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The reduced mileage you're getting blows my best mileage out of the water. I have an 03 Rav 4 I get maybe 300km (~185mi) to a tank, 350 if I'm lucky.
I've ran about 5 bottles of fuel injector cleaner thru the system, both Lucas and Seafoam, with no luck. I changed my PCV valve and hose and that helped a little bit bringing the mileage from 200-250km a tank to the aforementioned 300-350km. I have also plugged an exhaust leak after the resonator, changed the bank 2 downstream o2 sensor, drained and filled engine oil and atf twice, checked for a stuck caliper, cleaned the throttle body, maf sensor, and idle air control valve, and checked valve lifter clearance. I have no engine codes and the fuel system enters closed loop after warming up. I've even busted out the stethoscope and listened to the injectors which sound perfectly fine.
The only issue left I can think of is the catalytic converter has a rattle at idle, but never under load, and that could possibly reduce airflow and cause slightly worse mileage. But even that shouldn't cause it to be anywhere near as bad as it is.
I've scoured these forums and others for a possible cause and haven't come up with anything.
I am at a complete loss with this. Please keep us updated if you end up finding a cause.
 

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It certainly is running rich as evidenced by the black deposit where the exhaust leak was. What is causing it to run rich I have no clue and haven't been able to pinpoint it. I'm no mechanic, I've done all of the work with the help of my old man, but from the research I've done I reckon it could only really be the a/f ratio and 02 sensors causing a rich condition, possibly the cat as well?

To answer a few of your questions,
1) the air filter is in fair shape, definitely a little dirty but you can still see sunlight through it when held up. Gave it a few good smacks on the concrete along with a few gusts of compressed air.
2) I don't believe any of the injectors are stuck open, one of them was a tiny bit more loud that the other three, but cycled in the same manner. May be something to look into?
3) I wouldn't have the slightest clue how to check if the ECM is commanding the injectors to run rich. Is this something I can check with a bluetooth obd2 scanner? I was using torque lite to check fuel system loop status and clear any errant codes, but couldn't find a way to actually log any data, only real time info. I took a brief look at the fuel trim graphs, but again with no data logging it proved a fair bit dangerous/unruly to quickly glance at the graph on my phone while driving.

And to address the things to be aware of,
1a) Stock size Cooper Discovery with ~80% tread (P215/70R16)
2a) Recommended PSI of 29 all around
3a) No aftermarket accessories save a standard aftermarket deck & the factory roof rack

The mention of alignment may be something to check out as I'm unsure of when the alignment was last done on this vehicle. The previous owner seemed to at least try to take care of it with fairly thorough service records, but I never saw any record of an alignment. That being said, it doesn't pull to either side when my hands are off the wheel.

Thank you for the documents, I'm sure those will come in handy with troubleshooting.

I appreciate your response, you always have great information.
 

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OBDII information: the ST/LT fuel trims and EQ ratio (air:fuel) are what you want to look at. An EQ of 1.00 is supposed to be a perfect 14:1 stoichiometric ratio. Anything below 1.00 is rich, anything above is lean (counterintuitive to some).
In regard to the short term/long term fuel trims, what would be considered within tolerance? 0.95-1.05? Or should it always be exactly at 1?
And say it is off and running rich as I believe it to be, what might be the next step?
 

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@05RAV Thank you for all the info, I'm going to try to get a reading on the st/lt fuel trim sometime soon here. Maybe I'll try a different app.
Should the trim end up being off, it could be for any number of reasons, no? Likely culprit being the air fuel sensors upstream of the cat I suppose

@Commando Yes I did clean the maf, I didn't physically remove it from the intake hose but I did hit it with a fairly liberal spray of maf cleaner
 

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I’ll pop the air filter box off again later today and see if I can’t shimmy the MAF loose. If it is dirty, do you reckon it’ll be dirty as in bits of dirt that I can physically remove or will it be smaller bits that may be hard to see and thus need a good 360° spray of MAF cleaner to get rid of? Reason I ask is all I have at my place is brakleen and I don’t feel like driving across town to my dads today lol

