Help with bad gas mileage and no CEL

[stir_fry_boy]

HELP! My 98 cam 1mz is getting about 20 mpg mostly freeway. New airfilter, new spark plugs (dealership), running Mobil 1 0w-40 full synthetic oil with no intake or mods. My Bank 1 O2 sensor recently went out on it (fixed it) so I don't know if the other ones are going bad....(I hope not because they are almost 200/ea). But no CEL. This car has been dealer maintained with 128k and fuel system cleanings have been a ritual. What else can I do? I was initally getting almost 28 mpg now I get 20 about 8k later. I did clean out the throttle body completely at 125k. WTF!?!? Ideas?

[Obelix]

better gas and the cat. convertor maybe...

[stir_fry_boy]

It is making a weird noise out of the cat sometimes (at least I think it is the cat when you can feel it in the floorboards). When it rains or snows, it sounds like it has an exhaust leak only sometimes, then after about 30 sec it goes away only to come back speratically. How long should the cat coverter last? The car has 128k on it now. I always use 89 octane or above in it. Are they hard to put in? Ideas?

[stir_fry_boy]

Yep, air filter is clean, same set of tires all the time (street) with 35 psi. I put a 1/2 tank of supreme in it this time to see what it does. Any other ideas?

[Mike Gerber]

Check to see if the thermostat is stuck in the open position. If it is, the engine will not reach operating temperature and will run a richer mixture resulting in wasted fuel.

Also check the ECT (engine coolant temperature) sensor for being in spec. It may be telling the ECU that the engine is constantly cold, resulting again in a richer fuel mixture. You will need a DVM and the specs from any manual for this test.

If those two things check out OK, then check for a dragging brake. Jack up each wheel and see how freely they all spin. If you notice one wheel that is much harder to spin than the others, you likely have found your problem. Inspect that brake for a stuck caliper/wheel cylinder or stuck slider pin(s) on a disc brake. You could also have a stuck emergency brake cable in the rear.

Remember, in Winter months you can expect to get 10%-15% worse fuel mileage. Don't compare yor mileage now to what you get in the Spring and Summer. Colder ambient air temperatures require longer warm-up times and a slightly richer mixture when running once the engine does actually reach operating temperature.

Mike

[stir_fry_boy]

New thermostat from the dealership that I put in (including the gasket). I have a manual, I should test that ECT. I actually am running a tank of premium in right now and it seems to be doing better. ALso,I was thinking that I run my car a few minutes before driving off and it is running at almost 2k rpm (lots of fuel) but fuel is cheaper than a new engine. I will check the ect and the brakes. Any other suggestions?

[myungx714]

shouldnt you use 5w-30 for optimum gas milage?
cuz i dont think 0w-40 has energy saving logo on the oil bottle

[stir_fry_boy]

I was going for optimum protection, I can't believe that it would make that big of a difference on gas mileage. It may though.
UPDATE: Running Premium Chevron 91 octane and still 20 mpg.

[stir_fry_boy]

bump....

[moocow]

Yeah my 98 Camry 4cyl is getting not so good mileage. I know the engine check light is on, and a computer diagnostic said the Fuel Air controller is bad...

However, another issue is that the temperature gauge needle only goes up 25% of the way up, it used to always hang right in the middle 50% of the way there...

[tacojet]

moocow,

See above, your thermostat is stuck open. Replace it and it'll fix it. The needle will go back to it's happy place

[stir_fry_boy]

Still no one has any ideas about how I can get bad gas mileage and no CEL?

[73sport]

Hey Fry Boy,
Auto or Manual?
If Auto Trans, try this:
Take it out on flat highway, stop completely. Overdrive off, ECT up for economy.
Aceelerate moderatly you should feel 4 distinct bumps as the trans goes from 1-2, 2 to lockup, 2-3, 3 to lockup.

Now repeat w/OD on. You should feel 2 more bumps. Trans goes from 1-2, 2 to lockup, 2-3, 3 to lockup, 3 to OD, OD to lockup. When I say "lockup" that is the torque converter clutch engaging for added fuel economy (like a manual trans).

Also, check your trans fluid for burned smell and correct level....

Point being, if the trans never goes into lockup, your economy will be in the toilet. Same for running with the ECT off - cause that seems to effect the shift points, lockup points, fuel mixture and the V6 Acoustic Control Induction System (ACIS) and it is all tied to the Throttle Position Switch TPS. So if aint the gas, and you don't have error codes - unknown.......

Good luck,

RA

[stir_fry_boy]

Update: I changed the oil to 5w30 Mobil 1, drained and filled the tranny (looked red as red can be), put a K&N filter in, and seafoam in the gas tank and in the crankcase. Lets see what it does now.

[nodrogkam]

that should help it out. good luck man.