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Major Drop in Gas Mileage

27K views 54 replies 14 participants last post by  Jaymo 
#1 ·
I just noticed that I ran a full tank and I only drove 209 miles. I average around 240miles per tank.

I do have an SES light issue which is a Very Small EVAP Leak.

Would this cause the drop in gas mileage?

I haven't had a chance to diagnose the problem. It's either a leak in the hose(s) or a crack in the EVAP canister.

Any thoughts/suggestions?
 
#27 ·
cyclemut - thanks for all this. alot to take in. so, i have done the plugs recently. put platinum ngk's in. never done distributor or plug wires. but never had and issue with misfire. did have the KS code but took care of that with relocate mod. as far as timing on the vehicle, this is beyond my realm of current knowledge. never had any rough idle or anything. should i check the timing? also how do i do a hard learn reset? never did that after the new air filter. MAF is clean, i cleaned it not long ago thinking it was part of the problem but this issue has been there before i put the new intake on....didnt see much of a difference when new intake went on.
 
#28 ·
With the engine running, unplug the MAF sensor. The engine will die. Turn the key off, plug the MAF back in and then restart the truck.

This will reset the self learn in the computer, clearing learned A/F alphas in the system and returning the vehicle to base parameters.

The CEL should turn off, but if it doesn't, you can disconnect the battery for about a minute (if you don't have a code reader/eraser) to get the code to go away.

Double check your vacuum lines. Once you take an old line off, they can sometimes come back off if not put exactly back on like they were. You can also spray around the intake area with some brake cleaner to see if there's any vacuum leaks, maybe a cracked hose, etc.

I'd check your timing if you're not doing the timing belt right away. When the timing belt is done, then the timing should also be reset again, along with another hard reset of the computer. I like to bump the timing the two degrees that Nissan allows. Depending on altitude, I even bump it a little more (an additional two degrees). So up here at 8000 feet, I'm running four degrees above stock and passing emissions easily.
 
#34 · (Edited)
guys. i need some help off topic. ya'll have been very helpful so maybe you can help with this problem. i was driving home tonight and my oil light came on, as well as, the "SET" light for the cruise control started flashing. i was close to home and driving in a neighborhood so i was only doing about 20 - 25 so i decided to drive home, slowly, listening for knock or ticking. didn't hear any. i made it home and was confused about the problem because i just changed the oil a week ago filling with factory manual prescribed amount. so i put it in the garage and shut it off. turned it back on and oil light stayed on but no more flashing "SET" light. i check oil level and it looks good, so no leaking. my dad knows a thing or two about cars and said it could be a malfunctioning ECU or possibly a bad sensor or something scary, but he doesn't know enough about X's so he said to call our mechanic. i thought i would ask on here first. any ideas?

quick edit, i just washed the truck tonight at a hand wash place and i did blow some water underneath to clean all the salt off the underside. possible i got something wet?
 
#36 ·
yeah but could possibly be the oil pump going bad check that sending unit first as it would be a cheaper fix. check for leaks near that unit as it maybe causing low pressure
 
#37 ·
thanks for those suggestions. i'll check those first thing in the AM. is it possible i got water in the connection?

also any ideas on the flashing "SET" light?

i'm a little stressed out cause this is our only vehicle in the winter. i garage my tc cause i have a work car but can only use for work.
 
#39 · (Edited)
read those threads. "SET" light is off. I think i got the harness wet. the oil light, however, is still on. I checked the sending unit, no leaks. i'm going to just take it in. i don't have the time or tools to do anything major anyway. hope i didn't jack up the engine by driving it a couple miles......

all the mechanics i use aren't free til friday....so i need to figure this out myself if it's an easy fix. one guy said there's a physical gauge on the oil sender that i could verify if there is oil pressure or not, and thus suggesting an electrical problem (ie bad sensor) but i can't find the gauge on the sender....
 
#40 ·
You have to remove the idiot light sensor and place a gauge in it's place.
 
