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Where's the boost?

Discussion in '6.9L IH & 7.3L IDI Diesels' started by DrCharles, Feb 11, 2019.

  1. DrCharles

    DrCharles Full Access Member

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    I finally got a chance to drive my newly-turboed 7.3 IDI on the highway. I had been seeing 5-6 psi boost on the bumpy dirt roads, and figured once I did some fast runs it would make more. Pump turned up two flats, but it's of unknown age, as are the injectors (odometer has 189k but I think it's been swapped). It starts fine, hot or cold.

    But it won't make over 5.5 psi at all, even up to 3000 rpm in fourth and fifth gears (4.10 rear) with the pedal on the floor. Pyro was just under 900F during the wide-open runs. About 40F ambient. No load in the bed. Didn't see any smoke but the rain would make it hard to see a light haze. I don't hear any obvious boost leaks.

    Due to rain I have not checked to see if the wastegate is sticking open. What should I be thinking of first - boost leak, wastegate or turbo problems, or simply not enough fuel from old pump/injectors? :dunno
     
  2. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    At that temp, you could stand to have some more fuel but I’d be looking for signs of boost leaks.
     
    IDIBRONCO and Kevo1288 like this.
  3. 79jasper

    79jasper Chickenhawk

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    I would check fuel pressure under load, first thing.

    Sent from my SM-N960U using Tapatalk
     
  4. DrCharles

    DrCharles Full Access Member

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    Thanks, I'll check those once the weather improves ;)

    Wouldn't it take a huge (and therefore audible) leak to bleed off that much of the gases that are supposed to spin the turbine? The up-pipe is at least a couple inches in diameter... I will feel and look for soot.

    I do run a Facet 40285 pump which may not be maintaining 5 psi under max fuel. But even if the pressure is dropping to 1 psi at wide-open (the filter light does not come on), wouldn't that just retard the timing slightly (which tends to result in hotter EGT and therefore boost)?
     
  5. FordGuy100

    FordGuy100 Registered User

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    What turbo kit?
     
  6. DrCharles

    DrCharles Full Access Member

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    Factory except for Banks downpipe. 3" exhaust with Walker round truck muffler.
    Also, I shortened the wastegate rod three turns before installing the turbo.
     
    Last edited: Feb 11, 2019
  7. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    Go a quarter turn more if you find no boost leaks, do you know the filter light works? ie, it comes on when you first turn the key on? Mine doesnt in the shop truck, but vacuum sensor is long gone.
     
  8. pelky350

    pelky350 Full Access Member

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    Does it climb fast to 5.5 and stop or does it slowly get there?
     
  9. DrCharles

    DrCharles Full Access Member

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    Yes, all lights come on during cranking (the schematic confirms that the fuel filter and the overtemp light only test with the switch in Start, not Run).

    It seems to climb gradually to 5.5. No instant "jump" except maybe to 2 psi when I first hit it.
     
  10. FordGuy100

    FordGuy100 Registered User

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    Unhook the pressure line going to wastegate, plug it, and give her a rip.
     
  11. DrCharles

    DrCharles Full Access Member

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    Thanks. I'll definitely try that too, especially if I can't find an obvious boost leak. I think it's less likely to be the culprit though, because the wastegate lever is in the closed position at rest, and the spring feels plenty strong.
     
  12. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    It sounds like your muffler is probably fairly loud, so you may not be able to hear an exhaust leak. No judging on the muffler, I won't be running one at all when I turbo mine.
     
  13. DrCharles

    DrCharles Full Access Member

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    When I got the truck, it had just a straight 3" pipe (and no turbo) so it was obnoxiously loud. I must be too old ;) Actually it's very quiet, exits in the right rear. It's a pretty big muffler. The only time I can hear it over the IDI (and the old, hard Mud Terrain T/A's at any speed over 40 mph) is when the passenger side window is open and I'm next to a curb or wall on that side.

    I confess that the downpipe is still temporarily connected to the 3" pipe with a piece of 3" ID flex pipe, not even clamped. So there is an audible leak at the downpipe end since it's only 2.840" and there are some soot streaks there :D Not very loud though because it's post-turbo.

    How big a leak does it take to drop the turbo to a max of 5.5 psi? Pre-turbo leaks usually "blat" pretty loud!
     
  14. The_Josh_Bear

    The_Josh_Bear Full Access Member

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    I'll throw in my two cents:
    I have the XLT Lariat edition and it had a lot of sound-deadening stuff to keep engine bay noises down. It does a good enough job for me, and I never ever heard a hot-side leak even though I had a few.

    Thanks again to @Thewespaul for suggesting the use of copper RTV on all the joints in the up pipes-- it made a huge difference for me. With my wastegate unplugged I would eventually get up to like 12 psi at best. But the weird thing is that it would peak at 9psi some days. Under the same conditions as far as I could replicate it. (This was even before the charge air cooler)

    After sealing the up-pipes, pretty much anytime I punch it over 2k it races up to 13psi and blows the waste gate open, staying at 13-14psi through redline.

    This is with a maxed pump and charge air cooler too.
     
  15. Hagan

    Hagan Registered User

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    That's what I was thinking. Could be opening way too early.
     

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