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.