jt_2002_ta

iTrader: (3)

Ok,so I'm getting terrible gas mileage(150mi) per tank replaced o2 sensors in front they looked really black like they was fouled. After replacing them got great mpg for about 300 miles. Now I'm back to the same crappy mpg. Seems to run ok though, and also at my muffler there is a lot of black soot leaking out. Could the tune be off or is there something that will cause it to run really rich?

LS6427

iTrader: (2)

Oil blow-by is the biggest killer of an 02 sensor.

Thats why auto manufacturers are idiots for designing PCV systems the way they do, its pathetic. Yes, they work, but the smallest failure or a valve sticking open and it ruins other expensive parts and makes the engine run terrible.

Black soot means you are running rich.

Check for vacuum leaks that will make you run more fuel, foul the sensors, then they will cause you to run even richer.

OR

Oil blow-by or a PCV issue causing allot of oil to go through the intake.

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DisasterFormula

iTrader: (1)

Any mods to it?

jt_2002_ta

iTrader: (3)

Lid,long tubes,85mm mass air,stall,

LS6427

iTrader: (2)

Donn't know how long the LT's have been on there or if its a recent install....but they often melt 02 sensor wires if they're not tucked up properly.

PCM's fail as well, and usually cause lean/rich issues when they're on their way out.

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jt_2002_ta

iTrader: (3)

So I should replace pcv valve and see what happens, any way to clean the o2 and reuse or are they shot. It does use oil.

LS6427

iTrader: (2)

How much oil does it use? If the PCV is actually stuck open, you'll suck in allot of oil.

Is the oil burn because of high mileage?

They really can't be cleaned. They cost around $$35.00-45.00 each on rockauto.com or amazon.com. Denso 2344025 should have long enough wires. 16" long. They fit mine with my LT's.

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jt_2002_ta

iTrader: (3)

It just turned 100,000. It uses about a qt every 300-400 miles.

LS6427

iTrader: (2)

Well, a bad PCV issue cannot suck that much oil. Thats definitely whats killing the 02 sensors so early. Its also causing you to run very rich. But the PCV valve can be a heavy contributor of the oil burn. Get a new PCV valve in there. While you're at it, take a flashlight and peak inside the intake through the TB blade, see if its all wet with oil. Maybe stick something in there, see if it comes back with oil on it.

You could also have a bad ring(s) or your cylinders are way out of round.....but this would cause oil burning and smoke out the back pretty much all the time at all rpms.

OR.....you could have bad valve seals. If so, a valve job would stop pretty much all of your oil burning. A leakdown check will tell you that.
Or, do you get a quick cloud of white smoke when you nail the throttle from a 50-60mph roll, then as you stay on the throttle it clears up pretty quick?? That means bad valve seals.

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jt_2002_ta

iTrader: (3)

It doesn't smoke at all that I know of, so I will replace the pcv this weekend,hopefully that fixes it. If not I guess I got a excuse to do a h/c swap! Not a bad thing thanks ls6427.

LS6427

iTrader: (2)

Sure....

I had bad valve seals about 6 years into owning my 427ci....the oil burn was horrendous...just as bad as yours......I took them off, had them rebuilt. Zero oil burn.

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It'llrun

If your check engine light is on, take it to a local parts store and hook it up to the OBD II tester.... Once you've done that, you may choose to clear the light, but it's probably best if you leave it on and go to a real shop and have it tested again(for a more complete test), then have it fixed.

If your engine is going through a quart every 3-400 miles, you've got to be seeing smoke, or you've got to have a really oil covered chassis by now. Find the leak or blowby and repair it.

jt_2002_ta

iTrader: (3)

A qt may be a little much 1/2 qt for sure, no smoke no codes. Chassis is pretty clean, I have started to notice on exhaust after the cats and at the muffler there is a lot of soot.

LS6427

iTrader: (2)

You're burning the oil, its making you run rich. I went through this. Black soot on the tailpipes and it started getting on the bumper just above the pipes. Fix the oil burning and it will fix the 02 sensor issue telling the PCM to make things run rich, which is what the soot is from.

Too much oil throws off the 02 sensors air reading and they add fuel.

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