Cm870 random stumble
04-26-2022, (Subject: Cm870 random stumble ) 
Post: #1
Cm870 random stumble
My 870 has a random thump out of the exhaust that it will do every so often. It may do it under load or steady throttle. Have one fuel dampener that is seepy so ifsm will be comeing off for a rebuild but while it’s in the shop I would like to figure this out. I don’t believe it’s any air in fuel but will test that first.

My theory is a metering or timing acctuator is randomly hanging from a worn step in a plunger. As just one simple thump and that’s it, no stumble or running rough under load.
I will say that idle isn’t quite as smooth is I would like.
Anyone have similar symptoms?
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04-27-2022, (Subject: Cm870 random stumble ) 
Post: #2
RE: Cm870 random stumble
It can be a whole lot of things.. including even the head and/or valve train itself wearing out.

Inspecting the overhead cams, doing an overhead valve adjustment, etc. is one place to look too.


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 Thanks given by: Precisionw900
04-27-2022, (Subject: Cm870 random stumble ) 
Post: #3
RE: Cm870 random stumble
(04-27-2022 )Rawze Wrote:  It can be a whole lot of things.. including even the head and/or valve train itself wearing out.

Inspecting the overhead cams, doing an overhead valve adjustment, etc. is one place to look too.

Thanks Rawze. 398k miles and overhead just recently run and everything inspected. Injector leak and cutout test ran within 8k miles but plan on running test again to be sure nothing has changed. New hoses from tank to lift pump and Lift pump doesn’t cavitate but a air leak beyond that is always a possibility.

Another concern or symptom is a shortly after start up the truck will be idling and will jump up 50-75 rpm. and stay there. Is this normal? I recently read on another thread about a engine that would do that and I believe you stated the only reason for that is it’s getting fuel it shouldn’t be and pointed towards metering actuators.

Hopefully I will begin trouble shooting tonight.
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04-27-2022, (Subject: Cm870 random stumble ) 
Post: #4
RE: Cm870 random stumble
(04-27-2022 )Precisionw900 Wrote:  
(04-27-2022 )Rawze Wrote:  It can be a whole lot of things.. including even the head and/or valve train itself wearing out.

Inspecting the overhead cams, doing an overhead valve adjustment, etc. is one place to look too.

Thanks Rawze. 398k miles and overhead just recently run and everything inspected. Injector leak and cutout test ran within 8k miles but plan on running test again to be sure nothing has changed. New hoses from tank to lift pump and Lift pump doesn’t cavitate but a air leak beyond that is always a possibility.

Another concern or symptom is a shortly after start up the truck will be idling and will jump up 50-75 rpm. and stay there. Is this normal? I recently read on another thread about a engine that would do that and I believe you stated the only reason for that is it’s getting fuel it shouldn’t be and pointed towards metering actuators.

Hopefully I will begin trouble shooting tonight.

yes... you should do the things/tests talked about in this thread...
ref: http://rawze.com/forums/showthread.php?t...4#pid71234


User's Signature: ->: What I post is just my own thoughts and Opinions! --- I AM Full Of S__T!.
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 Thanks given by: Precisionw900
04-28-2022, (Subject: Cm870 random stumble ) 
Post: #5
RE: Cm870 random stumble
So I got some time to confirm injectors with a leak down test, tested good. Pulled the metering actuators and checked side play with dial indicator and have anywhere from 3.5 thou to 5.5 thou play and they will stick with any small amount of side pressure.
Since I’m sure I will be replacing them now and Out of curiosity I decided to throw the plungers in the lathe and polish the stem of plungers with fine scotch brite. After polish and a good cleaning upon reassembly I noticed a difference in slide fit but still not perfect. ReInstalled all actuators in original positions and fired up, it does idle noticeably better but still not perfectly smooth but the random idle up remains.

Looks like all actuators will be ordered . Ouch! 750$ a pop for new as I refuse to buy reman.
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04-28-2022, (Subject: Cm870 random stumble ) 
Post: #6
RE: Cm870 random stumble
(04-28-2022 )Precisionw900 Wrote:  So I got some time to confirm injectors with a leak down test, tested good. ...
it being a CM870..
was it the bubble test that you did, with the test plate?.

just making sure... as it is not really considered a 'leak-down test' for the older engines. It is more like an injector leakage bubble test instead. It has to be done mechanically, using a proper block plate where the actuators mount and checking for bubbles in a glass of water while cranking the engine with the ecm power connector unplugged, and not by using the Insite software.

ref: http://rawze.com/forums/showthread.php?t...8#pid67948

(just confirming, thats all).


User's Signature: ->: What I post is just my own thoughts and Opinions! --- I AM Full Of S__T!.
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04-28-2022, (Subject: Cm870 random stumble ) 
Post: #7
RE: Cm870 random stumble
(04-28-2022 )Rawze Wrote:  
(04-28-2022 )Precisionw900 Wrote:  So I got some time to confirm injectors with a leak down test, tested good. ...
it being a CM870..
was it the bubble test ...
just making sure...

Sorry bout that, wrong terminology on my part.

Yes, I performed a bubble test with the block plate in fueling actuator position on both banks. Both passed.

New compucheck fitting and gauge just came in tonight so will also check cranking and running fuel pressures tomorrow as I would like to give entire fuel system a once over.
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