Horrid gas mileage?

[Kingman]

My '87 has been averaging roughly 11mpg since I bought it 5 months ago. This seems pretty low, even though it's gutless and heavy. It runs perfectly fine, but the fuel economy is killing me. It's the 3.0 V6, auto, and has about 200,000 miles on it. When I bought it, I replaced the plugs, wires, cap and rotor, fuel filter, and air filter.

 

Any ideas on what might be wrong with it, or what I could do do improve this?

I've already ran a tank of Lucas Fuel Additive, and that didn't help the slightest bit...

[MY1PATH]

my mpg increased after I did headders, the challenge being gentler on the gass after you have them.

try filling @ half tank, the existing gass has allot more surface are to evaporate from once you go past half tank.

tire preassure @ 34

maybe change your o2 sensor and cat those got me improvements too.

I get over 20mpg higway (other minor mods; HEI coil, K&N drop in...) and 13-16 arround town depending on if its the 20mph military base housing area or the town arround it @ 35ish(people normally going 40).

 

oh! heres a good one! are your front hubs stuck in the locked position? jack the front end and spin each wheel if the other wheel & front drive shaft move then your hubs are stuck. thats allot of extra rotating weight turing CV's, differential and drivshaft that should otherwies be sitting stationary when driving in 2wd.

 

as far as calling it heavy... I disagree, the 2doors weigh in @ under 3600lbs and the 90-95 4 doors are only 3800lbs stock. thats light for IMO.

 

ALSO; feel free to post an intro in the "new people start here" so we know more about ya!

[GrimGreg]

Yeah, check the drive train for any drag, what size tires are you running? Might want to drain an refill your diffs with fresh fluid too.

 

On my last trip (400 round trip) I got 16mpg in my 88 (also the V6 auto) and it is overdue for a tune up, has 31MTs, and I was driving on some hilly windy country roads (lots of speed up, slow down stuff). My engine is also unadultered. I took a trip in early summer, strictly highway and at that point I was actually getting 20mpg. I know when I drive it as a DD carrying about 500lbs+ of tools I got about 16mpg also.

[Kingman]

Yeah, check the drive train for any drag, what size tires are you running? Might want to drain an refill your diffs with fresh fluid too.

 

On my last trip (400 round trip) I got 16mpg in my 88 (also the V6 auto) and it is overdue for a tune up, has 31MTs, and I was driving on some hilly windy country roads (lots of speed up, slow down stuff). My engine is also unadultered. I took a trip in early summer, strictly highway and at that point I was actually getting 20mpg. I know when I drive it as a DD carrying about 500lbs+ of tools I got about 16mpg also.

 

Thank you for both your replies, and I will check out the new members part. I had never thought about the hubs being a problem, though they aren't stuck. I'm also running 31x10.5x15s, which are all brand new. On the freeway doing about 65ish, my rpms are at about 3700. Is this comperable to yours?

 

It is also extremely slow, which is another question I actually had, if they are really this slow, or there's something up with mine. It seems that some days, it simply has more power than other days. Weird?

 

I'll check on the o2 sensor, and if it's in my budget, I'll replace it. It's probably well due =P

 

I also like to get it very, very muddy

[MY1PATH]

high rpms @ 65-70 is quite normal, the stock final drive ratio is 4.3 and 4.6. 4x4parts.com sells 4.1 gears if you want better mpg, lower rpm on the higway and maybe higer top speed but most ppl go the other way so they can turn bigger tires. Yeh, these are kinda slow, if it feels like its falling off over 75-80 than its normal. If that bugs you the most benificail thing I reccomend is swapping out a 3.3L (same engine w/ a bigger bore and cheap and easy to find) form one of the later model nissans but if your only looking for small gains, headders and full exhuast should pep it up a little.

[Precise1]

change your o2 sensor and cat those got me improvements too.

Clean the MAF...

 

Drive train fluids/drag is also a good one as per GG

try filling @ half tank, the existing gass has allot more surface are to evaporate from once you go past half tank.

