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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / January 2007Tacoma poor fuel mileage...........
Own a 1996 4wd Tacoma with 2.7 four. I had a friend hook up his OB II scanner while I drove vehicle. Level ground in high gear, light throttle, ignition timing was shown to be 20-22 degrees. If I hammered it from about 1800-2000rpm in high gear, timing dropped back to 6 degrees. Not having found anything else in my quest to solve this issue, I think this might be the reason. From my previous automotive background, the timing would normally be about 40-45 degrees for optimum fuel mileage under same light throttle scenario. Is the combustion chamber shape that efficient in this engine that Toyota can get buy with such low amounts of igntion advance? Bad knock sensor? Since knock sensor is a bitch to get to, are there conditions that would have a bad knock sensor and not have a code come up? Once again, any techs with experience along this line care to offer their input? i also had poor milage with my 99 tacoma, then the check engine light flicked on. i ended changing plugs and leads as well as the EGR valve. i get WAY better milage now than i was before and the light turned off. here is the problem... THE LIGHT CAME BACK ON AGAIN... wat should i consider next?  Signature.::Supra73::.. http://www.automotiveforums.com >i also had poor milage with my 99 tacoma, then the check engine light >flicked on. i ended changing plugs and leads as well as the EGR valve. >i get WAY better milage now than i was before and the light turned off. >here is the problem... THE LIGHT CAME BACK ON AGAIN... wat should i >consider next? That depends. What code is the system giving you? Steve B. > >i also had poor milage with my 99 tacoma, then the check engine light > >flicked on. i ended changing plugs and leads as well as the EGR valve. [quoted text clipped - 5 lines] > > Steve B. That's what is difficult.......................no codes to date. >> >i also had poor milage with my 99 tacoma, then the check engine light >> >flicked on. i ended changing plugs and leads as well as the EGR valve. [quoted text clipped - 7 lines] > >That's what is difficult.......................no codes to date. If the check engine light is on then there are codes stored in the computer. Many of the auto parts places will read them for you now for free... you might call around and see if someone in your area is doing it. Steve B. > >> >i also had poor milage with my 99 tacoma, then the check engine light > >> >flicked on. i ended changing plugs and leads as well as the EGR valve. [quoted text clipped - 14 lines] > > Steve B. The OP never said that his MIL was on. Toyota MDT in MO > Own a 1996 4wd Tacoma with 2.7 four. I had a friend hook up his OB II > scanner while I drove vehicle. Level ground in high gear, light [quoted text clipped - 4 lines] > background, the timing would normally be about 40-45 degrees for > optimum fuel mileage under same light throttle scenario. These look like normal snapshot values of the total timing PID on Toyotas at those load/RPM conditions. You aren't seeing every single adjustment to timing because Toyota's data rate and your scan tool's refresh rate are too slow. > Bad knock sensor? If it is retarding timing, it probably works properly. It could have failed and become "too sensitive", but I've never seen that happen. Do you hear any undesireable noises on acceleration, such as pinging, timing chain slap or rod knock? > Since knock sensor is a bitch to get to... Good thing you don't have a V6. Access on the 2.7 is easier than most. If I were tracking down a low gas mileage complaint, I would: *check base timing -- jumper TE and E1 in the diagnostic connector underhood, look for about 5 degrees at warm idle *Inspect spark plugs -- OE are NipponDenso K16R-U w/ 30k mile replacement interval. Get ND or NGK plugs in your engine if not currently equipped. Replace them if their mileage is suspect or the electrodes are rounded. *Using the scantool you have access to, check long term fuel trim and short term fuel trim at idle, at various RPMs, and while driving. Added together, these numbers should be between +10% and -10% at any given screenshot. *Using scantool, check main O2 sensor output for max and min voltage. It should range from .1 to .9 volts if healthy. You can max it out by accelerating and you can zero it by decelerating if you want to see it's maximum range of operation. *Keep in mind that winter fuel formulas and colder weather may be taking a small toll on mileage. *Make sure the trans shifts through all gears (though it's likely a standard trans with the 2.7 4X4) and there is nothing else binding in the powertrain. *Make sure the brakes aren't dragging. Toyota MDT in MO > > Own a 1996 4wd Tacoma with 2.7 four. I had a friend hook up his OB II > > scanner while I drove vehicle. Level ground in high gear, light [quoted text clipped - 44 lines] > > Toyota MDT in MO Thanks MDT in MO, I really appreciate your input! Have been on a steep learning curve attempting to solve this and need some input from people such as yourself with real world experience. I have been a motor head my entire life, but have neglected to stay up to speed on the new electronics used for emissions. I do know what detonation sounds like and have never heard even a hint from this engine. Although engine has 150,000 miles on it, it's mechanically quiet as a "church mouse". I suspected as you suggested that the response time between timing adjustments and the ECU would be on the slow side. The 20 degree reading at light throttle cruise was done for about a mile with almost no change in