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Fuel pressure dropping/ rough starts


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Hi all, I'm new here, just bought an apps sensor and saw this forum that looks very informative so I figured I would join! Anyways here are my issues:

1- P0122 code which I figure is the apps because I did the reset found on the cummins forum and it went away for a few days but now is back... Going to install new apps tonight.

2- Rough starts first thing in the morning. Seems like it cranks over for about 2-3 seconds before firing up, no issues starting any other time of day.

3- if I don't allow the truck to warm up and shift directly into drive it bogs down quite a bit, then RPMs bounce back up and I'm good. It has died a couple times. No issues at lights or stop signs at all tho.

4 - my last issue is one I noticed last weekend, I was driving along and noticed my attitude cs was beeping at me for low fuel pressure, it would go down to about 7 psi then jump back up to 15. This problem went away as soon as I filled it up with fuel, I was at about a quarter tank.

Truck is an 02 cummins with rebuilt tranny, attitude cs tuner, fass fuel system, etc.

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The low fuel pressure I think could maybe have something to do with a incorrectly installed draw straw. The quarter tank issue is very common with draw straws that are cut to short. If I were you I wouldnt let it get anywhere near or below a quarter tank until you figure out some other way to draw fuel from the tank. (Tank sump, liberty module, etc)

 

I agree that the apps is probably bad too. That is another fairly common issue with these trucks. Good luck on your project!!!

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1. Before doing all that do the diagnostic to verify it's not a wire or some other issue. If the APPS is truly bad then get a Timbo's APPS.

http://articles.mopar1973man.com/2nd-generation-24v-dodge-cummins/59-obdii-error-codes/151-p0122-apps-sensor-signal-voltage-too-low

 

2. Double check your cranking pressure. I know you Edge CS is worthless for cranking pressure testing and will require a mechanical gauge to test it. You should see 7-12 PSI bouncing while cranking. More than likely there is nothing for a short span then pops up which is a sign of loss prime.

 

3. Shift into Neutral for a short span to allow the trans pump to fill the torque converter and see if that help with the bogging out.

 

4. Once again electric gauges aren't worth much. Hook up a mechanical gauge and verify the gauge is accurate. I've seen way too many people install the fuel pressure sensor at the VP44 without protection of a needle valve or a high quality snubber and eat the senders. Another problem is people get paranoid of the draw straw sucking bottom so they cut way too much off and end up sucking air all the time. 1/16 to 1/8 inch max for clearance on the tip of the tube on a dry tank. It a must to cut the tube STRAIGHT not at a angle! As the fuel tank is filled the bottom will flex another 1/8 to 1/4 of inch depending on level and fuel temp. Hence why the original fuel pickup was spring loaded and allowed the tube remain at the bottom of the tank.

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Excellent information. Thank you very much. I am in the process of installing the new apps right now and just noticed that the blue wire with the black tracer has been tapped into at some point and it runs toward the firewall but then I can't tell where it goes... Any idea what this might be for?

Also, what is the easiest way to set up a regular guage to check if my cs fuel pressure is correct?

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Excellent information. Thank you very much. I am in the process of installing the new apps right now and just noticed that the blue wire with the black tracer has been tapped into at some point and it runs toward the firewall but then I can't tell where it goes... Any idea what this might be for?

Also, what is the easiest way to set up a regular guage to check if my cs fuel pressure is correct?

 

 

I think its a band-aid fix for torque convertor unlock issues

 

More than likely binding the ground wire to the PCM I suggest you remove it and return to stock wiring and fix the real issue which is typically a bad alternator diode.

 

http://articles.mopar1973man.com/2nd-generation-24v-dodge-cummins/28-transmission-systems/260-torque-converter-lock-unlock-issues

 

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  • 1 month later...

Ok well I'm not sure if anyone is looking at this thread any more but here goes. I replaced the apps sensor and that fixed my code issue! Been running great ever since! Also, I installed a mechanical gauge and realized I was getting only 12psi... Just under 10 wot so I called fass and they shipped me a new spring and ball. Installed that this morning and now I'm at 24 psi and 21 wot... Should I be worried?

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Bit too high I would try for another spring 17 PSI.

 

As for the previous bogging problem you might still check the alternator out. I just talked to Pepsi71Ocean yesterday and found out his transmission and alternator issues were caused by a $10 set of alternator brushes. This created enough AC noise to measure 0.7 and 0.8 AC volts which was enough to create lock up issues and stalling issues.

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Very interesting mopar1973man, I am going to be in contact with fass for a different spring this week.

How would you recommend testing my alternator without a setup like in that video? Maybe start the truck, unplug the alternator, then shift into gear to see if the problem still exists?

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Very interesting mopar1973man, I am going to be in contact with fass for a different spring this week.

How would you recommend testing my alternator without a setup like in that video? Maybe start the truck, unplug the alternator, then shift into gear to see if the problem still exists?

You need to unplug the alternator then start your truck and take it out on the road to see what happens.

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  • 3 weeks later...

Well guys sorry for taking so long to reply. My wife and I just had our first baby last Friday so I've been pretty busy lately!

Anyway, I changed out the spring and that worked great!

Next I disconnected the alternator and my issue of shifting and bogging down went away! You guys are amazing with this knowledge, thanks again! Now I'm looking to change that alternator out, any suggestions? Or will one from auto zone do just fine?

My last issue was rough starts. I notice now if I bump the started b4 firing it up there is no issue so from what I've read it seems like I just need to wire my FASS into the power window fuse.

Thanks again to all and any help would be appreciated!

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CONGRATULATIONS!!!!!! To hell with the truck, take care of your family first!! I can remember whe my first was born. The nurse that was there put her in my arms and said "Now if she were a walleye, how much do you think she would weigh?". The doctor then took her and weighed her. She was 10 pounds 1 1/2 ounces and the doctor's record at that time! That was 25 1/2 years ago and I can still hear the nurse and doctor! You will remember that day for the rest of your life!

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Congrat's indeed and the more i read about 17+ PSI fuel preesure the more i want to get a new spring and ball from FASS for mine since mine is only a year or 2 old and is 7 when cranking 13 at idle 13 at running speed and 13 at WOT.... same as my uncles truck his is only a couple months old 7 cranking, 13, 13, 13 ....odd lol But my fass line's are really really dry rotted and has a small pin hole leak aswell but i can't get any info from them on new line to and return line for it i want a part's number for the spring, ball, and hose to and from but all they tell me is they have it kinda making me mad to a point. lol Glad you got your's all lined out

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Thanks a lot guys! It's been exciting but busy for sure!

MoparMan - so any stock standard output alternator for a truck with my vin should work as long as the ac output is below .1V? What would you recommend to fix my issue with rough starts other than wiring it into the power window fuse?

GreenRiver- ya man I called fass and they send out the parts ASAP. I changed it out but now I'm up around 21 psi so I need to do a little tweeting on the spring. Maybe try stretching yours 1/8 - 1/4"... That's that fass recommended me to do if this new spring was too much

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Ya mine looks a little different but I do know it's the return line that u need to remove. There should be a little "R" stamped on the body of the FASS unit next to the hose.

Also, when I did mine, the guy from FASS told me to remove both the filter and water separator but keep them filled with fluid. Then take the hose off, stretch the spring, put the ball and spring back, reinstall the water separator then the fuel filter, prime the system and check your pressure!

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