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Old 05-25-2010, 04:33 PM   #1
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Default Black smoke from tail pipe?

Another question about this 99 GC I bought. Its only getting 7-10 mpg. I changed the plugs(they were black) and changed the coolant temp sensor.(the guy at autozone said a bad one would make it run like crap) And I changed the oil/air filter. But it still blows very black smoke/oil out the tail pipe. And under the hood were the fuses are, there are relays for the O2 sensors.(places fo them to go anyway)mine are not there. And there are NO little slots for a relay to plug into either. Should there be?

ps. When is warmed up the back smoke stops for the most part.

any help is appr. thanks
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Old 05-25-2010, 07:59 PM   #2
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The cathalitic converter my be bad. You will have black smoke and bad fuel millage.
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Old 05-25-2010, 09:06 PM   #3
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It has no cat. I bought it this way but Ive had other cars that didnt have a cat and they did not do this. I know I need to try a mass air flow sensor/throttle position sensor. But I really wondered about the missing O2 sensor relays.
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Old 05-27-2010, 08:43 AM   #4
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Originally Posted by XJ_FEVER View Post
It has no cat. I bought it this way but Ive had other cars that didnt have a cat and they did not do this. I know I need to try a mass air flow sensor/throttle position sensor. But I really wondered about the missing O2 sensor relays.
You really need the O2 sensors in and working before you do anything else. I would say that not having them is the problem.
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Old 05-27-2010, 03:32 PM   #5
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Black smoke? 7-9mpg? You're running awfully rich.

If the HEGO heater relay(s) are missing, that's probably your problem. Should repalce with standard DIN relays, from what I've seen (maybe mini-DIN relays - check the sockets. A DIN relay is about a 1" cube, not including terminals - while a mini-DIN is half as wide.) No HEGO signal will cause the ECU to fail over into a very rich "limp home mode." (Not quite rich enough to cause cylinder washdown, but it can be close.)

If you've been running like this for a while, you're probably going to be in for changing your HEGO sensors as well - they'll have become clogged up with carbon.

And by all means - replace the cat! There's really no good reason to run without one, we don't need the Greenies after us any more than they already are, and the good designs (like those by, say, Random Technologies) can actually improve exhaust airflow slightly!
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Old 05-27-2010, 03:38 PM   #6
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You really need the O2 sensors in and working before you do anything else. I would say that not having them is the problem.
your running rich and I bet it has to with the 02 sensros. Off the top of my head you should have 2 (could be wrong) one before the cat and one after
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Old 05-27-2010, 03:53 PM   #7
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yea it sounds like your getting a really rich a/f mixture
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Old 05-28-2010, 08:04 AM   #8
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your running rich and I bet it has to with the 02 sensros. Off the top of my head you should have 2 (could be wrong) one before the cat and one after
Yea there are 2 O2 sensors. One before the cat and one after.
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