CM 871 getting 3.6 MPG
01-30-2021, (Subject: CM 871 getting 3.6 MPG ) 
Post: #1
CM 871 getting 3.6 MPG
Picked up a 2007 Volvo VT800 in July. Has an ISX CM 871 500st stock motor and after treatment with 200,000 miles on it. I did a few things with the EGR system right away.
1. Baked the entire DPF for around $400. Paperwork showed a flow of 1.0 afterwards.
2. Replaced all sensors on Filter.
3. Replaced- differential pressure sensor on the horn. Imap sensor. 4928594 pressure sensor.
4.Pulled off and cleaned out the horn.
5.Pulled off and cleaned out the 7th injector.

The motor does have to work though. Its hooked to an 18 speed with 3.73 ratio rears and grosses about 117,000 with every load. No real hills. She can still average about 60 mph per trip. She wants to REGEN about every 230 miles. Oil temp stays around 210. She blows white puffs at a steady rhythm in the beginning of the regen and then slowly clears up until the regen is done. Regen takes about 40 minutes on the road.

I don't hear any air from the charged air cooler. As I am writing this I realize I should get it professionally checked. Assuming it is not the charged air cooler what steps do I need to take to get the MPG up? I know I pull heavy but 3.6 mpg is a killer and I feel something isn't right.

Sorry to be so long winded but I got to figure this out.
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01-30-2021, (Subject: CM 871 getting 3.6 MPG ) 
Post: #2
RE: CM 871 getting 3.6 MPG
Things someone can do to help eliminate fuel mileage problems after doing the normal stuffs...

* Replacing the Exhaust gas pressure sensor if it is more than 400k miles old, and ensuring its feed-line/circuit is not plugged up (this is common).

* Emnsuring the Altitude (Ambient pressure) sensor is reading correctly.

* Pressurize the entire engine with 30 psi of air using block-plates at both ends of the turbocharger. Check for boost, egr, and exhaust manifold leaks. You would be surprised the unusual places you can loose boost and fuel mileage, not just in the CAC.

* Check to ensure the Doser injector is not bleeding fuel all the time because it is leaky. Remove the Doser-pipe off the back of the turbo and crank the truck. See if the injector tip os getting wet with fuel. - Clean the tip well while your at it.

* Fuel line leaks that are hard to find. - Look for wet, oily spots under the truck or places that accumulate a lot of extra dirt on them.

* Adjust the overhead valves properly, inspect the cams to ensure they are not chewed up, and then perform an injector leak test. Leaky injector or out of adjustment valves cause all sorts of issues as well as can be fuel mileage problems.

* check the metering and fueling actuators so ensure they are not worn out. ref: http://rawze.com/forums/showthread.php?t...619#pid619


* ENSURE YOU HAVE THE RIGHT PROGRAM FOR THE WHOLE TRUCK and not just the engine. If someone changed the HP settings using a different program in the engine, this can be a big problem.

* Check for a slipped valve or injector cam ... may have bad cam sensor or crank position sensor, and all sorts of other places to look too.

* Wrong injectors installed into the engine vs the programming. Seen that too.

* verify the fuel mileage in the dash vs. what is measured at the pump. - if they are way off, then the fuel is not being accounted for in the ecm and this is bad. .. it has to be going somewhere if not onto the cylinders or Doser injector.

* turbo out of calibration or packed with soot.

* Intake or CAC restrictions or an aftermarket CAC or EGR cooler unit has been installed onto the truck... Doing Either of these are terrible choices.


maybe others can chime in as well.


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: hookliftpete , Signature620 , Pd6cas2
01-31-2021, (Subject: CM 871 getting 3.6 MPG ) 
Post: #3
RE: CM 871 getting 3.6 MPG
Make sure all tire pressures on the whole unit are up to snuff. With that weight your hauling id have every last one of them at the tires rated psi limit. That what I run on my truck with absolutely no ill effects. And i run FAK at all load ranges.

Check the air filters

How are your driving habits. Are you like 90% of drivers?. Hammering through the gears because you think its a race not a business? This one murders most drivers MPG. Slow down. 60 mph average sounds like your pushing it hard to keep it up there. Ease up and work with the terrain more and not against it. Average speed shouldn't matter very much to someone chasing MPG.
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 Thanks given by: Waterloo , Pd6cas2
01-31-2021, (Subject: CM 871 getting 3.6 MPG ) 
Post: #4
RE: CM 871 getting 3.6 MPG
Another thing that comes to mind is compression loss in the engine. Might not be a bad idea to loosen up all the rockers so that all the valves are closed and then pressurize the intake manifold to about 30 psi. See if the intake holds pressure and does not drop off. Listen for air flowing into a cylinder with all the valves closed. - Then do the same on the exhaust side. There should not be any pressure drop for several minutes (on either side) on a good head.


User's Signature: ->: What I post is just my own thoughts and Opinions! --- I AM Full Of S__T!.
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01-31-2021, (Subject: CM 871 getting 3.6 MPG ) 
Post: #5
RE: CM 871 getting 3.6 MPG
Sounds like the constant regens are killing your mpg, which is already lower due to heavy loads. So the question is why are you having so many regens? Here's my thoughts based on your description and having been through this myself. I would go down this list, checking each in order.