On the freeway doing about 65ish, my rpms are at about 3700. Is this comperable to yours?

No, that seems much higher than normal or desired. My old 95 MT runs at 2900max at 65+(31x10.5), and the new 95 auto is a bit higher at 3100 at 65+ (235/75/15). I'll have to check the gearing in both, but 3700 is WAY too high. Is your OD engaged/working correctly?

 

B

[Kingman]

Clean the MAF...

 

Drive train fluids/drag is also a good one as per GG

 

No, that seems much higher than normal or desired. My old 95 MT runs at 2900max at 65+(31x10.5), and the new 95 auto is a bit higher at 3100 at 65+ (235/75/15). I'll have to check the gearing in both, but 3700 is WAY too high. Is your OD engaged/working correctly?

 

B

 

 

I thought overdrive was avaliable after 1990, as far as I know, It's only a 3-speed automatic.

I forgot to mention that all the fluids were changed 6 months ago.

 

I'll take a look at the MAF tomorrow if I have time, thanks for the advice

[GrimGreg]

Yep, 87s (unless a late build) had the 3 speed autos. 88-90 had the 4 speed non OD autos.

[Precise1]

I thought overdrive was avaliable after 1990

Ahh, ok, I learned something. Sorry, this is my first auto tranny in 20 years...

 

B

[Simon]

Weird....my gas mileage has been pretty low on this truck since I got it. I'd attributed it to the AT vs my old trucks MT. (and the fact that I've got 6" of lift, 33's and a whole bunch of extra weight) That said, I've read your other thread regarding low idle, and mine exhibits that trait at times as well. I wonder if the two are related?

 

Out of curiosity, when you start your truck, are your lights dim until you hit about 2700RPMs? I've had that problem on my truck since day one as well, new battery, new alternator, and checked all my connections....yet, it won't charge until I've hit 2700RPMs once, then I can idle all day and it'll maintain a charge.

 

If I find my solution, I'll report back.

[adamzan]

Simon, when you turn on your a/c does your clock light dim? Also, do your windows go up kinda slow unless you're moving? Strange

[StatutoryApe]

i get the same mileage. around 13 on average. i've done all the tune up stuff and cleaned the MAF and yielded no increase in MPGs. i don't have much time to work on anything else tho. lol.

[Precise1]

TBI+33" tires+3"BL+SL+auto tranny=crap milage?

 

B

[Simon]

Simon, when you turn on your a/c does your clock light dim? Also, do your windows go up kinda slow unless you're moving? Strange

Nope. Everything works fine, once I've hit 2700RPM's once.

[Simon]

TBI+33" tires+3"BL+SL+auto tranny=crap milage?

 

B

Oddly, since I added the lift and tires and weight, my mileage hasn't suffered. It's slower than it was, but my mileage is around the same, might even be better, given that my speedo/odo shows 18% off now. It's very strange.

 

And mine's MPFI

[Simon]

On the topic of voltage, I will be replacing all the connections at the fusible links in the near future, as my positive terminal is less than fantastic at this point. I'm hoping some nice fresh clean connections will alleviate my charging issues, otherwise I have no idea where to look.

[GrimGreg]

Both of mine have enough get up and go (granted the 93 with it's mods has more), my only complaint is the trottle responce. The seasons change between the time I step on the peddle and when the engine goes, "HOLLY CRAP I'M SUPPOSED TO REV UP!"

[bigfisch1191]

Weird....my gas mileage has been pretty low on this truck since I got it. I'd attributed it to the AT vs my old trucks MT. (and the fact that I've got 6" of lift, 33's and a whole bunch of extra weight) That said, I've read your other thread regarding low idle, and mine exhibits that trait at times as well. I wonder if the two are related?

 

Out of curiosity, when you start your truck, are your lights dim until you hit about 2700RPMs? I've had that problem on my truck since day one as well, new battery, new alternator, and checked all my connections....yet, it won't charge until I've hit 2700RPMs once, then I can idle all day and it'll maintain a charge.

