I replaced the left head gasket and the timing belt (from Subaru Dealer) on my 1996 EJ22 and now get very bad gas mileage (20 mpg). The engine sounds good but is it very possible to be off on the timing belt a tooth or 1/2 tooth and it still sound like it is running good? Also would changing just 1 head gasket mess the whole timing belt process up? Thank you for your help.

Yes.

Ulinkely - 20 MPG is more likely to be a combination of tire pressure, old O2 sensors, etc. It wouldn't idle right if it were off - you can't be 1/2 tooth off.

 

GD

IT HAS 164,000 MILES ON IT. iS IT TIME TO REPLACE THE o2 SENSOR, iS THERE A TEST TO SEE IF IT IS BAD?

agreed i've never seen one a tooth off run "exactly like normal".

 

O2 could be bad and affect gas mileage, it would be the front one not the rear. but there's no gaurantee you'd have to analyze the O2 sensor signals to test.

So if I replace the front O2 sensor and notice mileage is normal again I don't need to replace the rear one? It's always nice to save a buck.

So if I replace the front O2 sensor and notice mileage is normal again I don't need to replace the rear one? It's always nice to save a buck.

yep the good news is that it doesn't matter if you replace the front one or leave the one that's in there, or install a known bad O2 front sensor with 2 million miles on it - the rear sensor has nothing to do with gas mileage so you can ignore it.

Thank you for the specific info. Just got done installing the front O2 sensor and will take the other back to the parts store.

...I'm no expert, but I have to disagree that the belt being off a tooth wouldn't hurt gas mileage. I had that happen to me before on a '98 Legacy L with the 2.2 VIN 3 motor.

 

From the time my wife bought the car to when I changed the timing belt, the best gas mileage the car ever got was in the low 20's mpg. We just figured it was because of high miles and the AWD. It seemed to run okay, for a 4-cyl...no noticeable missing or rough idle, etc. I ended up changing the timing belt while performing a repair that involved tearing down that far, anyway (crank pulley...long story...but had to check the timing gear while I was at it). The timing marks didn't line up. The left side cam was off one tooth on the belt. I installed a new belt, making sure that the marks were properly aligned, and slapped everything back together. Believe it or not, when we tracked the gas mileage the next week, the car was doing just about 30 mpg. I couldn't believe that one tooth made that kind of difference, but there it was.

 

I wouldn't rule out double checking your timing marks.

Edited February 5, 201115 yr by Camaro98

This leads back to the old observation, if it was fine before the work was done, and now it is not, it is the work taht was done that is suspect.

 

The true test is go out and buy a 19.00 vacum gauge and put it on the car and see what it is reading.

 

http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

 

Just telling someone to replace a part (which BTW is prbbly old enough to be replaced) without checking the original work is foolish.

 

Cam to crank ratios are 2 to 1, so it is very possible to have one of the three cogs off and the car run where it seems to be running well, but if it dumped the mpg it is not running well.

Wow...alot of information but all doable. I just replaced the O2 sensor and if the mileage does not go back to 30 mpg then, as was pointed out, my prior timing belt installation is probably off a tooth which is an easy fix. Thank you for all of the excellent input and problem solving.