View Full Version : Does anyone believe that exhaust leak can cause decrease mpg?
05-25-2010, 07:07 PM
I would say from my experience with this 95 F250
When I bought it only 9 mpg with 4.10. When I plug that exhaust leak mpg went 10.50-11 mpg.
But for some reason it keep pop exhaust leak where I patch.
I couldn't justify drive F250 daily due 9 mpg so I upgrade to 3.55 now I get 13 but I was wonder if I get 15 mpg.
I decided to replaced exhaust y pipe where it have hole. It turn to be Half of pipe are broke it leak tons. I haven't test with new y pipe because exhaust stud broke. I let you know in couple weeks when I get mpg.
I talk to several people they don't believe exhaust leak cause terrible mpg.
Can anyone prove it will cause terrible mpg when there are leak exhaust.
05-25-2010, 08:26 PM
If there is a leak before an o2 sensor, it can fool the sensor into thinking that you're running lean and then add fuel to compensate. Thus, lower mpg.
05-25-2010, 08:53 PM
In my experience with my Dodge ram 2500, I was getting 15.5 mpg. The muffler went bad and i had a leak before the cat. I didn't fix it right away because I don't drive it much but when i used a whole tank and did the math, it came up just above 14. I have since fixed the leak and replaced a sensor on the cat and am now getting 15.5 again. John
05-25-2010, 09:51 PM
It before oxygen sensor.
That why I remove to replaced one. Amazing it 15 years of plow get toll on it.
It was so thin it crack easily.
I hope mpg improve 2-4 mpg with new exhaust y pipe.
And cat converter is serious plugged. I don't see white screen it just full of carbon black.
Fun thing owner told they replaced cat converter 2 times because it commercial plow for 15 years. Why they replaced because they couldn't pass Federal inspector.
It only have 80K miles now with 18K miles on new engine.
I am consider remove cat converter because new one is $450.
05-26-2010, 06:15 AM
I am consider remove cat converter because new one is $450.
Removing a catalytic converter will send your emissions through the roof & fail any inspections you might need.
Can you clean yours or pick a used one up from a junkyard?
05-26-2010, 11:35 PM
Holy crap.......450$????.......I got the new one for the dodge for 175$. I used the generic version so i had to pick up an adapter and do some work, but it seemed to work just fine. Good luck. John :flag:
05-26-2010, 11:49 PM
It giant cat converter.
More close to golf club. Weigh about 30 pound. No reason to have another cat converter since it will get plugged again because I use truck for plow so that would waste my money if it was plugged.
My 91 bronco is only $180 for new y pipe with cat. It small cat like size of math book.
Was work and found #2 giant leaks. It was EGR valve's pipe to exhaust manifold were rot it leak so bad.
No wonder it so annoy on highway it sound like motorcycle.
05-27-2010, 12:23 PM
:biglol: You must be kidding ?!
In all seriousness, I don't know much about cars, but I learned when I was a kid that if you have a muffler that resembles a block of swiss cheeze, or in my case at te time, no muffler at all, you will not get as much power out of it due to a lack of back pressure.
If I understand this correctly, I think you would lose out on fuel efficiency for the same reason.
Perhaps if you have a torch you could cut a muffler & cat converter off one vehicle at the junk yard and then weld it onto yours ?
05-27-2010, 01:28 PM
Ok Why don't you put plow on that Subra you have.
And plow in 50 driveways which you can't do light on gas you need floor on gas to moving. It put much gas and stick to cat converter's screen.
If you check lot plow guys with Ford or GM truck they don't have cat because they get plugged fast. They only make like 50k miles in 7 years. -They only get 4-5 mpg with cat converter when they remove they get 7-8 mpg.
05-27-2010, 01:38 PM
Sorry, that wasn't meant as an insult or anything.
Put a plow on my Subaru ? Heh heh .. I probably wouldn't get very far .. for that matter I probably would not even be able to see over the plow :D
05-27-2010, 06:59 PM
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.
05-28-2010, 09:15 AM
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.
05-28-2010, 11:58 PM
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.
06-04-2010, 02:49 PM
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.
Hi Charlie and Phantom:
Thanks for the exquisite details...
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