poor gas mileage

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  #1  
Old 10-21-04, 08:10 AM
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poor gas mileage

I have a 1996 Chrysler Sebring Convertible(4.2 liter). I bought it used in 2000 and it had an oil leak that wound up being the head gasket. The dealer had it fixed at a shop that would fix it for half of what Chrysler wanted. Needless to say I had to have the head gasket replaced 8 months later by Chrysler. Chrysler also replaced my manifold and told me it was cracked and that I would get better gas mileage now that they replaced it. I'm honestly not sure what else they did if any thing at that time, but I went from getting about 215 miles to the tankfull to getting 315 miles to the tankfull. It was great. About a year or so later my gas mileage went back down to 215. When I asked my personal and trusted mechanic about checking my manifold, he told me that if the manifold was cracked I'd actually probably get better gas mileage (something about air getting in). Since they don't want to cost me unneeded repairs I haven't done anything further, but with the price of gas these days I'd like to figure out if there's anything I might look into to increase my mileage. I get 16 miles around town (which sounds really really bad to me, might as well have an SUV) and got around 30 on a highway trip recently (which sounds like an incredible difference to me). I had added a 3in1 additive before the trip, but I always get much much better highway miles even without it. And it went right back down after my trip.

Who is right about the manifold crack?

I hope I have given you enough information.

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  #2  
Old 10-21-04, 11:33 AM
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Which manifolds are we talking here.

intake manifold: worse gas mileage
exhaust manifold: possibly better gas mileage, Much louder car.
 
  #3  
Old 10-21-04, 11:54 AM
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cracked exhaust=better fuel mileage?

not so...fuel delivery corrections are accomplished by reading oxygen sensor voltage. o2's generate anywhere from about 100-900 millivolts (a good one anyways) with low voltage indicating a rich exhaust and high voltage the opposite. the corrective action taken by the controller would be the opposite of the indicated running condition...so...if you have a lean indication, then the controller "adds" fuel and vice versa. lot's of oxygen indicates a lean condition and fuel is added accordingly...if the manifold is cracked, atmospheric oxygen is pulled into the exhaust during negative exhaust pulses...this extra oxygen "tricks" the sensor into producing a false lean indication and the controller adds fuel...thereby lowering fuel economy
 
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Old 10-21-04, 01:15 PM
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I stand corrected.
 
  #5  
Old 10-21-04, 03:14 PM
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Wow! You're dealing with a woman who has no clue what you are talking about! LOL

My car is not "loud" and I don't really know which manifold they replaced a couple of years ago. Is there a way to check the manifold without disassembling the whole car? Something my mechanic can plug into it to check it? And if it is cracked, what might be causing this to keep happening? And if it's not cracked, do you have any other suggestions for what might be causing the terrible gas mileage?

Like I said earlier I have not persued this with my mechanic because he didn't seem overly concerned about it, but with the gas prices these days maybe the extra maintenance on the car would be worth it.
 
  #6  
Old 10-21-04, 06:13 PM
Dadealus
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How does the car idle? Stable? What RPM? How many miles on the car? Could be more than one thing...bad O2 sensor for example. For comparison my v8 truck gets 10-13mpg around town and 25 on the highway, partly because my daily commutes are fairly short.
 
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