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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 was hoping for the easy fix too, so I disconnected the wastegate line. No change. :( Relatively slow climb to just under 6 psi.

    It rained for over 48 straight hours and everything is saturated. Today there's a bone-chilling wind (and still no sunshine to dry anything out) so I probably won't be getting underneath today!

    My prime suspect (naturally, the most difficult to access) is the up-pipe slip joint. It did not feel very tight at all, although as the wye traveled further into the female end of the up-pipe it was more snug. I did apply some copper RTV at assembly.
     
  2. FordGuy100

    FordGuy100 Registered User

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    I've had up pipe leaks before that impacted boost and mine was fairly audible and I don't think I had as much of a boost issue. It sounded like a run of the mills exhaust leak. How much is your pump turned up?
     
  3. DrCharles

    DrCharles Full Access Member

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    Turned it up 2 flats per the usual recommendation.

    I'm going to see if the exhaust shop can make me a bend to properly (and removably) connect the Banks downpipe to the 3" tailpipe. The flex pipe is leaking audibly which is likely masking an up-pipe leak.

    One thing at a time :) Meanwhile the truck is definitely noticeably more powerful even with 5.5 psi boost!
     
  4. The_Josh_Bear

    The_Josh_Bear Full Access Member

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    Unless I'm misunderstanding you, the flex pipe is after the turbo and is irrelevant to your boost troubles. (By all means fix it right for your own sake but it won't change your boost)

    To this day my exhaust (post turbo) isn't sealed by a long shot, the only way it would be an issue is if it was choked down like to <2" or something goofy like that.

    I'll throw out one other thing that happened to me just in case:
    I bought a used turbo. Turns out it was run well past its service life and with a maxed out pump all I could get was 5psi. I took the intake off and saw the shaft was wobbling like crazy!

    So it was trash and a new CHRA was installed. Still no better than 6psi! Turned out to be the wastegate, once unplugged I got 9psi max but not snappy. Turned up fuel, 13 max psi.
    THEN I cleaned the up pipes good and added the copper RTV.

    So all things considered it's a little bit of an adventure to get the most out of a turbo setup but it's totally worth it. That's beside the timing and IP tweaks available now by @Thewespaul in his thread DB2 Tuning Guide.
     
  5. catbird7

    catbird7 Full Access Member

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    Didn't notice any mention of studs, so I assume this isn't a studded engine. I'm sure this has been answered about a million times on here however for clarity sake and while we're on the subject, "what's the max boost recommendation for non-studded engines"?
     
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  6. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    12 psi is very reliable, 15 psi is pushing it. Retorque the head bolts to 110 ft lbs and you can push it to high teens low twenty’s for some time.
     
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  7. FordGuy100

    FordGuy100 Registered User

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    ^ On a 7.3.
     
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  8. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    Correct. Should have made that distinction. When turboing one of these trucks it’s important to have the right mindset with the potential risks, you are adding from 50% to double the power output of the engine in its intended form, so things like a blown headgasket has to be taken into consideration with the potential total cost, even if you keep it tame at 10 psi, there’s always the chance of failure because of the age of these engines and the significant increase in power output a turbo makes. Some people have been unlucky enough to loose a headgasket on a 7.3 at stock boost levels, and some have been lucky with 20 psi on a stock engine with no issues. It’s just always a potential risk with these engines with boost until you stud them unfortunately.

    With that boost and egt temp it just sounds like you either have a boost leak or not enough fuel. It could be possible the cheap egt gauge isn’t reading accurately as well, but with no smoke your egts are probably fine. I’m leaning towards a tired pump that needs more adjusting.
     
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  9. DrCharles

    DrCharles Full Access Member

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    I am getting the downpipe-to-tailpipe connection fabricated tomorrow afternoon. Yes, it won't affect boost at all, but I don't want an audible leak, not least because I am trying to see if I have other boost leaks :)

    Today I removed the K&N filter that came with the airbox and put in a paper one. I've done enough reading to stick to dry filters especially on our dusty dirt roads. Then I got underneath and could not find any obvious soot marks (except at the loose flex downpipe, of course). Finally I turned up the pump another 2 flats and went for a drive.

