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I have been getting really lousy gas mileage in my '97 (16-17mpg, both city and freeway.) I've been wondering if I have a bad sensor or vacuum valve somewhere but I wanted to hook an OBD II code reader to see what I could find. My check engine light has never turned on, however. I was wondering: does the diagnostic system report any codes despite the fact that my "Check Engine" light is not on? If I hook up a diagnostic code reader will it tell me anything at all or do me any good? I'll do anything to fix this gas mileage problem. Thanks.
 

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If you have any luck, let us know, I believe this is a common problem for most of us here, I too have the 97 rav4 4door, and its the best car I have ever had, but I wish the fuel economy would be better, keep us posted,, thanks,,,Joe,,,
 
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I'm having the same problem. I bought a scan tool to try and diagnose the problem as well. My 97 4dr auto never had the check engine light come on, but am getting the same abd mileage. If the check engine light isn't on, it probably deosn't have any codes to read. I can scan for codes and pending codes. The only time anything has shown up was when I caused a code to make sure the reader worked right and when I accidently left a couple vacuum hoses disconnected. Oops.

As far as diagnosis, it works great. A code reader will do just that, read codes. If there are no codes, it probably won't help. With the scan tool, I am able to watch my O2 sensors, fuel trims, calculated load, intake manifold pressure, etc. I have been using it for a few months now trying to pinpoint a problem. No luck yet. I don't have any reference numbers to judge against to see which readings might be out of whack. The only out of ordinary things I've seen are that my calculated load is always really high @ 47% load sitiing still in drive (this number is calculated using the MAP sensor readings), and that it seems to switch out of closed loop mode anytime I'm over 65mph or even mildly accelerating.

So if anyone is famililar with what types of numbers I should be getting as far as intake manifold pressure, load, fuel trims, etc... let me know and I'll keep note of my findings. Or, if there's anyone in Oregon nearby with a RAV4 that's getting good mileage that wouldn't mind me checking a couple readings, let me know. Maybe we can figure out this mileage problem!
 

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One thing to consider is when the last maintenance was done, such as your regular tune-up stuff: spark plugs, wires, distributor cap and rotor, and also an oil change, if you haven't done an oil change for a while, having some fresh oil in there may help the performance of the vehicle as well....

--DJ
 

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Yeah, a tune up would do it. I doubt a dodgy O2 sensor would make that much difference to gas mileage. As suggested above, new spark plugs, wires, distributor, rotor and a new air filter and you will be running like a dream.
 

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ian g said:
I doubt a dodgy O2 sensor would make that much difference to gas mileage.
It did for me. I was getting a lean/fuel startvation condition. After swapping out for an OEM 02 sensor I picked up 60 miles/ tank full.
 
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I went in to Autozone to see if the OBD II system was reporting any codes. None whatsoever, which is actually kind of disappointing. If I was given an idea of what needed fixing I could fix it, but now I don't have any leads. I have changed the plugs (bosch platinums), distributor cap, rotor, fuel filter, air filter, and cleaned out the inside of the air intake manifold with carburetor cleaner. I haven't got around to changing the spark plug wires yet, but I'm planning on it. Do which brand/type of wires I use make that big of a difference?

I think all of the above has upped my MPG by 3 or 4. That's good but still not enough to please me.

I'm wondering if I don't have a sticky brake somewhere. I was told my rotors are "glazed". If I were to lube up any particular thing in the brake system that was stuck what would it be and how would I go about doing it?
 

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Just jack up the car with the parking brake off, if oyu can easily spin all the wheels be hand oyu do not have a problem. Front lower slider pins in the calipers do seize, periodic removeal and greasing can save them from getting bad enough to require replaceing the caliper.

Also, if you have the all too common oil buring at startup problem, you may have a clogged EGR valve and heavily carboned throttle body. It's not difficult to clean both with throttle body cleaner and a steel pipe cleaner if you have a service manual.
 

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Chuck said he replaced his o2 sensor, and it fixed his problem, I was wondering if you replaced both, or just the front one, as their are two on there, and there both very expensive, dealer wants $229.00 each, and aftermarket still wants 175.00 for each one, anyways let us know if you get a chance, thanks,,,,,Joe,,,
 

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Sorry about all the questions, But, how would you even know when it is the o2 sensor that needs replacing, I would hate to spend all that money, and it isnt faulty, thanks,,,Joe,,,
 
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Well, I finally got around to replacing the spark plug wires. Wow, what a difference. The original ones must have been in awful shape. I swear the engine is running a lot cleaner and it most definitely has more pickup. I had no idea a new set of wires would make that big of a difference. Whether or not this will have any effect on my gas mileage is something I have yet to see.

I'll probably clean the EGR valve sometime soon. I've already cleaned out the intake manifold. What's involved in removing the EGR to clean it out?
 
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Also, what kind of oil is everyone putting in their RAVs. 5W-30? What about really cold weather? Synthetic? "Normal"? High mileage? Good experiences, bad ones?
 

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I am using 5W-30, and I have been very happy with it, I change it every 3months, and I have never had a motor problem in any of my cars,I too just did my plugs,wires, cap and rotor, and it too made a world of difference, total cost was $176.00, but I can feel the difference also,.....
 

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I am really looking forward to seeing what mileage you get having done the tune up (and without replacing the expensive O2 sensors). What wires did you buy? I have opted to replace mine with Densos but they cost an awful lot.

For the EGR I would recommend buying a manual. It is not a 'servicable item', like the plugs, wires, PCV valve etc. so I am not sure whether you would need to do this. I think you need to remove the throttle body and remember where all the pipes go etc.....not easy for a Saturday mechanic like me.

And let's remember that O2 sensors should be good for at least 100,000 miles before they need replacing. And then, they may only need replacing because their signal starts to slow.
 
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As far as what kind of wires I got I just went to the local auto parts store and bought what they had. For the RAV they had a $45 dollar set and a $55 dollar set, both Borg-Warner, I believe. I got the more expensive set.
 
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BTW in the wire set there were six wires. 4 for the plugs, one to go from the distributor cap to another box (is it the coil?), then another that looked similar to the dist. cap-coil wire except it was slightly longer. Am I missing something here. Are there six wires that I need to replace and I'm not seeing the 6th somewhere or did the set just come with an extra wire that is a little longer just in case you need the length? Some insight please.

I'm also thinking of changing out my stock air filter with a "drop-in" K & N filter to see if it will give me better fuel mileage and possibly power. Anyone have experiences with that? Is it worth it?
 

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I had the same in my new wire box, I looked allaround and couldnt find anything that resembled a sixth wire to replace, so I just kept in the box and stored it, there was nothing coming from the coil that I could see, that resembled it , I was going to go back to the parts guy, and ask him, just havent had a chance yet, let me know if you figure it out though,, thanks,,,,
 

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Hmm.. I must be the only person who has had a failed O2 (front) sensor, and am getting better milage.

My check engine light was cycling on/off about every 2 weeks (very heavy driving). I noticed that when it was on, it ran better and got slightly better milage. When it went off, it idled rough, and got worse mileage. It finally came on and stayed on.. Its running the best its ever ran though. But I'm gonna order one this weekend (NAPA claimed they had one for $111. )

My open loop settings must be right on. :D
 
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