silverton Posted December 25, 2011 Share Posted December 25, 2011 My truck gets pretty bad gas mileage, I know it's just how it is, but this is a bit off the wall. I have 33" BFG KM2's on lego's. I could get 13-15 around town and 17-19 if it was all highway. So... not bad right? That's what the EPA estimated was when it was new! But since the pathy has become my dedicated daily driver (weekend wheeler) I decided it would be a good idea to get a set of "drivin on the street" rubber. I picked up a good set of 29" (235/75) BFG AT KO's on lego's. That's 14 pounds per tire lost in rotational weight. I expected a gain of some mpg's, seems reasonable right? nope, it is EXACTLY the same. I get 13-15 around town, and 17 on the highway. When calculating, I am correcting for the tire size error, so that's not it. It recently went through emissions with what I would consider flying colors for it being 16 years old and having 190,000'ish miles. 5/220 idle hydrocarbons, 10/200 "2525" hydrocarbons. 0% CO count. There was some O2, but it is a sniffer test and I have an exhaust leak a bit behind the catalytic converter. I have a K&N air filter. Unknown on O2 age. fairly free flowing exhaust with a good cat, I inspected it with my eye balls. It has power when I need/want it. I drive it easy for the most part, especially when I'm trying to get a good MPG reading. The only funkyness about the truck that I notice, is that under light to medium throttle, the RPMs... oscillate. they don't steadily increase like they should, like the ECU is getting an alternating rich/lean signal. I have cleaned the MAF sensor, I don't know if I should replace the MAF, inspect the coolant temp sensor, or inspect/replace the O2. I imagine if any of the components were failing, that I wouldn't have gotten such a good score on my emissions test. Link to comment Share on other sites More sharing options...
Tungsten Posted December 25, 2011 Share Posted December 25, 2011 Have you tried a tune-up and have you checked your timing with a light? Link to comment Share on other sites More sharing options...
silverton Posted December 25, 2011 Author Share Posted December 25, 2011 Spark plugs, cap, rotor, and air filter have all been replaced in the last 7500 miles. the air filter is a K&N and has been recently cleaned and oiled. I was doing some googling and came up with a suggestion about the TPS. Link to comment Share on other sites More sharing options...
Precise1 Posted December 26, 2011 Share Posted December 26, 2011 I was just going to recommend checking the TPS... What you gained by removing 14 lbs of rotational weight, I suspect you lost with higher RPM. The difference in tire circumference is about 15% so what speed was 2500 rpm before is now about 2875. You'd probably get the best mileage with 31" tires; I'm guessing you have 4.6 gears? Hows your U joints, are you running heavy oils, any brake drag? All that said, the Pathy has the aerodynamics of dumpster (and damn near the weight of one) and an underpowered motor with tall gearing. There isn't much you can do to get huge gains in mileage... B Link to comment Share on other sites More sharing options...
adamzan Posted December 26, 2011 Share Posted December 26, 2011 Mine used to have that wavy acceleration as well. It was the o2 sensor. Unplug it and see if it doesn't do it. Someone else here told me it was the TPS and I changed it with no effect. Link to comment Share on other sites More sharing options...
Tungsten Posted December 26, 2011 Share Posted December 26, 2011 I still manage to get decent MPGs for having the drive-ability of a dumpster. The oxygen sensor is a big factor too but it has even more effect if you are running a performance exhaust. Of course, the other thing that matters is correct timing. There is a diagnostic mode for the oxygen sensor in the FSM. Link to comment Share on other sites More sharing options...
silverton Posted December 27, 2011 Author Share Posted December 27, 2011 I was just going to recommend checking the TPS... What you gained by removing 14 lbs of rotational weight, I suspect you lost with higher RPM. The difference in tire circumference is about 15% so what speed was 2500 rpm before is now about 2875. You'd probably get the best mileage with 31" tires; I'm guessing you have 4.6 gears? Hows your U joints, are you running heavy oils, any brake drag? All that said, the Pathy has the aerodynamics of dumpster (and damn near the weight of one) and an underpowered motor with tall gearing. There isn't much you can do to get huge gains in mileage... B I really lost 52 pounds of rotational weight, and you are correct in the rpm gain. I do have 4.6 gears and I'm going to try to find a set of 31's, this was just a good deal I couldn't pass up on these 235's. U joints are unknown, no heavy oils, and I believe I do have a sticking front caliper. Weighed my pathfinder today, 4360 pounds at the transfer station scale. Just tested the TPS, it checks out fine. .98k ohms closed, 8.3k ohms WOT. Checked the timing and the crank pulley is WAY off, can't even line it up with the marks. once winter is over and my car is drivable, I'll be doing a timing belt job on it and fix the pulley, but I "advanced" it as far as it would allow, and it runs WAY better now. it has an intermittent miss now though, so I'm probably going to try backing the advance off a little. Link to comment Share on other sites More sharing options...
