My scanner says the fuel pressure is 57-60 at idle, but the pressure dropped to 42 at the time of the DTC incident.

 

The P2635 sets when your lift pump pressure (actual) deviates more than 5.8 psi from lift pump pressure (desired) for more than 30 seconds. Not unheard of for them to go bad. If you have a way to either monitor or data log, I would watch the lift pump pressure, and see if it’s only doing it under medium/heavy acceleration, as well as how much it is dropping. IIRC, between 50-70 psi is considered spec, but it’s usually right around 60, idling or under load.


DTC P1089
Fuel Rail Pressure Performance During Deceleration Fuel Cut-Off

I’ve seen this come up multiple times as well. Usually what happens is when you take your foot off of the accelerator, the actual fuel rail pressure doesn’t drop quick enough, and sets a code. Monitor your actual vs desired rail pressure on deceleration, and see if that is the case. Actual will lag behind desired. The only thing that will fix that is a new driver side fuel rail. A new rail comes with a new pressure regulator as well.


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Thanks. I don’t have a way to watch real time. I can just see the data captured by my scanner at the time the code goes off. Could a cts2 do that? I was thinking about the fuel rail and/or fuel pressure regulator being bad as well because I read a thread about that. Is that common? How does the rail go bad? Or is it the regulator?

 

Can’t you watch in on that same scanner? Yes, a CTS2 would work to watch it.

It’s not common, but I’ve seen it a few times. It’s one of the things that sometimes the regulator fixes it, and sometimes it took a a new rail and regulator before the code went away.


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This scanner will log off as soon as vehicle speed is detected. It’s one that uses Bluetooth and I view the data on my phone. It’s great for the basics; clearing/viewing codes and seeing data at idle.

Fuel rail is on back order. I Think I might just take it in for a diagnosis. The $75 diagnostic may point to the problem. Then I can order the parts and fix it myself. I didn’t realize these motors were so un-shade tree mechanic friendly.

Thanks again for your time.

 

I noticed a couple more things this weekend. First, the drop in fuel pressure happens when in park, engine rev’ing. It dropped to 41-53 several times within a few minutes. Second, the truck doesn’t run poorly when the light comes on. It runs the same all the time. Plenty of power, and no drop in mpg. Lastly, I found this TSB reference on this site.

MAJOR ECM L5P engine Calibration released see below...

I have 3 of these issues going on, on my own personal truck. Hopefully it fixes them. #17-NA-171: Hesitation, Stalling, Engine Speed Flare, Unintended Accelerations Feel, Reduced Surge in Low Speed Maneuvers, Stumble, Loud Ticking Noise While Coasting Down, Harsh Tip-In/Tip-Out Shifting, Lack...

www.duramaxforum.com

I’m not sure if the flash has been done on my truck. I guess it’s time to make an appointment with

 

Now steering wheel controls for the cruise control and forward collision alert don’t operate those systems. The FCA is still on but the button doesn’t control its operation. I’m starting to think this may be a related electrical problem. Can someone send me digital copies of the wiring diagram for these trucks? Thanks.

 

Hello, all. I believe the truck is fixed, and P2635 code is no more. Since there aren’t any tutorials on lift pump repairs, I thought I’d document my repair in this thread.

Here’s what I did to fix the fuel flow performance low, P2635 DTC code. I first ordered a new lift pump assembly from Amazon; AC Delco part number 13465400.

Next, I chose to raise the driver’s side of the bed. I youtube’d a video about it. The towel is in place to protect the cab because the bed will contact the cab when you lift it.

The fuel lines clips come off easy. Just pry them up with a flat head screw driver. Remove the over flow hose. Next use a brass punch and hammer to remove the lock ring. The pump assembly comes right out.

The inlet on my pump was clogged with debris, which I’m certain cause the low flow problems.

The tank was clean, so the new pump went in with a new o-ring.

I reconnected everything, cleared the code, drove easy, moderate, and heavy for 50 miles. No code. Message me if you have questions.
Travis

 

Glad to see you found the culprit and fixed the problem. This reminds me of a trap door I once saw in the pickup bed to easily access the lift pump.

 

Thanks for updating. What kind of debris was clogging the inlet, any idea where it came from? Also, what’s the price point on the lift pump? Do you think the original lift pump would have been fine if you had just cleaned the inlet?


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It looks like some sort of organic matter like grass, or even hair. Hard to tell.

The pump cost $250. The original might have lasted, but I wasn’t taking any chances. I can’t hear this pump running over the engine in the truck. The original made a rhythmic whining noise. My guess is that it has been struggling to push fuel with that restriction for a while, and it can’t be good for its life expectancy.

 

Hey I’m having the same problems and I’m unable to find the same pump