (or similar device) installed on the air intake somewhere,, do you?

    Fuel filter has been replaced?

    Can you duplicate the condition while park braking and have someone look at the spray pattern from the injectors?

    If the EGR is one of the electronic ones, they can be suspect.

    You might want to pop out a "pre" cat O2 sensor and take it for a short test drive and see how it does.
     
  19. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    Fuel filter is new. No tornado installed.

    I checked the spray pattern of the injectors while I had a buddy in the car on the brakes. Looks good.

    The EGR valve is not electronic.

    The O2 sensor is also new.
     
  20. BUICKRAT

    BUICKRAT Torque Rules!

    Have you checked/replaced the throttle position sensor yet?
     

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