Transcription
I've got a 2018 F-450 with a P20EE, which is SCR NOx catalyst efficiency low. So, it's telling me I have poor NOx reduction on this truck. So, we're going to look at a couple of pieces of data here. On the bottom, I just have DEF pressure and DEF injector duty cycles. So, you can see when the DEF injector is squirting and that has good pressure.
The numbers I'm really going to be watching are these two on the top. And that's NOx out of the engine on top, and NOx out the tailpipe on the second line there. So, you can see as I drag this cursor forwards and backwards, I've got some numbers that change in the top right. So, I have current data number, and then it also shows me the highest number it saw under max, and lowest number it saw under minimum. So, I can see the ranges here.
So, if I start at the beginning of this recording here, you can see that I have, you know, roughly double or a little more NOx, uh, on the front than I do on the back. And so, this would be bad, right? So, I can see that, uh, I've got crummy NOx reduction there, right? So, I should have about a 90% reduction in NOx from front to back. So, if I had 500 parts per million out the motor, I should have 50 parts per million out the tailpipe. And so, as we scroll through this recording, we see this stays fairly steady, no matter where I go in this recording, it acts like that.
So, if we're going to use these NOx sensors in order to diagnose my SCR, I need to make sure that these things are reading correctly. So, one of the things we do is de-cell fuel cutouts. We're going to decelerate hard enough that the fuel injectors shut off, and you see the DEF injector is turned off. And look at my NOx numbers, they dive really low. So, single digits. And if I can get these things under 10, I'm happy. Notice they both get down to, you know, zero or two at some point, uh, but nice low numbers. If they stick around, you know, probably 20, I really stop liking them at that point. And, uh, and you're gonna have to replace the NOx sensors before you go any further. But either way, on our de-cell fuel cutout on this truck, we get nice low single digits.
So, before I'm ready to call this thing and say, "All right, we need, we need an SCR," there's one other check I like to do. And so, uh, here's the scanner that we're using. You can see again, inlet and outlet NOx in the first and second position. And now I'm also taking a look at true NOx out the tailpipe. So, this is used, just using a NOx tester, uh, to sniff the tailpipe. And so, notice what we've got here. You know, we're sort of steady state, 175 parts per million out of the motor on the scanner, and it says 67 parts per million out the tailpipe. And again, that's some reduction, but not enough. Notice my gas bench shows truly 50 parts per million of NOx out the tailpipe. So, why the discrepancy here?
So, uh, so this is interesting. The NOx sensors that are on the truck can't tell the difference between NOx and ammonia. And DEF turns into ammonia in the exhaust. Then we inject it in there. And so, what we see on the vehicle is actually a combination of NOx and ammonia out the back. So, this, uh, extra 17 parts per million is, uh, is a little bit of ammonia that's not getting caught in the slip catalyst.
So, when you have a failed SCR, these numbers range widely, right? So, if this was a totally failed SCR, on the scan tool, I'd see 175 parts per million out of the engine, but I'd see double that out the back. And again, the reason for that is that the rear NOx sensor sees both the NOx out of the engine and the ammonia. So, if we're injecting DEF at a ratio of one to one, uh, if I'm injecting, you know, 175 parts per million DEF in the exhaust in order to get rid of 175 parts per million of NOx, again, I would see double on that NOx sensor on the vehicle because, again, it can't tell the difference between the two.
Now, the gas bench doesn't have that problem with sensor crosstalk. So, I can use the gas bench in order to see true NOx out the tailpipe and see what my failure level is on the SCR. So, in this case, I have a partial failure. I got 175 parts per million out of the engine, almost 70 out of the tailpipe. Again, that's bad. If I look at the true number, it's 50 parts per million. Even that's still bad, it's too high. And the difference between the gas bench and the NOx out the tailpipe is, uh, is a little bit of ammonia slip out of the back of the vehicle. So, again, uh, failed SCR on this vehicle, and not terribly hard to see in the scanner data, you know.