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  • Diesel Tech Questions: Duramax Losing Power, 6.0L Ford Engine Ramping Up, and P0299 Codes

Diesel Tech Questions: Duramax Losing Power, 6.0L Ford Engine Ramping Up, and P0299 Codes

Top Tech: You've Got Questions? We've Got Answers!

Bruce W. Smith
Sep 1, 2016
Photographers: Bruce W. Smith
Towing Into Limp Home
Last winter, I put a Garrett PowerMax Stage 1 turbocharger on my ’07 truck’s 6.6L Duramax engine. The truck ran great until I slipped my 30-foot toy hauler on the gooseneck. On the first long grade I came to, the engine started losing power and then the ECM activated the limp-home mode and triggered a P0299 (turbo underboost) code. Thinking it was an exhaust leak, I checked every connection and fitting. Everything is tight. The truck is otherwise stock. Garrett’s instructions don't say a tune is necessary, but I’m wondering what else it could be?
Mark Killingsworth
-via the Internet
We’ve seen this happen a few times with mild turbocharger upgrades. The problem usually requires reflashing the ECM with a custom tune. The Duramax tuning experts at Adrenaline Truck Performance (atptrucks.com; 855-370-4678) explain the problem you are experiencing this way:
The 6.6L Duramax LMM (’07½ to ’10) and LML (’11 to ’16) are very finicky when it comes to changing turbos, even a mild change like the Stage 1 Garrett units. The strict emissions requirements for the newer models use ECM calibrations that control turbo parameters that ensure the backpressure is sufficient enough to operate the EGR effectively, i.e., more boost = more backpressure.
When the ECM sees lower boost numbers that do not correlate with the desired factory-programmed parameters, the ECM will send the engine into limp mode. With modified or slightly larger turbos, the factory vane position and boost tables within the ECM need a bit of “tuning” to bring the boost numbers back in line with the requirements of the processor. Our inside source at ATP also says earlier Duramax engines do not have this problem with the Garrett Stage 1 turbos, as their ECMs aren’t as picky. However, running a “large modded” variable-geometry ’charger on any Duramax does require custom tuning to optimize the vane position and boost tables for the best driveability, efficiency, and power possible.

Photo 2/3   |   A “wear spot” on the accelerator-pedal-position-sensor rheostat is one common issue that affects 24-valve Cummins engines that run primarily at constant highway speed for long periods of time as they wear.
Pulsing Concerns
My ’00 Dodge Ram 2500 has less than 160,000 miles on its 5.9L Cummins engine, which pulses while cruising down the highway. The engine stops pulsing when I roll into the throttle going uphill. I have heard this is a sign the turbocharger is going bad. Is this true? Everything is stock except for aftermarket VP44 injection and lift pumps.
Stan Cerek
-via the Internet
It’s difficult to diagnose “bucking” or “pulsing” without knowing boost and fuel pressures. However, for trucks with 24-valve 5.9L Cummins engines like yours, Mobile Diesel Service of Oakland, Oregon, says recalibrating the accelerator pedal position sensor might resolve the issue. According to Mobile Diesel, there is an area on the APPS rheostat that is known for becoming worn out from continued usage.
Here’s how they do it. They disconnect both batteries and turn the ignition key to “Run” for 30 minutes to fully deplete the starting circuit capacitors. The key is switched to “Off,” the batteries reconnected, and the key switched back to the “Run” position (not Start). Then the throttle pedal is slowly depressed until it’s floored, and then slowly released. This lets APPS re-learn the limits of the throttle. The key is then turned to “Off,” and the engine restarted immediately after that.
MDS’ diesel techs say if that doesn’t cure the pulsing, the APPS might need to be adjusted. This is a delicate operation that should be performed by a good diesel mechanic. In a nutshell, the voltage going to the #23 pin on the ECM’s C1 connector should be around.0.5 volts.
The APPS is then carefully rotated (clockwise/counterclockwise) until the voltage at pin #23 matches what’s noted on the tag (again, this should be 0.5 volts).
Another possible cause of the pulsing could be fuel-related; sometimes the lift pump isn’t supplying the VP44 injection pump with enough fuel pressure at cruising rpm, causing the engine to pulse or buck. The VP44 needs at least 10 psi to operate correctly at cruising throttle position. But keep in mind, it’s hard to pinpoint a fuel issue without gauges.

Photo 3/3   |   A diesel engine’s sudden rpm ramp up and return to normal is often a sign of an accelerant being induced into the fuel stream. Tracking down the exact source, however, would be impossible without data-logging engine activity.
Squirrelly Throttle
My ’05 Ford Super Duty’s 6.0L engine acts odd when I cruise the highway or drive on city streets. At random, it will ramp up the rpm and run rough. Normal idle is about 650 rpm, and when it gets erratic, the engine speed climbs to around 900 rpm. So far, my mechanic and I have run all the scans and tests, and nothing has popped up. We also checked the injection-control pressure sensor to make sure it is putting out correct voltage and it’s working fine. The truck has 360,000 miles on it, it’s already had all the typical 6.0L “problems” addressed, and it’s maintained religiously.
Corey Smith
-via email
If you have ruled out electrical and ECM programming issues, then "ramping up" is usually caused by an excess of fuel or an outside accelerant. But that isn’t really a common issue for 6.0L Ford engines. “If you are seeing excessive blow-by and the charge-air cooler has oil in it, then periods of higher boost could push the extra oil out of the ’cooler and ramp up the rpm. Once the oil is depleted, the engine will run normally, says Anthony Youngblood, of Super Duty Service in Grain Valley, Ohio (sdutys.com; 816-548-6970).
Either way, in order for the rpm to increase the way it is with your truck’s engine, there needs to be an accelerant causing it. Tracking down the exact source, however, would be impossible without data-logging the engine’s activity.

Muffled Worries
If I remove the factory muffler from my ’07 Dodge Ram and replace it with an aftermarket unit, will the 6.7L Cummins engine throw codes, or will the new muffler mess anything up?
Edwin Werx
-via email
Change away! For now, at least, everything downstream of the diesel particulate filter (from DPF to the exhaust tip) is free from pollution-control sensors. So, no codes will be activated. By the time the exhaust goes through the turbocharger, catalytic converter, and DPF, there’s not much of an exhaust note left to muffle—or make louder, in your case.
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