Unexpained MPG drop

[TSimonson]

So I plugged the PRV for troubleshooting - it won't be the permanent solution, but before replacing the the whole rail (w/sensor and PRV) I wanted to see if there was actually an issue with the PRV. It's tough to say if power/response has improved since winter has arrived and its dropped 30 degrees outside from previous driving without the PRV plug, but anecdotally I think it's better. I've attached a before and after graph which I think shows a lot better correlation between RPM and RP with the PRV plugged, which I think is a positive pointing to a bad PRV. I'll drive it this way for a couple weeks, hawking the RP so I don't overpressurize anything on or after the rail. If performance is back, I'll replace the whole rail. If not, back to the drawing board. BTW, when I pulled the PRV there was significant "mung" on the inside of the valve (pic also attached).

Looks like left over metal, but I haven't checked it - waiting for the fuel to evaporate in the 40 degree garage.

 

[TSimonson]

Didn't realize showing the thumbnail would delete the attachment...

 

[TSimonson]

So it's been more than a "couple" weeks running with the plug. Truck felt great for maybe two days after changing out the PRV for the plug. Overhead display MPG read 17.8 for those first couple days (similar to pre-2014 conditions - overhead usually about 2 MPG above hand-calculated). But after the first couple days, overhead MPG dropped back down to 15-ish and hand-calculated back to 12.58 for this last tank. Same with performance - back to pre-plug sluggishness, with the exception of boost. Boost is still really good (11-14 psi @ 70 MPH steady). So I'm leaning toward the ECM being goofy. As soon as I get a chance, I'm going to pull off all the connectors from the ECM and clean them up. Maybe some corrosion is causing the ECM to continue to cause problems? If that doesn't help, I'm going to look at getting a re-flash or a programmer.

 

[Ozymandias]

I think you spend a whole lot of money to safe some cheap fuel.
Maybe you are hunting a ghost.

That's my thought after re-reading all this Thread.

What I learned with my Truck is that if I feather the throttle for a long time, weeks, the throttle responsiveness gets worse and worse, it feels then like grandpa's Truck. Horrible, all my Mopars act that way.

 

[TSimonson]

Ozy,
I sure feel like I'm chasing a ghost. I don't understand what you mean about cheap fuel. That was my immediate thought back in 2014, but the tank-to-tank drop (17.27 to 14.72) occurred on a tank I filled in central Nebraska. Haven't been back since and the problem has persisted. I Use 3 different stations around Omaha for 90% of my fuel, but no change if filling up in Iowa, Minnesota, the Dakotas, or Missouri.
I rarely feather the throttle, I've gotten used to the long reach between 3rd and 4th on the 48RE.

 

[Ozymandias]

What I wanted to say is that fuel is so cheap, especially in the US, that it doesn't make sense to spend thousands of dollars into replacing random parts that haven't been (knowing afterwards) faulty.

Let me do some math, assuming 2000$ in parts, that gives roughly 900 Gallon of fuel, you need 2mpg more now then before.
900×14.5mpg=13050mi÷15%=87000mi of driving to compensate the 2000$ investment in parts.


But I'm with you, that mpg drop is irritating but if one can't point a finger on a faulty part then it is a hard to solve problem.

Why I talk about it, i had exactly the same peoblem happen years ago with my Grand Cherokee Gasser, the mpg dropped as your Rams now. Then I started changing parts, and i really changed EVERYTHING that has the slightest relationship with with it, really everything, spent thousands and thousands of dollars - and guess what - gained nothing, absolutely nothing.
It was and is so frustrating.

 

[Mark31]

What I wanted to say is that fuel is so cheap, especially in the US, that it doesn't make sense to spend thousands of dollars into replacing random parts that haven't been (knowing afterwards) faulty.

Let me do some math, assuming 2000$ in parts, that gives roughly 900 Gallon of fuel, you need 2mpg more now then before.
900×14.5mpg=13050mi÷15%=87000mi of driving to compensate the 2000$ investment in parts.