    Same as before. Boost comes up promptly to 4 psi if I floor it at 2000 rpm in 4th, and then inches up to 5.5 psi as I keep my foot in 4th and 5th to 3000+ rpm. EGT max around 900-950F doing 80 mph in 5th floored. No smoke that I could see in the mirrors.

    I'm starting to think that my IP is tired and just can't put out any more fuel than it's doing! Although there's no problem with hot-starting. With the fuel turned up that much and only 5-5.5 psi boost, I would expect to see smoke if a boost leak was limiting the air with excess fuel :dunno

    Josh, why would loose turbo bearings cause a lack of boost? You went from 5 to at best 6 with the new cartridge. The defective wastegate seems to have made all the difference and then the increased fueling made even more boost). If the impellers were scraping the housing that should be obvious and a horrendous noise, or maybe it had clearanced itself by the time you got the used turbo...

    Catbird, this is not a studded engine. I'm shooting for 10 psi max. I want longevity more than I need every last hp. Originally I hadn't even planned to turbo it, but someday I want a camper and want the hill-pulling capability. It was leaking coolant from both head gaskets when I got it, and after pulling the heads they had significant cracks so I got rebuilt heads too.
    :fan:

    Anyhow I may just have to live with it for a while. It's an improvement over stock, although not a $1000 worth, and a good rebuilt pump and injectors from our favorite suppliers is about another grand and a lot of elbow grease :rolleyes:
     
  10. FordGuy100

    FordGuy100 Registered User

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    When my turbo grenaded it had a hard time boosting, to the point where it barely would make positive pressure. Shaft play seems good on it? Compressor wheel look alright?

    IIRC you should be able to rev it to the governor and make a few psi, even in neutral.
     
  11. DrCharles

    DrCharles Full Access Member

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    I remember there being some axial play, but I'd expect a little with no oil pressure since the shaft floats in the bearing. No noticeable radial play. Compressor wheel intact.

    I do not see any boost when revving at least to 2500 in neutral. Has to have a load on the engine, then it'll make boost as described.

    Again, since I turned up the fuel, wouldn't I get high EGT's and black smoke if the turbo can't put in air to match the increased fueling?
     
  12. FordGuy100

    FordGuy100 Registered User

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    Yes you would. No pinched line going to your boost gauge right?
     
  13. DrCharles

    DrCharles Full Access Member

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    Right. ;)

    I was expecting that turning up a properly working pump 4 flats would look like a mosquito-fogger in the mirror...

    Too bad there isn't a way to check the IP output with it on the truck... if I have to go to the trouble of removing it, a good rebuilt one will definitely be going back on! :)
     
  14. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    If you’re not making boost just reving the engine you either have a boost leak or a worn out turbo. With a good sealed kit and decent turbo, you should start seeing boost around 2000-2300 rpms in neutral just slowly bringing the rpms up. Look at the base of the intake hat and the o ring in the back of the intake for oil.

    Worn bearings in a turbo will hamper performance in a number of ways, one being additional drag due to the worn surfaces on the bearings. With axial play the shaft is allowed to “wobble” from its fixed location which acts as leverage against the bearings which makes additional restance with the leverage multiplication when you try to push a worn out turbo hard, even if the wheels aren’t touching the housings. There’s a lot of fascinating physics in what makes a turbo work the way it does, are you sure you actually turned the fuel another two flats or could your Allen have been slipping?
     
  15. DrCharles

    DrCharles Full Access Member

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    Yes, I'm sure that I turned the screw. It's quite stiff but turns smoothly, with none of that unpleasant slipping/popping feeling from a rounded off female hex, and it's a snug fit. I am just surprised that it seemed to have no effect at all, and no increased smoke.

    The turbo is definitely well-worn, and may well be in need of a rebuild. No idea how many miles on the engine it came from. There was a fair bit of oil in the wastegate actuator and line but I'd expect that from the CDR oil vapors being shoved in there under boost. Today I looked but did not see any stains around the snail o-rings (which I replaced with new ones during the install).

    I suspect this truck either had the odometer swapped, since everything seems more worn than it should be at 189k miles, or it was just abused by previous owners who didn't fix a thing unless the truck wouldn't run at all. No idea how old the IP is but it may be an original...

    Edit to add: what turbo rebuild kit should I be looking for? thanks.
     
    Last edited: Feb 12, 2019

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