What is causing MPG drop and power loss?

subaru
outback
fuel-economy

#1

My 2001 L.L.Bean Edition H6-3.0 is losing power & MPG on the fwy. Sometimes it will feel like something is holding it back or dragging. I’ve checked the brake calipers and they’re not sticking, changed the air filter, oil & filter, transmission fluid, and still cannot find the problem.



I’ve overfilled the gas tank and read this can damage the charcoal canister.



Will a saturated charcoal canister cause a drop in power and MPG?



What should I check out next?


#2

Check engine light on?


#3

If your check engine light is on have the car scanned for error codes and post the exact codes (e.g. “P1234”) here. Many auto parts chain stores will read them for free.

Report age/condition of spark plugs & wires & fuel filter.

Have your fuel pressure checked and your exhaust system checked for restrictions.

Don’t overfill your tank anymore but this isn’t a charcoal canister issue.


#4

CEL is not on, the car now has about 149,500 miles on the odometer

spark plugs & spark plug seals were last changed at odom=141,000 (8,500mi ago in May 2010)
but no spark plug wires were changed

fuel pump module failed and was replaced at odom=131,000 (18,500mi ago in Aug 2009)

fuel filter was changed at odom=120,000 (30,000mi ago in Dec 2008)


#5

Nope, the CEL is not on, the car now has about 149,500 miles on the odometer

spark plugs & spark plug seals were last changed at odom=141,000 (8,500mi ago in May 2010)
but no spark plug wires were changed

fuel pump module failed and was replaced at odom=131,000 (18,500mi ago in Aug 2009)

fuel filter was changed at odom=120,000 (30,000mi ago in Dec 2008)


#6

Having had the fuel pump & filter replaced recently does mean that all is well with the fuel pressure. One would like to think it should be but for this kind of problem the pressure must be checked - along with the exhaust.

Short of that, a knowledgeable tech with a scantool should be able to drive it and get some idea of what is going on.

How is it at idle? Is the engine temp normal?


#7

Idle is about 750 RPM and engine temp seems just a touch high.

Not that it makes much difference, I forgot to add that at 141,000mi
I had the valve cover gaskets replaced, and when I got it back there
was a slight vibration at idle.


#8

No MIL? Really? Well in no particular order:

  • Perhaps some service person unplugged the MIL light rather than fixing something. Scan for codes anyway.
  • Does it tend to happen only in one gear or within a certain range of speeds? If so, check the transmission (slipping band in an automatic transmission, for example) and/or clutch.
  • Are you sure you’re not just driving into a headwind? :smiley:

I guess it could be fuel related, but I’d expect the MIL to be illuminated if you had a significant fuel supply problem more than one or two times within a few days. Maybe not. If you’re looking to diagnose it yourself, you should look into either buying, borrowing, or renting a decent scan tool. Some of the better scan tools can log telemetry while you drive. That would provide helpful info about how the engine is running, including fuel pressure, airflow, various temperature readings, etc. when the problem is actually occurring. Then, you can compare it with similar data when the problem is not occurring, and you might be able to spot the problem. I’m not sure if the loaners from Auto Zone can do that logging or not, but it would be worth asking.


#9

Get a compression test.

The check engine light should come on for a second or two when the ignition is first turned on.


#10

hey thanks, I’m taking it to get scanned at AutoZone later this week. Funny you should say so, but it can feel just like driving into a headwind - w/out the headwind. It may have been occurring in one gear, the “headwind” effect was usually when driving 50-70mph.


#11

My Camry behaved like that and the O2 sensor was bad. I replaced it and everything was much better; more zip, better mileage.

Rich