Subaru seems to run better on 89 octane?

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Apr 29, 2012
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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.
 
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You seem onto this problem or condition....you have a lot of data and observations swirling about. I've read that water in gas is a thing of the past with the ethanol. Ethanol carries the water away. I suppose gas stationscan take advantage of the water attracting properties of ethanol and add water to gas but that seems like it could backfire. Could the higher octane fuel cause the engine to run in a "happier place", you ask? Sounds like it could to me. I always thought the 2.2l in my '95 Subaru was always straining. That engine had NO GUTS at all. No problem with that except that if it had no guts then why didn't it get higher mileage? It ran like a watch. After 138,000 miles (easily 3/4 open highway driving) it averaged 24.24mpg. Fairly low in my opinion.
 
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I'd go to Walmart and buy 3 bottles of Gumout Regane High Mileage. Buy THIS!! Dump two bottles in your tank, and fill it up (use 87, 89, or premium). Run this down to a half tank. Dump in 3rd bottle and re-fill, and run it down to a half tank. Than refill with just gas and run to near empty. Then see how your Subie runs on 87. My guess is that you have some build up in your combustion chambers and you've artificially increased the compression or created a "hot spot". This will remove the carbon build up (if there is any) and return your combustion chambers back to "normal" compression. it's my theory.
 
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My saturn did better on high-test though spec'd for 87. It burned oil which lowers the octane of the fuel/oil mix substantially. It remembered a particular RPM/ MAP pressure that knocked 100 miles ago and would give itself "flat spots" in acceleration. Modern cars use the knock sensor as a normal part of the feedback loop; it's not an "emergency timing pull" like the 1st generations were in the 80s. So you'll sometimes see a car advertised as 87 spec'd with "200 peak hp" where the peak is achieved with higher octane. That all said you might need a general engine douching. If you have an EGR system roto-root that out. Maybe a water drizzle in the throttle body. Reset the computer and see if you still have "flat spots".
 
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our 02 MDX had the same, very minor symptoms at very light throttle cruise, such as 35 mph. I found it to be the EGR kicking in and out since throttle input was so low. cleaning out the EGR valve and passages helped. I also cast a vote for carbon buildup in cylinders & valves. -m
 
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Originally Posted By: meep
our 02 MDX had the same, very minor symptoms at very light throttle cruise, such as 35 mph. I found it to be the EGR kicking in and out since throttle input was so low. cleaning out the EGR valve and passages helped. I also cast a vote for carbon buildup in cylinders & valves. -m
My Taurus used to do it along with pinging. Was the DFPE sensor causing it to open the EGR at the wrong times, creating a lean condition
 
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Cleaning the fuel system and EGR seem like a good place to start. Do you have a way to monitor engine data as you drive? With the Torque app and a Bluetooth adapter, I discovered that my 2001 Silverado has more knock retard on 87, a little less on 89, and still less with 93 octane. I compared the three cruising at highway speed in overdrive.
 
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Same issue here on the Rogue. Ran some Gumout with PEA and it ran better for 1 tank after the treatment. Have been exploring different stations and different octanes. Sometimes I get a combo/ mix that runs pretty good. I think I have a MAF issues as the tailpipe is coal black with carbon evem spreading down the outside if the pipe for a 1/4 inch back. Before the carbon tailpipe issue I got 27 -28 MPG and again had no carbon. I ask the dealer to scan for LT/ST trim but they say all is good. Well I rack this up to usual worthless dealer service wont fix car under warranty unless a part is falling off. Now I think the cat is plugged as the engine sounds like it has potato up the tail! Event leading up to black pipe was super heavy pollen for a month then I got a tank of bad gas. Never recovered. Can you read fuel pressure on a scangauge or is this a mechanical check?
 

TurboFiat124

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Originally Posted By: Ohle_Manezzini
89 from what brand you put in? Some have friction modifiers and your subie injectors stop jamming out in the light pulses range.
I have no idea. Petrol stations in the US are franchises. The don't sell their own brand of gas. From my understanding the oil companies that drill their oil here in the US, consolidate what comes off the refinery and enters a big pipeline from the gulf that runs throughout the US. One branches off in Knoxville, TN. So every station in my area get's their gasoline from the same source whether it's Exxon, BP, Shell, etc.
 

TurboFiat124

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What if I disconnected the vacuum hose on the EGR valve and plugged the hose off to prevent it from opening? I'm sure it would set off a code like on my 96 Ford Explorer. With the Explorer there was no issue other than the check engine light. There is a sensor that actually detects whether it's completely closed. The EGR was coked up and the valve would not close all the way. I removed it, hooked a vacuum hose on it, sucked on the hose to keep it open, threw it in my beadblast cabinet and cleaned it up. Problem solved.
Originally Posted By: meep
our 02 MDX had the same, very minor symptoms at very light throttle cruise, such as 35 mph. I found it to be the EGR kicking in and out since throttle input was so low. cleaning out the EGR valve and passages helped. I also cast a vote for carbon buildup in cylinders & valves. -m
What if I disconnected the vacuum hose on the EGR valve and plugged the hose off to prevent it from opening to see if that makes any difference? I'm sure it would set off a code like on my 96 Ford Explorer. With the Explorer there was no issue other than the check engine light. There is a sensor that actually detects whether it's completely closed. The EGR was coked up and the valve would not close all the way. I removed it, hooked a vacuum hose on it, sucked on the hose to keep it open, threw it in my bead blast cabinet and cleaned it up. Problem solved.
 