 

If I find my solution, I'll report back.

i have that problem with the power. my voltage is at 12-11 untill it is reved to 2700-3000 and then it jumps to 13 volts. if you find a solution fill me in. i think it is a bad connection with the alternator. btw i have a new alternator and batt.

 

and on gas mileage i get 8-17 mpg depending on how i drive. 8 being 4-5 grand shifts in mud bogs 17 being 55 mph down the highway on overdrive. v6 auto auto hubs truck weight 4300 pounds

Edited October 10, 2008 by bigfisch1191

[adamzan]

Nope. Everything works fine, once I've hit 2700RPM's once.

Strange, for me, even if I'm on the highway doing 110 at 2700~ and hit the a/c, not that I need it now, it will cause the clock light to dim and you can feel the load when the clutch engages.

 

Oddly, since I added the lift and tires and weight, my mileage hasn't suffered. It's slower than it was, but my mileage is around the same, might even be better, given that my speedo/odo shows 18% off now. It's very strange.

 

And mine's MPFI

My mileage actually went up with the 31s on, as opposed to the 235 75 r15's. Maybe he should get bigger tires

[Kingman]

Weird....my gas mileage has been pretty low on this truck since I got it. I'd attributed it to the AT vs my old trucks MT. (and the fact that I've got 6" of lift, 33's and a whole bunch of extra weight) That said, I've read your other thread regarding low idle, and mine exhibits that trait at times as well. I wonder if the two are related?

 

Out of curiosity, when you start your truck, are your lights dim until you hit about 2700RPMs? I've had that problem on my truck since day one as well, new battery, new alternator, and checked all my connections....yet, it won't charge until I've hit 2700RPMs once, then I can idle all day and it'll maintain a charge.

 

If I find my solution, I'll report back.

 

 

No, everything is constant, the only time anything dims is if i switch my blower fan from off to high quickly, and even then its only for a split second. My voltage also stays at a constant 14 volts.

 

I would also doubt the idle problem and bad gas mileage are related, but it's worth a thought.

Edited October 11, 2008 by kingman92010

[Kingman]

Yep, 87s (unless a late build) had the 3 speed autos. 88-90 had the 4 speed non OD autos.

 

I would think mine is also a late build, because after the 3 initial shifts, a 4th seems to happen shortly afterwords, it's only about a 200rpm difference, but its noticeable.

It also disengages when I go up hills and it bogs down enough, then the rpms raise and I can make it up the hill fine. This isn't mistaken with kicking down to second gear, because if I manually kick it down, it really goes, then there's first.

 

That was actually something I was wondering about, whether or not there was a tiny 4th gear. Like 3 and a half...lol.

[Simon]

That's your torque converter lockup.

[GrimGreg]

That's your torque converter lockup.

 

Pre 90 didn't have the lockup TCs.

[89_trailboss]

Pre 90 didn't have the lockup TCs.

 

my 89 had lock up.

 

and it had O/D as well. I belive 87 was 3 speed and 88+ all had the 3 speed with OD. I put a 97 tranny into the 89 and there was no difference in RPM's on the hiway.

[GrimGreg]

my 89 had lock up.

 

and it had O/D as well. I belive 87 was 3 speed and 88+ all had the 3 speed with OD. I put a 97 tranny into the 89 and there was no difference in RPM's on the hiway.

 

88+ have a 4speed non OD final gear, 90+ have a 4speed with OD final gear. Same exact tranny, just different electronics and gears in it. All the 88+ VG V6s had the RE4R01A automatic tranny, and they can be cross swapped, but they aren't all the same. They will fit the 87s also, but require the tranny ECU to be swapped(which is not in the tranny BTW) because they used a L3N71B. 2wd models used a 4 speed variant of that, the E4N71B until 89, in 90 they started using the RE4R01A also.

 

If your 89 had lockup and OD then it was a late 89 build and one of those weird inbetween models like we were talking about earlier this week in POHO. Or it could be that a PO replaced your TC or tranny at some point before.