#43 ·
just a quick update on the oil light and set light issue in case anyone runs into this again. my mechanic started it up this afternoon....no lights on! what?! so my wife picked it up and drove it 8 miles home. i installed a new pcv valve and started her up for a drive, oil light back on. but i noticed that it was VERY dim....odd....so my lovely wife found the below thread and found out that it will happen sometimes when connections underneath get wet. the description in this thread is EXACTLY what i was dealing with. hope this helps someone else and thanks for all your advice guys!

http://www.clubxterra.org/forums/archive/index.php/t-21036.html
 
#44 ·
odd, i wash my truck once a week (at least) i like to keep it clean! and have never had any wiring issues then again mine is a 2000 not a 2004 so the way they did the wires is most likely different or maybe on the 04s the wiring can slip loose from a holder somewhere it wont get wet and get to where it can, might check that out, or wrap it in some electrical tape
 
#45 ·
So I replaced the PCV valve yesterday, along with Seafoaming 1/3 the Gas, EVAP line and oil. The old PCV valve was FILTHY to say the least.

I noticed some smoke/exhaust coming from what I believe is the Valve Cover (passenger side where the oil spout is). I tightened down the 8 or so screws, but there still seems to be a bit of exhaust coming from there.

I'm going to assume that the valve cover gasket is shot and that it's not a crack.

I also noticed my car seems to sputter when idling. Almost like it wants to stall, but it keeps running.

I am going to do an oil change tomorrow along with the fuel filter.

Does Seafoaming cause a sputtering until it's flushed out of the system?

I did a seafoam treatment about 6 mos ago during a previous oil change, and didn't notice a sputtering. But I also changed the oil out the same day.

Any thoughts on the sputtering? I changed the Alternator too, but I don't believe that has anything to do with it.

Thanks
 
#46 · (Edited)
did the sputtering start when you did the seafoam in vacuum line? if so did it stop after you stopped pouring it in or no? if no check the vacuum line you used to pour it in and make sure its tight(if tight check your spark plugs, wires and cap and rotor on distributor or it may have the first signs of a failing distributor, my dizzy gave me a bad idle for about 3months before (not hardly noticeable but the occasional, rhythmic off feel in the idle then the other day the dizzy quit on me so just a thought, dont freak out and go buy a distributor until you have narrowed it down). if it was just when your pouring it in thats normal as you are basically creating a massive vacuum leak to do it. the smoke in the engine bay is not your valve covers its the exhaust manifold that has cracked (most of us have that, both sides on mine are) if its not throwing a check engine light for o2 sensors or cats then dont worry about that as it wont hurt anything (or hasnt on mine, and mine still passes inspection and runs just fine with it that way)
 
#47 ·
I'm not sure if the sputtering was caused by the Seafoam. I don't believe the car started sputtering til sometime after, not immediately.

I also replaced the PCV valve, but didn't replace the hose connected to it, nor did I replace the clamps. Maybe it's creating a vacuum leak in those areas.

No codes have been thrown. It can't be plugs or wires. I just changed those in July. It could be the Dizzy. I don't believe that was ever replaced. But it just seems too coincidental that it would be the Dizzy. I also didn't spend much time on the top of the engine to knock something loose. Most if the work on the Alt was done underneath or from the wheel well.

Thanks for the suggestions.
 
#48 ·
i would suspect a vacuum leak then check all the hoses you touched for tightness and replace that hose clamp should fix er right up
 
#50 ·
i would suspect a vacuum leak then check all the hoses you touched for tightness and replace that hose clamp should fix er right up
Thanks for the tip. I'll check it out.
 
#51 ·
Do you have airconditioning and if so do you use the window defrost option? Putting it to defrost, air to the glass only will turn the air conditioner. Turing on the air conditioner uses alot of power. For mine it does.
Yes, I have used the AC, but not since maybe Sept. It's winter here in the North East, in the 30's and 40's. No need for AC. ;)
 
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