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Discussion Starter · #1 ·
For the past 3 tanks of gas, I've been getting worse mpg than normal. Not just off by 1 or so because of the extremely low temps outside. I'm talking 5mpg off what I normally get. I'm not getting any codes for this, or any real signs of anything. However, I talked with a couple people I know, and I'm thinking that perhaps my catalytic converter might be on its way out. Does this seem like a logical conclusion, or should I look elsewhere for my issue? The car does seem a tad slower than normal, but obviously I've no numbers to prove this so that's kinda moot.
 

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Discussion Starter · #3 ·
No intermediate pipe on it, well nothing aside from stock resonator anyway. It's a bit loud, but that's due to not having a rear muffler anymore. Small children only flee when I drive down the sidewalk after them.
 

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You could try pouring a whole can of "Seafoam" into your tank when you fill up to clean out the fuel system (pump, injectors, throttle body, etc.) to see if that works and maybe follow that up with adding a bit of it to your oil (following the directions on the can on both accounts) to help clear out the carbon from your oiling system and the potential varnish build up in your head (which should be followed by a complete oil change after a 1000 mis. or so).

You can get the stuff by the can at almost any auto parts store.
 

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Discussion Starter · #5 ·
Yeah I'm aware of the wonders of Seafoam, but with an oil cooled turbo, I'm not too keen on adding anything that might compromise the bearings in the turbo. I've done this before with a non turbo engine and it worked, but I don't think this is what the issue is.
 

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Yeah, the no asterisk means I was being serious.

When temperature decreases, so does the pressure on a control volume. Low tire pressure, more rolling resistance, consumes more gas.

As always, its just one more thing to check out. Especially if its been really cold like you said its been.
 

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If the cat is clogged enough to restrict performance, it usually breaks apart inside and the pieces get all lodged in it, that's what restricts the flow.

If you smack the bottom of the car with a rubber mallet and hear it all clinky and rattly inside, then yeah, you got a problem, otherwise you're gonna have a hard time finding anything wrong from an electronic standpoint without being able to view the data in the ECU as the car is running. O2 sensor out of whack, MAP sending signal off, coolant temp sensor, inlet air temp sensor, will all have an effect on driveability and on gas mileage, but without being able to see what they're doing (and knowing what they should be doing to compare that to) you might just get yourself frustrated trying to find something wrong.

Vacuum leaks will do this too.

Severely clogged cat will drive normally when driven gently and not accelerating hard, but will give you rough running / misfiring / stumbling on larger throttle openings and under boost - if you can build full boost correctly and it feels smooth, that's not your issue.

Drew
 

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Discussion Starter · #11 · (Edited)
Yeah, the no asterisk means I was being serious.

When temperature decreases, so does the pressure on a control volume. Low tire pressure, more rolling resistance, consumes more gas.

As always, its just one more thing to check out. Especially if its been really cold like you said its been.
Ok wasn't sure if you were serious, or just being an ass and didn't care. I'm usually the type to check my pressure every couple days, but what the hell, I'll look at it again. I know I've checked them within the last tank of gas at least.

Perhaps for the MA area, its not been terrible, but a consistent 18-20 degrees at night is unusual for VA. Also I'm not sure what they do to gasoline here in winter, so I've no idea if that is as much of a factor as it could be.
 

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Discussion Starter · #12 ·
If the cat is clogged enough to restrict performance, it usually breaks apart inside and the pieces get all lodged in it, that's what restricts the flow.

If you smack the bottom of the car with a rubber mallet and hear it all clinky and rattly inside, then yeah, you got a problem, otherwise you're gonna have a hard time finding anything wrong from an electronic standpoint without being able to view the data in the ECU as the car is running. O2 sensor out of whack, MAP sending signal off, coolant temp sensor, inlet air temp sensor, will all have an effect on driveability and on gas mileage, but without being able to see what they're doing (and knowing what they should be doing to compare that to) you might just get yourself frustrated trying to find something wrong.

