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Old 09-11-2019, 03:33 PM   #1
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Question Loss of Power on Cummins ISB 350 6.7 L (CM-2150)

I have a 2008 Newmar VTDP 3332. I bought it in 2016 and it ran great when I first got it, it would easily go up hills at 65 to 70 mph. After about 6 months of ownership I got a CEL going up a big hill on the way to San Diego. I took the rig to a Cummins Service Center and they said they needed to replace the turbo because the sector gear had seized. I made them show me the turbo and verified that the sector gear was in fact seized.

After the repair was completed I noticed that I didn't have as much power and took it back to Cummins to have them troubleshoot it. They said they couldn't find anything wrong. They dyno'd and it and it peaked at 240 hp, which I think is low for a 350. People I've talked to said it should be closer to 280 hp. It turns out that the loss of 40 hp makes a big difference. Anyway, Cummins said that there wasn't anything else they could do.

I was not happy with that diagnosis because I knew that it had more power before the turbo was replaced. I took it to Massey's Diesel in Phoenix and they replaced the exhaust manifold. That made a huge difference in the engine braking performance but didn't help with the power issue. So I took the rig to a MHC Kenworth in Denver. At the direction of Cummins they took the intake system apart piece by piece and cleaned it. Still no help. This all happened over the last 2 years.

I decided to take it on one last trip to see if I could live with it like this or not. About 6,000 miles into a 7,000 mile trip I got another CEL going uphill in the mountains on a hot day towing a CR-V. This time it shut down the engine to where there was very little power (limp home mode I think). Not knowing what else to do I turned off the A/C and the power immediately came back. When I got to Denver I took it back to MHC Kenworth and they replaced the water pump. That didn't help with the power but it got me back home. I took it back to Massey's and they installed a software update to the ECU using Insite.

I took the rig on another trip to see how it was doing and it ran OK, but not like it did before the turbo replacement. It would struggle to make it up steep hills and even lost a little speed on not so big hills. About 1,200 miles into this trip I noticed that at the top of a big hill I was still doing 67 mph. So I started looking at the speed and rpms on the big hills and sure enough all the power it had before was restored. I drove it a couple hundred miles like this until I got home with a big smile on my face.

So, my questions are:
Has anyone ever experienced anything like this before?
Is the software update responsible for the power increase? Cummins said that it was intended to clear out the CEL codes from the previous trip. Why would it take 1,200 miles to restore power (if that's what really happened).
Does the ECU incrementally cut back on power if there are stored CEL codes or are the only options full power or limp home?
Will the power still be there next time I drive it?


Sorry this is so long, but it's a long story. I would appreciate any thoughts or comments you may have.

Thanks.
--Jerry
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Old 09-11-2019, 04:46 PM   #2
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I was going to ask if you have a turbo boost gauge and if so is the max boost psi the same as the old turbo or lower. If lower power will be less and the wastegate actuator on the turbo can be adjusted to change boost or there could be a leak in the piping downstream of the turbo. But it sounds like that is not your problem since u say power is back to normal.

I do know i had an intermittent bad crankshaft sensor on mine and the ECM will derate fuel when that sensor is bad. It will also derate fuel for all kinds of reasons looking at the service manual for my ‘98 5.9L ISB. I have a scangauge D and the sensor thru a code but it would run at full power even with the code not cleared, power would come and go as the sensor went good /bad. New sensor solved problem

Most power problems on diesel are either turbo /boost or fuel related due to derate by ECM or clogged filters etc.