Tungsten Posted December 27, 2011 Share Posted December 27, 2011 You are getting that because the fuel injectors are not synchronizing properly. This is why it is so important that the injector timing matches the spark timing and that will only happen when the timing is set at 15 BTDC. Link to comment Share on other sites More sharing options...
1994SEV6 Posted December 27, 2011 Share Posted December 27, 2011 (edited) You are getting that because the fuel injectors are not synchronizing properly. This is why it is so important that the injector timing matches the spark timing and that will only happen when the timing is set at 15 BTDC. how does timing get messed up anyway? I thought the timing belt dictated the timing. oh wait, that's for the valves. So the distributor controls the timing of the spark plugs? How would this timing get off? Edited December 27, 2011 by 1994SEV6 Link to comment Share on other sites More sharing options...
Kingman Posted December 27, 2011 Share Posted December 27, 2011 how does timing get messed up anyway? I thought the timing belt dictated the timing. oh wait, that's for the valves. So the distributor controls the timing of the spark plugs? How would this timing get off? If the crank pulley is installed incorrectly you'll never get the engine timed right. Link to comment Share on other sites More sharing options...
Kingman Posted December 27, 2011 Share Posted December 27, 2011 like the ECU is getting an alternating rich/lean signal An O2 sensor will do that, you can watch the ECU switch between lean and rich with a scanner. So will a funky Coolant Temp Sensor. I remember you asking me about it but don't remember what came of it. Link to comment Share on other sites More sharing options...
silverton Posted December 27, 2011 Author Share Posted December 27, 2011 An O2 sensor will do that, you can watch the ECU switch between lean and rich with a scanner. So will a funky Coolant Temp Sensor. I remember you asking me about it but don't remember what came of it. Unplugging the CTS threw a code and made it run even worse. Today I'm going to back the advance off til the intermittent back firing stops. Probably won't get around to fixing the crank pulley position until after winter. Link to comment Share on other sites More sharing options...
Tungsten Posted December 27, 2011 Share Posted December 27, 2011 Those pulleys will usually get messed up when a non Nissan tech does your timing belt. Those marks are very hard to see so if they take that pulley off for whatever reason they only have a 1 in 6 chance of putting it back on the right way. Link to comment Share on other sites More sharing options...
1994SEV6 Posted December 28, 2011 Share Posted December 28, 2011 Those pulleys will usually get messed up when a non Nissan tech does your timing belt. Those marks are very hard to see so if they take that pulley off for whatever reason they only have a 1 in 6 chance of putting it back on the right way. the crank pulley? the thing that all the belts run off of? How would that pulley affect how your engine fires? the crank will spin no matter what. Link to comment Share on other sites More sharing options...
Trogdor636 Posted December 28, 2011 Share Posted December 28, 2011 (edited) the crank pulley? the thing that all the belts run off of? How would that pulley affect how your engine fires? the crank will spin no matter what. He's talking about the timing marks on the pulley. If the pulley is not put back on correctly, the timing marks on the pulley aren't in the proper place, so reading the proper timing is impossible. Edited December 28, 2011 by Trogdor636 Link to comment Share on other sites More sharing options...
adamzan Posted December 28, 2011 Share Posted December 28, 2011 Or you just don't be a moron and take the pulley off the proper way, you don't have to remove all 6 bolts. Just 2 for the puller if you use one and the big crank bolt. It is keyed to the crankshaft. Link to comment Share on other sites More sharing options...
tmoore4512 Posted January 2, 2012 Share Posted January 2, 2012 All that said, the Pathy has the aerodynamics of dumpster (and damn near the weight of one) and an underpowered motor with tall gearing. There isn't much you can do to get huge gains in mileage... Wow that is REALLY funny. Made my day... Very true though... Mine weighs in at 4770 lbs on the local CAT scale with no front bumper whatsoever. I average 16 to 19 mpg, and have 225+ miles... What these guys have mentioned is spot on... i would only add to check your gap, I know it sounds silly but it can make a difference... Link to comment Share on other sites More sharing options...
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