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1999 Subaru Impreza Problem:

  • Drastic reduction in mileage (about 50% of normal). It only happens on long trips after car has burned through about ¼ tank of gas. So for the first ¼ tank it gets 100% normal fuel economy, then we start to see a reduction in the next ¼ to about 75% or so, then for the last ½ tank it drops out completely to 50% of normal mileage.
  • The problem can be slightly mitigated by stopping and letting things cool down a bit. Stopping for 30 minutes on a long trip gets me back up to 75% mileage for a little bit.
  • It does not matter if we start the trip with a full tank or not, the problem seems to be more of a warm up issue

Things checked already:

  • Not a fuel leak; the gauge does not go down when the car is sitting
  • Just replaced the spark plugs and plug cables
  • New fuel and air filters
  • Does need an oil change but fluid is not too dirty and its barely overdue (just over 300 miles). I seriously doubt this could be it.
  • Front pads and rotors were replaced and did not help the problem. It also coasts in neutral the same as ever so I don’t think it is a brake issue
  • The car is slightly out of alignment, which needs to be fixed but I don’t think this would be the issue either. Alignment has been an issue for longer than the mileage problem.
  • All fluids seem to be normal, including transmission fluid.
  • Put in fuel injector cleaner and ran a full tank with that in but it didn’t help. (This doesn’t mean its not the injectors though.)
  • No sign of clutch slippage, plus it was replaced only 2 years ago

Things not checked:

  • There seems to be a leak in the exhaust somewhere, not sure when this happened. Maybe this could lead to one of the O2 sensors misreading and causing the fuel to be too rich? Haven't checked the exhaust to see if it burns black. It doesn’t seem this theory works with the whole warm up thing though. I would also think that this would throw a warning on the check engine light. I guess it depends on where the exhaust leak is; I haven’t looked into this yet.
  • If it is a "long" trip and the car has burnt through a 1/4 tank , then it is well-past warm-up... – Solar Mike Dec 3 '18 at 13:48
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    How are you measuring MPG? You do know that the fuel gauge over-reads when over 50% full and under-reads when under 50% with the last 25% of the gauge being significantly inarticulate. – Steve Matthews Dec 3 '18 at 13:52
  • I have heard of that but I dont think that is it. I have been measuring it for years the same way and its always been fine until now ...unless the gauge is broken. But when I go to fill up and put 15 gallons in, then divide the odometer reading by that, it is way less than it should be. – Gavin Graeper Dec 3 '18 at 14:07
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    "I dont think that is it." Please don't think. Measure – mike65535 Dec 3 '18 at 14:26
  • Only way to be sure is the "brim to brim" method... – Solar Mike Dec 3 '18 at 14:43
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Another answer gave the following advice:

I would increase the weight of the oil you use

Don't do this.

Manufacturers specify an oil viscosity for a reason! Running a "heavier" (i.e. thicker oil) than intended is not a good thing.

The thicker viscosity oils flow slower (pretty much by definition!), and this causes problems -

  • Localised high-oil pressures in the oilways - since it's designed for a certain viscosity if it's thicker than that certain parts won't cope with that and you'll get variances in oil pressure across the system. Bad things ensue.

  • Reduced cooling - one of the key things oil does for an engine is actually cooling it. A slower-flowing oil means that it's taking heat away at a slower rate, which gives you higher oil temps, which gives you worse cooling performance.. see where I'm going with this? In the extreme case you can end up with cavitation in the oil around the bearings!

  • Increased wear at cold crank - the thicker oil takes longer to circulate properly, increasing the time the engine spends without proper lubrication and therefore wear.

There are occasions to run slightly higher weight oils in Imprezas - but under specific circumstances such as very high ambient temperatures or in the case of modified cars doing extensive track time. As a blanket recommendation? It's pure idiocy.

Also:

if the car calls for 10W30 use 20W50 in summer and 10W40 in Winter.

Modern engines use multi-grade oils - changing between different oils in summer and winter is no longer required unless the temperature extremes take it beyond the operating temperatures specified by the manufacturer.

As for the causes for your issue:

  • Low oil level, low oil quality - particularly the latter could be a problem. If the oil has degraded to the point where it doesn't reach it's proper lubrication levels even when up to temperature then this can result in significantly reduced fuel economy because it's no longer cooling the engine properly. Effectively giving you the symptoms of heatsoak, which may explain why it takes a while to manifest and why it improves when given chance to cool down.

  • Reduced oil cooling - as above really, the oil gets rid of the heat through the sump as air passes over it. If something is preventing this transfer or reducing the airflow of cool air over the sump oil temps rise, engine cooling reduces, fuel economy drops. Check to make sure the undertray for the engine is properly seated and nothing is obstructing airflow.

  • Failing Mass Air Flow sensor - the MAFs on '99 Imprezas fail if you look at them wrong, it's a bit odd for one to be temperature sensitive but not completely unheard of. A bad MAF being at the root of so many strange problems for this age of Impreza is so common it's practically a standing joke!

  • Failing O2 sensor/inaccurate O2 reading - you mentioned an exhaust leak, the properties of exhaust leaks change depending on temperature (courtesy of thermal expansion) so if your leak is acting in such a way as to give incorrect readings from the O2 sensor in the exhaust when it gets hot.. well that would certainly torch economy.

  • Splits/leaks in the air intake - again thermal expansion can play a part here. Also worth looking to make sure there is nothing causing the air intake to suck in air from within the engine bay - this is significantly hotter than air from outside and leads to heatsoak and poor economy.

My recommendations at this point would be to get the oil changed and the exhaust leak sorted first. Then see where you are at.

  • About the MAF, always try cleaning them with MAF sensor cleaner or no residue contact cleaner before replacing them. They eventually get dirty and start reading lower airflow values than before. – Al_ Dec 7 '18 at 17:55
  • @Al_ if cleaning the MAF the OP would need to be extremely careful.. They are stupidly fragile, tbh cleaning an Impreza MAF is 50/50 whether you damage it in the process. – motosubatsu Dec 7 '18 at 20:20
  • When cleaning a MAF, the keys are using the proper cleaner and never touching the sensing element with anything at all. Just spray the cleaner so that it reaches the sensing element (and the IAT if included in the assembly), let it drip away and then dry. – Al_ Dec 7 '18 at 20:43
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Provided you are measuring correctly your consumption which means full tank to full tank an correct km measurement in between!!!

The sensor reading wrong values could actually be an issue here bu i would not imagine it to have such an impact. On the other hand it's one of the easiest things to rule out with a visit to a repair shop.

Also have you checked your intake system? A slightly cut, or damaged plastic pipe getting softer when warmed up by the engine could lead to a way bigger opening thus air not been measured-delivered properly in your motor.

Another unlikely thing could be your timing chain??? I believe your car has a hydraulic timing chain tension gear. Have you checked that there are no skipped teeth on your chain? This could affect consumption as well but would probably kill your engine way before...

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    '99 Imprezas have timing belts not chains - and you're correct in that a timing belt failure or skipping would be doing a lot worse than merely effecting consumption. Slipping aux belt could cause issues with consumption though (and would also potentially be temperature dependent) but I would expect some other symptoms (noise!) if that were the case. – motosubatsu Dec 4 '18 at 16:45

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