But I'm with you, that mpg drop is irritating but if one can't point a finger on a faulty part then it is a hard to solve problem.

Why I talk about it, i had exactly the same peoblem happen years ago with my Grand Cherokee Gasser, the mpg dropped as your Rams now. Then I started changing parts, and i really changed EVERYTHING that has the slightest relationship with with it, really everything, spent thousands and thousands of dollars - and guess what - gained nothing, absolutely nothing.
It was and is so frustrating.

Ozy, I agree with you, except for one small detail: How is burning too much fuel affecting the long term health of an engine?

 

[Ozymandias]

14.5mpg isn't out of spec, many of us use that amount including me.

 

[cerberusiam]

A 305\555 should make better than that in good conditions. If you were driving it in cold conditions all the time maybe nothing to worry about but that is low. It could be an ECM issue on an 04, not the brightest one ever made but changing it is problematic also.

 

[TSimonson]

Ozy,
Thanks for the clarification - I totally agree with you that chasing 2 MPG by replacing $1,000's in parts is silly. But that's not what's happened.
- 48RE transmission problem I found on my own doing scheduled maintenance changing the fluid and finding a sh*t ton of metal in the pan.
- CP3 was initially diagnosed by the local dealer after 6 hours of them telling me nothing was wrong, then their "miracle" diagnosis. But I had it independently verified by another shop, then I checked the old CP3 when they pulled it out. I'm certainly no expert, but I don't think I should be able to physically wobble the input shaft on a 22 kPSI pump. I thought it was bad, but maybe not, and I maybe wasted a LOT of cash. But...

The only part I've changed out solely to troubleshoot this issue is the PRV - changing the valve for the plug - less than $40. That's less than a half-tank of fuel here. And just like the 48RE and CP3 replacement, power & MPG (at least on the overhead) were back to pre-2014 initially. But after about 2 days of driving, truck was back to sluggish power and poor MPG - last tank was 12.75 (winter fuel) - no towing, no hauling, half city, half highway.

So back to the original question - What causes a sudden drop in power and MPG (from 16.0 pre-2014 to 13.6 post-2014)?
Further, when some integral systems are replaced that cause a very temporary (~2 driving days) increase in power and MPG, what's left as a variable?

I think it comes down to 1 of 2 things:
1) As cerb has described, I've might have a long-term problem with CP3/injectors and I'm still totally f***ed because the fuel system is contaminated with metal particles
2) The "learning" algorithm of the ECM/PCM keeps bringing the truck back to some sub-optimal state

Going back to Ozy's point, and given previous problems with the ECM, combined with the empirical data on engine response from the earlier engine logs I posted, I think I'll start with 2) and get the ECM/PCM re-flashed for $80. If that seems to work, I'll get a true DRB-III reader/programmer and download that firmware version so that I can upload/change what the computer is telling the truck and make other changes as time passes.

Thanks again to the whole TDR community for your help. Once I get this figured out I will definitely post.

Happy New Year!

 

[User113]

When you found the metal shavings, did you flush the entire fuel system including the tank?

 

[TSimonson]

Yep, flushed all of it including the tank.

 

[TSimonson]

Update - local dealer was unable to re-flash the ECM. They don't have the OE software to upload, and they couldn't get their "new" DRBIII to talk to the ECM other than to retrieve the VIN. I'm looking for other dealers within driving distance to check with them. The local Cummins place won't even look at it since it's a Gen 3. So while I'm looking at ECM options, I'm thinking maybe I should look at the inlet sensor (the one on the air filter box). As described above, I replaced the IAT on the manifold along with the harness, but maybe the ambient pressure sensor is bad. I'm assuming ECM reported boost is a calculation based on the difference between inlet and intake sensor pressure data. My boost gauge reports data from an Edge banjo bolt replacement sensor on the manifold. Logs show ECM vs. gauge boost is different. So maybe the inlet sensor is bad? This is way out there - but for the previous 4 years I was driving in the mountains - the truck was never below 4,000 feet above sea level. The poo hit the fan once I got back down to about ~1,000 feet above sea level. Fill-up in Cheyenne: 17.6 MPG. Fill-up in Grand Island: 14.7. 13.5 ever since moving to the flat lands. Maybe something goofy with altitude compensation? I know that's a big deal for a gasser. I would assume the same for diesel.
Still looking for options to re-flash, maybe replace the inlet air/pressure sensor after I try to read what the ECM is reporting as ambient.

