I own a 2009 Subaru Forester with a 5 speed manual having 125,000 miles on it. The EPA fuel economy rating for this car is 22 mpg avg. 20 mpg city and 27 mpg highway; I have always been able to do much better than that using the cruise control, until just recently after having the timing belt replaced.
Our daughter lives in Pittsburgh and we make the trip from central Connecticut to Pittsburgh at least 4 times a year using two basic routes I-84 to I-81 to I-80 to I-99 to US-22 or I-84 to I-81 to I-80 to PA-28 using the cruise control set between 60 to 65 mph average 32 to 33.5 mpg on the way to Pittsburgh and 34.5 to 37 mpg on the way home except for the last 2 trips which were after the timing belt was replaced. On the way to Pittsburg 26.2 and 26.6 mpg and the way home 26.9 and 27.5 mpg, the second numbers were after a complete tune-up (plugs, wires, air cleaner and also an oxygen sensor) suggested by my mechanic to fix the problem, even though the engine diagnostics did not indicate a problem with the sensor, the engine computer had absolutely no error codes displayed.
Since retiring in 2011 my wife and I have made the trip from central Connecticut to eastern Colorado twice a year to spend two and a half month to help take care of my 99 year old father and give my two sisters a little vacation.
We have made this trip 8 times using the I-40 once and the rest of the time using either I-80 or I-70 just to break-up the monotony. This winter before making the trip I took my Subaru to have the oil changed and the tires rotated.
The mechanic at the discount oil service informed me that I had a broken passenger outer tie rod end, so I took it to the shop I have been doing business with for the last 25 years to have the tie rod end replaced and a front end alignment. One of the mechanics that normally works on my car asked if I have had the timing belt replaced as it was due at 105,000 mi and I had 115,000. I said no so go for it. On the first 7 trips I averages 32.4 to 34.1 mpg on the way to Colorado and 34.8 to 37.9 mpg on the way back, the lower numbers were during the cold winter trips. The last trip I got 25.9 on the way out and 26.6 on the way back. All these trips were made using the cruise control and a speed of 60 to 65 mpg as above.
While in Colorado I called my mechanic and he told me that it couldn’t be the timing belt as if it was only one tooth off the engine would eat itself up, but why then did this 20+ percent drop in fuel economy happen immediately after changing the timing belt?
It sounds like “The mechanic at the discount oil service” is probably something less than a real mechanic. You need a real pro.
Mechanic’s wrong, could be a tooth off and still work.
Its possible nothing’s wrong at all, and you are misinterpreting the mpg measurements you are getting. For example, outside air temperature has an effect on mpg. Warmer air produces better mpg. So if you got better mpg during the summer trips, and worse during the winter trips, that would be expected.
The next thing to look at for mpg problems is the coolant temperature. mpg is very sensitive to that. Make sure your thermostat is working correctly and the coolant is reaching the proper operating temperature. If it is cooler even by a few degrees, it will affect the mpg. Another possibility, the coolant temp could be ok, but the engine coolant temperature sensor could be off. The ECM will inject more gas if it thinks the coolant is lower than the optimum temperature. Ask your shop to check that sensor. They might have had to disconnect it to do the timing belt job. Maybe they forgot to reconnect it or not connected securely.
After that, anything else the shop forgot to reconnect could cause this, including grounds. Quite a bit of stuff has to be removed in order to change the timing belt usually, so there’s lots of chances to make a mistake by not reconnecting everything. On my Corolla there’s a ground wire between the engine and chassis for example I have to disconnect to change the timing belt, and it would be easy to forget to reconnect it.
Last would be the timing belt alignment is off a cog. It’s difficult to believe it could be off a cog, b/c setting the belt alignment isn’t much more difficult than dialing one of those 3 digit combination locks. Its hard to believe a good mechanic would make that mistake. That’s possible though, but I doubt it would produce much in the way of mpg differences. Might cause some pinging or some lag in accelerating, but mpg differences, not so much. The above posts are correct, a timing belt off one cog wouldn’t damage the engine. At least I can’t imagine an engine design where that would happen. Maybe in a Formula 1 car, but not likely a Forester.
If you want to be certain the timing belt alignment is correct, you have that right. It’s your car after all. I’m not sure what is involved in doing that on the Forrester, but on my Corolla it would be maybe a 20 minute job. Not that big of a deal. It’s a lot more difficult to change a timing belt than just checking the alignment.
I don,t know how hard it is to check the valve timing marks on a Subaru but it is the first thing I would want to do.
A cam can be off a tooth or two and the engine still operate but possibly not as efficently.
A legitimate severe drop in fuel economy (assuming it’s related to engine management) should sow a CEL or set codes for running too rich.
That being said, how is it that your fuel mileage is 7 to 10 MPG higher than the EPA figures before going in the tank so to speak?
Way back when my Chevette jumped a tooth, I’d estimate it lost 20% power. It then jumped another, and would barely move.
Have you changed the fuel filter? It made a difference for me. And are your valve gaskets tight?
If the driver’s side cam pulley is 1 notch retarded the ignition will be 8* retarded which will significantly reduce performance and mileage while not necessarilly causing any driveability problems or tripping the check engine light. And being off that far or even 3 notches won’t cause the piston and valves to collide.