Fuel system work - RPM Drop

[Porterave]

Engine DD15, 2 Filters fuel system, Odometer: 714 273 miles, Engine hours: 17115, No deletes

After working on the fuel system the truck has an rpm drop after 1min to 1min 30sec on a cold start. A cold start, meaning truck is parked for more then 2h to 4h. Also this only happens on high idle, for example when I set it to high idle to build up air quicker on cold starts. If I turn the engine on and leave it as is on low idle, it won't drop rpm's.

No fault codes related. When driving no issues. No cranking issues truck starts right away. Only happens once on a cold start cycle.

I am attaching the Diagnostic Link Software graphs of the rpm drop and corresponding rail pressure drop, desired rail pressure and low pressure side readings.

Work done on fuel system, at first:
-Water separator filter
-Primary and Secondary filter
-Leaking Injector fuel lines 5 and 6 replaced
-2 Rail feeding lines replaced (As per manual when replacing injector fuel line 6)
-Banjo bolt washers and banjo bolt (As per manual when replacing injector fuel line 6)
-Reprimed system using manual prime pump only, started right away no issues.

Work done troubleshooting rpm drop on high idle:
-Replaced water separator dome, vent cap, collar
-Replaced primary filter cap
-PLV (pressure limiting valve) open counter found >50 (95) and replaced, didn't solve the issue
-Replaced again O-rings of 2 filter caps and O-rings of water separator dome and vent cap.
-Replaced again the 2 filters and water separator filter
-Re-torqued rail feeding lines and injector fuel feeding lines (not replaced)
-Fuel system integrity test with Diagnostic Link software is successful, no leaks detected
-Rail pressure bleed down when engine off is in specs, 1 min +- to bleed down to 9 bars default.
-Low pressure values in specs, minor drop when rpm dip - it is found on the graphs.
-Repriming with the hand pump on a cold start didn't prevent the rpm dip.


I am focusing on what I disturbed when working on my fuel lines and filters, hoping that I didn't inadvertently damage anything else. It first seemed to me an air ingest on the low pressure side, not sure now. I'm wondering if this is a low pressure side or high pressure side issue?
Can the injector fuel lines or rail feeding lines cause this issue, should I replace them again?
I still didn't replace the high pressure side filter cap, should I?

Any suggestion would be welcome.

Thanks for all of this.

[irontom10]

What year is the engine?

I remember some of the older Detroit engines had an issue where, if the fuel system was not primed through a specific quick-disconnect fitting after service, the fuel pump could be damaged or destroyed from running dry.

[Porterave]

(06-12-2026 )irontom10 Wrote:  What year is the engine?

I remember some of the older Detroit engines had an issue where, if the fuel system was not primed through a specific quick-disconnect fitting after service, the fuel pump could be damaged or destroyed from running dry.

It is a 2016 Freightliner Cascadia. It is a one time thing on startup. No loss of power when driving, it is really precise always 1 min to 1 min 30 sec only one dip. No fault codes.

[Rawze]

there is also some related info...
https://rawze.com/forums/showthread.php?...9#pid12489

hopefully it did not spin a bearing.

[Porterave]

(06-13-2026 )Rawze Wrote:  there is also some related info...
https://rawze.com/forums/showthread.php?...9#pid12489

hopefully it did not spin a bearing.

After your comment I checked the prefilter for metal shavings. No metal shavings but I found dark brown spots in the prefilter that accumulated in just 2 weeks of changing it, is that possibly related to a faulty fuel pump? (See picture)

Also could a injector fuel line not properly seated causes a RPM dip after a 1 min to 1 min 30 sec? Is it worth replacing them to reseat them?

[Rawze]

also do a full egr system tune-up on it.. for all anyone knows, it could be surging due to poor egr gas flow / clogging sensors.

[mikkhh]

Not related to your problem, but since the topic of egr tuneup came up.

Keep a close eye on dpf differential pressure readings. Aftertreatment Control Modules located on the cans go bad on them quite often, first symptom is diff pressure faults. After that usually diff pressure sensor is replaced (or somebody decides to "deman" the thing and then not long after acm itself dies, but that "deman" is joke which requires acm to be connected, unlike the EURO3 conversion which is meant for high sulphur diesel countries), also wiring, then dpf itself. And still faults no fix no avail, until there are no communications with acm module.

Also egr coolers do crack on them and cause leaks into engine, again just a thing to keep in mind when you have a coolant loss, on those I would not start from oil pan pressurizing when looking for internal coolant loss.

Spun bearings wise DD13 15 engines have hard material two metal bearings (aluminium-steel), lots of others have tri metal (lead/babitt - copper - steel), that wear quicker, but are more forgiving for dry starts etc. Same exact story is on Mercedes cars, since DD is owned by daimler since 2000s lots of design things carried over, mercedes cars also have bearings that are very hard and do spin, even mains. Just doing long life oil change intervals and having abrasive soot form up in oil galleys can spun mains on benz cars, does not take much. I have measured crank runout on ones that got locked up after couple minutes of idling, all checked out fine.

I have also heard about plastic oil pickup tubes cracking (where it is ultrasonically welded together) on DD13 and DD15, therefore creating catastrophic damage.