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Bigmart, Service Manager
Category: Heavy Equipment
Satisfied Customers: 81
Experience:  35 Years working on Class 8 Trucks and Construction Equipment
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I have an ISX cm870 And it has a miss, after a cylinder performance

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I have an ISX cm870 And it has a miss, after a cylinder performance test cylinder 1 reads high and cylinder 5 reads low, what is wrong?
HelloCustomer What service tool are you using to get performance test results? Next would be to do cylinder cut out to isolate the cylinder thats missing, you may have a bad injector, to compensate for one cylinder missing, the ECM will fuel other cylinders to maintaine a certain level of performance, based on driver input and load on engine-Hope this helps you, if you need more info, just ask.
Customer: replied 8 years ago.

I used Cummins Insite for the cylinder performance test. I ran the cylinder cutout test and it come back as cylinder 1 low and cylinder 3 high. My mistake it was 3 high not cylinder 5, sorry about that. It shows cylinder 1 @ 84% and cylinder 3 @ 132% out put.

What should I do now?

ok, how bad does it miss and whats your mileage?
Customer: replied 8 years ago.

the miss is 84% on cylinder one, cylinder 3 is 132% the current milage is 445,673 I don't understand why the millage would have anything to do with an engine miss? An engine miss can happen at any milage. I have had engine misses before on my other truck with only 25,000 miles. Any help is greatfull

as for the mileage were looking to see if your due for top end set, usualy when you do performance test the results are cylinder output rather than cylinder miss, therfore 84% could simply mean that its out of adjustment, I would do cut out test manualy, verify which cylinder is weak, than check over head settings on that cylinder, if there within spec. then proceed from there, pull injector(s) and inspect, if you have accsess to a borescope check the cylinders at the same time. Good luck
Customer: replied 8 years ago.

None of these answers are correct. One, the ECM does not compensate for a missing cylinder, that statement is false! The problem is the number 1 cylinder injector is not firing! Due to the fact that a ISX CM870 uses only 4 solenoids for all 6 injectors 2 metering and two timing. the front bank 1,2,3 and the rear bank 4,5,6 . When the 1 injector doesn't fire the fuel it is trapped in the fuel rail so the next injector in firing order in that bank gets the un injected fuel causing the 3 injector to have a high output.


Adjustment also would not cause a output of lower then 90% due to the fact that ISX injectors are adjusted at botton stop, or outer base circle and at normal ware all the injectors would somewhat ware the same and wouldn't cause a 84% 132% split.


I just stumbled on to this site and wanted to see how the "experts" would answer something a little harder.


Well I already had to spend the 15 dollars to check out how this thing works so here is an easy one so you can have the money.


My truck has slowly lost power over a few weeks, it shows no other signs beside a loss of power, No check engine lights and no fault codes. What is my problem?


Hello Querty and welcome to JA. Just so you don't think this site is completely bad, I am going to jump here and work with you on this.


You seem to know much about what's going on with your engine, but I do have to clarify a few things. First of all, the cylinder performance test can not verify a dead injector since it only measures the time it takes for the piston to make its travel, there is no such sensor within the cylinders to measure the amount of explosion or force, or even temperature of the burn. Cylinder 1 is 16% slower the rest, but this does not necessarily mean the injector is dead. In a diesel engine there are more factors involved in the power stroke. One of them, and most important is air/fuel ratio. This is accomplished by the amount of air and temperature of the air that is let into the cylinder vs. the amount of fuel injected into the cylinder. If you indeed have an intake valve that is not adjusted properly this can affect the air/fuel ratio thus causing a cylinder to be stronger or weaker than the others. This is why step 5C-1 of the Cummins guided diagnostic tree requires that you check you overhead adjustment. The only absolute way to isolate a bad injector is to perform a cylinder cut-out test. An ISX engine will run on 1 cylinder at a time and still maintain the RPM. If you cut-out 2-6 and the engine is still running at 700 RMP (or whatever speed is preset for the test) than this would mean that 1 is still firing.

But, to throw another wrench into it, an injector may be working but not at full capacity. Injectors will wear unevenly due to the fact that they consist of moving parts at close tolerances. Change this tolerance only slightly and you can have an injector that does not perform as the rest. Heat and also the machining of the individual parts can have a factor in their life. In my many years as a mechanic I have seen many times when only 1 injector will fail at a time. In regards XXXXX XXXXX adjustment of an injector, there are many factors that can change this. The spring becomes weak, the roller wears slightly, the cam wears slightly. I have seen cams where only one lobe is worn and all the rest are good. This can only be found by checking the adjustment of the injector.


In regards XXXXX XXXXX fuel being trapped, this is not true, the fuel in the rail is maintaining a pressure of 230 - 274 psi(w/EGR) and 225-274 (wo/EGR) at an idle, and the amount the injectors use will not affect this. Fuel is circulated through the rail in a continuous motion and is controlled by the Pressure Regulator.


Back to the original post, I assume you have a CM870 and not a CM875? Although they do have a CM871. Can you give me the S/N and the current ECM calibration code? There are several factors that can cause low power, but since you say it has had a quick drop, over a short period, we can eliminate a few of them.

The ones that jump out first would be a leak in your charge air circuit, turbo going out, and a clogged air or fuel filters. I assume you have checked these, correct? What is the measure fuel pressure? Not the pressure that is indicated in the ECM as a bad sensor can give a false reading, but the actual fuel pressure measured by a mechanical gauge. Now if all these check out, we can get into checking other more technical problems that could cause low power.


Thanks for coming to JA and I hope we can work together to solve your problem.


Bigmart, Service Manager
Category: Heavy Equipment
Satisfied Customers: 81
Experience: 35 Years working on Class 8 Trucks and Construction Equipment
Bigmart and other Heavy Equipment Specialists are ready to help you
Customer: replied 8 years ago.

Big mart,


Sorry I meant fuel rifle not rail!

Fuel is trapped in the fuel rifle between the metering solenoid and the injectors of the front or rear bank! And yes The ECM does in fact calculate the amount of force from each and every combustion cycle. If you know what amount of force it takes to spin a crankshaft of known weight to a certain speed then with a lot of math you will know exactly how much out put the cylinder is putting out! The ECM uses algorithms stored in the ECM, inputs from various sensors and commanded out put out to metering and timing actuators and calculates the exact amount of force.


So where is it your employed?

Yes, it is a calculation, but not a true or shall I say a physical means of measuring the output of an injector.

Sorry my reply was cut short, I had to tend to a minor family problem.


Yes, you are correct and very knowledgeable on this. I didn't go into to detail with you because most of our customers don't know what an algorithm is yet alone mathematics.


But I still have to maintain my answer regarding the trapped fuel. It is not possible for it to be used to increase the output of another cylinder particularly if it is of a different bank of injectors, and it is still a regulated pressure, and it can not be increased.


As far as where I work, I really can not divulge that information as it may conflict with my position and besides that, I may not be there very much longer. I am looking for a more stable place of employment.




Customer: replied 8 years ago.
And so is horse power! Just a calculation
Customer: replied 8 years ago.
I ment like a dealership? Factory tech? Own your own shop? I realy don't own a truck, I'm a factory trained Tech. But I got tired of making everyone else's businesses grow so I opened my own! One thing lead to an other, now I sale software.

Interesting, tell me more!


[email protected]




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