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Diesel Tech Q&A: Hard-Starting Cummins

Originally Published in Trailer Life Magazine

Hard-Starting Cummins

Red letter QI have a 2005 Dodge Ram dually Cummins that has begun having starting problems when the weather gets below 45 degrees Fahrenheit. It started tripping the Check Engine light, even when the block heater was plugged in overnight. The codes were MAP sensor, IAT sensor and battery-temp sensor. The air-intake heater works fine, and replacement of all sensors was performed. The block heater makes it easier to start, but why is the minimally cold weather here in southern Arizona affecting it starting?
John Arcari

Red letter QWhat is the main cause of a 2003 Dodge Ram Cummins with 150,000 miles that starts really well in warm weather or with the heater plugged in but is very hard to start in cool or cold weather? The engine cranks well, and no codes show up when checked, with the ignition switch on and off using the “three-times” method.
Ralph Rickstrew

One of the most common hard-start-when-cold issues on the 5.9-liter, 24-valve Cummins is that the batteries are no longer supplying the load needed to spin the starter as it should. Bad or weak batteries begin rearing their ugly cells when temps drop down below 50 degrees Fahrenheit. Have the batteries load tested. If they don’t pass muster, replace them as a pair.

The second common issue that could be causing hard starts in cooler temps is that the grid-heater relay or the grid heater itself goes bad. The Cummins 5.9-liter doesn’t have glow plugs but rather a single heater that mounts between the air-intake horn and the intake manifold. When temps are between 15 and 59 degrees F, the two heating elements inside cycle on for 10 seconds, providing about 500 degrees of air heating. Each heater is controlled by its own relay, and each is controlled separately by the engine control module (ECM).

The grid heater is both a cold-weather starting aid and an emissions control device. Normally, a bad grid heater will throw a P0380/P0382 code. To see if it is working, watch your voltage during startup. If the grid heater is working properly, “the headlights will dim as the grid heater cycles on, and the volt gauge will read about 12 volts at start, 13.5 volts during grid cycling, and then jump back to around 14.5 volts after the heater cycling stops,” according to Shawn Smalley, owner of Mobile Diesel Service in Oakland, Oregon.

Smalley says when his techs don’t see any change in voltage during startup, they pull the intake horn and take a close look at the terminals where they bolt to the battery cables. Vibration and age can fatigue the ends, and sometimes they aren’t making contact and never heat up. Same with checking the battery-terminal connections. Grid heaters take a lot of juice to operate correctly.

If the batteries and grid heater aren’t the issues, the third possibility is the injectors. What generally happens is the injectors will not fire unless the rail pressure has reached 5,000 psi, and excessive return flow from a worn-out or cracked injector(s) can make it take longer or prevent this rail pressure from being reached while cranking. If the injectors are the problem, it could be about a $3,000 repair, depending on whether or not you use new injectors or remans.

One last thought: On the 2003 to early 2005 Cummins, there was also an ECM reflash that fixed an injector-supply timing issue with the number-6 injector, which had caused rough-idle issues.

A certified diesel tech should be able to diagnose and fix any of the above with ease.

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To ask Bruce W. Smith a question about maintenance, repairs or upgrades to your diesel truck, SUV or van, go to the Diesel Tech Q&A home page, scroll down to the Leave a Reply box and type in your question. Please include your full name and hometown.

A respected automotive and RV journalist and longtime Trailer Life contributor, Bruce W. Smith has held numerous editorial titles at automotive and boating magazines, and authored more than 1,000 articles, from tech to trailering. He considers his home state of Oregon a paradise for RVing and outdoor adventure.

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