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Fuel Economy Discuss how to achieve better fuel economy.

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Does anyone believe that exhaust leak can cause decrease mpg?

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Old 05-27-2010, 06:59 PM
makita makita is offline
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Join Date: Apr 2010
Posts: 37
Re: Does anyone believe that exhaust leak can cause decrease mpg?

Sorry but I have hard time explain why I remove cat converter.



Predicted you are plow but you can't do anything to save gas because it wouldn't push snow.

Floor gas pedal half way rpm would be around 3k then stop then put in reverse then 3K rpm.

Think keep repeat do that would put excess fuel unburnt and tons carbon.


Oil I change will not last 2k miles it get very very black sludge so I only can change 1K miles while those miles were PLOW. No highway drive.

I will try get video how plow guy drive their plow truck you will be surprised that they are worst on gas. I try use little on gas but sometime it can't moving heavy snow so end floor to roll it.
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Old 05-28-2010, 09:15 AM
phoebeisis phoebeisis is offline
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Re: Does anyone believe that exhaust leak can cause decrease mpg?

On another forum-GM trucks- a very mechanically adept member suddenly lost 3 mpg(from 13mpg to 9 mpg with a ~96 Suburban 4x4).

He very systematically went thru a VERY LONG diagnosis and fix.
This is what happened, and yes at the core-the cause-was an exhaust leak.

1)Dropped 3 mpg(about 25%)
2)Discovered exhaust manifold leak
3)Fixed leak-no improvement.
4) He was getting a code that indicated knock, and that the knock sensor was sending a message to the ECU retarding the timing waaaay back. The timing wasn't as advanced as it should be so at part throttle the relatively small amount of precombustion mixture wasn't getting "touched off" soon enough-so it wasn't being burned very well, or giving as good a push as it should.
5)He also had what I considered "too good' compression readings.I forget the particulars but his numbers-on a 150,000 mile tow vehicle- either dead perfect or maybe 10 psi better than perfect.

6)Long story short the exhaust leak was read as too lean-too much O2 in exhaust gas,
7) So the EFI was 'told" to spray in more fuel.
8)The too rich mixture and too little ignition advance meant this extra fuel wasn't burned, and it just carboned up the compression chamber-raising the compression and also making it knock.

9) He fianlly used some GM top end cleaner-he sprayed it in the spark plug holes(he could actually see the buildup thru the holes) and let it sit-then started the engine and watched it blow horrendous smoke for 5-10 minutes or so.

10)MPG finally restored to normal.

Big aside-yes 13 mpg isn't great-but in a hilly area-NE- NH maybe-with a 4x4 it isn't too bad.
The same setup-2WD 1998 Suburban 1/2 ton will get an honest 21 mpg on 3000 mile all hy trips with the CC set to 65 mph, but all city, town and road work limits observed. Actual average driving speed is probably 60-62 mph.The only hypermiling tricks are 45 psi, and d/cing the cc uphill and DWL, and motor on P&G thru small towns and cities.
This is with 1200 lbs of passengers and cargo with a cargo basket on a 2" receiver hitch.

Yes, exhaust leaks can kill mpg, and fixing the leak doesn't always cure the mpg drop if the combustion chamber has become carboned up.
Charlie
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Old 05-28-2010, 11:58 PM
makita makita is offline
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Re: Does anyone believe that exhaust leak can cause decrease mpg?

Plan clean carbon after I get gas tank to E for calculate mpg.

But for some reason that fuel gauge seem to be slower than normal. Before that Half tank it would be 90 miles. Now it 154 miles half now. Before that It was real E it only 145-150 miles out 14-16 gallons.


I hope I see 15-18 mpg that be awesome.
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Old 06-04-2010, 02:49 PM
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Phantom Phantom is offline
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Re: Does anyone believe that exhaust leak can cause decrease mpg?

Yes an exhaust leak can hurt gas mileage if before the O2 sensor it will think it is lean and add fuel, if after the O2 but before the cat the extra clean air and cause the cat to heat up so the vehicle will go into cat protection mode where it dumps fuel to cool the cat.

If the leak is post cat it COULD effect the mileage because the leak CAN cause turbulence in the exhaust and that will slow the velocity of the gases causing a reduction in exhaust scavenging that will increase backpressure and result in a loss of power that can attribute to worse mileage. Backpressure in never good people think it is needed because they will go to a 4in exhaust and lose performance and they think it is due to the lack of backpressure but in reality it is because of the lack of exhaust scavenging.
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Old 06-11-2010, 03:20 PM
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xcel xcel is offline
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Re: Does anyone believe that exhaust leak can cause decrease mpg?

Hi Charlie and Phantom:

Thanks for the exquisite details...

Wayne
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