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#1 2020-01-03 10:13:33

rayjay
Member
From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 42

Sudden decrease in fuel mileage

I have a 2005 Linear with 137K miles. In November I replaced the alternator, battery, and accessory belts. In early December I had the valve cover gaskets, catalytic converters, exhaust system, and oxygen sensors replaced along with new spark plugs and wires. The coil had been replaced late summer. FYI - the timing belt was replaced four and a half years ago at 106K by the dealer I purchased the car from. As you can tell, I am retired and do not drive very much. But since I live in Killingworth CT my short trips are 10 to 15 miles round trip and trips to doctors or to Yale for procedures are 40 - 60 miles round trip. I also take occasional trips to North Shore MA, Northern NJ (my old stomping ground), and Silver Spring MD to visit friends. I use synthetic oil and a good oil filter designed for synthetic oil, which is changed every fall and spring by time not miles. I raced cars and motorcycles so I believe in preventive maintenance and changing consumables before they become a problem.

When the alternator and battery was changed I experienced a temporary reduction in fuel mileage, which I attribute to the ECU relearning how I drive. It dropped from 25 to 23 for the first tank of gas returning to 25 with the second tank. Winter gas seems to drop my mileage to 24, but since I do not have to drive very much in winter this does not seem abnormal. Now the problem, since the cat, exhaust, and plug change my mileage has dropped to 22 and does not seem to be coming back up.

I purchased name brand cats, Bosch Ox sensors, NGK plugs, and a "quiet tone" stainless exhaust which was just a little bigger (and throatier) than stock linear. The size difference between the cats and exhaust addressed by an adapter placed between the cats and exhaust that was linear size on one side and flared to the exhaust size on the other to hopefully reduce any turbulence caused by the sudden increase in exhaust pipe size. The only other modifications or deviations from stock are a K&N filter and a 2005 WRX intake snorkle which picks up cold air from between the hood and radiator cross member rather than from behind the cross member. These have been installed for some time and did not seemed to change anything except to make the car run a bit better, with better fuel mileage, at high highway speeds.

The thermostat was replaced just prior to my purchase of the car, and I am about to replace it again. My car seems to warm up surprisingly quick (to me) but never overheats in summer. The local Subaru mechanics have told me that Subaru's tend to warm up quickly and that it was not overheating in summer it was not anything to worry about.

I'd like to find out why my fuel mileage has dropped since the exhaust / cat change. Any ideas? What am I missing? Is there anything else that is going wrong>

Thanks, rj

Last edited by rayjay (2020-01-03 10:18:21)

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#2 2020-01-03 10:17:13

bentmettle
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From: Ann Arbor
Registered: 2005-11-16
User Number: 897
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Re: Sudden decrease in fuel mileage

How are you calculating fuel mileage?  Physical measurements based on how much fuel you use over a couple tanks? or some scangauge like tool?

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#3 2020-01-03 10:26:17

rayjay
Member
From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 42

Re: Sudden decrease in fuel mileage

I use an Excel Spreadsheet I found on line to record gas purchases ($'s and gallons) and mileage at purchase. It also does an overall calculation of gas purchases since I purchased the car. I know based on how you fill up the tank the mileage can change for one tank, but then looking at a few tanks you can get a pretty good idea of what the mileage is. I try to use the same gas station (one I trust to always have good gas) and try to use the same pump at least once a month (i.e. car in same position) to maintain consistency and comparability.

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#4 2020-01-03 10:32:15

bentmettle
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Re: Sudden decrease in fuel mileage

Since you are measuring how much fuel you are actually using, unless you've been letting the car warm up idling longer than previously, I don't have any serious insight to what is happening.

I try to not worry too much about FE in the winter since the fuel blends change, longer warmup and idle periods, etc.

I don't see any reason to touch your T-stat unless you are due for a coolant change.

What precipitated the replacement of the cats?

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#5 2020-01-03 10:51:51

rayjay
Member
From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 42

Re: Sudden decrease in fuel mileage

I like to replace t-stats with every other coolant change, just as a precaution. That's not from racing as when you race you rarely use a thermostat as it is just one more thing to go wrong and cause an engine to expire.

Cats were replaced based on ORB codes. I have an ORB reader as part of my tools. I had purchased the stainless exhaust because the now 14 year old exhaust was still on the car and was beginning to rust figuring that since I do not drive much the stainless exhaust was a way to have it last longer. At 137K I figured the Ox sensors were due for a change, and the ORB codes said the cats were failing (although I do not remember the exact wording). To replace the sensors easily you need to drop the cats and exhaust, so I figured it was a good time to replace everything. I now have a zipper (scar) up my back from an operation, so for things like replacing cats, sensors, and exhaust I have to also figure shop time as part of the cost. Cheaper to get it all done at once.

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#6 2020-01-03 11:28:10

bentmettle
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Re: Sudden decrease in fuel mileage

How does the car feel? same power as before- i.e. it didn't pull timing with the plug change?

Again, totally spitballing, but I'd probably try an ECU reset by popping off the battery terminal, and seeing if I noticed any change, and if I didn't, I'd just blame winter big_smile

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#7 2020-01-03 12:44:57

rayjay
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From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 42

Re: Sudden decrease in fuel mileage

To be honest it felt pretty much the same. At first I though it was a bit softer but I had come to the conclusion that I may have not been using the gas as hard as before due to slightly louder exhaust sound. You could be on to something as it does feel like a bit of timing has been pulled back. Can changing plugs on a '05 Linear change the timing?

