We did this in the 6.0 forum, so lets try it here. I'm only going to ask questions, and not make any statements. When you reply, I don't want to see guesswork or conjecture, I want information you can back up. READY?...... If the thoughts are that a cracked DPF is the cause of sooty pipes, why then doesn't the pressure sensor that is between the DPF and the cat see a lessened amount of backpressure?

 

On the high pressure fuel pump, you have a pressure control valve, and a volume control valve. Conceivably, can you have a catastrophic situation that leads to excessive fuel usage/regens? And will this lead to sooty pipes?

 

Ok, so ask a question that the only answer to is conjecture, and say no conjecture...whatever. Here is the simple scientific hypothesis. In a clean DPF a crack would allow higher airflow resulting in lower back pressure. However, the DPF will still trap some soot, and it might already have soot trapped prior to cracking. Because of this back pressure can very easily be higher with a crack than a clean DPF that is uncracked.

 

There is a range that the dpf sensor can be in and be acceptable. If a DPF is cracked sufficiently to have sooty pipes, the "hole" will be significant. Why then isn't the check engine light on with a dpf sensor code?

 

We have an EGRT (egr cooler outlet temperature sensor). The pcm monitors exhaust temperature from the egrt outlet sensor signal to aid in controlling the egr valve position and throttle position. With all the radiator and coolant issues present in the 6.4, can air bubbles, coolant quality/strength, contribute to an overfueling issue involving this sensor?

 

It is impossible to answer without conjecture. The DPF is too new with no reliable historical test data in the real world.

As I have stated in the past, or at least implied, the 6.4 and the DPF system will drive you nuts if you try to figure it out or attempt to establish a pattern of behavior. Trying to guess when and what it will do is insane.

Sooty pipes are caused by soot. The DPF is supposed to capture and incinerate soot. If there is a condition that makes so much soot that the DPF can't incinerate it then you look at over fueling, lack of air, lack of heat, whatever makes soot.

 

NOPE! Thats a cop out! We're supposed to figure it out........lets keep going!

 

There are only 2 things that ultimately contribute to fueling- injection pressure and injection pulsewidth (added up over the multiple injection shots). The diagnostics that control the pump are pretty *tight*, ie it doesn't take significant error to throw codes. Any difference enough to add up to an overfueling condition from either the pressure or volume control valve will throw the truck into limp and DTC's long before any DPF damage comes about.

It only takes a small crack in the DPF to allow soot by; even a very tiny amount will build up soot in the pipes and not much more to see visible smoke. These engines run low injection timing (and correspondingly more particulates) in order to keep NOX emissions down; the exhaust in the downpipe contains a visible amount of soot the majority of the time under load.

The calibrated error thresholds for the DPF pressure sensor to detect an actual leak aren't as stringent as one might think, simply because there are fluctuations in average pressure over the lifepsan of the DPF and simply in the allowed tolerances as the DPF's are manufactured. The tolerances allowed would not detect the small leaks that are necessary to build up soot or visible smoke.

If it gets bad enough, obviously, codes will be thrown and limp mode will likely ensue, but not all the time. Most DPF cracks happen diagonally across the substrate filter media between the "squares" inside or down the length of the ceramic mesh that binds the substrate squares together. Given the average volume of exhaust passing through the DPF at any given point in time, it takes a pretty hefty leak to cause a detectable amount of pressure drop- let alone one large enough to exceed the calibrated thresholds. The tolerances are more than enough to allow soot buildup and smoking if the DPF contains small cracks.

I can also definitively tell you from looking at the calibration that fueling is in no way directly controlled by the DPF pressure sensor; even for the POST2 "regen" injection. The pressure sensor can request small percentage changes in fueling based on large differentials but normal fueling functions are controlled simply by mass fueling tables.

As for the EGRTB sensor, it being out of calibration could allow the EGR valve to open farther and for longer than needed, decreasing oxygen to the cylinders and increasing particulate production- this would increase overall catalyst and DPF temperatures on average and affect lifespan over time, but once again there are error thresholds for how much duty cycle change is allowed based on EGR inlet/exit temperature differentials. As for coolant quality- anything that affects the function of the EGR coolers- be it air bubbles from low coolant level or whatever else will affect the efficiency of the coolers and provide less temperature drop in the incoming exhaust to the intake manifold. This could also affect soot production as high EGR temperatures require more EGR flow in order to do the same job (and the calibration is aware of this). However, just like the last scenario, there are numerous plausibility diagnostics in place to ensure that these tolerances don't pile up on each other to create a condition that shortens the lifespan of any of the components or cause a drivability concern.

No DPF can allow soot to leak by because it is "overwhelmed" or the presence to too much soot. It can only leak by physical failure; ie cracking; which can be accelerated by too much soot or regens that are too hot or too long due to a malfunction. I see this myth float around the forums a lot.

To answer vloney's question directly, yes, there are situations with hardware control such as the pump valves that can accelerate failure of the DPF under the right conditions. However, I firmly believe none of the conditions could occur in a situation without tripping the diagnostics and correspondingly causing the truck to run like absolute ***. DPF's that so happen to fail and leak soot without diagnostic warning or indications were, IMO, simply faulty components to start with. They are prone to failure inherently by current design.

 

My cop out is made because folks like Mr. Geouge. All his points make sense to me and as I thought about vloney's questions I cam up with many of the same things but I don't have the experience/knowledge to back it up with PCM theory. Obviously, he does.

Thanks for the info Matt!

 

Great info Matt. Makes sense to me. My DPF is still clean after 21K miles and alot of it towing hard. So the DPF does not get overwelmed. If the soot passes by then there must be a crack.

The crack is caused by other issues or as we have seen a faulty one from the start.

 

As usual, Matt's information is very good. My contention all along has been, can the DPF have a crack causing sooty pipes, sure. Is a cracked DPF the only cause, no. I've repaired many with sooty pipes with injector repairs, fitting repairs, component replacement, without dpf replacement, and the pipes clear up. In my opinion, dpf replacement is still the LAST thing you do for a repair, not the first.

 

vloney
I would sure hope that FORD has a procedure of steps to take to check out all other possibilities before replacing the DPF. If there is such a series of checks my dealer does not know them. I have had my DPF replaced and the tail pipes are black again after about three months. Do you know if there is a TSB or something in place for other mechanics to know what to check? Is Ford addressing this issue?

 

Yep! 07-8-9 at first that outlined all steps to take, now it is superceded by 09-10-2 that directs you to the on-line manuals for proper procedures to repair the problem.

 

To be clear, a DPF can just crack on its own.

A DPF can be cracked by other systems (fuel or cooling).

The only way to have sooty pipes is a cracked DPF so it must be replaced regardless, right?

I'm glad that Matt and vloney are in on this. Your info is backed by what you do. Most of us here just kick around ideas without ever really knowing if they are good or not.

 

No. Overfueling is the major cause of sooty pipes. A cracked DPF may or may not be involved as well. Thats why its the LAST thing you replace when dealing with this issue.