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​Cold Starts, Rough Idle, And The DDEC Oil Temperature Sensor

By Fernando DeMoura

November, 2019

The start of October is usually warm in Pittsburgh but by the month’s end I’m back to plugging in my block heater. I buy the nicest 1500w block heaters I can find but I consider myself lucky if I can get more than two winters out of a starter. Before I knew better I blamed my fuel, or quality of the starters but I soon realized the issue was because my 1999 7.3 ECM didn’t support the cold start functions of the 2002 7.3 that I repowered my truck with. 

The cold start functions on a DDEC are activated and deactivated by oil temperature. A lot of Series 60 owners don’t know this. Some guys unplug their oil temp sensor if there’s an issue with the sensor or the wiring to avoid a derate or shutdown. You can get away with this on many Caterpillar engines because most Cats don’t do much if anything with oil temperature but that’s not the case with a Detroit. Detroit’s use oil temp for engine fan functions and engine protection derates/shutdowns just like the other guys but it also uses oil temp to alter fueling and timing, especially during cranking and at idle. When cranking at 200 rpm a DDEC can provide a Series 60 with as much as 3 degrees of timing advance and almost double the amount of fuel if oil temperature is reading below -30 degrees Fahrenheit compared to if it’s reading 200 degrees Fahrenheit. If your oil is at -31 degrees you need extra fuel to overcome the thick viscosity of the oil and get the crank to 600rpm but if your oil is at 200 degrees F and you have the oil temp sensor unplugged then you’re getting a lot more fuel and timing then you need. The voltage difference between a valid -31 degree Fahrenheit oil temp reading and an unplugged oil temp sensor is only a few tenths of a volt and sometimes a DDEC won’t catch this. Back when I was training new techs I’d wait till they were looking at their laptop and then unplug the oil temp sensor and see if they’d notice. I’d come back about 10 minutes later and if they hadn’t figured it out I’d ask them why the coolant temp was reading 190 degrees and the oil temp was reading -31 degrees. Some guys said they didn’t think there was a problem because there wasn’t an active fault code. Some said they thought oil temp was like EGR temp and had an assigned value on non EGR engines. The computer doesn’t see a problem with 190 degrees coolant temp and -31 degrees oil temp. I’d tell them not to let the computer do the thinking for you.

The cold start feature most DDEC IV owners notice is the higher minimum idle. Anything below 60 degrees oil temp will set minimum idle rpm to 900 unless it’s been specifically programmed out. As oil temperature comes up minimum idle rpm comes down till oil temp is at 120 degrees. At that point idle stays at 600 rpm. The most obvious symptom of a bad oil temp sensor on a DDEC IV Series 60 is the engine won’t idle below 900 rpm after it’s warmed up. DDEC III programs didn’t have this feature until October 1997 so unless the GK60 has a reman DDEC IV on it then you have to look with a datalink device to see what the oil temperature sensor reading is. 

For those of you who are thinking about unplugging your oil temp sensor just to get more fuel during cranking don’t…All you have to do is turn the ignition on, push the throttle to the floor and release it twice, then hold it to the floor while cranking. That’ll give you even more then you’d get unplugging the oil temp sensor.

Written by Fernando DeMoura, Diesel Control Service. Phone 412-327-9400. Website: www.dieselcontrolservice.com