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Old 04-21-2016, 05:39 PM   #1
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6BT Good power at lower RPMs but no so much at higher

New to me 6BT, Allision 3060 powered 1994 Allegro Bus. All fluids, filters, hoses replaced. Seems to run and pull ok under light/moderate throttle settings. Pulls good through around 1700-1800 rpms. As it gets above 1800 (or so) rpm's it doesn't seem to pull (accelerate) well. It well accelerate better if it shifts to next gear.

My first thought was to check fuel pressure to see if overflow valve is bad (have to purchase the gauge setup first). If that's ok, then maybe add boost gauge to confirm proper boost.

I recall reading about a 3200 spring in the injection pump. Any anyone give a little insight on how that differs from stock? Thanks.
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Old 04-21-2016, 07:45 PM   #2
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Cummins engines , failing to pull to max RPM's does sound like a fuel supply issue; but before you go and buy gauges.
Are you sure that ALL fuel filters have been replaced , all coaches will have 1 fuel filter , most will have 2, and some will have 3, the third being a small one in the fuel line almost more like a pre-filter screen.
There have been members chase power problems for weeks on end at different repair facilities , only to find the third filter.
The filter can be hidden in the wiring harness on the frame like the picture below.
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Old 04-21-2016, 07:54 PM   #3
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First off I am guessing you have the P-7100 pump? The 3200 spring will allow the pump to full fuel up to about 3000 RPMs, but what you are describing sounds like a low fuel situation. If your overflow valve has a 10mm bolt head on top of the valve then try taking that bolt out and under it you will find a small spring. DON'T loose the little ball in there!!! Stretch that spring out to 1/2" or maybe a little longer. Reassemble the valve. What you just did was increase the pressure to the pump. Another thing that people don't remember is there is a pre-screen on the fuel heater just in front of the lift pump. Here is instructions on how and where the screen is:
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Old 04-23-2016, 10:14 PM   #4
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Quote:
Originally Posted by Fugitive861 View Post
Pulls good through around 1700-1800 rpms. As it gets above 1800 (or so) rpm's it doesn't seem to pull (accelerate) well. It well accelerate better if it shifts to next gear.

I recall reading about a 3200 spring in the injection pump. Any anyone give a little insight on how that differs from stock? Thanks.
Accelerating "well" is kind of a subjective idea, so it's difficult to say if the engine has an issue, or if it is just operating as it was designed. Your particular 230hp Cummins is tuned a little different than the ones put in the pickup trucks back then, as it makes more hp and torque at slightly lower rpms. It produces peak torque (600lb-ft) at around 1450-1500 rpms. This is where it will "feel" like it pulls the hardest, up to about 2000 rpm. Peak HP starts at around 1900 rpm and stays flat to about 2200rpm, but the engine is governed to rapidly defuel by 2500 rpm.

My experience with this engine in it's stock configuration, is that beyond 2000 rpm, it's starting to get out of it's "zone" and by 2500 it's pretty much dead. It's actually designed this way so that it produces the best power way down low in the rpm range. This adds to longevity, and helps get a big load like an rv moving with relative ease.

You can increase the useful rpm of the engine and produce more HP with the governor spring kits without damaging the engine. You will, however, find yourself probably needing to reprogram the Allison to respond correctly to the increased rpm.

-cheers
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Old 04-28-2016, 02:23 AM   #5
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Is it missing or just not 'going' ?
Ours was a duel filter and air filter issue

Was fine 95% of the time and heavy loading like Towing the jeep up and over the hills in our trip from DFW to Hilton head showed some dragging that went away after filter changes
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Old 05-06-2016, 12:44 PM   #6
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I agree with all of the above. Step one in any purchase of a used vehicle is basic maintenance...check all your fuel filters and air filter. My bet is that you'll still be disappointed with the performance of this set-up.

We have the same set-up. Ours was listed as a 230hp version. This engine will pull harder than your transmission will take if you wanted to. It's very easy to "tweak/adjust" your fuel pump to end up with a big ol' grin on your face and not burn-up your trans.

This topic usually gets people all up in arms about reliability/factory setttings/blah blah...the cummings 5.9 is one of the best engines in its class every made, my opinion.

Okay, so if you're willing to do a little work and research, you'll be stoked on this engine.

The two things holding you back the most right now are the governor springs(starts to defuel around 1800-2000) and your fuel plate. The P7100 has a fuel plate that acts as a fuel profile...this much fuel at this rpm...the one I removed was extremely flat and restrictive(pict attached).

*******AT YOUR OWN RISK!!!*******
Do some reading about removing your fuel plate...fairly easy to access(4 screws on the top of the pump) once you remove the tamper screws. Take the plate out. Slide that housing that you had to remove to get to the plate at little(like 1/16" to start-this increases fuel) towards the rear of the coach. There is a small metal tube running from your turbo to your fuel pump. It has a rubber hose that connects the two, remove that hose, cap the side of the fuel pump.

This is half the overall process that will wake this thing up. You're going to want to get some gauges on it though. A pyro meter in the exhaust manifold and a boost gauge. Honestly, you can do the above and just take it easy but before you start pulling any real hills or really "getting into it" you SHOULD have gauges.

Gauges MUST be installed before moving on.

The second half is to add a 3000rpm governor spring kit(3k GSK) or even 4k(some say its less twitchy. The 3k can be installed alone while the 4k they recommend swapping intake and exhaust valve springs(not a terrible job. Took me a few hours to do on my first try). You may want to consider this since if you decide, as I did, that you want an exhaust brake, they are going to tell you to renew your exhaust valve springs. There are some additional adjustments/screws to turn on the fuel injection pump. They will be used to "dial" it in. This will be trial and error...Make an adjustment, climb a hill or get on a freeway...then, adjust and repeat....the final piece of the puzzle is timing adjust. You'll need some special tools. You can buy or borrow. Join some cummins forums, someone may lend them to you. You'll only need them once. The consensus is 16-16.5 degrees of timing is a good, safe number with stock injectors and stock head gaskets/bolts.

With this set-up you will be amazed at the difference. You will pull hills like never before. It will get on the freeway with ease. It really made a huge difference on ours. I've pulled the grape vine on I-5 with my foot down, pulling hard, no issues. I've adjusted it for a max of 30psi of boost(a little conservative) and I keep the pre-turbo exhaust temp(pyro) below 1250(also a little conservative).

As for the programming the trans, it's my understanding based on my reading that the transmission has adaptive programming and will adjust. I did not experience any issues with shift points or anything else.


There's a lot of info out there on this engine, do some reading, watch some youtube and get to it.

Feel free to message with any specifics.
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Old 05-06-2016, 05:45 PM   #7
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I had an Allegro with a front engine 5.9 1st gen Cummins, with a 4 speed. As noted lots of info out there. The Turbo Diesel Register web site is a great source.
I now have a 8.3 350 in a Newmar pushed so I haven't used TDR much.
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