Low, LOW mpg

[bryanedp9]

I've got an 87 lwb with the 4.3 in it, TBI and whatnot. Exhaust down to flexpipe, no muffler right now, so it's a little harder to tell for me, but I think I'm not finding my top gear.

It was a family-owned thing and had a trans rebuild right before I got it. I fiddled with it a little because it shifted hard but I ended up figuring my grandfather just had a shift kit put in and didn't think to mention it, or the shop took liberties on it. Anyhow, the shifting is smoother these days or I've gotten used to it. I've owned it a little over 3 years now. Early on, I think the trans dipstick got broken at the tip and I was told not to worry about it. Obviously I've learned better than to just forget about things like that now, and I'm not too afraid to pull the trans out if I have to.

Anyhow, I got 101 miles out of 8.75-9.35 gallons, which comes out to around 11mpg if you just figure I used 9 gallons even. This has been over the past few days on 65mph highway, and the first time in quite a few months that it's seen those speeds. New plugs, wires, rotor, and cap a couple months ago. I'm gonna try a little injector cleaner but I don't have much faith in it, and check plug color to see if that tells me anything.

But back in the day, at about 67mph I'd hit top gear, and it was smoother shifting at higher speeds. Coupled with the fact that I have no tach or muffler now, it's hard to tell if that's actually happening. I'm really wanting to blame it on this, but I'd like to know what to look for if I drop the pan on the trans and have a look.

As well, any other suggestions to figure out the low mpg would be great, and as always, sorry for the long post.

[bake74]

     Don't blame not having a muffler.  (free exhaust means no back pressure), though not too good on your ears. 
     When was the last time a good tune up was done on your truck ?  You could always take off the intake and visually inspect it and throttle body for carbon build up to see how the rest of your engine could possible work.  Your right, using those injector cleaners do no work, if you use a cheap one and only do it once.  But if you use a good one and use it every 2 - 3 fill ups over a few months, it will work. ( I have seen proof with my wife's car from carbon build up in intake and after how the carbon build up was decreased).
     I would check into the transmission problem.  1: check with family/shop to see if shift kit was installed, and adjust accordingly.  Or find the cause of the shift problem, maybe you lost top gear and it is not even shifting into that gear. ( I assume you have a auto trans. since you did not state what you had).
     In the end, I agree, you should get better mileage than that out of a 4.3 L engine. 

[lukieg]

i bet its running rich... fix you're exhaust... start with the obvious.. go from there!

[lukieg]

oh and adjust your tv cable...

[Irish_Alley]

my 4.3 swb 1994 gets about 16 but that was if i kept it below 60 and i know with my 04 impala if i keep it below 60 i can get 28-30 if i go above 60 it drops to 25. your speed plays a big role in mpgs even in my 79 i get i think 11 if i stay around 55 if i get to 60 its a couple mpgs under that

[bryanedp9]

Sorry, it's a TH400. The only reason I'm not 100% certain that top gear is out is because of the lack of a muffler now. I don't feel the shift, and I don't hear it.

I would imagine with the TBI of '87, exhaust pressure wouldn't matter for fuel mix. I'm often wrong, though.

Do I just pull the can and TB off the check the intake out? I'll look at fuel possibilities first, but I'd like to know how to identify the problem if it's in the tranny.

[zieg85]

You should be able to manually shift it and hear a difference.  4.3L are gas hogs if they aren't running efficiently.  My 86 is a carb'd 4.3L and was a pig on gas if there was any lugging of the engine having to mash the go pedal to keep the speed up on a hill or due to a head wind.  Maybe it was just me but the torque was lacking and just not enough to move a 1/2 ton effectively with the gearing my truck has.  My $.02

[bryanedp9]

You should be able to manually shift it and hear a difference....

So if 3rd usually came in at 65-67mph, you're saying I should get up to that in 2nd and then go into Drive? If I'm wrong, could pushing 2nd gear that high put me in a world of trouble? I most certainly don't want to downshift from D into 2 at that speed if I'm wrong.

[zieg85]

You can try manually shifting from 2 to D at 50.  If you don't hear a difference 3rd gear may be gone.

[bryanedp9]

Well, I'll give it a go come next commute and report back. Thanks for the advice.

[gto109]

I had a 4.3 in a 99 Blazer.  I always thought that thing was a gas hog and didn't have enough power for it's size and that thing had a 5 speed stick.  I had nothing but problems with that thing to the point of when i seen my 85 for sale I almost didn't get it because of that thing.

[bryanedp9]

Well if anybody's just throwing out a 4bt and an NV4500, I'll be right over.

[Irish_Alley]

you should see top gear way before 67 on a th400. how hard on the right pedal are you?

[bobcooter]

Well if anybody's just throwing out a 4bt and an NV4500, I'll be right over.

X2 If you're going 65 MPH in 2nd gear your engine will be screaming to the point where you'll know you have a problem.

[fitz]

What are the rear end gears?
A 4.3 with a turbo 400 is an odd combo, maybe it has some steep gears killing the mpg's