Vacuum leaks will do this too.

Severely clogged cat will drive normally when driven gently and not accelerating hard, but will give you rough running / misfiring / stumbling on larger throttle openings and under boost - if you can build full boost correctly and it feels smooth, that's not your issue.

Drew
I've seen how a severely clogged cat makes a car run, and this isn't it. So maybe its just the shitty winter gas, combined with other stuff. I have a lot of new sensors on the car, although no new o2 sensors, so who knows. Wish I had an easy and free way to get a Tech2 plugged into the car and really see whats going on.
 

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try something simpler. Check your thermostat. Check the temp needle when ur going fast on the highway. It might be lower than you expect. If your t-stat is stuck open your car is never entering closed loop, and running super rich, burning fuel like a mother to try to warm up.
 

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Discussion Starter · #15 ·
Nope, tstat is not the issue. Temp comes up as quickly as ever, and coolant temp sensor is new. I checked some hoses Drew, per your suggestion. Well that was the problem. Had a tiny leak around the BPV, but not enough to hear it. Obviously I can only trust the SID but so much, however it did show a gain back to around where it should have been. That and did some research, and where I'm at in VA is famous for winter gas blends being less efficient.
 

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I had this problem. First, I had the thermostat stuck and replaced that. Next, I'm a fan of marvel mystery oil in the fuel which has helped clean the fuel system and injectors in many cars and cause my check engine light to go out. Next, check your vacuum lines for looseness. I had a loose old vacuum hose on the PCV valve cover bleed line that cause me this problem until I replaced it and then all my check engine and fuel problems have gone away. Vacuum leaks can cause this problem, but sometimes not register the check engine light. The car just think it's running lean so it compensates rich. How is the air filter? Does you car need the computer engine management sequence reset. I've driven my car in the sequence as described and this has also helped my gas mileage. It's as if sometimes my computer resets itself, or after time in storage I'll perform the ECM driving sequence. This assures my computer has all diagnostics and sensors checked.

I also like 30 mile or greater highways drives as they seem to clean my engine of carbon buildup.

Gas mileage should be better in the winter because of density altitude and that fuel and air are denser when it is cold. Consumption is higher during warmup as the engine runs rich to warm the engine quicker. Short trips will cause higher consumption.
 

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Yeah I'm aware of the wonders of Seafoam, but with an oil cooled turbo, I'm not too keen on adding anything that might compromise the bearings in the turbo. I've done this before with a non turbo engine and it worked, but I don't think this is what the issue is.
Following the directions on the can concerning the appropriate amount to add to your crankcase generally keeps the everyday commuter from screwing up their ride, be it turboed or not, so I believe you're safe.

If you wanted to be real particular about it, you could always pull your valve cover off and have a look inside to see what's what and if it requires any attention (the Seafoam for example) other than the usual maintenance (routine oil changes).

Short of that, I've never had a problem with pulling the turbos' oil feeder tube and pushing/pulling a pipe cleaner or two through it to make sure there's nothing restricting the flow.

The only other thing that occurs to me is what someone else mentioned about there being a vacuum leak, perhaps around the PCV nipple??

Good luck with the diagnosis and too bad that you're in Virginia, if you were closer I could pull codes for you to see if that came up with anything.
 

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Try 25 mis. x 22, @ about 91/2 hours, to make it to where I am which, I believe, is a bit out of the way for you.

Maybe you should try checking with a local AutoZone or PepBoys or some other auto parts stores to see if they'll pull the codes for you, some do.
 

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Discussion Starter · #20 ·
Try 25 mis. x 22, @ about 91/2 hours, to make it to where I am which, I believe, is a bit out of the way for you.

Maybe you should try checking with a local AutoZone or PepBoys or some other auto parts stores to see if they'll pull the codes for you, some do.
I have a scanner, which I used. Hence the part in the first post where I said there were no codes pertaining to the problem.
 
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