Black exhaust

Something I noticed today while washing my XJ(which rarely happens). The end of my tailpipe is charcoal black and when running water ( which is normal) comes out but it is filled with black stuff. I'm pretty sure its carbon but why so much? I changed my old spark plugs and they were black as well.

What causes this? Should I run a higher quality fuel? I use BP 87 octane.

Thanks in advance. (99 sport 4.0 115k miles

Dripping/slightly running water is fairly normal - when hydrocarbons are burned, the "ash" is water and carbon dioxide.

The carbon deposts aren't normal - that's unburned fuel. Excessive carbon fouling in the plugs gets you pointed in the right direction - you're getting it because you're running rich. Check your air filter (dumb, I know - but it does affect things,) and clean your fuel system (crap in the injectors can keep the pintle open slightly when it's supposed to be shut, causing excess fuel delivery.) Also, check the signals from your MAP and IAT - since they're the primary sensors used for fuel metering. Check your HEGO as well, since it's responsible for "trimming" fuel delivery in a feedback look.

MAP - Manifold Absolute Pressure sensor
IAT - Intake Air Temperature sensor
HEGO - Heated Exhaust Gas Oxygen sensor.

Another common cause is running cold. Do you have a temp gage? Does it show 210 degrees normally? Do you have a 195 tstat installed?

Forgot to mention - some carbon deposits around the tailpipe exit are normal, since you'll always get some from the cold starts. However, you should not have visible particulates in the exhaust once you are at operating temperature.

I have a OEM filter ( it was like 5 bucks or so). I was looking at a spectre High flow perf. air filter that's like 20 bucks.

I routinely use fuel system cleaners like Gumout Regane complete fuel system cleaner.

As for how to check signals from those sensors - you got me. I assume it takes a volt/ohm meter or something. I'm more of a mechanic than an electrician.
I'm sure that if I changed/adjusted something i wouldn't burn off a quarter tank of gas driving probably 20 miles or so in town. Even flooring it it shouldn't be that bad.

winterbeater said:

Another common cause is running cold. Do you have a temp gage? Does it show 210 degrees normally? Do you have a 195 tstat installed?

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lots of cold starts. But since it rarely reaches freezing now shouldn't it not happen to that degree? Faithfully shows 210 (or just a hair over). Now when it gets hot out it gets so hot that the AC fan kicks on regardless if the AC is on or off. I attribute that to my water pump dilemma. 4 water pumps in 7 months. No Idea why they are going bad. Yea probably a 195 t stat. Whatever the stealership put in.

99_XJ_ said:

I have a OEM filter ( it was like 5 bucks or so). I was looking at a spectre High flow perf. air filter that's like 20 bucks.

I routinely use fuel system cleaners like Gumout Regane complete fuel system cleaner.

As for how to check signals from those sensors - you got me. I assume it takes a volt/ohm meter or something. I'm more of a mechanic than an electrician.
I'm sure that if I changed/adjusted something i wouldn't burn off a quarter tank of gas driving probably 20 miles or so in town. Even flooring it it shouldn't be that bad.

Click to expand...

Just make sure the filter is clean. I don't use anything high-tech - just Wix.

You'll need an DMM, a thermometer, and probably a vacuum gage to test those sensors. The thermometer will be to take the temperature of the manifold (for the IAT, you compare the value at a given temperature with a table of known values,) you'll want the vacuum to check the MAP (I don't recall seeing a list of values for a given vacuum, but you can check to make sure it's changing,) and the HEGO is usually just checked by checking continuity/resistance in the heating element, but you should be able to read the voltage signal from it as well.

The tables you need will be in the FSM (Factory Service Manual,) but I believe you'll find them in a Chilton's manual as well. Get a service manual! It's money well spent, and it's the best tool you can buy for working on your rig! You should be able to find a Chilton's locally, or you can check eBay and other sources online for an FSM or a reprint.

99_XJ_ said:

lots of cold starts. But since it rarely reaches freezing now shouldn't it not happen to that degree? Faithfully shows 210 (or just a hair over). Now when it gets hot out it gets so hot that the AC fan kicks on regardless if the AC is on or off. I attribute that to my water pump dilemma. 4 water pumps in 7 months. No Idea why they are going bad. Yea probably a 195 t stat. Whatever the stealership put in.

Click to expand...

Check your fan clutch. Water pump fails, wobbling water pump shaft damages fan clutch, new water pump installed, wobbling fan clutch takes out water pump, and again, and again. The fan clutch not working properly will contribute to the overheating.

5-90 said:

Get a service manual! It's money well spent, and it's the best tool you can buy for working on your rig! You should be able to find a Chilton's locally, or you can check eBay and other sources online for an FSM or a reprint.

Click to expand...

Mostly agree. If you can't get ahold of an FSM, I'd recommend a Haynes over a Chilton. Close in price, but the Chilton I bought isn't worth the space it takes up. Haynes write ups make more sense (to me) and have better illustrations/photos imho. The Chilton pages do soak up a good bit of oil though; they're not good for much else.