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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Cowman85 on January 26, 2009, 12:27:57 AM
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I was contemplating switching my th400 our for a 700r for the OD and gas mileage, would it be worth the effort? would it be difficult?
what about a 4L80E, or is there any that will just bolt right up in place of the th400?
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keep in mind that if your rear gear ratio is too low you may have to change gear ratio's. You should see improved acceleration and gas mileage though.
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What do you use the truck for? And what is your rear ends final drive ratio? What size tires do you have? What is the rest of your driveline configuration?
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i would imagine all stock, i have bought the truck but i pick it up on Feb 6th so i dont have access to the info i need, been trying to look stuff up but there is so much crap on the internet. All i've found was its a LS9 or LT9(hopefully the LS9) 350 4bbl th400 16x6 wheels(if the net was correct) not sure about the gearing there were a couple of different ones on the chart but i couldn't decipher without more info.
i was doing research early in case i found something good at the ol' wreckers.
The truck will be used for city transportation, some light off road/camping (2wd style), and some highway, but not really wanting to hit the highway until i replace the 3spd! the most it would be pulling is a tent trailer, but i drive ALOT, runs to the groc store etc, and i have a habitual heavy foot.
thats all the info i have until i pick the truck up in 1.5 weeks, i'll check back periodically and give the information i can, thanks for the input and time guys, if is greatly appreciated!
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I'd see how it runs as is before I did anything. If your th400 is in good running shape it should be very reliable for a good long time and my first choice would be to leave it alone. If you do decide to go with a different transmission, I would go with the 4L80E. There will be more work to get this transmission into the truck and working properly but the extra strength and reliability will be worth it in the end in my opinion if you really want a 4 speed OD transmission. I really don't think you'll notice much difference around the city between any of these transmissions. I guess the OD transmissions have a little lower first gear but that truck has enough power to smoke the tires without needing a lower first gear. My brother had a similar truck with 4.10 gears and he had no trouble on the highway with the th400....engine turned about 2800 rpm at 70 but still got 12 mpg or so (that thing was awesome for towing). How much highway driving will you do with this truck? Even with today's fuel prices I bet it would take a long time to pay for a tranny swap with the money you save on gas just from having OD. Maybe someone else here who has swapped from a 3 speed auto to a 4 speed OD auto can chime in with the difference in fuel mileage.
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I have yet to take that first highway trip, that will come friday evening if all goes well. If the 3 spd is ok at 115-120 km/hr then i will keep it but if not then i will switch, the number of camping trips and errands from calgary to edmonton would make a difference. I wont be looking for a new transmission, probably a used and get a rebuild kit to learn the experience.
Is there anything with OD that would bold right up in place of the TH400
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Every once and a while the engine will kinda miss then stall, sometimes i can catch it if i pump the gas early, the fuel & air filer was replaced, no difference. Once stalled it will not start again unless i put a little bit of gas in the carb, i checked out the fuel pump and there is just enough seepage to color the ends of the lines and clamps, i carb is pretty dirty too however gave it 1/4 of a can of carb cleaner to try and clear it a bit. There does not seem to be a pattern aside from travelling over 60 km/hr when this happens, however it does not hit all the time, every once in a while.
I am hoping that this is the fuel pump, i do not have the time to strip and rebuild the carb right now.
Also my secondaries on my carb dont seem to open up at all, there is real punch when i hit the gas, it will climb ok but only around 4000rpm. This truck only has 152,000 km and the heads were replaced within the last couple of years, something is not right here.
Anyone have any suggestions?
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As far as transmissions, keep in mind that a 4l80e is a great one, but, to use one, you will need to rob the computer from a donor truck....they are computer controlled.
Fuel pumps are cheap. I'd take the end of the fuel line off and put it in a mason jar and turn the engine over see how much volume of gas pumps out. That'll give you an idea if the pump is shot.
Will the engine idle properly? Could have a vacuum leak.
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Welcome to the site! I have a couple opinions & thoughts on your truck. The overdrive trans. won't be worth the COST/bother, if your not driving it for allot of long trips. Around town, with a heavy truck & a lo-po 350...as the trans goes into OD, the truck will lug, then need to downshift unless your on nice level ground, it just won't be in OD long enough to add up to any real gain.
Dropping down on the rear gear slightly (4.56 to 4.11) would be a better (more cost effective plan) if you are doing open road driving on a regular basis. However, for the sole purpose to "save fuel", this may not produce the results you think. I would think of it as more of a "not working the engine so hard to keep up with traffic" plan.