I dont think anyone except Cummins would know what that new flash in your ECM changed but I doubt it would include an increase in power JMHO...
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Old 09-11-2019, 06:13 PM   #3
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Yes I have a boost gauge on the dash and I get right around 30 psi max boost pressure, both before and after turbo replacement. One shop had a manifold that they could insert into the plumbing between the turbo output and charge air cooler inlet that they could pressurize and check for leaks. It turned out that a couple of the clamps were loose/worn out and needed to be replaced. They did that repair and rechecked it for leaks and found none so I am confident that is not an issue now. Especially after power coming back while I was driving it.
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Old 09-11-2019, 06:20 PM   #4
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Would it be the learning curve of the new software really learning the new parts and engine? I know it took my truck 500 miles to learn new stuff on mine to get to optimal
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Old 09-11-2019, 06:48 PM   #5
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Don't forget the basics. How's that air filter? Mine was within the "gauge" OK zone, but near the red line, and I was losing power. New air filter did an amazing job to restore that power. Sometimes it's best to replace the AF early, as it puts stress on the turbo as well as diminishing power.
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Old 09-11-2019, 07:00 PM   #6
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Interesting question about the "learning curve of the software". That's what I thought at first, but when I mentioned it to other people they either didn't know or thought it wasn't possible. Does anyone know for sure if the ECU can learn new things (like transmission shift points) and then increase power when it's satisfied that all parameters are met?

Good point about the air filter so I'll replace it just to be sure.
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Old 09-11-2019, 08:00 PM   #7
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What do you mean by "derate fuel"? How does that work?
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Old 09-11-2019, 09:45 PM   #8
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The reason I ask is because just about the same time the power came back I was very low on fuel to the point where the "low fuel" sensors triggered some warnings. I've never run it that low on fuel before. After that I went another 50 miles and then put in 25 gallons (at an outrageous price) to get me home. It ran as good or better with the new fuel. Do you think running it down to the bottom of the tank did anything?
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Old 09-12-2019, 04:26 AM   #9
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The term "de-rate the fuel" means the ECU is cutting back on the fuel/timing to reduce engine power.

What caught my eye is you mentioned your ISB was making 30psi boost before and after the turbo fix. Well, as I understand, in a diesel your boost IS your horsepower so if you felt the coach lacking power even at 30psi boost it had to be some parasitic loss after the crankshaft. IE- accessories, drivetrain. Strong winds will also make a coach feel doggy obviously.
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Old 09-12-2019, 04:48 AM   #10
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You say power came back after running tank to near empty and then adding 25 gal of fuel? When was the last time you changed both fuel filters or added treatment to your fuel tank? You could have possibly had contaminated fuel or a dirty filter and by running it so low could have possibly burned up most of the bad fuel and by adding fresh to a mostly empty tank could possibly flushed your filter with clean fuel. A clogged filter will create a power loss...no fuel no power
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Old 09-12-2019, 07:01 AM   #11
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Yep your correct on derate. ive never had my motor lose power from running tank low, the ECM derates ( think lowers) the fuel mainly to protect the rnotor from damage (bad sensor etc) but still allow it to run.

If your making 30# of boost the fuel is not being derated for sure. Thats near full engine load and it takes significant fuel to create that much boost.

im sorry but I cant explain what your experiencing.

If it were me id check all the basics change both fuel filters and air filter and if you dont have a fuel pressure gauge at inlet to injector pump its worth it to install one , helps greatly to know when ur filters are clogging and for troubleshooting.

And a code reader like scangauge D is invaluable for finding sensors that go bad or are intermittent like my crank sensor did. If u find issue please let us know were all in this to learn and help each other.
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Old 09-12-2019, 07:13 AM   #12
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Id also check / clean the charge air cooler, it cools the hot compressed air coming off the turbo and looks like a radiator but its air to air. Just follow the turbo outlet and it should have orange boots as thats the hot side, the outlet should thave blue boots thats the cool side ( assuming the cooler is working properly.

30# of hot boost creates much less power than 30# of “ cool” boost due to the cooler air is higher density and hi air density is what makes power not pressure.
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Old 09-12-2019, 04:14 PM   #13
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DPF may have been clogged due to faulty turbo...would be my bet
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Old 09-13-2019, 12:32 PM   #14
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A clogged DPF makes sense to me. Perhaps the original turbo failed because there was soot in the DPF which caused additional heat, back pressure and a loss of power even though the turbo was putting out 30 psi. When they replaced the turbo they didn't clean out the soot and it ran at low power for 2 years because of it. After the recent software update which cleared the CEL codes it was able to regenerate the DPF and burn out the soot over 1,200 miles of driving up and down mountains in CO.

Does anyone believe that could be the case? How can I test the DPF to see if it needs additional cleaning/replacement?
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