Eratic high pressure readings

[DE Jeeper]

I am trying to diagnose what seems to be a high pressure fuel delivery problem. I have an electronic Pricol guage and the readings are all over the place. It started on a 300 mile trip while pulling about 9k on the trailer. Usually at crusing speed 2200 rpm it reads around 10k psi and fluxes with right pedal input. After leaving from a fuel station it threw a 193 code(Fuel pressure sensor voltage high) and a 642(sensor voltage low). I was able to clear out the codes with the Smarty, but the idle started getting a little eratic. As I was crusing the HP rail droped to 3-5k for a while then back to 10k. On my way home it was all over the place. It even was hanging around 25k and pegged the guage at 30k. I don't think I have a problem with the guage because the truck just doesn't feel right. I am getting some reduce MPG and the acceleration feels light.



Today I started the truck and it threw the codes again and this tiime it was almost stalling out at idle. The computer kept ramping the idle to keep it going and the guage wasn't even reading a pressure over the 3k min on the gauge. This time I changed the filter and cleared the codes, but I had to clear them 3 times before it started fine and the codes stopped coming back. I thought that the filter may have been the problem because my 2 hour trip home the gauge looked normal, but when driving around town the idle when "jumpy" on me again and the hp gauge was all over the place. It still feels like there is something wrong.



Now I am going on a 500 mile trip next weekend and I need to figure out what is going on. The truck does have 160k with power mods so something failing is not unexpected.



Any thoughts on how to diagnose if I have a failing CPS or FCA. With my mods I have to ugrade the fuel evenually. I wonder if it will be sooner then later.



All theories are welcome.



Thanks,

Aaron

 

[DE Jeeper]

Any thoughts out there?



Aaron

 

[RMarkey]

good luck, no idea here, I'm still trying to find out why my high pressure drops sharply under full acceleration then comes on strong, and I did the same thing, replaced the filter to no avail.

 

[SShafranko]

When you raise the revs from idle do you hear a slight knock coming from the engine?

 

[JStieger]

There are 5 possibilities based on my own experiences - some as recent as last week :-laf. The possibilities are:



1) Does your TST have the rail pressure feature? If yes, can you disconnect it? For that matter, can you install the TST bypass plug to take the TST out of the picture? How does the truck run now after the TST is bypassed?



2) Do you have a pressure box installed? If yes, disconnect it. Last week my VA C3. 1 pressure box harness shorted out, which caused a P0193 code, white smoke, erratic idle as you described, and horrible missing. Details here:



Dual CP3, PO193, and engine miss - Dodge Diesel - Diesel Truck Resource Forums



Note that the initial troubleshooting I mentioned in this above thread pointed to a bad rail pressure sensor, but in fact it was a wiring harness.



3) I've had these same symptons in terms of widely fluctuating rail pressure when the FCA starting going FUBAR. However, at the same time I never got the rail pressure codes when this happened. The FCA receives its orders from the ECM via the rail pressure sensor. The FCA is only $110 versus the ~$200 for the rail pressure sensor. Do you know someone else with a common rail Cummins? If so, ask nicely to see if they will let you swap their FCA for a test.



4) If the rail pressure sensor is bad, then it sends bad signals to the FCA, which in turn regulates the rail pressure. $200 if you want to throw parts at it.



5) If the rail pressure harness to the ECM plug is shorted anywhere, it can cause bad or shorted 5 volt supply, sensor signal, and/or sensor return readings that can mimic a bad rail pressure sensor.



If you have a good digital multimeter and the 05 shop manual, which has detailed and illustrated trouble shooting procedures for various "P" codes, you can find in fairly short order whether the harness or sensor is bad.



If I was at home instead of on this business trip and currently stuck in Kentucky/Ohio due to weather, I could walk you through the test procedure step-by-step by doing it on my truck again.

 

[DE Jeeper]

Good info. The TST doesn't have rail pressure and I have no pressure box. I guess I have to test the FCA and sensor. I have to see if I can get a copy of the test proceedure. I think I may get a FCA just in case. It seems to make the most sense.



Aaron