Intermittent rough idle/rail pressure fluctuation - Dodge Cummins Diesel Forum
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Old 09-03-2011, 05:00 AM   #1 (permalink)
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Intermittent rough idle/rail pressure fluctuation

I searched the forum for a specific cause to my problem and found bits and pieces on a possible fix but didn't find the exact issue as mine

My mods are in my sig. Hot or cold I can have intermittent rough idle. My rail pressure is usually at 5k at idle and when the problem appears it will go to 4k back to 5k with slight rough idle. Or it can just have rough idle with no rail pressure change. ONE time I noticed my fuel pressure gauge fluctuate from 14psi to 13psi and then back up to 14 when the problem appeared but it's usually the rail pressure. The problem could last like a couple minutes and then be fine the rest of the day or not even show up for a few days at a time. And yes I did check for any bare wires!!!

I did read a post on where a gentleman had intermittent rough idle and it was his cascade overflow valve. I changed mine because the part was inexpensive but it did not solve my problem. Other than the rough idle issue the truck runs great. FCA maybe????
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Last edited by monster diesel; 09-03-2011 at 05:02 AM.
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Old 09-03-2011, 08:07 AM   #2 (permalink)
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FCA's my bet.
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Old 09-03-2011, 08:23 AM   #3 (permalink)
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A little more info I didn't post the first time, it always starts easy and I never have any smoke while it's acting up. After work I'll try and unplugging the FCA to see if the rail pressure goes to max. Anyone else have an opinion??
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Old 09-03-2011, 07:54 PM   #4 (permalink)
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Update!!

Ok guys I had time to do some more trouble shooting. I disconnected the FCA harness and started the truck and rail pressure went to max. So, I would say it's not that. Next I disconnected the banjo bolt from the PRV and started the truck. The fuel started gushing and I mean gushing out of the top. So, that's it right? My relief valve is shot? Comments please. I just want to verify its that before ordering it.
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Old 09-03-2011, 11:04 PM   #5 (permalink)
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People viewing but no opinions??
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Old 09-04-2011, 12:16 AM   #6 (permalink)
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If its gushing out the valve then yes it needs to be replaced.
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Old 09-04-2011, 02:02 AM   #7 (permalink)
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If this relief valve goes out because of high rail pressure I'm trying to figure out how this happened. I have a smarty jr but I drive it like an old man, never beat on it or do boost launches. It's set on the 70HP setting since I bought it. From what I've read it sounds like the FCA is/has gone bad and that is why it blew my rail pressure valve. Does that sound logical? Should I replace the FCA before I install the new pressure valve?
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Old 09-04-2011, 06:52 AM   #8 (permalink)
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a leaker PRV will usually cause hard/no start depending on severity of leak.

you sure it's the end of the PRV that has diesel coming out or is it the diesel return line that connects to the PRV which is normal?

FCAs can become sticky from deposits/water in fuel corrosion at any point in their travel range. idle position is common. sticky FCA causes rpm fluctuations because the fueling system in a HPCR Cummins is digital hysteresis and is always tracking an ideal programed setting which it never finds, always slight changes up and down as computer searches. old analog foot throttle systems directly controlled rpm
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Old 09-04-2011, 07:37 AM   #9 (permalink)
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After work I will double check to see if the fuel is coming out of the line or the valve itself. Thanks for the response
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Old 09-04-2011, 11:09 AM   #10 (permalink)
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Ok I disconnected the relief valve again and the fuel doesn't appear to be coming out of the valve but out through the banjo line. Ok so now what? Is unplugging the FCA and having it go to max psi a solid way of knowing if it's good? The truck runs and starts solid with no smoke. Maybe a little black smoke when I get on it. Idle rail pressure is 5500 to 6k and fuel pressure is 13 at idle. Fuel filters are about 6 months old and I have been running 2cycle oil almost every time I get fuel. Need some opinions on the next step.
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Old 09-04-2011, 12:03 PM   #11 (permalink)
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How many miles on 6 mo filters? Might change them for piece of mind. You can't tell the FCA is sticking at the idle position by unplugging the elec conn--it just goes to full open to verify that it is moving.
You would probably have to hook the truck up to the computer to see desired vs. actual rail pressure to see if it is sticking. Even then with the normal variation in pressure, it might be hard to tell.

You can get a new FCA from Cummins for ~ $100.

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Old 09-04-2011, 02:54 PM   #12 (permalink)
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