8.1l tail pipes - is this a problem

[MrMark52]

As stated above, 2002 W22 chassis that we bought in January with 36k miles on her. We’ve since put ~2k on her since.
I didn’t notice this issue of carbon on one of the two tailpipes as they exit in front of the driver rear tire until just before I changed plugs before this last trip - Dallas to MO, back thru AR and back to Dallas (picture should be attached).

Since we got her, mechanically related things I’ve done are -

Changed the oil
Switched tranny fluid to TranSynd
As mentioned, new iridium plugs and high performance wires (Bravels)
New air filter
Flushed coolant system refilled with Dexcool

Engine runs great! Doesn’t lope, doesn’t consume coolant, no detectable sign of leaking head gasket. The only thing I’ve noticed is a little lifter noise or wrist pin slap until the engine warms for 1-2 minutes (if that long), then gone until the engine has had an overnight rest (and may not do it then).

I have not cleaned the MAF.
This last trip, pulling a Jeep Wrangler and dash air on, we got from 6 - 6.5 mpg (6 mpg is more typical over the total miles driven).
No codes.

I find it interesting that carbon can be seen off the left bank only. My concern is the computer may be compensating the fuel mix due to possibly a more catastrophic problem (burned valve or head gasket) that doesn’t show up without running a compression check.
But before I go to that trouble, thought I would get advice from the experts on the board.

[F4Gary]

I think the ethanol in the gas causes the black soot these days. Not sure why one tailpipe is different from the other.

[VanDiemen23]

Just a couple of guesses here:

Front pipe sees less flow out of the muffler. I have an 8.1 truck, just one pipe.

8.1s tend to burn oil. Low tension rings and some other issues. The cats can clean it up a bit, but there will still be soot. Also, gas cars are programmed to run a bit rich. It's easier to clean up HC in the cat than the NOx that's produced in a lean condition. Over time, this will produce soot.

[MrMark52]

Originally Posted by VanDiemen23

Just a couple of guesses here:

Front pipe sees less flow out of the muffler. I have an 8.1 truck, just one pipe.

8.1s tend to burn oil. Low tension rings and some other issues. The cats can clean it up a bit, but there will still be soot. Also, gas cars are programmed to run a bit rich. It's easier to clean up HC in the cat than the NOx that's produced in a lean condition. Over time, this will produce soot.

Thanks VD23!

With no crossover between the two banks, I’m still confused. The only difference is the right bank has to be a little longer to go across the the left side. Up to the point where that 90 degree bend happens, from the exhaust headers thru the mufflers and cats, everything is identical.

You think maybe there could be an O2 sensor running right on the ragged edge of lambda issue that could be causing a lean condition on the right side?

My other 2 vehicles, an ‘03 Jeep Wrangler and a ‘14 Buick both have a black tailpipes, and the Jeep I just did a complete header to bumper replacement last year with new O2 sensors (6 cylinder split to 2 precats, then tied together to a single cat, then muffler - required 4 O2 sensors).
I think the V6 Buick combines both banks before the cat but would have to confirm.
Point is, all the exhaust is combined so if one side was running lean or rich is impossible to see at the tailpipe as it can be seen here.

Bottom line - I just want to head off any major failure before it happens if I can.

[CamJam1]

Do you have a way to measure fuel trims? A ScanGauge, for example…

[Oemtech]

Part if the soot problem is probably caused by the extra fuel used to cool the cats.

[Arch Hoagland]

You need to clean your MAF.

You should be seeing 7mpg.

[MrMark52]

Thanks CamJam1! Yes, I can measure fuel trims - sometimes I forget the obvious. I’ll measure both before and after I clean the MAF.

Thanks OEMTech! Do you think this is an indication that one cat is going out versus the other?
Am assuming I will be able to see what’s going on there by monitoring either the O2 sensors activity or maybe a direct readout of cat temp if my reader will provide that info - via live data from scanner.

Thanks Arch! See above - looks like I have a full day of testing/cleaning/testing in front of me today (although will have to depend on live data for the moment to get a feel for fuel consumption until we make another trip).

[Mark_K5LXP]

The only thing that will leave soot in the tailpipe is a rich mixture. I have no idea where the idea catalytic converters are cooled by fuel comes from. The fact it's coming from one bank and not the other indicates there's a mixture problem on one side, whether that's an O2 sensor going south, vacuum leak, bad injector or other issue. Watching short and long term trim will reveal this for sure, from there it's identifying the failing components.

Mark B.
Albuquerque, NM

[Arch Hoagland]

On my 2004 W22 8.1 I have dual exhaust, one exhaust pipe on each side of the coach.

On the drivers side the generator exhaust pipe is right beside the engine exhaust pipe.

Are you sure those are both main engine exhaust pipes in your picture and not the generator exhaust pipe?

[MrMark52]

Originally Posted by Arch Hoagland

On my 2004 W22 8.1 I have dual exhaust, one exhaust pipe on each side of the coach.

On the drivers side the generator exhaust pipe is right beside the engine exhaust pipe.

Are you sure those are both main engine exhaust pipes in your picture and not the generator exhaust pipe?

😂😂😂😂😂 Thanks Arch! 😂 Good thought, but no, the picture is of the right and left banks of the 8.1l.

The Onan Genset exhaust is behind the driver rear wheels and is only about an inch in diameter..

[MrMark52]

Originally Posted by Mark_K5LXP

The only thing that will leave soot in the tailpipe is a rich mixture. I have no idea where the idea catalytic converters are cooled by fuel comes from. The fact it's coming from one bank and not the other indicates there's a mixture problem on one side, whether that's an O2 sensor going south, vacuum leak, bad injector or other issue. Watching short and long term trim will reveal this for sure, from there it's identifying the failing components.

Mark B.
Albuquerque, NM

Thanks Mark!

You an I are thinking somewhat alike. Was hoping to get fuel trims today - it may not be until next week.

But it thinking about it combined with your comments -
1) if it was vacuum related I would think it be an issue to both banks - unless the intake manifold to head seal was compromised on what I would think would be the right side (since it’s showing “dry” for lack of a better term).
2) it would be weird that (considering the lack of soot on the right side) that all the injectors on the right side were firing lean, or all the injectors on the left side were firing rich, unless -
3) the one common thread to each bank (since there isn’t a crossover on the exhaust) is/are the O2 sensors for that bank. So 1 each pre and post-cat for each bank.
If that’s it - that’ll be a nice piece of change out of my pocket. 😡

Considering that, Im with you - trims will very well tell the tale. I just hope that the valves on the right side bank haven’t been compromised from running lean.

[ktoutoor]

What if you swapped o2 sensors and see if the dry pipe becomes black like the other?

[MrMark52]

Originally Posted by ktoutoor

What if you swapped o2 sensors and see if the dry pipe becomes black like the other?

Good point! I may try that - 1) read fuel trims, 2) swap sensors, 3) read fuel trims again.

Then to figure out which of the O2 sensors are bad - I would guess the ones on the side that the fuel trim range is broadest (reading in double digits).