Author Topic: What causes cats to smolder on a 6.9.  (Read 127 times)

secondslc

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What causes cats to smolder on a 6.9.
« on: 21 September 2020, 07:30 PM »
Hi Guys,

In my next adventure, I'm playing the game of, are my cats bad, self healing, or is the transmission leaking on them.

I know I'm a ways out from getting my fueling tuned properly (a gauged cold start to full warm test is happening this weekend). But I do have a concern before I start taking the car for drives further than around the block.

-Letting the car get to operating temp is ok, but as soon as I start upping the revs and parking it in the driveway, the cats smoke like crazy (lots of white smoke). At first I thought it was 20 years of "stuff" burning up, so I pressure washed and degreased the underside of the car and it's still happening.

-I know I was dumping an extraordinary amount of fuel while I was getting it running, I have a massive amount of soot unburnt fuel on my driveway as a result once I got the thing running and was able to rev the engine (as evidenced by the picture). I am not dumping soot/fuel from the tailpipe anymore, but I wonder...are the cats still burning up a lot of fuel or are they so clogged that they're dead?

-My transmission leaks while sitting parked. More than likely its the front pump seal. But my question is, does the transmission usually leak on the cats? I dripped some ATF on a hot manifold, and it smoked white just like the smoke coming from the cats... If this is the case, then I need to not drive this car and fix the leak.

Anyone have this experience or do I just need to keep poking around the car and finding any external leaks once I get the fueling dialed in?

1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

daantjie

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Re: What causes cats to smolder on a 6.9.
« Reply #1 on: 21 September 2020, 07:57 PM »
Highly unlikely the smoke is from the transmission dripping on the exhaust but I suppose it's possible.   You would think then that you will see a lot of wetness in general around the transmission like around the pan as the lowest point, also around the mount and exhaust brace.
Best to give the whole area a good clean and then place some cardboard underneath overnight then you can pinpoint the leak location.
Daniel
1977 450 SEL 6.9 - Astralsilber

rumb

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Re: What causes cats to smolder on a 6.9.
« Reply #2 on: 22 September 2020, 04:32 AM »
If the engine is running decent it's time for an Italian tuneup. Go drive the snot out of it for 20 minutes.
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

secondslc

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Re: What causes cats to smolder on a 6.9.
« Reply #3 on: 22 September 2020, 07:31 AM »
I will lean it out slightly more (backfires in the tailpipe) and then go from there.

I fixed a passat with the italian tuneup once... but thats a much less expensive car to break!
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8