Mechanical Cummins - power issue

[jimhfurlong]

Does anyone know if on the mechanical cummins (97 8.3-300) there is anything that limits engine power by the transmission? We've replaced the maniforld, turbo, filters (all) and checked for leaks, but still have an issue when the engine goes under load, the power drops off exponentially. Lots of boost, seems the engine is being limited by something. Runs fine on the flat, the minute it begins to climb, revs fall and speed drops.

Thoughts?

[allegro5]

Hi Jim, I bought my 96 Dynasty last April. It is 40 ft. no slides. Its my first diesel of the 7 coaches we have had! It runs good and drives nice, but hasnt had the power I expected in an 8.3. Is it possible you need to change the water/fuel filter? If they sit a time they do get a flow problem. Also, use some fuel addative, like Lukas, that helps clean things up! I love this beautiful rig, but they are a challenge! allegro5 PS My motto is try the easy and cheap fixes first!

[skipwil]

Look at my posts:

"'01 ISB 300 Cummins- boost gone"
Posts#8 & 10
I pulled my hair nearly out hunting. An air compressor hose was broken. $55.00
solved my problem.
I know very little about a diesel but with the help here on the forum I've learned more.
Good luck...

[94-Newmar]

Jim,

--Both Fuel filters? (There are 2)

--Lift pump?

--May have a bad rack in the injection pump... the 8.3 pumps are notorious for having issues burning them up. Had to replace mine, although my symptoms weren't anything like yours, I had idling problems.

You say there is plenty of boost... do you have a dash gauge or something? How many PSI is it making? Any stumbling?

[csp]

Jim
How much boost is lots of boost? If it is stock you should be getting around 20 to 23 pounds of boost when under a hard load.
Have you checked your exhaust brake? Wire it in the open position and then take it for a drive.
Chris

[94-Newmar]

Another thought....check the cruise control cable as well... ours got hung up once and even though the pedal up front was on the floor the throttle assembly on the pump wasn't going past 1/2 way.

[deandec]

Originally Posted by jimhfurlong

Does anyone know if on the mechanical cummins (97 8.3-300) Runs fine on the flat, the minute it begins to climb, revs fall and speed drops.

Thoughts?

Just on the chance the problem is your expectations, I will outline my rig's performance profiles (in layman terms..... that's me.)

My rig weighs 31,000 lbs, I tow a Jeep that weighs 4,500 lbs. But the engine is a C8.3L mechanical 300 hp.

So it is less than 1 HP per 100 lbs. It is underpowered compared to most newer rigs.

We travel mostly around the Sierra and Siskiyous Mountain areas from the Central Valley of Ca.


The observations over the past eight years are:

A cummulative fuel mileage of 7.25 including moderate generator use.

35 MPH max when climbing 6% grades with or without a toad. Typically I have to downshift to 4th or 3rd gear to keep the rpms up and the heat down in 90 degree temps. The fall off of speed and power is very rapid as the ascent begins.

First gear is really slow until the turbo kicks in. It is much closer to a turtle than a jackrabbit from the stop sign.

Flat and rolling hills travel can reach 80 mph without great effort.

If the exhaust brake is sticking, you will know it immediately as there will be very little power above 20 mph (my actual experience).

The guy who used to manage multiple shops for over 500 diesel trucks indicated that my engine was running good (about 4 years ago but the performance has not changed).

So, I have concluded that with my weight to HP ratio and traveling in mountain country, I should not expect much in the way of power performance when going up grades.

[Phil R]

I have a 97 8.3 that I think is 275hp. The performance your seeing is pretty much the same as mine. I pull a jeep on a trailer and get 8.5-9 mpg. It will slow to about 40 on most mountain passes. I'm not going to change anything on it I like the mpg's.
So I'd guess that what we see is what we get until we change rigs.

[Always-RVing]

That's is so strange... I have a Cummins 2008 8.3 ISC and its horsepower rating is 360 with 1050 lbs of torque. How can your 8.3 be 275? Something is weird.

[jimhfurlong]

Ok, here's the low down (no pun intended). We solved the problem today and have restored the power issue. For the record, our rig (C8.3-300) would climb any grade between Virginia and Canada with no hesitation on our first trip, but mid-way back lost power. Three days in a truck shop got the power back, although all they did was replace filters (Which I had already done a few weeks previous) and install a new lift pump because it was only producing 8-10 lbs to the injection pump. Problem wasn;t gone, but we made a second trip to Canada, again, ont he way back we had a hard time even climbing any grades (What a long 300 miles that was!). This time, I noticed a manifold leak at #5 & #6 cylinders so I replaced the manifold, turbo charger, gaskets and anything else I could find that remotely looked wore. (I have a really good source for rebuilt turbos if anyone needs one). After all that, still wouldn't work. Truck shop couldn't find the problem (AUTHORIZED CUMMINS????) and I was skeptical about taking it back (They told me they couldn't plug it into a computer, so may take replacing everything to fix it). Couldn't limp to the distributor outside Balitimore, but my train guys came through for me today!!!

Seems Cummins has had several (3) campaigns on return check valves on the injection pump (returning to the fuel tank) and a friend of one of the guys recommended clamping off the return line to about 30 PSI with some vice grips to check if that was the problem. Once we did, coach ran better than new, had to ease off the throttle climbing 6% grade outside of town. Fix? $80 check valve from a diesel shop. The "Authorized" Cummins shop, the Cummins support line and dealers, not one suggested it.

Moral of this story? My next coach will be CAT powered!!!

[94-Newmar]

Glad you got it sorted out, will be good info for someone else with similar problems. However.. if you think Cummins is expensive? CAT is even more outrageous.... 1st hand experience.

[bsedor]

You also won't be buying a newer rig with a CAT becasue they are no longer suppling them for over the road use.

[deandec]

Originally Posted by jimhfurlong

Seems Cummins has had several (3) campaigns on return check valves on the injection pump (returning to the fuel tank) and a friend of one of the guys recommended clamping off the return line to about 30 PSI with some vice grips to check if that was the problem. Once we did, coach ran better than new, had to ease off the throttle climbing 6% grade outside of town. Fix? $80 check valve from a diesel shop. The "Authorized" Cummins shop, the Cummins support line and dealers, not one suggested it.

I have $80 to throw at my rig!!

Where is the check valve located?
Who did you buy the valve from?
Can you describe the visegrips to 30 psi trick?

Any clues would be appreciated as I would like to see if my rig responds better.

[deandec]

Ok, having time on my hands, I did a web search on this check valve test process. There is a good explanation and diagrams on the test process on a dodge ram truck web page involving a Bosch P7100 injection pump.

I believe that pump is the one on my C8.3L mechanical engine

TSB 14-07-96 Rev. A


Basically, one inserts a pressure guage at the fuel supply banjo fitting on the injector pump. Pressure readings are taken to match against the manufacturer's parameters.

Then the return line is crimped slightly to see if the new input pressure readings meet the preset parameters.

If crimping the return line improves the pressure reading result, then the check valve should be replaced because it is leaking fuel back to the fuel tank.

It appears the check valve may also create hard start problems if leaking.

I also found an after market adjustable check valve for $60 that seems to cater to the check valve problem.

Thank you JimHFurlong for the results of your power loss solution. I had never heard of this issue.

Dean