Fuel Pressure Fluctuation

[B model]

I have a 06 cm870 500/1850 917k miles on it and it has recently developed a kind of miss when idling. I just checked and got a few bubbles out of the back 3, so it has a injector bad. Anyway my real question is, how stable should my fuel pressure be? It fluctuates so much at idle you cant even read the gauge, and at about 875 rpm it will kind stabilize enough to read about 250 psi. As for cranking pressure it just goes up instantly and starts when it gets close to 100. Is it supposed to show a even pressure or is this normal. I guess it should also be noted this truck has never had valve cover off or actuators off or any of the normal stuff until now. My mechanic assured me that you never needed to set overhead unless the cam or injector went bad. But after reading on here I decided to get in there and have a look.

[Lonestar10]

takes about 100psi cranking for a 870/871 to start and should run between 250-280 at idle and 270-300 under throttle.

[B model]

I was more asking if the needle fluctuating so fast you cant read it was normal? It only settles down enough to read at about 875-900. Then starts fluctuating again and will kind of slow down fluttering at about 1600 again. But I get no steady readings. Is this normal?

[Rawze]

(05-31-2020 )B model Wrote:  ... My mechanic assured me that you never needed to set overhead unless the cam or injector went bad.
...

FIRE YOUR MECHANIC!!! - NOT ADJUSTING THE OVERHEAD VALVES OFTEN LEADS DIRECTLY TO A WIPED OUT CAM!!!

Cummins says 500k maximum, but realisticly, it should be done every 250k miles (or equivalent hours) if you value the life of your cams and engine.


* The injectors leaking. They get leaky if no one adjusts the overhead valves and injeoctr rockers often enough. - Always do an overhead adjustment Both valves and injectors before doing an injector leak test using the test block. This is because the injector rockers back off (get loose from wear) over time and the needles do not seat 100% closed, giving a false reading as a leaky injector if they are out of adjustment.

* The exact opposite happens on the valve cam side rockers. They get tighter over time due to valve seat wear and the rockers start riding the cvam all the time and wipe it out. - 250k adjustment intervals are your friend towards preventing wiped cams.

=================
Fluctuating idle will be usually one or more of ...

* Leaky injectors.

* Air in fuel system.

* Worn our metering (fuelling) actuators where they have developed slight step-wear in the shanks of the plungers (very common). ref: http://rawze.com/forums/showthread.php?t...619#pid619

* worn out head, leaky valve seats and guides.

==================
Wiped out cams...

* NOT adjusting your overhead valves often enough.
* Not changing oil often enough (oil changes should be based on fuel mileage, not any set interval). - related info: http://rawze.com/forums/showthread.php?t...4#pid48014

* Not keeping after your EGR and other systems (regular EGR tune-up, etc.) , the result of which is excess soot in the oil all the time.

[B model]

I didnt really have to, just got tired of waiting on him. So I am starting on my own path of fixing it myself, btw my cams looked great for 900k. But I change my oil at 10k and I dont pick my oil based on advertising. Any thoughts on the fuel pressure thing??

[Rawze]

(05-31-2020 )B model Wrote:  ... Any thoughts on the fuel pressure thing??

the fuel pressure will vary at idle with rpm. Solve the rpm fluctuation issue and you will likely see a lot less variation.

[Rawze]

(05-31-2020 )B model Wrote:  ...
917k miles on it and it has recently developed a kind of miss when idling.
...

My engine at 985k was loping at idle a bit, especially when cold. My head was worn out. - When i did the inframe, I literally replaced no components in the top end except the head itself. - i re-used all of the million mile parts, rockers, cams, injectors, all the IFSM components,,, everything. - The engine ran smooth as a button with no lope, no misses when cold,.. like a new engine again. - In my case, all the lope, miss, etc. was simply the worn out head (non-lubricated parts like valve seats, guides, etc).

You having 900+k on the head, it could be exactly the same thing. - Personally, i would not worry about it if it runs good, given that many miles on it unless you find worn out injectors or actuators.

-- Next Q. would be -- Did you do the rod and main bearings at 800k? -- if not then in a year or 2 more, if it makes it much past the 1 m,il mark, it will be a ticking time bomb ready to send a piston thru the block with zero warning as a wrist pin lets go because no one did the rod/main bearings when they were due. - just saying.

[B model]

Well after a bit of digging around on the internet, cummins parts says my head is not available. They also didnt have any supersecsion info. I find that hard to believe, but was able to find what i believe to be off brand heads?? Also did a leakdown and did have 1 leaky injector. I am planning on taking apart the actuator to inspect also. The injectors had about 50 inch/lbs. preload on them and the valves felt like they were pretty close, but could tell they had a little bit more movement after adjusted. And there hasnt been any egr work done for 500k miles. Its been unplugged. Also it has good oil pressure and the oil is clean uses no water or oil and very little blowby.

[Rawze]

(05-31-2020 )B model Wrote:  ... cummins parts says my head is not available.
.. off brand heads??
...

as far as the head goes, when its ready for an inframe ...
call around. they are out there, I see them get ordered every day around here.

ONLY USE OEM certified HEAD!!! - OR YOU WILL REGRET IT IN ABOUT 2 OR 3 YEARS AFTER REPLACING IT! -- SEEN IT A HUNDRED TIMES OR MORE!