TurboFiat124

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Originally Posted By: eljefino
My saturn did better on high-test though spec'd for 87. It burned oil which lowers the octane of the fuel/oil mix substantially. It remembered a particular RPM/ MAP pressure that knocked 100 miles ago and would give itself "flat spots" in acceleration. Modern cars use the knock sensor as a normal part of the feedback loop; it's not an "emergency timing pull" like the 1st generations were in the 80s. So you'll sometimes see a car advertised as 87 spec'd with "200 peak hp" where the peak is achieved with higher octane. That all said you might need a general engine douching. If you have an EGR system roto-root that out. Maybe a water drizzle in the throttle body. Reset the computer and see if you still have "flat spots".
This car has always burned oil from day I bought it brand new. I was told this was normal for boxer engines due to the pistons being mounted horizontally. On a V8 or inline motor, Instead of oil draining back into the oil pan, it puddles in the cylinder.
 

TurboFiat124

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Originally Posted By: Rhymingmechanic
Cleaning the fuel system and EGR seem like a good place to start. Do you have a way to monitor engine data as you drive? With the Torque app and a Bluetooth adapter, I discovered that my 2001 Silverado has more knock retard on 87, a little less on 89, and still less with 93 octane. I compared the three cruising at highway speed in overdrive.
I've got one of those code readers that is suppose to monitor the engine while it's running then store it but I don't know how to use it! I printed off the manual to it, it's quite long!
 
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I had a similar issue with my 2005 Malibu Maxx with the 3.5 V6. I felt like I had a tail wind, then a head wind, then a tail wind, etc. The Germans call it "riding a horse" as it speeds up and slows down. It drove me nuts on long cruises so I was trying just about everything to get rid of it. It was more noticeable when I was in 4th gear with the converter locked. I didn't notice it in third gear hardly at all. I ran every fuel injector cleaner I could find through it, put new plugs and wires on it. Changed all the oxygen sensors, cleaned the throttle body, etc. This made it "less worse" than before. I was riding with a friend of mine who said: "That's easy, the MAF sensor was dirty! Just buy the MAF cleaner spray at AutoZone/Advance Auto/NAPA/Your Store of Choice and spray it out." He was absolutely right! The problem would come back every now and then, so I made a note when I changed oil every other time to remove the MAF sensor and spray it out. It takes just a few minutes on that car and would keep that issue from manifesting itself again. I also looked into different air filters to see if I could find one that might allow less super-fine material through, but as I just resorted to cleaning the thing every other change, I never really found any difference. It might be worth a try to spray out the MAF sensor with MAF sensor cleaner and see if it helps.
 
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I have 2 Subaru vehicles. One is a 2003 2.5 naturally aspirated engine. It states "87 octane or higher", the other is a 2002 3.0 engine which says "91 octane or higher recommended". I've run 87, 89, and 93 in the 2.5 engine, it does not seem to list any difference in values via looking at ignition on my Scangauge II or return differences in MPG that are outside of what could be margin of error. It may or may not run smoother on 89+, my butt dyno is not that finely calibrated. So, it usually gets 87 octane, and 93 on the occasional long interstate trip for the extra detergents (advertised by Shell for their premium) because...well, why not. The 3.0 definitely runs better on premium, you can tell the timing is pulled on 87. MPG stays within the same bracket, but the car runs so much better on it that I've only run 93 since I got it this year.
 
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Originally Posted By: TurboFiat124
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.
Depending on conditions, my 2.5 gets between 16-22 MPG on a city tank. Stop and go, weather, traffic, etc.
 

JTK

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I know with my 5spd equipped 2014 XV Crosstrek, the A/C compressor cycling was like flipping a power robbing toggle switch. You could REALLY feel it cycling on/off. With my CVT equipped Legacy and Forester, you don't notice it at all. In regards to fuel economy, newer non-turbo Soobs are great. I'm at 15K miles with my 2016 Forester 2.5i CVT and average a bit over 31mpg tank/tank. 5 or 6spd manuals reduce your MPGs a bit.
 
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TurboFiat124

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Originally Posted By: MichiganMadMan
I had a similar issue with my 2005 Malibu Maxx with the 3.5 V6. I felt like I had a tail wind, then a head wind, then a tail wind, etc. The Germans call it "riding a horse" as it speeds up and slows down. It drove me nuts on long cruises so I was trying just about everything to get rid of it. It was more noticeable when I was in 4th gear with the converter locked. I didn't notice it in third gear hardly at all. I ran every fuel injector cleaner I could find through it, put new plugs and wires on it. Changed all the oxygen sensors, cleaned the throttle body, etc. This made it "less worse" than before. I was riding with a friend of mine who said: "That's easy, the MAF sensor was dirty! Just buy the MAF cleaner spray at AutoZone/Advance Auto/NAPA/Your Store of Choice and spray it out." He was absolutely right! The problem would come back every now and then, so I made a note when I changed oil every other time to remove the MAF sensor and spray it out. It takes just a few minutes on that car and would keep that issue from manifesting itself again. I also looked into different air filters to see if I could find one that might allow less super-fine material through, but as I just resorted to cleaning the thing every other change, I never really found any difference. It might be worth a try to spray out the MAF sensor with MAF sensor cleaner and see if it helps.
I'll will give that a shot as well. I would imagine with the crankcase vapors being fed back into the engine, something is bound to clog up. By the way. I have a K&N air cleaner and someone said that the oil from the filter can coat something (MAP sensor perhaps)?
 

TurboFiat124

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What tells the EGR valve to open? I know it's vacuum but is it just manifold vacuum? Or rather when does it open? Under deceleration? I could clean it like I did on my Explorer but if something is malfunctioning telling it to open and slam shut then looks like if I plug the vacuum hose off to it, it would remain closed. I could at least rule that out.
 
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