Another rough idle thread...

LiftedBlackHD · May 13, 2014

Truck is becoming hard to start and idles rough. While idling the cab bounces up and down mildly.

I did some tests today and the findings are as follows:

At idle I am getting 13.8-13.9 volts at the batteries and 13.9 volts at the alternator measured with the voltmeter, with NO acessories on. These numbers drop to 13.2-13.3 and 13.4 respectively with all acessories on.

According to my SCT at idle with everything on I am getting 13.0. With acessories off it jumps to 13.7.

I tested the FICM and I am getting about 47.5 with key on and at idle.

I then pulled the ICP sensor and there is no noticeable change in sound or RPM at idle. Plug it back in and again no change. I tried this several times with negative results. The ICP was replaced a few years ago. When it went bad the first time the idle was jumping up and down and the truck would die. It is not currently experiencing those symptoms. Just a steady rough idle.

According to my SCT there are no DTC's.

Any opinions?

Thank You

6speedsd · May 14, 2014

First off you need to put a GOOD scanner on it and watch some things. SCT won't read all the codes that could be stored. Watch what ICP, IPR are doing when you crank it, and while it's running. Are we talking about hard starts when hot or cold?

LiftedBlackHD · May 14, 2014

What scanner do you recommend? These are cold starts.

6speedsd · May 14, 2014

AE (auto Enginuity) is very popular. I want to say Snapon makes one that works well, but don't hold me to that. I have AE and love it.

Monster12valve · May 15, 2014

I had the same problem, it was the ficm. Was bouncing between 47-48 volts. Took it to the dealership, they changed it and now it reads 49 volts all the time and runs perfect. My battery's were reading the same as yours except it drops to 12.9 with my camper hooked up.