Totally befuddled by drastic / fast drop in gas milage??

yabadabadoo1026 · Post by **yabadabadoo1026** » 26 Jun 2024, 10:51

got 2001 S60-----my norm was around 21.5 mpg and 8 or 9 months ago i over a very short period dropped to about 17 mpg and cant make sense of it. No check engine lites to suggest some equipment failure, runs good with no misses, idles pretty smoothly. I've check the air cleaner, sprayed the MAF and throttle body and check tire pressures which could only have negligible effect anyway. Have not changed plugs because again no misses and running /idling smoothly. My thoughts only a check engine like situation could have such major impact on mileage and ive had no light on. ANY THOUGHTS AT ALL??. Thanks folks

Post by **volvolugnut** » 26 Jun 2024, 13:17

Let's think about this. Your MPG dropped about 20 percent. This indicates the efficiency dropped 20 % or the load increase 20 %.
Have you added weight to the car? Tires under inflated? Steering alignment bad? Added a cargo box on top?
Engine drag may have increased: Dragging steering pump, alternator, idler pulley, AC compressor?
Changed to thick oil? Over filled with oil? Low oil?
You might have a loss of power in one cylinder (1/5 = 20%). Bad valve? Low compression? Bad plug, coil or trigger wiring?
Some things will not throw codes. It is back to old school investigation.
volvolugnut

yabadabadoo1026 · Post by **yabadabadoo1026** » 26 Jun 2024, 14:58

thanks for the thoughts but none of those you mentioned-----if it were anything mentioned in paragragh 2 it would reflect noticeably in how its running, just fine with no loss in power or misses as a bad plug or coil would suggest. Guess ill remove them at least and take a look though its good performance now suggests Ill find no issue

Post by **scot850** » 26 Jun 2024, 15:18

Have you checked if any of your brakes are dragging, especially the parking brakes? Check the shoe condition if they are the originals as they may have separated and causing a drag. Rear caliper can cause a drag and not be as noticeable as the fronts.

When were your O2 sensors last changed?

Is your thermostat jammed open making the car take longer to heat up?

Have tested your coolant temp sensor to make sure it is reading correctly (and no, not at the gauge)?

Neil.

Post by **jonesg** » 26 Jun 2024, 15:38

yabadabadoo1026 wrote: ↑26 Jun 2024, 10:51 got 2001 S60-----my norm was around 21.5 mpg and 8 or 9 months ago i over a very short period dropped to about 17 mpg and cant make sense of it. No check engine lites to suggest some equipment failure, runs good with no misses, idles pretty smoothly. I've check the air cleaner, sprayed the MAF and throttle body and check tire pressures which could only have negligible effect anyway. Have not changed plugs because again no misses and running /idling smoothly. My thoughts only a check engine like situation could have such major impact on mileage and ive had no light on. ANY THOUGHTS AT ALL??. Thanks folks

a vacuum leak is countered by adding fuel to the mix by the ecm.
theres a need to read fuel trims with vida or other capable reader.

deano1 · Post by **deano1** » 30 Jun 2024, 06:37

I had to replace those "L" shaped boots at the intake manifold on my 2002 S60 n/a as they were totally dry rotted and causing unmetered air into the engine.
I believe they are for warm air recirculation at the air box.
I had been chasing the reason for my poor idle for a long time and it was for something as simple as that.

Post by **Krons** » 30 Jun 2024, 07:33

You don’t list the miles but I’d do a thorough preventive maintenance: fuel filter, plugs, coils, replace EVAP hoses under the intake, Chemtool B12 for a few tanks to clean the fuel system. That would rule out the easy stuff and you could proceed from there.