A good alternative is a used Gearvendor unit (An add-on OD unit) They bolt on to your existing TH400 (all you need to do is shorten the driveshaft) & lock it in only on the highway. I'm sure with the gas prices in the past year...the price has gone up on them. I would like to find one for my motorhome, but haven't really hunted very hard. If you do a little figuring, you'll see that even if you go from 10 mpg to 12, which is believable (15 would not be) if changing just to the OD, it's going to take allot of trips to add up to $1800 (about the cost of a rebuilt OD trans).
As for the secondaries not opening...Are you looking at the air door on the top, while opening the throttle? If so...they will never open, it is controlled by demand vacuum. If you look at the baseplate on the Left side...you should be able to see the linkage that opens the secondary throttle valves in the base plate once 1/2 or so, throttle is achieved. Lorne
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Well so much for that, changed the fuel pump, filter, wires and spark plugs, the flow from the new pump was checked and is good, the filter is in the right way, also used some carb cleaner to try and clean it a bit to no avail, and added some gas treatment/water remover.
Right now the truck will not start without adding gas to the carb, even then it will only run till it burns that off then it will stall again,
i dont know a dam thing about carbs...yet.
If anybody has rebuilt or knows carbs and is interested in some trouble shooting i would be able to pay a small fee for a fix, if this person knows his carbs then it shouldnt take long. I know its not a real complicated system but starting off from not knowing anything makes matters worse for me.
as for the od it will not be necessary as you said, it seemed to do ok on the highway.
thanks for the input so far guys, it is appreciated.
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Did you save the gas you put into the jar that the pump shot out? Put it in a clear jar, leave it sit over night & have a look. Lorne
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Sounds like a float problem to me...but I'm no expert on carbs. If you still have the Q-jet I know there are some good reference books out there that give good rebuild instructions if you're interested in learning about it and have a little time.
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i did a double check this morning, looks like the fuel that shot out the pump was the stuff left somewhere in the line, there is no more gas coming out of the pump, therefore its either the line or the tank. Considering the other tank is rusted out my guess is the actual tank, the junky part is that every nut and bolt down there is rusted to heck, so i guess i'm breaking them off and going with some stainless steel.
the gas isnt the problem, i drained some out into a clear water bottle and let it sit overnight and there were no sediments.
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Ok, My thought at this point would be, get a gas can (or suitable container) & a couple feet of rubber hose. Take the hose off that is coming into the pump from the tank, put the new, long hose on & stick it in the can...if it pumps fuel now, it's the tank, hoses, line, ect. & work from there, if it doesn't shoot fuel then...it's the pump. Process of elimination at it's finest, Lorne
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I had a similar problem years ago and it turned out to be the sock filter at the end of the pickup tube in the fuel tank. I pulled the fuel cap off, and put some short bursts of air back through the line from the pump, to the tank, and got it cleared. I then put an inline fuel filter before the fuel pump. I rempoved the tank later, and found the sock filter laying in the bottom of the tank. I guess I blew it off the tube.
May be worth a try, just dont get carried away with the air, and make sure the cap is off.
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Thats what i thought of today, dont know why it didnt hit me earlier but hindsight is 20/20 right.
I bought 10ft of 1/4inch fuel hose, disconnected the smaller inline on the fuel pump and stuck the other end in a 30L gas can. put a bit of gas in the carb and fired her up, it fired ran for about a second then started choking, if will not idle proper for the 3-5 seconds it usually does when you dump some fuel in it.
I guess there could be air in the line, but wouldnt the pressure suck it through though?
I double checked the pump when installing and made sure the spring was under the bolt, i'm hoping its just a lemon of a new pump but we'll see, i will be testing it this week by taking it off and trying to hand pump it.
initially when i first bought the truck it died twice on the way back, once at the end of the block, 2 km down the road, and then again a couple of km after that. Each time it took some gas in the carb and some pumping to get her fired up again, and it would last around 5-10 minutes. Once in a while it would kinda lag out or miss on the highway but a quick pump or 2 and it was ok. These are the symptoms that i can describe.
