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Old 05-18-2017, 09:36 PM   #1
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Cummins 8.3 Bogs Down Under a Load

OK Friends...I about at my wits end here. I have a 1989 Bluebird All American FE with a 8.3 Cummins/4 spd Allison Automatic drivetrain with a 4/11 rear end that bogs down HARD when pulling a hill - never did that before. In fact the thing normally runs like a dream, and pulls hill at 70mph - no problem. The engine is strong, smooth, and purrs like a kitten at idol, revs up and down perfectly, does not blow bad smoke - at all. All pressures look right, all temps look right. It's just a bog bitch now, I'm stumped!!!!

Here's a list of things I have done in an attempt to remedy my problem.
1. Drained Diesel tank...YUK - NasTee, flushed out sludge.
2. Replaced ALL fuel/cooling/hydraulic/air filters I could find.
3. Replaced all fuel/hydraulic/cooling lines and clamps that looked the slightest compromised.
4. Flushed out cooling system.
5. Pulled the Turbo, it had some warping and blade damage, replaced it.
6. Pulled the fuel injector lines, blew them out, and pulled the injectors for inspection and cleaning.
7. I considered pulling the lift pump BUT when I did a fuel flow check the system gushes clean fuel.

Any ideas?? Short of taking it into a Bluebird dealership and getting ripped I'm out of ideas. I will most certainly and greatly appreciate any advice concerning what I may have overlooked.

Thanks Kids!!...Happy Day - Mark
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Old 05-18-2017, 09:48 PM   #2
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Have you tried injector cleaner? I know you said you pulled them. That is the last thing. Or injector timing. You seem to have covered everything else

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Old 05-18-2017, 10:02 PM   #3
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That made a very notable difference in my 5.9. Runs better, sounds better, smokes less.
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Old 05-18-2017, 10:03 PM   #4
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In fact I will be cleaning the injectors out tomorrow. A local diesel repair/supply house here in Nashville offered to put them on their machine to clean out for $15 each. I'm gonna try that. The fuel lines have been blown own out and or replaced. Like I said...I'm STUMPED!

Thanks for the response.
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Old 05-18-2017, 10:12 PM   #5
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Get a gauge and check fuel pressure at the pump while pulling your hill. It sounds like you're running out of fuel. So short of actually checking it, everything else would be just a guess.
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Old 05-18-2017, 10:16 PM   #6
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Are you thinking the lift pump, or the main injector pump??
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Old 05-19-2017, 12:20 AM   #7
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I strongly recommend having a good fuel pressure gauge on the output of the secondary filter - it will show the main pump's health. Between the primary and secondary filters I also have a Racor fuel filter restriction gauge that will show if the primary 5-micron filter is getting blocked.

Have you checked if there are any pinholes in the fuel suction tubes from the tank to the primary filter and from there to the pump? A tiny pinhole will introduce air into the fuel, but obviously won't leak fuel out.

John
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Old 05-19-2017, 12:31 AM   #8
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Do you have a boost gauge?

I started to ask if you could feel the torque converter unlocking, but realized you mentioned previously pulling hills at 70, which would mean there was no downshift involved and the converter stays locked. Are the hills big enough that the engine's lugging because the transmission should be downshifting and it isn't? Or is the engine just low on power?
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Old 05-19-2017, 12:34 AM   #9
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Hi John...Thanks for the advice about the pressure gauge monitoring! I will most definitely check those things out. Incidentally, how to you spot pin holes that may be sucking air under a load if they don't leak "out"??

Thanks...Mark
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Old 05-19-2017, 12:38 AM   #10
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This is a new issue. Pulling hills easily used to be the norm, now the engine bogs down and chokes out. It's not a shift problem, or a tranny problem. It is quite maddening though! I almost had to be towed home the other day...limped home.
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Old 05-19-2017, 03:58 AM   #11
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Is it possible something can flap down over the air intake for the breather? I assume we are talking about a FE bus and not an RE bus. The air intake for FE buses goes through a real maze before it gets to the turbo.

Another lack of air could be a hose that has collapsed on the inside and is not able to deliver enough air volume at speed. The same could apply to a an intercooler or charge air cooler.

I know that if you were short on air the exhaust would be black as sin.

But lack of air is the only other possibility if you are getting plenty of fuel.

My vote, after checking the easy air blockage, is for a lift pump that can't produce enough volume when it is working hard. At low engine speed it can produce enough volume and pressure but as the engine speed goes up it can't keep up with the volume.
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Old 05-19-2017, 07:36 AM   #12
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since you said there was nastiness in the tank, im assuming you cleaned the pickup screen and changed the water separator filter?
-Christopher
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Old 05-19-2017, 09:39 AM   #13
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Quote:
Originally Posted by Mark Antony Wray View Post
Are you thinking the lift pump, or the main injector pump??
Lift pump. There might be a port on the side of the injection pump that you can easily install a gauge to. That'd be my first check. My thoughts on the injection pump/injectors is that if one of those were bad, you'd have a misfire. Unless all of them went bad, and that's pretty unlikely.
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Old 05-19-2017, 10:37 AM   #14
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I made sure to not reconnect the short maze of ducting to the turbo except the filter so I could get straight air. There is nothing "air-wise" to collapse.

At this point I am convinced it has got to be fuel flow related, but holy sheeit it's getting expensive throwing stuff against the wall to see if something sticks!!...LOL!

I do now consider the lift pump to be my next target. I will put the injectors and injector lines back on this weekend. After that, if injectors aren't the deal then I'll go after the lift pump. (deeeeep sigh)
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Old 05-19-2017, 10:43 AM   #15
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All filters have been changed....all lines have been swapped out for new...all connectors replaced..........BUT, I honestly can not say I am sure about the "pickup screen"???? Hmmm....interesting...I'll have to research that. The bus had sat 3 years with old fuel in the tank before I bought it, and even then I don't think the tank had EVER been cleaned out by the church that owned it.
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Old 05-19-2017, 10:47 AM   #16
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Exactly...that's was my thinking as well - it idols and revs like a dream, will cruise at lower speeds on flat ground like a champ, but you load her up with fuel to pull a hill and she'll sit down like a stubborn jackass on ya!

I pulled the injectors and the injector supply lines expressly for the purpose of cleaning and piece of mind. At this point I'm thinking Lift Pump (which is on the side of the engine block on my 8.3.
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Old 05-19-2017, 10:50 AM   #17
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Quote:
Originally Posted by cadillackid View Post
since you said there was nastiness in the tank, im assuming you cleaned the pickup screen and changed the water separator filter?
-Christopher
Where would the "pickup screen" commonly be located??? I wonder, is the pickup screen called by another name??
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Old 05-19-2017, 11:41 AM   #18
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Quote:
Originally Posted by Mark Antony Wray View Post
Where would the "pickup screen" commonly be located??? I wonder, is the pickup screen called by another name??

Look in the tank for the pickup screen.

It can be accessed through the access plate on the floor of the bus over the top of the tank. If your conversion didn't leave the access plate accessible you will need to drop your tank.
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