I will try the ECU reset as soon as I can get to it.

I went to your website, and now realize I do not need to explain anything to do with racing.

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#8 2020-08-26 22:31:46

rayjay
Member
From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 42

Re: Sudden decrease in fuel mileage

I know it has been a while, but with staying home due to Covid I really haven't been going anywhere. Gas mileage is still down, but I have found something. The new gasket between the cats and exhaust has a slight leak. Since that is by the second oxygen sensor could that cause the drop in mileage by making the second sensor read too much oxygen?

I'm also about to clean and drain the coolant system and change the thermostat. I know I need to use a coolant that is kind to aluminum, but is there any advantage to going to water less coolants or something else?

rayjay

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#9 2020-08-27 12:02:45

snowphun
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From: Central CT
Registered: 2005-11-09
User Number: 890
Posts: 613

Re: Sudden decrease in fuel mileage

I would use Subaru concentrated coolant, the green stuff (not the blue 50/50). I can't see a small exhaust leak (which would blow out) causing such a drop in economy.

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#10 2020-08-28 16:20:35

rayjay
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From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 42

Re: Sudden decrease in fuel mileage

Thanks!!

rj

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#11 2020-09-01 09:59:23

bentmettle
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From: Ann Arbor
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Re: Sudden decrease in fuel mileage

did your fuel economy ever come back to normal now that it's summer?

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#12 2020-09-01 10:32:22

rayjay
Member
From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 42

Re: Sudden decrease in fuel mileage

bentmettle wrote:

did your fuel economy ever come back to normal now that it's summer?

Not really. It picked up a bit but it's still not where it was before. Even my highway mileage is down after thinking maybe it was just down around town. I am still checking things out, but with the "new normal" I haven't been driving that much.

I'm running 23 combined and 26 on the highway, where I ran 25 combined and 28 - 29 highway like clockwork. Winter blends here in CT seemed to only take 1 mpg from my usual average.

rj

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#13 2020-09-01 11:00:13

Rich-N-AZ
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From: Arizona
Registered: 2013-10-25
User Number: 11897
Posts: 90

Re: Sudden decrease in fuel mileage

You may have already covered this, but have you looked at cleaning the MAF sensor?

Have you checked tire pressure, alignment and wheels etc. for possible issues that could cause drag there?


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#14 2020-09-01 11:49:04

rayjay
Member
From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 42

Re: Sudden decrease in fuel mileage

Rich-N-AZ wrote:

You may have already covered this, but have you looked at cleaning the MAF sensor?

Have you checked tire pressure, alignment and wheels etc. for possible issues that could cause drag there?

Next on my list is to replace previously mentioned gasket between cats and exhaust, check the oxygen sensors, and then run Seafoam through intake to clean MAF and the rest of the intake system. Early spring and late fall I spray brake cleaner at MAF and around butterfly to keep them clean. I run fuel injection system cleaner through my gas every other month. I fill up pretty much every week, or every other week depending on if I use the car or how far I've driven. I rarely let my tank get below 1/2 except on long trips. So cleaner every other month is fine.

My tire pressures, and tire wear, are fine. I check the tires once a week, whether I've driven the car or not. Alignment was done ~year and a half ago when I had shocks and springs changed and new tires put on. Nothing in the front  or rear end is loose. Bearings seem okay and will probably be replaced over next year just as a precaution.

I have a K&N air filter, which I keep clean and lightly oiled. I have been using K&N's in the 9-2X pretty much since I purchased the car. I have grown in to spring and fall pollen and mold allergies which means I run the a/c on recirculate with a carbon cabin filter almost year round, but that is not different from when I bought the car.

The only thing that I haven't accounted for is changing the tires to 205/60x16 from 205/55x16. Slightly taller, and now I notice my odometer aligns with mile markers on highways and my speedo matches the digital readout on the speed warning setups towns around here like to set up. Before the new tires, odometer and speedo read high. The change in rolling diameter is only a few inches, which should not have been enough to cause engine lugging or anything else to change fuel mileage.

This change in fuel mileage happened right after the exhaust, cats, sensors, and spark plugs were changed. And unlike after the time when the alternator and battery were changed, meaning the battery was disconnected which reset the ECU, the mileage has not come back. I have also disconnected the battery a second time to see if resetting the ECU would effect anything.

rj

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#15 2020-09-01 12:05:49

bentmettle
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From: Ann Arbor
Registered: 2005-11-16
User Number: 897
Posts: 2387
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Re: Sudden decrease in fuel mileage

rayjay wrote:

Bearings seem okay and will probably be replaced over next year just as a precaution.

rj

This has always been a bit of a trigger for me, and I found a series of books and articles by an aviation maintenance leader that tends to agree. 
https://blog.aopa.org/aopa/2014/06/10/d … intenance/

In the case of wheel bearings, there is no failure mode that typically actually leads to any real peril.  The labor cost to replacing them is the same after they're making noise, so you don't really get any benefit, other than the privilege of throwing out parts that were still working fine and providing you value.

Likewise, except in very rare instances, there is no such thing as PM on clutches, and I'd argue for many people, rotors and pads.

Check out the link, it hinges back to some really interesting work done in WW2...

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