 

[Ozymandias]

Not for the Cummins, up to 9000ft you have 100% warrant power.

I can't think of any influence on the Milage in higher altitudes.

When I updated mine the dealer had to use an adapter cable to the DRBIII, I think he said something about a red plug, he had to buy it just for my truck. DRBIII only didn't work out.

 

[TSimonson]

Thanks Ozy,
I wasn't trying to say there was less power at elevation, only that inlet air pressure and temperature are inputs to the ECM. I'm not sure if they make a bit of difference in fuel schedule (and resulting engine performance), but I'm assuming both inlet and intake pressure are used in some algorithm to do more than compute boost for the ECM - especially since that boost isn't displayed anywhere and a simple banjo bolt sensor in the manifold would report actual boost.

I'm only curious because I found the PID for ambient pressure at the ECM, read it out, and it was at 29.20 inHg today but the barometer was at 30.34 inHg. That's a pretty significant difference. I can't truly tell if it's the sensor since I couldn't find the PID for inlet air temperature (same sensor) only ambient air (sensor on the driver side of the radiator). But I'm betting sensor since readings remained continuous (not intermittent) and changed as I would expect going up and down in elevation - .1 inHg /100 feet - which was a challenge to check here in Nebraska!

So I guess I'll keep looking, but it might be worth a shot. I'm sure that sensor isn't too much $$$

 

[cerberusiam]

You want the MAP sensor PID for comparison.

 

[petersonj]

I'm only curious because I found the PID for ambient pressure at the ECM, read it out, and it was at 29.20 inHg today but the barometer was at 30.34 inHg. That's a pretty significant difference.

I don't know if this matters, but barometers are usually calibrated to compensate for elevation at a specific location, so readings can be compared at locations with different elevations - for example, to track a low pressure system. I believe the ECM is reading absolute pressure which would likely differ from barometric pressure.

- John

 

[cerberusiam]

MAP is absolute pressure so it sees boost as just an increase in pressure over ambient. The AIT in the TQ tube is reading ambient UNLESS you have a restriction in the filter and them it starts dropping ambient below what it should be and that will foul up fueling, your foot keeps adding fuel to bring boost up to what it should be based on APPS and load.

 

[TSimonson]

Completely agree with all of the above relating to ambient vs. MAP. I had the MAP PID previously - that's what I had posted earlier in the logs regarding boost. That was the whole source of my question regarding computed boost vs. actual, as well as any insight as to what the inlet air sensor (temp and press) is actually providing the ECM to influence fuel schedule.

Air filter looks good and filter minder is at 0%. Inlet air pressure doesn't change when I turn the engine on, so I think the filter is fine.

One point of clarification for pete - barometers aren't calibrated for elevation, they just show ambient pressure. Barometric altimeters are calibrated through the Kollsman window so you know your MSL altitude (or elevation if you're on the ground) even when the atmospheric pressure changes.

That's why I was concerned that mine's reading so much lower than ambient - It goes right to cerb's point. ECM thinks the ambient air is less dense, schedules insufficient fuel for actual air density, builds inefficiency, I add fuel with my foot, and voila, degraded performance and fuel economy.

At least I hope. I'll let y'all know when I get the sensor - hopefully Friday.

 

[TSimonson]

Replaced the sensor - same pressure reported engine off or engine on Still about 1" Hg low. Anyone know the nominal supply voltage to the sensor? I'm guessing 5V.