I won’t have an opportunity to check the plug cylinders for a minute here but that’ll be the first thing on the list
 

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Ok so I've got the OBD2 logs finally. I thoroughly cleaned the MAF with sensor cleaner and blew some more dust out of the air box.
I'm not entirely sure what I'm looking at here, it'd be great if someone could take a quick glance at the file.
Might make my own thread here considering I basically just stole this other guys thread
For the file, I started the engine from cold, idled for a moment, drove down the street, idled to look something up, drove a bit, idled to look another thing up, drove and drove normally. gunned it a few times as well as you can see with the associated rpm’s
 

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Thanks for the write up, everything you said is clear to me and I appreciate it.
I’m curious about the standalone o2 probe you had mentioned earlier, I may look into that. In light of everything I’ve done to the car so far with few results, I reckon you’re onto something with the upstream o2/air fuel sensors. If I ever get this working as it should, I’ll post a follow up thread
 

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So I tossed in the two new upstream o2/airfuel sensors and the car has never ran better. It feels like it's brand new, but the gas mileage is still abhorrent. No better, ~250-300km a tank maybe 350 if I'm lucky. I'm going to give it a few more tanks to see if it straightens itself out and if it doesn't I'll have to sell it which I really don't want to do
 

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Tire Motor vehicle White Automotive tire Hood

Photograph Automotive tire Motor vehicle White Hood


these are all new plugs put in when I bought the car about 2 months ago. The cylinder and threads are certainly wet and a few of the plugs look by no means fouled but definitely dirty, especially the one that has the funky sounding injector. It’s difficult to see in the photos but you could certainly see in person that the plugs and cylinders were wet.
I don’t know if they’re supposed to be a little wet, or if being wet at all is indicative of a problem.
I did this shortly after getting home, so the engine was just beginning to cool down if that makes a difference.
 

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I don’t have the original plugs, but I do remember them looking a bit dirtier than the new ones but not by much. It’s possible that maybe the previous owner changed them shortly before I bought it. Changing the plugs didn’t have much of an effect if anything. Nothing noticeable at least.

What’s odd about the new plugs being clean is that I know the vehicle is running rich. At least I did before I plugged the exhaust leak a month ago. The area where the hole was was completely black with soot. Changes I’ve made and parts I’ve swapped since then may have effected just how rich it runs, but I can say with certainty that it was running very rich recently. The proper end of the exhaust is still black now but I haven’t gone and cleaned it to compare if it gets black again.

I went ahead and ordered 4 remanufactured injectors from rockauto and they should be here shortly. Even if the injectors are working as intended, they’re likely just as old as the car and are probably due for a service. Seeing as I don’t have an injector cleaning shop in town and I don’t have the luxury of sending them off and being without a car for a week/two weeks, I figured what the hell. Even if they’re not the problem, it may help the engine run better who knows. It’s only $150 anyway.

I haven’t noticed any fuel leaking nor a smell of gas anywhere around the car. I reckon if there’s a hole in the filler neck I’d likely be able to smell at least a faint smell of gas, no? I’m unsure of while driving, I guess it could be possible but I wouldn’t know what to look for or what signs there would be that it’s leaking while driving. Again, never any smell of gas at any time. Seems to be plugged up pretty tight.

As for the downstream data, I know with almost complete certainty that the catalytic converter is toast, so is it not possible that the downstream sensors are simply reading the wrong data?
 

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You figure a bad cat can have that big of an impact of fuel economy?
This is one repair I’m not looking forward to footing the bill for haha

The reason I reckon it’s gone bad is there is a rattle out of the cat at idle but never under load. Strangely enough after replacing the aftermarket pcv valve for an oem one, the rattle has gone away. I’m not sure how those are linked. Now that I think about it it has me second guessing the state of my cat. I’d likely be well served to get a smog test done similar to the device you posted earlier in this thread. Might tell me with 100% certainty that the cat has actually gone bad before I spend ~$600
 
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