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Thread: The case of disappearing boost

  1. #1
    Join Date
    Apr 2001
    Location
    Festus, MO.
    Posts
    173

    Default The case of disappearing boost

    This one has me stumped.

    The truck:
    '95 6.5TD KD chip, gear drive, KD exhaust, air box opened, clean air/fuel filters, and the required boost/egt gauges. New vacuum pump and belt/tensioner, and newer OPS and lift pump (both seem to be working fine), oem original turbo with approx 200k miles.

    It seems that when I'm running under 2,000 rpm I have almost 0 boost. It never had a lot but would easily pull 5-7# or more if I leaned on it at all. Once I get closer to 2200-2400 rpm boost pressure shoots to what I'll call normal, 11-12# with a bit of the skinny pedal applied. Obviously, with the KD chip and no boost it rolls the black smoke like an old Mack dump truck so I would think the fuel is there, just no boost. The actuator rod on the turbo is nice and tight at an idle too as it should be.

    I haven't tested the new pump but the old finger on the hose trick says that it's pulling pretty hard and smooth. Testing is in the plan for tonight.

    Thoughts?

    Thanks in advance
    Big Green

  2. #2
    Join Date
    Sep 2001
    Location
    Lubbock TX, USA
    Posts
    4,194

    Default

    Remove the actuator on the turbo to see that the actuator moves properly not binding and has no excessive play pulling the flat part towards you.

    Check for exhaust leaks, especially on the turbo crossover pipe, a blown donut can cause these issues.

    Speaking of leaks, check for intake leaks as well, especially the boot between the turbo and the upper intake manifold. Then check the gasket between the upper and lower intake manifold, these can blow out.

    J
    1982 C10 SCSWB 6.5TD, mods too extensive to list. (13.69 1/4 mile @94.6 MPH) RACE TRUCK
    1982 C10 SCSWB 6.2NA, 2.73 700
    1986 C10 SCLWB 6.2TD 3.73 700
    1989 V20 SUB 6.2NA, 3.73 400
    1994 G20 VAN 6.5NA, 3.42 60E
    1994 K20LD ECSWB 6.5TD, 3.42 80E
    1995 K20 SUB 6.5TD, Wrecked, ran into by stupid teen.
    1995 C3500HD DRW 6.5TD, 12' Flatbed 5.13 80E
    1995 C3500HD DRW 6.5TD, 18' Rollback Wrecker 4.63 80E
    1994 C20HD ECLWB 6.5TD 3.73 80E Wifes Truck.
    1995 C20LD ECSWB 6.5TD 3.73 80E
    1995 K20LD SCLWB 6.5TD 3.73 80E
    1996 K30 DRW 6.5TD 4.10 80E
    1997 C10 Tahoe 2Door 2WD 5.7L to 6.5 Conversion Underway

  3. #3
    Join Date
    Apr 2001
    Location
    Festus, MO.
    Posts
    173

    Default

    Thanks John,
    I'll check the items you suggested.

    I would have thought that a blown donut would make enough noise that you'd notice it but I'll check it none the less. The crossover is oem original too so while I'm under there I'll give it the good once-over.

    The boot was replaced when I added the gauges, still worth the double check. Same goes for the upper to lower intake gasket.

  4. #4
    Join Date
    Dec 2003
    Location
    SC
    Posts
    939

    Default

    I'd tee into the vaccum line and watch vacuum at the wastegate and compare to the pump itself. Sounds like a waste gate solenoid. It might not be regulating well to make the lower boost but can at higher rpm and more boost call out. Gotta know for sure the pump can hold steady 26"Hg iirc all the time and at the actuator it varies. Stock at 2000 rpm light throttle boost falls off and so will vacuum at actuator. KD program might be slightly different but is still PCM controlled I believe; so the solenoid must function and so must the vacuum pump / lines.
    97 5spd K2500 Ext Cab short Bed ~160K miles.
    TM, 3" downpipe & 4" exhaust, remote FSD, remote oilfilter, Gauges: EGT, Boost, Fuel Pressure, B&W Gooseneck Turnover ball, Prodigy Brake Controller. Hi-Temp Hydraulic Oil Cooler Lines.

  5. #5
    Join Date
    Apr 2001
    Location
    Festus, MO.
    Posts
    173

    Default

    Well, I check over the crossover, donuts, vacuum lines, actuator, etc and all appears well and good.

    Looks like the issue may be the vacuum pump. Gauge shows 23-24 inches at the solenoid.

    So out to the garage to pull the brand new pump and back to the junk store (local autoparts) it goes.

    Will keep you updated...

    Thanks

  6. #6
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

    Arrow

    23-24" is a GOOD pump. Optimum is about 25-26", but they more typically produce 22-25". 20"+ is more than sufficient. The modulated vacuum at the WG actuator is only 15" (or should be). If it's showing 15" at the actuator, your pump is not the problem.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  7. #7
    Join Date
    Apr 2001
    Location
    Festus, MO.
    Posts
    173

    Default

    Well new vacuum pump helped but still seems low to me.

    I wd-40'd the solenoid and cleaned the manifold pressure sensor while I was there.

    So, the search goes on...

  8. #8
    Join Date
    Jan 2001
    Location
    New Hampshire - Live Free or Die
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    6,057

    Default

    Under 2000 RPM the PCM doesn't monitor the boost. It runs the solenoid at some default duty cycle (maybe 70%) and takes whatever it gets. If your wastegate was leaking or had some deposits holding it off the seat this could account for the low boost. Once it reaches 2000 (actually 1800) and the fuel rate is high enough that it could overboost (20mm3, IIRC), the PCM starts paying attention. If the wastegate leaks it will crank up the duty cycle until it sees the desired pressure. (Also, the leak becomes less significant.)

    So, I'd try to get an eyeball on the wastegate seat and valve and confirm it is closing all the way.
    The Constitution needs to be re-read, not re-written!

    If you can't handle Dr. Seuss, how will you handle real life?

    Current oil burners: MB GLK250 BlueTEC, John Deere X758
    New ride: MB GLS450 - most stately
    Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510

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