saab92x.com - Over 10% of all 9-2x owners registered

Saab92x.com is dedicated to the discussion of the Saab 9-2x sport wagon, based on the Subaru WRX.
Please register or login to post.

Welcome to Saab92x.com!
Saab92X.com is the premier Saab92X forum on the internet. Registered Users do not see the above ads.Please Register - It's Free!

#1 2020-01-03 10:13:33

rayjay
Member
From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 38

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)

Offline

 

Remove Advertisements Advertisement

Saab92X.com

Offline

 

#2 2020-01-03 10:17:13

bentmettle
Supporter
Supporter
From: Ann Arbor
Registered: 2005-11-16
User Number: 897
Posts: 2382
Website

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?

Offline

 

#3 2020-01-03 10:26:17

rayjay
Member
From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 38

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.

Offline

 

#4 2020-01-03 10:32:15

bentmettle
Supporter
Supporter
From: Ann Arbor
Registered: 2005-11-16
User Number: 897
Posts: 2382
Website

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?

Offline

 

#5 2020-01-03 10:51:51

rayjay
Member
From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 38

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.

Offline

 

#6 2020-01-03 11:28:10

bentmettle
Supporter
Supporter
From: Ann Arbor
Registered: 2005-11-16
User Number: 897
Posts: 2382
Website

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

Offline

 

#7 2020-01-03 12:44:57

rayjay
Member
From: Killingworth, CT 06419
Registered: 2015-05-31
User Number: 31513
Posts: 38

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.

Offline

 

Board footer

Site problems, bugs, or questions? Contact Administrator
All content copyright Saab92x.com 2005-2010
SAAB and the SAAB logos are trademarks of Saab Cars USA, Inc.