Need help recovering 2 MPG
09-20-18, 02:08 PM
#1
Driver
Thread Starter
Need help recovering 2 MPG
I've own my '93 for almost 2 years now and am guessing the it has about 115,000 miles on it. The car has always run exceptionally well but suddenly dropped from the mid 15 mpg to the mid 13 level. I know everyone is thinking caps but I just changed them out resulting in marginal improvement in acceleration. My driving habits haven't changed, I'm a oldster more concerned with mileage than performance. Also changed out that often mentioned temp sensor last winter with no improvement. Just had the emissions checked at DEQ and passed with flying colors. On top of that, I add Lucas to every fill up.
09-20-18, 02:19 PM
#2
The pre-cat O2 sensors are also key contributors to the efficiency of the car. Toyota recommends 100K intervals for both banks. I do fairly regular runs when the LS is out and for whatever reason this may vary 1-2 MPG. My usual trips to San Diego 100 miles one way I return home averaging 24.4 MPG. Yesterday's run was at 21.3 MPG. Disappointing because the previous trip was really good at 25.6 MPG in December. Colder air temp, winter gas formulation... With the A/C on it usually does the trip in the 23-24 MPG range. May be that particular gas fill too.
Last edited by RA40; 09-22-18 at 10:18 AM. Reason: terminology update
09-20-18, 03:29 PM
#3
The bank 1 O2 sensors are also key contributors to the efficiency of the car. Toyota recommends 100K intervals for both banks. I do fairly regular runs when the LS is out and for whatever reason this may vary 1-2 MPG. My usual trips to San Diego 100 miles one way I return home averaging 24.4 MPG. Yesterday's run was at 21.3 MPG. Disappointing because the previous trip was really good at 25.6 MPG in December. Colder air temp, winter gas formulation... With the A/C on it usually does the trip in the 23-24 MPG range. May be that particular gas fill too.
09-20-18, 07:14 PM
#4
Pre-cat O2 sensors analyze the exhaust oxygen content and adjusts fuel trim delivery to the cylinders determining rich-lean or on conditions. Malfunctioning pre-cat sensors can misinterpret the content and adjust the air fuel ratio incorrectly. That often is a rich condition as they age. That rotten egg or if too much for to long an overwhelmed cat that ceases to function properly. Post cat sensors will relay a catalyst function error. Both are important for proper engine function, the pre-cat came to mind first.
Last edited by RA40; 09-20-18 at 11:29 PM. Reason: update terminology
09-20-18, 08:34 PM
#5
Moderator
Bank one analyzes the exhaust oxygen content and adjusts fuel trim delivery to the cylinders determining rich-lean or on conditions. Malfunctioning bank 1 sensors can misinterpret the content and adjust the air fuel ratio incorrectly. That often is a rich condition as they age. That rotten egg or if too much for to long an overwhelmed cat that ceases to function properly. At that point the Bank 2 will relay a catalyst function error. Both are important for proper engine function, the bank 1 came to mind first.
The air to fuel ratio of each bank is checked by the pre cat O2 sensors. The efficiency of the catalytic converters are checked by the post cat O2 sensors.
Have come to regard "bank-1" as pre cat from the I-4 and I-6 layouts.
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09-22-18, 09:38 AM
#8
I've own my '93 for almost 2 years now and am guessing the it has about 115,000 miles on it. The car has always run exceptionally well but suddenly dropped from the mid 15 mpg to the mid 13 level. I know everyone is thinking caps but I just changed them out resulting in marginal improvement in acceleration. My driving habits haven't changed, I'm a oldster more concerned with mileage than performance. Also changed out that often mentioned temp sensor last winter with no improvement. Just had the emissions checked at DEQ and passed with flying colors. On top of that, I add Lucas to every fill up.
Any chance your gas supply has changed to cause this drop?
Has tire wear increased?
09-22-18, 10:13 AM
#9
Racer
I get about 18mpg driving about 75% city. I was getting a P0420 code so I did the o2 sensor antifouler stacking and spaced them out from the exhaust stream and it got rid of it. If I do purely highway I think I can get 27mpg
09-22-18, 11:25 AM
#10
Driver
Thread Starter
Okay, maybe it's a environment issue having 90 degree summer and the occasional air conditioner blaring. But so far I haven't seen a return to the old mpg with the milder fall temps. Just reviewing the emission report and with numbers like 18 ppm HC and .0107 ppm CO I can't imagine there is a mechanical issue.
09-22-18, 11:39 AM
#11
Have you gone from non ethanol gas to ethanol infused gas? The smog results can also be interpreted that the cats are still within normal function not necessarily the air fuel ratio is drastically off to be a noticeable problem. For the sake of it may want to test the O2 sensors now that they are over 100K and well aged. When I changed the O2 sensors the emissions didn't change much but the MPG did go up.
09-22-18, 04:18 PM
#12
Driver
Thread Starter
Alright you talked me into replacing the upstream sensors. Unfortunately the best price I've found is $52.79 from RockAuto. Did a search on this sight but couldn't find how to test them.
09-22-18, 07:42 PM
#13
That is the going rate. Amazon varies at times for the Denso versions, the upstream for ours was $45 (sale) and downstream was $57 (regular). O2 test procedure is on other sites if you want to do that. For the hassle to get under the car you'll save time to swap them and be done. One less maintenance item.
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