- Apr 29, 2012
- East Tennessee
For about four years, my 2003 Subaru Baja has developed this bucking condition under cruise. It idles fine, does not stall, does not throw check engine codes and runs fine under acceleration. It tends to do this in between 2500 and 3000 RPMS. If I shift into 4th or 5th gear it seems to go away. I can mimic this by tapping the accelerator. It seems to do this when my foot is barely rested on the accelerator. I initially thought I had water in my gas, E10 or a clogged fuel filter but I'm leaning toward a dead spot in the TPS. This maybe apples to oranges but I've heard talk here at work about people having issues with their 2 cycle weed wackers and leaf blowers not running well on 87 E10 and they run better on 93 non ethanol. I always run non ethanol 87 in my Trabant, lawnmowers and anything 2 cycle with no problems. I've always ran E10 87 in my Subaru because the owner's manual says to. Well used to run pure 87 until E10 became the norm. So yesterday I decided to top my '03 Baja (non turbo) off with 89 octane just for kicks. It's too early to tell but this morning while driving to work I did not notice any bucking at cruise and the car seems to run smoother. I realize higher octane fuel will NOT give you more power. High octane fuel is somewhat of a misconception. The higher the octane the more the fuel will resist detonation. Particularly in a high compression or turbocharged engines. I do run 93 in my turbocharged Fiat Spider and my turbocharged Yugo. Not sure if I really need it but typically 93 is usually mandatory in turbocharged engines. However modern engines do have an engine management system. If the knock sensor detects a "ping" it will back the timing off. Despite the fact we had this Ford truck at work which never saw more than 30 miles per hour and it pinged like mad, probably from excessive carbon buildup. Unless the knock sensor was bad or it had so much carbon buildup that was the best it could do. But I wonder if the system will advance the timing further when running higher octane fuel until a knock is detected. I'm sure it doesn't go crazy because the EPA has limits on things like this. Any chance running 89 has allowed the computer to advance the timing a bit more causing it to run smoother? Another apples to oranges story, My Fiat Spider, if the engine does not have enough advance, it will exhibit basically the same symptoms ,except worse like under acceleration. Back in the 1970s, the EPA said these cars had to run at TDC to pass emissions. But these engines are intended to run at 10 BTDC by the factory and even better at 15 BTDC. However while tinkering with the timing on my Spider and Yugo during the tuning phases after installing my turbos, I found these cars will buck and carry on if the timing is set below 10 BTDC. So I wonder if I am on to something. I'm not experiencing any kind of knock with my Subaru. Any of you guys had any experience running higher octane fuel in you Subarus. Could the 89 be masking something? I run 87 in my 1998 Chevy van with 142,000 miles on it and it runs smoother than my Subaru. My Subaru shouldn't need higher octane fuel because it's only got 77,000 miles on it and the owner's manual says to run a "minimum" of 87 octane. On a side note. My Subaru once needed new tires and just for kicks on a 400 (one way) trip to the beach I decided to over inflate the tires to 45 PSI and run 93 octane. I'd always top off before leaving the house and 300 miles later would top off again. I'd always get 28 mpg. This time I gained a whopping 3 extra MPG! Even if running that much tire pressure and high octane fuel did in fact improved the fuel economy, wearing out tires and the higher cost of fuel would not offset gaining 3 miles per gallon. One more thing. Last two times I've driven this car, I've noticed it is getting 20 MPG around town. Seems like it used to get 23 mpg around town and 28 on the interstate. It may just be my driving habits but does 20 mpg around town sound normal or a bit low for a 2.5 liter Subaru? I got new plugs, plug wires and fuel filter.