* Excessive soot production from the engine leading to the need for more frequent regens. This one is easy to check, just disconnect the exhaust pipe from the back of the turbo and observe the exhaust. Even if it isn't noticably sooty hold up a piece of white paper behind the exhaust coming out of the turbo and see if any oil or fuel residue collects on the paper. If so then this is also what's collecting on the DOC and DPF. Causing exhaust restriction and poor oxidation, leading to more frequent regens and the white smoke at the start of a regen.

* Exhaust leaks. This leads to inaccurate sensor readings which screw everything up. From the back of the turbo to the end of the DPF it all needs to be free of exhaust leaks. Could be the cause of the white smoke at start of regen. Could also lead to unneeded regens due to inaccurate sensor readings.

* Leaky DPF injector, bad/leaky purge air actuator. Some extra fuel leaks or drips into the exhaust when regens are NOT being performed. This coats the DOC and maybe the DPF. Simultaneously reduces the oxidation ability of the DOC while also raising the exhaust restriction. This residue smoldering while regen temps are being raised could cause the white smoke at the start of a regen. Also reduces the longevity of the DOC and DPF.

* DOC and DPF are shot. For whatever reasons the DOC and DPF have reached the end of their functional life and need to be replaced. The white smoke at the start of the regen is a good sign that the DOC isn't properly oxidizing the exhaust and the injected fuel is not burning properly until temps have risen enough. It starts small but after time the smoke will get worse and worse and worse. Eventually active regens won't ever get up to the right temp to burn clean. Then parked regens will be needed. All the while further reducing the remaining life of the DOC.

* Sticky EGR valve. Last on my list, but any irregularities in EGR gas volume can lead to improper fuel/air mixture in cylinders leading to poor combustion, extra soot production and exhaust temps that vary from what the ECM expects. Could be the cause of the white smoke at start of regen. Could be causing excess soot which clogs up the DPF (and maybe starts to faceplug the DOC) faster then normal. *** This could be a cause of excessive soot production, but is fairly expensive to replace so make sure there isn't a different cause first ***



If you only see a few puffs of white smoke right now then it isn't bad, but IT WILL continue to get worse. Eventually the DOC and DPF will need to be replaced, regardless what the underlying cause was. It took my tractor about a year and a half (about 120,000mi) from the first white smoke signs to complete inability to regen. I tried many things in the process and the above info is built upon my hindsight. I was averaging 5-6mpg while I had these issues, but I typically ran on the light side. Sometimes I would have all day long regen attempts while driving. Once I rid my truck of this system the fuel mileage averaged mid 7's. Up and down depending on my day to day driving habits. So yeah, those regens are absolutely killing your mpg!


User's Signature: "...And as we wind on down the road, Our Shadows taller than our Soul..."
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 Thanks given by: Steel Dragon09
02-02-2021, (Subject: CM 871 getting 3.6 MPG ) 
Post: #6
RE: CM 871 getting 3.6 MPG
Okay. I am going to start with the Charge Air Cooler this weekend. Going to get it tested for leaks. I will post whatever I find. Thanks again.
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02-03-2021, (Subject: CM 871 getting 3.6 MPG ) 
Post: #7
RE: CM 871 getting 3.6 MPG
(02-02-2021 )Steel Dragon09 Wrote:  Okay. I am going to start with the Charge Air Cooler this weekend. Going to get it tested for leaks. I will post whatever I find. Thanks again.

Test the entire engine including the cac all at the same time. You need to block off both sides of the turbo and put 30 psi in through the turbo inlet temp sensor port.

Or get one of these for the clean side

https://www.ebay.com/itm/VOLVO-TRUCK-Tur...890.l49292

And maybe one of these for the dirty side

https://amzn.to/3tqxDOj

This one is on Rawze's Amazon page so buy it through his link at the top of the home page if you go this route.


User's Signature: Warranty??? Yeah right, I am the warranty!!!
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02-06-2021, (Subject: CM 871 getting 3.6 MPG ) 
Post: #8
RE: CM 871 getting 3.6 MPG
My CAC was leaking in 2 spots. The clamp over the hot side flange and also the top corner also on the hot side. So fingers crossed this will solve my problem. I took a thermal imaging video with my phones thermal camera during my regen this morning. Just curious to see if there will be a difference with the new CAC. I posted it here so anyone can look at it. Also if anyone sees anything wrong with those temperatures let me know. Or if you just like looking at thermal images thats cool too.


CM 871 Parked regen thermal imaging video
https://youtu.be/pa-OkHtD5DQ
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 Thanks given by: JimT
06-18-2022, (Subject: CM 871 getting 3.6 MPG ) 
Post: #9
RE: CM 871 getting 3.6 MPG
Turns out #6 exhaust manifold gasket had small leak. The one behind the heat shield that makes it easy to spot (sarcasm). I changed the entire manifold and now get between 4 and 4.2 mpg with my 3.9 rears heavy foot (65-70mph) and average gross of 117,000 lbs.
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