73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: bryanedp9 on January 20, 2012, 11:40:47 pm
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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.
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Don't blame not having a muffler. (free exhaust means no back pressure), though not too good on your ears. :o
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.
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i bet its running rich... fix you're exhaust... start with the obvious.. go from there!
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oh and adjust your tv cable...
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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
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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.
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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
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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.
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You can try manually shifting from 2 to D at 50. If you don't hear a difference 3rd gear may be gone.
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Well, I'll give it a go come next commute and report back. Thanks for the advice.
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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.
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Well if anybody's just throwing out a 4bt and an NV4500, I'll be right over.
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you should see top gear way before 67 on a th400. how hard on the right pedal are you?
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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.
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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
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Actually the 4.3L/TH400 was common in the later years as the TH350 wasn't offered, and if you didn't want a overdrive trans you could get the TH400 with the 4.3 and the 5.0L.
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I try to be pretty easy on the pedal, and it does like I'd imagine it should city driving, but I put cruise on at 65 usually on the commute. That's when it starts to tank. Picked up a bike today so hopefully I won't be in a rush to get it all taken care of.
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Okay, at about 40mph in 2nd it was begging for another gear, so I went to D and third came right in. Can I discard trans problems now and go straight to fuel/exhaust or could it still be a gearing thing?
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Okay, I have a couple ideas as to low mpg so far, so chime in with input. I haven't yet had a look at the intake, but this is what I know:
1) What I think is the charcoal canister has rotted lines. I'd imagine that's setting me back. If so, what size hose do I need?
2) Tank selector valve is either burnt out or has a wiring problem. I've only been able to use the right side tank since taking ownership. New selector switch did nothing to help. This might relate to...
3) I think my fuel pump is going. After coasting downhill, it might sputter before the uphill climb and sometimes doesn't want to start at all unless I tap the framerail in that area.
So any advice you have pertaining to this any other causes of low mileage, please let me know. It's a family vehicle but the money for pumps and an exhaust and whatnot might put me into sell mode instead.
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fom what u are saying and from my personal exsperiances with TBI systems, i would say that your biggest problem is not having a muffler for back pressure. My dads 87 454 was getting about almost 16-17 mpg and about a month ago the mufflers fell off and it is lucky if it get 10mpg. what i would do if u are trying to go cheep is stick a cheep muffler on it. how much of the exaust is left on it?
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fom what u are saying and from my personal exsperiances with TBI systems, i would say that your biggest problem is not having a muffler for back pressure. My dads 87 454 was getting about almost 16-17 mpg and about a month ago the mufflers fell off and it is lucky if it get 10mpg. what i would do if u are trying to go cheep is stick a cheep muffler on it. how much of the exaust is left on it?
It has one of those flexpipe deals from the headers. Someone (probably Pa) used bailing wire and a U-clamp to keep the muffler on so I lost it.
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most states are ok if you have the exhaust run past the cab so put some straight pipe in then the muffler. unless you want to run the whole thing where you will see the most back pressure is a cat. but its illegal to use used cats from another vehicle so i think you would have to buy them new
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What about dual glasspacks behind the cab? Would that give reasonable backpressure?
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it should, would be better than nothing. that is what i had on our 87 454 before they rusted out and fell off. turbo muffler are cheaper and quieter.
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glasspacks do give you something but not not a muffler will do. i love load trucks but wouldnt want to hear it everyday
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Alright, so backpressure is needed and a clean intake. What about the weak fuel pump?
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sound like u have a loose ground if u have to wiggle or tap tank.
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Anything that is a problem to how the truck is supposed to run normally is going to affect it. If you have to tap/hit/beat the side of the frame to wake up your fuel pump than it is time to replace it. Could be a bad ground but more than likely it is in need of replacement. I used to know of a guy who had a 4.3L with a 5 speed in a full sized 1990 extended cab C1500 and he was around 22mpg. My $.02
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yah. now that i think about it, it could be a bad fuel pump.
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Ugh, I need a job guys. Or better bike weather.
Should I drop the tank or lift the bed for a pump replacement?
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Dang. That's the worst mileage yet. Hole in the tank? You think I'm joking but I actually had that problem once on a 93 cavalier.
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What's your fuel integrator and block learn at?
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Dang. That's the worst mileage yet. Hole in the tank? You think I'm joking but I actually had that problem once on a 93 cavalier.
I think I had a similar problem, on the same year/model once. Hated that car.
What's your fuel integrator and block learn at?
I'm sorry, but you're gonna have to tell me how to obtain that info.
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You need scan data so either using a scan tool or software such as winaldl
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Well heck. Something I could get on a parts loan?
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Also, I want to thank you guys for being helpful and patient with me. I know basics but I've got a long way to go.
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ogh. one thing that might make a difference, does it have an o2 sensor?
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ogh. one thing that might make a difference, does it have an o2 sensor?
I imagine it does, being TBI. I had the throttle body replaced not long after getting it though with a used one. No telling if it's the wrong one. At least for me, I can't tell.
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is there an oxygen sensor on the exaust? if there isnt that might exspain some of the issues.
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with the in tank pumps theres a rubber fuel line between the pump and assembly. it can and will rot out causing the truck to run like crap. new pump comes with this line its thicker for the "new" gas. on replacing the pump its easier to just slide the bed back its only 8 bolts and a couple for grounds one near the tank and another on the left rear near the lights.
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Dang. That's the worst mileage yet. Hole in the tank? You think I'm joking but I actually had that problem once on a 93 cavalier.
I think I had a similar problem, on the same year/model once. Hated that car.
They're garbage. As were all the later ones.
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I think, given my funds, bills, and lack of employment right now, I might have to just part ways with the family truck. If somebody in my area wanted to do some driveway maintenance, I'm all for it, but I don't have the space and tools for it right now. Definitely not enough money to hop into a shop.