86 C2500 350 4bbl

[Cowman85]

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?

[dumbucket1]

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.

[VileZambonie]

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?

[Cowman85]

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!

[hilton850]

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.

[Cowman85]

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

[Cowman85]

 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?

[Lt.Del]

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.

[HAULIN IT]

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

[Cowman85]

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.

[HAULIN IT]

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

[hilton850]

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.

[Cowman85]

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.
 

[HAULIN IT]

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

[choptop]

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.