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P129F P2635 Codes 2018 3500HD L5P Duramax

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2.9K views 6 replies 3 participants last post by  reido113  
#1 · (Edited)
2018 L5P GMC 3500HD Denali CrewCab Dually Long Bed 68,889 miles.

While 1300 miles away from Home Base and a couple turns after exiting the Interstate, the check engine light illuminated. I was able to view DTC with my Edge CTS3 Monitor. Codes are:

P129F - Fuel Pump Driver Control Module Fuel Pump Speed Signal Incorrect

P2635 - Fuel Pump “A” Flow Performance

I am creating this thread to document my experiences.

Currently my status is:
Out Of Service
Engine will Start
Idle is rough

Everything seemed to be running fine. After exiting the Interstate. Stopping, turning, driving a mile or two, stopping, turning - The Check Engine light came on.
Shortly afterwards (~2 miles) I thought I felt a slight skip. Almost felt like a jerk. Then about 10 miles later the feeling was confirmed as it felt a little heavier this time and I was ready for it.
~5 miles later after stopping. I noticed this skip during early acceleration. Now that I was honed in on this, I could notice a slight difference in performance.
I only had ~15 more miles to go. The few times that I had to accelerate from a stop, I would get the skip/sputter - Once I felt the skip/spitter while going ~25 mph and depressing accelerator.

Upon arrival at my destination the engine was running rough at idle speed. When shifting from Reverse to Drive the engine shutoff. After engine shutoff, it would not restart. It would only crank.

After ~ 30 minutes of being shutdown, the engine started, but the idle was rough.
It has only been started for a moment to review codes since. Idle remains rough.

I have found info on the forum which points me in the Fuel Pump Control Module direction and I have a few more thoughts to expand on in later additions to the thread.

Currently I have located and ordered Fuel Pump Control Module part #13558813 which seems to have replaced the discontinued part #13540026. It has just arrived via FedEx.

After removing my spare tire and locating the part I noticed part #13518065 which I have found no information on via the web as of yet. I do see some non GM parts on Amazon that have that number and ordered one just for the heck of it right now.

*A little over a month ago I did the unthinkable and let a Valvoline Oil Change do the oil change because I was very short on time. I said what the heck and asked them to change the fuel filter as well. I normally do these myself. After a long period of time they said they were sorry they could not get the fuel filter cap off. (I currently have a VSE Fuel Saver on order for ~6 weeks now).
I mention this because I don’t put it passed them to have damaged the filter housing at some capacity. So, I am picking up a new OEM Fuel filter and Filter/water separator housing today and will replace that just to be safe. *Someday the folks at VSE will update me and send the real replacement!

Anyways… Time for me to stop typing and get to work. I look forward to following and updating this thread. Cheers!
 
#2 ·
Update 1

Today I replaced the entire Fuel Filter and Water Separator with AC Delco Part TP1014.

The filter had 30% life remaining (for whatever that is worth) and was the typical dirty color. I did notice a very small amount of dust like debris in the bottom of the filter housing. I poured the remaining fuel and debris into a small clear plastic container. Placing a small magnet on the bottom of the container did attract some of the very small particles. This scared me into thinking I might have the dreaded High Pressure Injector Pump issue that I’ve heard about where the pumps cam sheds metal shavings throughout the entire fuel system. Fortunately some quick reassurance came to the rescue as I re verified that the L5P has the new Denzo HP4 and not the Bosch CP4 which was a big culprit for this. Whew! But no lie, I was scared! Anyways…

I primed the fuel system by turning the key on for 30 seconds. Fuel Pump did make the usual gulping sounds at first but ran solid and pressurized the system. I started the engine. The Check Engine light remained on, but the vehicle did seem to have a better idle. I turned the vehicle off.

Next I went under the spare tire to the location of the Fuel Pump Control Module. I simply unplugged the OEM Part #13518065 by unlocking the red tab and raising the harness release lever. I immediately plugged in the new OEM Part #13558813.
I jumped in the truck and started the Engine as normal. The check engine light was no longer illuminated! I held my breath and counted to 120 in hopes that the vehicle would not shut down thus leading me to think it would need programmed into the ECM by Service. It did not shutoff. Engine idle was great.
The DTC’s were still in the memory but I was able to clear them with the Edge CTS3.

I went back under the spare tire area and removed the old part by removing the two 10mm nuts and mounted the new part in its place.

I restarted multiple times and took the vehicle for a 5 mile ride before it got dark. All seems normal again.

Talk about timing, Brady over at VSE sent me an email that a Black Anodized Fuel System Saver was in stock now and he could ship immediately if I wanted. He quickly changed my original shipping address to my current location of breakdown and overnighted the Fuel System Saver and Filters to me! Super cool gesture. Tomorrow I will get that new OEM Fuel Filter and Water Seperarator off and replaced with the new VSE Fuel System Saver. *The fact that filtration alone will be greatly increased by this eases my mind. All the other benefits excite me.
 
#4 ·
Today I replaced the Stock Fuel Water Separator and Filter with the VSE Fuel System Saver that arrived this morning. After the install I ran the truck for a test drive and Whoa! I was not expecting the throttle response I was suddenly getting. Not only is the truck running fine with the engine codes cleared. The performance has noticeably improved. Especially at lower throttle positions.

So, all and all, the Replacement of the Fuel Pump Control Module seems to be the fix for these DTC’s. Lots of other threads and info have said that the dealer would have to program this Module into the ECM. I did see others that said it does not need programmed.

In my experience, this part was plug and play. It immediately fixed my Check Engine light and DTC. I am back in Service and I now carry a new spare Fuel Pump Control Module with me.

Did I mention that I Love this VSE Fuel System Saver! Much better filtration thank stock and gave me a performance boost. Knowing that this is also helping prevent me from other issues surrounding stock filtration is a nice piece of mind.


Cheers!
 
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