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William, Fleet Manager
Category: Heavy Equipment
Satisfied Customers: 46
Experience:  OEM trained Allison, Detroit, Cummins, OEM ONAN capability, OEM Carrier AC Dealer, 27 years exp. DOT
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I have a 2004 Cummins ISX 435ST set at 500hp with 820k miles

This answer was rated:

I have a 2004 Cummins ISX 435ST set at 500hp with 820k miles in an Int. 9400i. I maintain the vehicle very well, oil every 15k, etc. This year I have lost approx. 1mpg and some power yet the engine runs well(other than noticable power loss). I had an overhead done 8 months ago. I have a PDI intake with the block-off plate for the EGR. That helped tremendously with low end. I had Cummins read the ECM code history and there are none other than oil temp., coolant temp, coolant level, etc. and none recent. I recently had my CAC welded when I found it to be leaking but that didn't solve the issue. Wouldn't I have codes if I had a turbo or fuel issue? I am looking for the fuel mileage/power issue as I am using 10% more fuel on the same route as I used to last year. The truck runs so good many have a hard time believing that the engine has 820k on it but it doesn't have 500hp available anymore, something has changed.

I would recheck the CAC as well as all the hoses and feed hose to the compressor again, maybe you are still low on boost. Do you have your engine SN you can provide? I can look and see if there is any OEM info. relating to the issue specific to your engine.



Customer: replied 5 years ago.

S/N 79022575

I was wondering if the variable geometry turbo can act up and still not throw a engine code? Seems like any issue that relates to my issue SHOULD throw a code?

What are the expectations based on? (Original mileage, previous mileage, competitive engine, other units, VE/VMS®/other expectations, or demonstration truck)Run a VE/VMS® report to determine if expectations are unreasonable.
Are the drive train specifications within Cummins Inc. recommendations? (Verify with a VE/VMS® run)Correct for gearing or compensate by setting a lower road speed governor (RSG). Explain the trade-off of fuel economy versus performance with present gearing.
Evaluate fuel consumption over an extended period of time (3 to 6 months)This is necessary to determine the effects of seasonal and/or operational changes.
Was the calculation of fuel consumption done correctly?Evaluate the method for validity. Correct and reevaluate, if necessary. Also, be aware of fuel practices (use of reefers, lubricating oil blenders, additives, etc.).
Were there any changes in the vehicle's operating environment that contributed to the fuel consumption complaint? (Change of route, change of loads, cold ambient temperatures, windy conditions, snow/rain)Explain the effects of the operating environment on the measured fuel consumption. A fuel consumption complaint may be based on adverse weather (up to one mile per gallon) or changes in routes and/or loads.
What grade or blend of fuel is being used when the complaint is generated? (Diesel number 1, diesel number 2, or winter fuel (P40, P50, etc.))Diesel number 1 and lighter winter fuels are lower in heat content and result in higher fuel consumption. Reevaluate using number 2 diesel fuel.
Does the engine coolant temperature reach normal operating temperature?Repair as necessary.
Have the tractor or trailer brakes been recently serviced?Check for dragging brakes and repair as necessary.
Is the trailer alignment correct?Repair as necessary.
Has the complaint occurred after new tires were installed either on the tractor or the trailer?Consider the effects of new tires and reevaluate fuel consumption. (There is an approximate 3 percent difference in the rolling radius between new and old tires.)
Are the hubometer and odometer calibrated correctly?See the Tire Revolutions Per Mile section to determine the actual tire revolutions per mile, in calibration of hubometer and tire revolutions per mile parameter in the electronic control module (ECM). See the Hubometer Verification section for hubometer verification. Repair or calibrate as necessary and reevaluate fuel consumption.
Check the tread condition and the inflation pressure on both the tractor and the trailer tires.Repair and inflate the tires as necessary and reevaluate fuel consumption.
Has the fuel consumption complaint occurred after an engine repair?Investigate the repair to determine its effect on fuel consumption (incorrect injectors, incorrect turbocharger, incorrect SC code, etc.).
Can the fuel consumption complaint be correlated to a change in drivers?If yes, evaluate the need for driver training.
Are there any signs of external or internal (crankcase) fuel leakage?Repair as necessary and reevaluate fuel consumption.
Are there signs of tampering with the CELECTTM system (inspect the vehicle speed sensor and the corresponding harness)?Repair or correct as necessary and reevaluate fuel consumption.
Are there indications of high exhaust temperatures (cracked exhaust manifolds, turbine casings, mounting hardware, etc.)?This is an indication of an incorrect air-to-fuel ratio. This must be verified on a chassis dynamometer.
Is the oil level correct?High oil levels can cause crank or rod dipping. Drain the engine oil and verify the dipstick calibration. Reevaluate fuel consumption with the correct oil level.
Check for active fault codes or high counts of inactive fault codes on CELECTTM engines.Repair as necessary and reevaluate fuel consumption.
Are adjustable parameters and optional features (gear down protection) set to their correct/expected values, and are they consistent with vehicles being compared to in the fleet?Set the parameters to the correct values and make sure that gear down protection is active and set correctly. Reevaluate fuel consumption.
Is the ECM calibration correct?Calibrate again, as necessary, and reevaluate fuel consumption.
Are fuel rate, boost, intake and exhaust restrictions, etc., correct?Verify these measurements using a chassis dynamometer. Repair as necessary and reevaluate fuel consumption.
Are there leaks in the intake air system between the turbocharger and the intake manifold? Does the charge air cooler have leaks?Repair the intake system as necessary. Check the charge air cooler. Use the leak down procedure in the Troubleshooting and Repair Manual, N14 Engines, Bulletin(NNN) NNN-NNNN Repair as necessary and reevaluate fuel consumption.

