What is an acceptable amount of blow by?

Ridgeline · Dec 22, 2025

Josiah Harder said: ↑

Im actually not mechanically inclined at all haha Ive done some research now on my specific issue though so I understand a little bit. So I had high crank case pressure codes on my way home from a trip which gave me a stop engine warning when the truck would regen. I was still 3000 kms from home so I went to Kenworth in the middle of nowhere Northern Ontario. They told me they measured my crank case pressure at idle and under load while driving and then they isolated both air compressor and Turbo to make sure those werent causing the issue. They determined I needed to tear the engine down so I went straight home to the shop who originally did my rebuild. On my way home I get another issue where the truck is telling me a regen is required despite JUST cleaning my DPF 3000kms prior and this DPF is only 1 year old. So something is wrong because ive never seen "regen required" even when my DPF was old. I let the truck do a regen for 2 hours before giving up and then I stopped at a Peterbilt in Quebec and they also forced a regen with the computer for 2 hours but it wouldnt complete. So I might have several issues going on. Now that im home again the shop here that did my rebuild replaced my crank case filter and is telling me that they are not getting any stop engine warnings anymore and that as per Cummins the crank case pressure readings are all within spec. Ive ordered a new turbo now to address the constant regen since they told me I wasnt getting enough closure rate on the Vanes to generate enough heat for the regen to complete. Maybe the regen issue really was a turbo fault but I am concerned I have an issue with a piston ring and they are trying to avoid tearing the engine down again. I asked them to hook my trailer up and record the crank case pressure values they read when the truck is under load so I can compare it to what Kenworth in Ontario told me. I will actually attach a photo of the notes the tech at Kenworth wrote on my invoice there. Im surprised they didnt change the crank case filter first thing there before doing anymore testing.
Click to expand...

Call Cummins customer service and ask them to get involved.

KDHCryo · Jan 3, 2026

Josiah Harder said: ↑

Im actually not mechanically inclined at all haha Ive done some research now on my specific issue though so I understand a little bit. So I had high crank case pressure codes on my way home from a trip which gave me a stop engine warning when the truck would regen. I was still 3000 kms from home so I went to Kenworth in the middle of nowhere Northern Ontario. They told me they measured my crank case pressure at idle and under load while driving and then they isolated both air compressor and Turbo to make sure those werent causing the issue. They determined I needed to tear the engine down so I went straight home to the shop who originally did my rebuild. On my way home I get another issue where the truck is telling me a regen is required despite JUST cleaning my DPF 3000kms prior and this DPF is only 1 year old. So something is wrong because ive never seen "regen required" even when my DPF was old. I let the truck do a regen for 2 hours before giving up and then I stopped at a Peterbilt in Quebec and they also forced a regen with the computer for 2 hours but it wouldnt complete. So I might have several issues going on. Now that im home again the shop here that did my rebuild replaced my crank case filter and is telling me that they are not getting any stop engine warnings anymore and that as per Cummins the crank case pressure readings are all within spec. Ive ordered a new turbo now to address the constant regen since they told me I wasnt getting enough closure rate on the Vanes to generate enough heat for the regen to complete. Maybe the regen issue really was a turbo fault but I am concerned I have an issue with a piston ring and they are trying to avoid tearing the engine down again. I asked them to hook my trailer up and record the crank case pressure values they read when the truck is under load so I can compare it to what Kenworth in Ontario told me. I will actually attach a photo of the notes the tech at Kenworth wrote on my invoice there. Im surprised they didnt change the crank case filter first thing there before doing anymore testing.
Click to expand...

NO, not crankcase breather.

Engine can't breathe out the exhaust.

You have a face baked DPF filter from excessive idle and a bad nox sensor, that's why it can't complete regen.

Solution is to bake the DPF filters out of truck, replace NOX sensors, then not idle truck anymore unless it's driving.

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