engine load. Shouldn't this allowed enough time for an accurate reading? If so, 20 degrees sounds WAY retarded to me........not so? The base timing on the scan tool shows about 7-8 degrees. Didn't use jumper. Is this needed when reading timing off scan tool? If so, is the jumper simply a piece of wire, or does it incorporate a resistor or something. The reason I ask is because my TOYOTA shop manual shows a Toyota part number for this "special tool". I recently replaced spark plugs. Soon after took a 1200 mile trip and performance and mileage were great. Problem developed several months later. For this reason I have ruled out the plugs. Long and short term fuel trim are well within the 10% values at any given time. Don't know if the tech helping me checked for minimum and maximum values, although he said O2 sensor function appeared normal. The sensor is a new TOYOTA item. Will check again as you suggested though. In regards to winter gas formulas, I am aware of the need to change formulations for different ambient temps. Have owned this vehicle since new (ten years) and have checked fuel mileage every after evey fill up. Have noticed a slight drop during cooler weather, but nothing of the magnitude I have had recently. Have tried several tanks of fuel from a Big Name (high priced) company and mileage/performance hasn't improved. Vehicle is standard trans model. The parking brake isn't dragging or anything along that line, but it tells me you are looking at all the angles..........Thanks! > I suspected as you suggested that the response time between timing > adjustments and the ECU would be on the slow side. The 20 degree > reading at light throttle cruise was done for about a mile with almost > no change in engine load. Shouldn't this allowed enough time for an > accurate reading? If so, 20 degrees sounds WAY retarded to > me........not so? Your interpretation of light throttle is no doubt different than mine. For me, 20 degrees wouldn't raise a flag during a road test with scantool. Sure I've seen higher, but I can't tell if this is a problem on *your* truck with you driving it. > The base timing on the scan tool shows about 7-8 degrees. Didn't use > jumper. Is this needed when reading timing off scan tool? If so, is the > jumper simply a piece of wire, or does it incorporate a resistor or > something. The reason I ask is because my TOYOTA shop manual shows a > Toyota part number for this "special tool". 7-8 degrees is fine at idle, but what I wanted you to check is actual timing at the timing marks with TE and E1 jumped at the diagnostic connector (using a timing light as outlined in the service manual). The book spec for actual base timing is 3-7 degrees IIRC. After reading actual timing, now disconnect the jumper; note that the timing has move slightly as it is now under continuous control by the ECM. The ECM *assumes* base timing is correct when it displays the total timing PID. If base timing is off, then the total timing PID will not reflect true spark timing. It is unlikely that base timing is out of spec since the distributor is fixed, but it's wortth a look to be sure. In place of the "special service tool" you can use a paperclip as long as you are careful to make proper contact with the two terminals. Even better, if you still have your old O2 sensor, you can cut off two of it's wires, back their terminal ends out of the connector, and splice them together to create the special tool. That terminal size fits correctly into the diagnostic terminals. > I recently replaced spark plugs. Soon after took a 1200 mile trip and > performance and mileage were great. Problem developed several months [quoted text clipped - 13 lines] > anything along that line, but it tells me you are looking at all the > angles..........Thanks! Poor mileage is a hard thing to diagnose once you've checked all of the basics and have found nothing wrong. Situations such as changed driving habits, seasonal weather changes, and recent implimentation of oxygenated fuels should be considered before condemning the truck. It sounds like you have taken these things into consideration. Anything *could* cause a loss of performance/mileage. Weak ignition coil, timing chain slack, poor supply voltage or ground to any electronic engine control component, a worn valve train component, out of calibration MAF/CTS/TPS sensor(s), etc... These examples are hard to nail down without a code or obvious performance issue. In a rare case, you could have excessive carbon buildup on the pistons causing preignition and knock sensor retard. This is not to rule out the possibility of non engine issues, such as parasitic energy loss from dragging brakes, low tire pressure, front end way out of alignment, etc. And finally, the super obvious to all but those who slept through 9th grade physics -- poor mileage experienced only while towing a trailer. Toyota MDT in MO well the last time i had a diagnostic done only one thing came up: EGR overflow. Therefore i replaced the EGR valve. There was a significant improvement in gas consumption. How much can weather affect consumption? everything was going well at first (70 degrees) then as the temperature dropped (33 degrees) performance dropped as well. Now if my O2 sensor went bad then thats a problem. Signature.::Supra73::.. http://www.automotiveforums.com the diognostic i had done before said EGR EXCESSIVE FLOW... therefore i changed the EGR valve... everything was fine... the check engine light came off... then about 5 days later the light came back on with the same code...WHAT DOES THAT MEAN? Signature.::Supra73::.. http://www.automotiveforums.com |
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