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good news, i bought 14ft of 3/8" and 1/4" and ran them from the box of the truck to the fuel pump, i filled the lines with gas to start and then fed them into the 30L gas can. She works like a dream, too bad its not a tercel then i wouldnt have to change the tank!
would it be worth a new tank @ $150 or a used one from the wreckers?
the only problem with the wreckers is that the tank is probably dirty and living in an apartment find it not one the most desirable tasks to deal with. Is there an efficient and low cost solution to this?
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Dirt, rust & crud in a tank can be a real pain. Why go through the trouble of changing it to end up with similar problems? Buy a new one & be done with it. Lorne
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A new one or a fuel cell.
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i figured as much, oh well, i guess better to get it done now than down the road.
as for the fuel line configuration (i'm still waiting on the other guy to drop my manual off), there is the 3/8 (main) then the 1/4 (pinhole size) as far as i can tell they run from the pump then bridge near the rear axle splitting to its L or R tank, this is where the tank switch is right?
would it do much to stick an inline fuel filter in before the fuel pump? if so what kind etc?
is it worth rebuilding your carb or should i just get a new one? the one that i have is really dirty, carb cleaner didnt do much to it, i cant really afford the downtime but if its quick enough then it would be worth it for me.
thanks for the input guys
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I put my inline fuel filter between the steel line in front(3/8) ,where it comes out the frame, and the fuel pump. As far as the carb, its cheaper to rebuild than replace, but your ability. Some cant rebuild carbs. Im retarded, I can rebuild a Q-Jet, but Holleys scare me????
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does anyone know if the haynes manuals are actually good for this? i know in some areas they are quite brief and non descriptive.
i do believe it to be a rochester but she is so dirty i would have to double check, carb rebuild kit would run me how much do you know?
what model of filter did you use? does it slow the flow down at all? does it make a good difference in the end?
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I had a neighbor teach me how to rebuild them years ago, but I did find a book on q jets alone that shows some pretty good tips. Key thing is just remembering howto put it back the way you took it apart. You basically are just replacing the gaskets, needle and seat and setting the fuel float level. I am not sure how well the haynes covers it. I will look later today.
As far as the filter, I just bought an A/C Delco inline filter with the right ends for thehoses. It doesnt have to be anything special. It helps keep trach from entering your pump, doesnt restrict fuel flow, and is alot easier to replace than the one in the carb. I removed the one in the carb when I put the inline in. The problem with the one in the carb, is it is real easy to cross thread the filter housing, and then you are in trouble.
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right on, i will go get one later today, since this truck only has 152 original km on it i am trying to keep her in the best shape possible. i almost crossed that a few times but managed not to. So you only stuck one on the 3/8 line right, the 1/4 line hole is so small it looks like a bleeder or vacuum hole, you just left that one as is right?
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The 1/4 inch line is a return line to the tank, no need to worry about it. I forgot to answer a question earlier, the rebuild kit I bought the last time was about $25. The only thing is you need the number off of the carb to get the right kit. (The parts guys around here really freak out if you dont have it. If I remember right it is on the front corner of the carb on the throttle linkage side. I dont remember for sure. One of you guys please help me out here, I am having a senior moment.
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thanks as always!
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ok i have yet to pull the tanks and it was -40 here so i just fed enough line up to the front of the box and put a 30L jerry can strapped in, strapped the hoses down and poof i am in business.
The truck ran like a top for a while but now when you hit the gas sometimes it will cut out completely and almost stall if you give it more gas, if you let off the peddle it will idle with no issue, i pull over let it sit for a couple a seconds and i'm good to go. But it happens during almost all heavy acceleration and hills.
When i got home i took the air housing off and looked at the line, sure enough there was a wet spot under the line on the fuel intake, then a drip, and a couple seconds later another drip. The thing is it only does it when i'm driving, if its sitting there idling parked, doesnt matter how much you rev it on and off it doesnt cut out, only when a load is introduced.
I will go out there and take it off again, and add some thread tape to it to make sure it is sealed properly, but in the meantime anyone every run into this problem?
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The stalling out could be the float being set too low or the accelerator pump in the carberator. Is the original filter still in the carb? When I put my inline filter in, I removed the filter in the carb completely. Also, make sure none of the fuel lines are kinked.
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The original filter is still in the carb, i have changed it twice so far just in case, i will be able to to put an inline filter before the fuel pump in the next few days. As for the carb i will need to get a book and also clean it off so i can find out the model.
The line in from the fuel pump to the carb is twisted a little bit at the top, i have yet to find a replacement. The truck ran fine for 2 weeks with that slight twist so i dont think it is that.
