Exhaust smoke, dark; Towing Related Maybe
 

Ok, not sure if this will end up Tech related, so feel free to move if that ends up the case.

This past weekend I hit the Rubicon Trail. No issues this time, unlike 2 weeks ago. hahaha

Drove what is called the Wentworth Springs Loop as my club puts on a Poker Run for this portion.


Issue: I ended up towing the following Heep on a trailer down some decent grades/turns. Total about 30 miles. OD was off. Kept the slush box in 2nd on the downgrades. Trailer had surge brakes. No issues coming all the way down until the last mile or so. The trailing support team noticed blackish smoke from the rear.

We stopped on the shoulder where appropriate thinking it was hot brakes from passenger rear. Engine was off and no smoke visible. Started the engine and sure enough smoke from the exhaust. It was blackish and maybe hint of rotten eggs, which would indicate catalytic converter. However, not long after getting back underway...no more smoke. Speeds were higher as was on highway now. Previously in 2nd gear I conservatively kept thing between 25-35mph, engine RPM ~2000ish.


No further smoke issue for remaining 50 miles to drop Jeep and trailer, then back home. No CEL, no engine or tranny weirdness. All things nominal.


Pics...and let me know any thoughts that come to mind. Thanks

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Some of the support crew, Diesels, with Jeeps on trailers.

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I suspect it was fuel buildup from the downhills that gave you the smoke. Black smoke is rich, rotten egg smell is unburned fuel running thru the cat, and burning there instead. Not really good for your cat.

If the truck runs fine, doesn't do it again, and doesn't seem to develop any issues. Chalk it up to your truck not like being pushed downhill by a jeep on a trailer. :D And call it a day.

Awesome pics. (as always.)

I had a dakota that used to do that. It was a manual tranny but when going down long grades with engine break holding high it would build up fuel for some reason. Then it would look like a diesel after I got back in the throttle. Black smoke I'm pretty sure is fuel related. Blue is usually oil related and white is usually burning water/antifreeze. From my experience

ahhh ok. So a little thinking this through...the pushing meaning the pistons getting reverse mechanical load rather than the combustion. Pressure lower in the cylinder....leaving the unburnt fuel...which gets pushed out into the headers/exhaust...?

The problem with that: The PCM should notice that you've got the throttle closed, RPM is well above idle, and load is nothing -- so should inhibit injector pulses. If you're getting fuel under those conditions, something's not telling the PCM the truth.

You might want to hook a scanner to it to see what the TPS and MAP are doing during the fault condition.

I wouldn't call it "no load" since there is engine braking going on. I was just differentiating which direction the load was coming from, at least attempting to. I don't see how the injector pulses should change. They are just following the CPS on the dizzy, right?

This towing thing is not a regular occurrence. It was purely to help out this instance, so reproduction is pretty much nil, zip, nada.....or very low probability. lol

However, I definitely could run the Torque App and see if I can watch the TPS/MAP just driving around. As said, the issue (black smoke) went away shortly after the long descent.

Oh, I do have the Superchips programmer set to Tow mode. Forgot that detail.

Wrong: You'll find in your FSM...
That should have no effect. I run around with mine set to towing mode all the time, and routinely see negative torque (wheels driving engine) with no evidence of smoke and a definite seat of the pants feel of fuel cutoff and a slight exhaust note change when it occurs. My wife can't perceive either effect, but she's never been one who could.

I engine brake all the time with the manual. It's pretty obvious to me. It starts going more of a "blurp blurp blurp" instead of a "boom boom boom"

I disagree that the O2 sensor would be ignored. However, for comparison (since you offered) you've towed 6000 lbs down winding 30ish mile grades to NOT have this "negative torque"? I only noticed the smoke after it was mentioned over the radio and caught a glimpse making a right hand turn.

Definitely no other operational off-nominal was occurring in this experience (e.g., exhaust sound, engine sounds, etc.). Everything was nominal with exception of the short duration black smoke.

One thing that comes to mind is oil being pushed by the rings to make the black smoke. I do need to perform a compression and leak down check for the heck of it. Check the plugs again as I replaced them all about a month+ ago.


Reproducing this event is few and far between, but still would like to understand it better, if possible.



Whhoops, not CPS, but CMP for Camshaft Position Sensor

From the FSM Camshaft Position Sensor section 8L-6

This implies either +5 volts or 0 volts.



Wheeling involves a good bit of engine braking also. Descents and such using the gears, etc.

Cam position sensor only clues the PCM in to which half of the firing order it is on. When the signal switches, (on to off, or off to on) that tells the PCM exactly which injector should be firing. It doesn't modify pulse-width at all. It is purely so the PCM know where in the firing order it is.

In a 'negative torque' situation, the PCM will drastically cut back on fuel, to the point of providing none at all. This will provide engine braking to help control vehicle speed. It's actually a pretty good idea. :)

Wonder if your tow tune doesn't completely cut off fuel.... or, if there is a bit of a flaw in the programming, that was feeding to much, hence, your black smoke.