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Old 07-05-2022, 11:16 PM   #1
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ISM Low Boost Pressure

Everything was running normally, no error codes, and I would routinely get 36-37 psi boost. Recently, I got an "ambient air density" error. After getting that error I also noticed that I wasn't getting boost above 30 psi. There weren't any additional codes about this though. Feeling under-powered I limped to my destination.

I thought that it must be time to change my ambient air pressure sensor and my intake manifold temperature/pressure sensor. They're inexpensive and have about 75,000 miles on them. So, I changed the sensors and on startup at the campground I wasn't showing any active codes.

Feeling confident, I took off on another leg of our journey. It was a hilly, mountainous, trip that certainly challenged my engine. This time I got a couple of error codes: A 102 - turbo boost pressure low, another 372 ambient air density error and a check engine amber light. The turbo boost remains at a max of 30 psi and I can definitely feel the reduced power.

So, the two sensor changes didn't fix anything but I'm glad they're new. I had to replace my turbo actuator abut a year ago and got the opportunity to inspect the turbo at that time and it looked and has been performing great.

I did a visual inspection of my flexible boots and they "appear" okay. Nothing visually obvious in the plumbing, clamps, or visual cracks in the CAC. I'm not ruling out any of those though. I'm just wondering what are the most logical trouble shooting techniques now? I'm leaning toward a pressure test while using my ears and soapy water. Any and all suggestions are welcome!
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Old 07-06-2022, 08:30 AM   #2
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Egr is the first thing I would go to. At high altitudes it should remain closed at all times as far as I understand.
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Old 07-06-2022, 09:03 AM   #3
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First thing I would check is the port the intake manifold air temp / pressure sensor is tapped into. It's possible there is a build up of carbon in the port preventing a correct reading.
Second item to check is that the "ambient air density" also known a Barometric air pressure sensor (BAP) is reading correctly. The ECM uses this reading to set how much boost pressure is allowed. With the new sensor the wire harness could be causing the problem.
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Old 07-06-2022, 09:47 AM   #4
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Egr is the first thing I would go to. At high altitudes it should remain closed at all times as far as I understand.
Yes, good call, that's on the short list. If the EGR valve is not closing/sealing, but not out of position enough to throw a code, then the extra boost can just leak by and blow out the exhaust. The EGR valve has never been serviced so it's time that the valve and cooler are cleaned. I have a few hundred miles to go before I get to a park where I have a 30-day stay and can get that done.
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Old 07-06-2022, 09:59 AM   #5
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Quote:
Originally Posted by Caliber View Post
First thing I would check is the port the intake manifold air temp / pressure sensor is tapped into. It's possible there is a build up of carbon in the port preventing a correct reading.
Second item to check is that the "ambient air density" also known a Barometric air pressure sensor (BAP) is reading correctly. The ECM uses this reading to set how much boost pressure is allowed. With the new sensor the wire harness could be causing the problem.
Well, changing the pressure/temperature sensor didn't change anything so it could the port itself. I'll certainly clean all the plumbing when I go after the EGR valve and cooler. Thanks for the tip.

I changed the BAP as well. It's certainly well hidden in the harness. No difference before or after the swap, unfortunately. I do have the Inline-6 and Cummins software so I can monitor the readings to see if it's behaving. I am operating at about 7K feet. So should I expect the BAP to be commanding the EGR closed?
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Old 07-06-2022, 01:51 PM   #6
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Yes, good call, that's on the short list. If the EGR valve is not closing/sealing, but not out of position enough to throw a code, then the extra boost can just leak by and blow out the exhaust. The EGR valve has never been serviced so it's time that the valve and cooler are cleaned. I have a few hundred miles to go before I get to a park where I have a 30-day stay and can get that done.
Unfortunately the sensor does actually read the gas passing it just tells shaft position. Got a 6.0 powerstroke sitting here doing just that. Good part is the sensor also doesn't know where the egr actually is.
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Old 07-06-2022, 04:45 PM   #7
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I meant doesn't read
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Old 07-08-2022, 06:34 AM   #8
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BonS,

I know you've inspected your air piping from turbo to CAC, but if you have any heat shielding on the piping like my coach (see attached), cut the stainless tie wraps on each end and inspect the solid pipe underneath.

This is what was uncovered on a similar coach that was losing turbo boost.

Hopefully, not your issue, but wanted to share in your process of illumination.Click image for larger version

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ID:	370473Click image for larger version

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ID:	370474
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Old 07-08-2022, 06:36 AM   #9
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Suppose to have read "process of elimination"

Darn auto suggest!
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Old 07-08-2022, 09:22 PM   #10
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Suppose to have read "process of elimination"

Darn auto suggest!
Thanks for the suggestion I'll keep that in mind. I like "process of illumination" better!
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Old 07-09-2022, 03:25 PM   #11
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A 300 mile update

We drove another 300 relative easy miles today so I had more time to watch the boost gauge. It's somewhat predictable and I hope the pattern will remind someone of a failure mode that they've seen. First, the boost is slow coming on from a standstill. Second, the boost runs up to about 30-31 psi and then rapidly drops to around 12 and begins it climb toward 30 again. This cycle repeats if demand remains. I was hoping to get it to 20 and try to hold it there but the terrain was rolling and I couldn't get the test done. Maybe in a couple of days when we do another 230 miles. Does this ring anyone's bell??

To me, it seems like something is being pushed open by the pressure and burping the boost, only to close again for the next cycle. I hate to think it's a crack in the CAC that opens under pressure. Buy maybe?
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Old 07-09-2022, 08:12 PM   #12
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Maybe this question might drill down to it: Given that the RPM of the turbo is monitored by the ECM, will the ECM alter the VGT vanes to reduce the RPM if it senses an over-speed condition of the turbo? It does seem that a simple leak could lead to a turbo over-speed condition given low back pressure. What say you?
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Old 07-09-2022, 09:35 PM   #13
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ISM Low Boost Pressure

Raise the rear engine access cover. Get someone behind the wheel. Have them put it in a forward gear with the parking brake on. Step on both the service brakes and throttle just enough to build boost while you stand behind and listen for a leak. Usually they are loud enough to hear. Hopefully you have a side radiator.
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Old 07-09-2022, 11:11 PM   #14
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I used my Inline-6 and Insite to take a closer look at the codes. I don't have an over speed code so that issue is off the table. I do have a 629 which is a "Turbocharger 1 Compressor Inlet Pressure - Data Valid But Below Normal Operating Range - Moderately Severe Level". I can now see that the complaint is about the compressor inlet pressure. Clogged air filter?! This all started when I was boondocking at Mexican Hat, UT. Anyone that knows the area knows that the dust is incredibly fine. I have a filter minder that says the filter is fine but I think the Filter Minder is pretty much junk. I've ordered a new filter but in the meantime I vacuumed and knocked the filter to attempt to release the fines, best I can, until I get the new filter next week. Here's hoping!
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