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ok, so a more apparent symptom now. When i step on the gas (or just cruising) the engine will cut out before like i said before, but if you keep your foot in the same place it stops accelerating (almost stalls/low idle) then goes back to accelerating, almost like a clock if you catch my drift. accel pause pause accel pause pause accel, until finally it stops for a few blocks. Hopefully somebody knows what i am talking about cause i am in the middle of school right now and cant afford much if at all. :'(
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gee i hate being right (when its bad) she stalled in the school parking lot and will not start again.
fuel filter was removed, there is sufficient flow from the pump, it will only run when you put gas in the carb.
So looks like the carb after all, and i dont know enough to fix this.
I do not have the tools or space to pull the carb off and clean/rebuild it, kinda iffy about pulling one out of the wreckers cause look at my luck an 86 with 153km and the quadra jet is crapped out.
Is it worth buying a new one (that is pretuned i hope) or should i just roll the dice with a used one?
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I bought a new one from Autozone when I wanted a new one, came with a warranty.
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You sadu you removed the filter in the carb. Did you put the inline filter in first and run it?
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not yet, but i have the lines sitting in the middle of a clean jerry can thats sealed at the top.
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That may have been what caused the final failure. If any trash was left in the line, it would now be stopping up the needle and seat in the carb. You may be able to remove the filter housing on the carb, and lightly ( ever so lightly) bolw air back through the vent and dislodge anything that may be stuck in the seat. Use extreme caution and protect your eyes and face. It seems tho that your carb is just one of the many problems with it that need fixing anyway. Always have a filter on your fuel line. I did remove the filter in my carb, after I installed the inline filter before the pump and ran it a while before I removed the carb filter to finish cleaning out the fuel line. I am sorry if I didnt clarify this.A little peice of rubber from the new hose can cause the needle and seat on the fuel inlet to stick open and flood the engine.
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The truck always had a clean fuel filter before and after the line change, until this afternoon when it wouldnt run unless you dump gas in the carb, thats when i removed the filter to see if there was anything in there. There was a metal ring that used to be in there, maybe a small piece of that was behind the filter in the first place??
I will definitely try clearing it, it wouldnt surprise me if a kitchen sink came tumbling out now.
Will be picking up a quadra jet book tomorrow and going over the trouble shooting in that as well, if she aint fixed by the end of the week i think i will just buy a performance carb and wait to rebuild this one when i have the means to do so. An inline fuel filter will be bought tomorrow before any more gas gets pumped through there.
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Get a gallon of Chem-Dip and a $30 rebuild kit, take pics when you take it apart and rebuilt it. The are REALLY simple.
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Captkaos ir right. They really arent as hard to rebuild as they look. Key thing is to take lots of pictures, even if you have a book, and clean everything real well. The hardest part may be getting the float set, but most carb kits come with a measuring tool, and good instructions. Cleanliness is the main thing. You can take it apart and let is soak in the carb cleaner while it sits in the back of your truck. Weather permitting of course.
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i know what i need and what to do, but i drive my truck everyday so it is not feasible to store in the back of my truck and i dont have a second carb yet either. Im not intimidated by it, i just want to be prepared.
You guys ready for a good laugh???
i tracked down an auto advisor at nait today and told him the situation, he also thought it was the float so i asked how to mess with it without taking the carb off. He said to gently poke something down there to nudge the float, i thanked him and went on my way.
I ended up using a friggin zip tie to fix the problem, after poking it 3 times it fired up as if it was new!
I would just like to say i have all the respect in the world for these older guys who are willing to pass on their knowledge
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thanks for the help though guys :)
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I have an '88 r-20 burb with a tbi 7.4l and a "compushift" 4l80e trans. it has a 4.10 gear and it kicks ass! great fuel mileage on the hwy ( i think around 14-15, and i can pull a 7X14 tandem trailer at 70 mph. in overdrive like it's nothing!
Compushift has a kit (trans, wiring harness, and controller) but it's not cheap! The trans they build are bullet proof, tho! All billit parts and they have severial different one's for differnt needs. Check em out...http://www.phoenixtrans.com/html/gmtrans.html.
Hope that helps,....i love my set up. works great.
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Wow, that is a nice setup! once i'm done school i will be able to afford some of the things i've been craving to put in.
i'll be sure to check that out too, i really miss my muncie 4 spd though, this th400 aint the same.