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Old 06-23-2021, 02:51 PM   #1
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Low power, no lights or codes

Hey friends, I have a '98 freightliner xc with the 5.9 Cummins and Allison six speed auto (Fleetwood Discovery) that I bought with a "transmission" problem, they said it was slipping in 4th. An Allison shop has confirmed for me that there is no problem with the transmission, but it's not getting enough power. He said "it seems like the turbo isn't coming on." (Can't rule out that this is intermittent, previous owner said "sometimes")

I can't for the life of me find someone who will work on it, but I'm not scared to do diagnostics and wrenching on my own... I just only have experience with stuff I've run into before, and 95% of that is not diesel. What should I be checking for and how do I do it?
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Old 06-23-2021, 03:18 PM   #2
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Tuckerstrong,
Given the information so far hear are my thoughts:
If you have a turbo boost gauges what is it reading? Given age and model of coach I doubt you do. So you have a couple of options - probably the easiest would be a shop that has a 3rd party tool that can be plugged into the obd2 or ecm diagnostics port that will read what the ECM is reporting as turbo boost. The prelim to that is are you seeing heavier than normal black smoke? if yes you have a turbo not spinning, and it could also be as simple as an exhaust leak. is this a front or rear engine diesel? If front you should hear the turbo spinning up or have someone follow close by to see if they hear the turbo whistle. could also be wastegate is stuck open not allowing the boost to build up or a boot that is leaking. That is the exhaust side of it in a nutshell.
If it is down on power then the other areas could be fuel as in filters, pressure (pump), or injectors, then we move to compression and that's a whole other path.
If the engine seems to be smooth and not jerking I would be looking at exhaust and turbo to start with. Open the engine compartment and look for heavy black soot as that will be a eyeball indicator of a exhaust leak. Especially around exhaust manifold and turbo connections.

JP
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Old 06-23-2021, 11:09 PM   #3
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Can you confirm that your engine is the 24 valve ISB version of the 5.9l Cummins ?

When it comes to fuel issues , you need to be aware that the Cummins / Freightliner chassis , of our vintage , come equipped with a fuel strainer , inline before the first filter /water separator , and having that strainer plugged up can leave you with all sorts of performance issues .
It's above the passenger rear air suspension bag.
Diagram below , it's well hidden in the wiring harness .
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Old 06-24-2021, 10:24 AM   #4
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Rear engine, I haven't noticed any black smoke at all, no boost gauges.



Yeah, the engine feels smooth, I've only had time to lift the bed and take a look around, but here is the turbo, I'm not seeing soot from this vantage point. I will continue researching the turbo things you mentioned so I can begin to learn my way around it.
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Old 06-24-2021, 10:25 AM   #5
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Tried to post a photo with that last reply... Trying again with a link

https://photos.app.goo.gl/QjPkr4wxGDtELQaN9

https://photos.app.goo.gl/QjPkr4wxGDtELQaN9
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1998 Fleetwood Discovery 36ft
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Just smart enough to be dangerous...
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Old 06-24-2021, 10:28 AM   #6
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Yes, it's the 24 valve ISB. I will certainly take a look over there check it out and familiarize myself!
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1998 Fleetwood Discovery 36ft
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Just smart enough to be dangerous...
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Old 06-24-2021, 09:09 PM   #7
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Two members report failed air brake compressor air supply hose from the intake manifold to the compressor, another possible boost leak to check out.
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