I ran your S/N and dont see anything that stands out, I agree that the turbo would set a code, but only if it did not pass an initial test by the ECM, low boost would have to be monitored under load.

Going to post several entries here from the Fuel Mileage section of Cummins for your review.

Hope it helps......


New Turbo Options:


New Turbochargers for ISX CM870 and ISM CM875 Engines

Warranty Statement

The information in this document has no effect on present warranty coverage or repair practices, nor does it authorize TRP or Campaign actions.


This document announces new turbochargers with improved turbocharger shroud plate robustness.

This new turbocharger release introduces new part numbers that obsolete and supersede the present part numbers. The new part numbers are documented in the table below.

New ID21-1 Short TurbochargerOld ID21-1 Short Turbocharger New Turbocharger KitOld Turbocharger Kit
376826828438892881994 NX/RX4955306 NX/RX
376826728438872881993 NX/RX4955305 NX/RX
376826628438902881992 NX/RX4955304 NX/RX

Interesting info. for low power....

Low Power Troubleshooting

When operating a vehicle on a chassis dynamometer or operating under fully loaded conditions, the User Fueling State (Engine Operating State) parameter must indicate Maximum Throttle or EGR Fueling Limit State when operating under peak torque conditions. If these states are active during maximum torque conditions (100 percent throttle operation and the engine is fully loaded and operating on the torque curve), this indicates that the air handling system is providing the desired boost pressure and no derates are presently active. If a performance complaint still exists, follow the engine performance troubleshooting tree to isolate the potential cause of the low power complaint.

If the User Fueling State (Engine Operating State) reads Air Fuel Control Derate for an extended period of time (greater than 10 seconds) during a hard acceleration or during normal operation, troubleshoot the turbocharger and air handling system. Air Fuel Control Derate indicates that there is not enough oxygen (boost pressure) entering the engine for complete combustion. The electronic control system then limits the amount of fuel being injected, to prevent excessive black smoke. Air Fuel Control Derate will be active for a few seconds during hard accelerations. It should not be active under steady state operating conditions. Failure modes that can cause excessive time operating in the Air Fuel Control Derate state include charge-air cooler leaks, charge-air cooler plumbing leaks, VGT turbochargers not meeting commanded position, a malfunctioning in-range intake manifold pressure sensor, a malfunctioning in-range ambient air pressure sensor, air intake restrictions, excessive exhaust restriction, worn or malfunctioning turbocharger, malfunctioning fuel injectors, and fuel system restrictions. To isolate the potential cause of the low power complaint, see the engine performance troubleshooting tree.

Intermittent Derates or Loss of Power

There are various power derates that can become active during extreme environmental operating conditions. Engines built in 2002 and later will log fault codes for these conditions, but it is possible for a derate to be active without a fault code. Confirm these User Fueling States (Engine Operating States) are not active under normal operating conditions:

  • Air Density Limit - This state will be active if the vehicle is being operated in a high altitude condition. Confirm the altitude of the vehicle and verify correct ambient air pressure sensor operation. Engines built in 2002 and later will activate a fault code under these conditions.
  • Turbocharger Fuel Control - Under extreme operating conditions, the engine will limit fueling to prevent turbocharger damage due to high turbine inlet temperatures. By reducing the torque output of the engine, exhaust temperatures are reduced. This state should only be active during hard accelerations. Confirm the altitude of the vehicle and verify correct ambient air pressure sensor operation. Engines built in 2002 and later will activate a fault code under these conditions.
  • Vehicle Acceleration Management - Engine acceleration is being limited by the Acceleration Management feature.

Last one I can find, hope some of this helps.....

© 2008 Cummins Inc., Box 3005, Columbus, INNNN-NN-NNNNU.S.A. Printed from QuickServe® Online.

Page 1 of 3

Fuel Consumption Excessive

Cause Correction


Fuel leak ............

Check the fuel lines, fuel connections, and fuel filters

for leaks. Check the fuel lines to the supply tanks.

Refer to the OEM service manual .


Go To Next Step


Hubometer or odometer is miscalibrated ............

Check the hubometer and odometer calibrations.

Calibrate or replace the hubometer or odometer, if

necessary. Calculate fuel consumption with new

mileage figures.


Go To Next Step


Operator technique is not correct ............ Refer to Section 1, Operating Instructions .


Go To Next Step


Equipment and environmental factors are affecting

fuel consumption


Consider ambient temperatures, wind, tire size, axle

alignment, routes, and use of aerodynamic aids

when evaluating fuel consumption.


Go To Next Step


Drivetrain is not correctly matched to the engine ............

Check for correct gearing and drivetrain

components. Refer to the OEM vehicle

specifications .


Go To Next Step


Electronic fault codes active or high counts of

inactive fault codes

............ Refer to Section 1 .


Go To Next Step


Fuel grade is not correct for the application, or the

fuel quality is poor


Operate the engine from a tank of high-quality fuel.

Refer to Fuel Recommendations and Specifications

in Section V .


Go To Next Step


Lubricating oil level is above specification ............

Check the oil level. Verify the dipstick calibration and

oil pan capacity. Fill the system to the specified

level. Refer to Section 3 .


Go To Next Step


Air intake system restriction is above specification ............

Check the air intake system for restriction. Clean or

replace the air filter and inlet piping as necessary.

Refer to Section 3 .


Go To Next Step

© 2008 Cummins Inc., Box 3005, Columbus, INNNN-NN-NNNNU.S.A. Printed from QuickServe® Online.

All Rights Reserved. Last Modified: 15-Mar-2002

File:(NNN) NNN-NNNNt087-om Page 2 of 3

Fuel Consumption Excessive

Cause Correction


Exhaust system restriction is above specification ............ Check the exhaust system for restrictions. Refer to

the OEM service manual .


Go To Next Step


Overhead adjustments are not correct ............ Measure and adjust the overhead settings. Refer to

Section 7 .


Go To Next Step


Contact a Cummins Authorized Repair Facility

William and other Heavy Equipment Specialists are ready to help you
Hello Jim,

I wanted to do a quick follow up and see if any of the info I provided helped with your fuel mileage, keep me in mind for any future Cummins info., will always need the Engine SN.

Thanks for the positive feedback....

Customer: replied 5 years ago.

Well, i finally took it to Cummins in Phoenix. I must say that I was wrong about the troubleshooting process and accompanying costs. They charge a flat $115/hr and it can take upwards of 8 hours. Mine took three. They also start at the top of a flow chart regardless of what info you give them and go from there. Actually, they were very effecient. They just called to say its ready for pickup.


I had some minor issues worth a 1/10th or two and also had two major issues.


1. A manifold pressure sensor stuck in range and always showing 30 lbs. or so boost. It turns out that my intake had a lot of carbon buildup from the EGR gasses and was on/in the sensors, too. No fault code.ECM thought it had boost to work with when it really didn't.


2. I had two weak injectors that at low RPM put out enough spray that idle, etc. was fine and only produced a miss at around 1400 RPM and up. Wasn't really noticeable and speed. Again, no fault code. They say it probably would have started throwing a code soon.


I will keep you in mind for future questions and will let others know about you. Your info was right on but my issues required a technician with the proper software, as do so many nowadays.


Thanks again for your help.


James Broulette

Touchdown Transport



James, glad it all worked out. Take care.