TBI replacing carb
 

Has anyone fitted throttle body fuel injection on an 70s chevy V8? My truck guzzles fuel and I'dd like to ditch the carburator. It's not funny, maybe 8 mpg:(

first of all, I know it isn't a V8 but my buddy is fuel injecting his chevette with a system called mega squirt. it is a very indepth system that you build yourself. he just got the kit and says it will take him about a month to put on because he needs to find other random bits to use like the throttle body, sensors, high pressure fuel pump. he already has another gas tank.

he is trying to make it to where there is not downtime with the car when he does the change over. he his hoping for more power and better FE.

I'll talk to him about the system and ask him what is different if it is done on a V8 vs his 4.

What type of 70's V8 Chevy truck are you talking about? With some relatively minor mods I was able to get my old 1974 Chevy C-10 pickup with a 350 V-8, Turbo 350 automatic tranny, and a Rochester Quadrajet carb turning almost 30 on the highway (with the a/c on!!!). They were great because they didn't have all the emissions crap on the engines to reduce mileage.

Here's the mods I made on my old truck:

Added cruise control

I flipped the bonnet on the air cleaner upside down so the engine can breathe easier.

I replaced the points type distributor with an electronic one from a 1980 Chevy truck with a 350 V-8 (from junkyard)

Switched to Platinum spark plugs, and increased spark plug gap when I switched to electronic distributor.

Even with over 300,000 miles on it that truck ran great, got great mileage when I was kind to the accelerator, but when you wanted to move in a hurry that thing would nail you in the back of your seat.

-Jay

I put a Holley Projection on my Buick 455 back in 1992. The worst part was the extra fuel lines- my original car didn't have a return line, and the supply line was too small. So I put in a new 3/8" supply line and 5/16" return line.

For me, the wiring part wasn't too bad, but I'm better at wiring than at plumbing.

-Bob C.

With mileage that bad, I'd look for something obvious, like a stuck choke. A stuck choke will make a truck gulp fuel like crazy.

You didn't say how big the engine is, or what the axle ratio is. A complete motor or axle swap might be easier than trying to modify an engine.

No kidding, I had a 1.3L car with an auto choke that screwed up, it got 25mpg, then I ripped the auto out and put a manual kit on it and got 35mpg.... then I later has some other issues with that carb and replaced it with a very low miles original carb, with the auto choke intact, and that got me 40mpg.

The auto choke on my 74 chevy was real simple. It was a flat, coiled spring bolted to the intake manifold. The spring would expand and contract with the temperature of the manifold, shutting off the choke when the engine warmed up. Adjustments were easy, I just bent the pushrod connecting the choke to the spring. Check your spring and see if it isn't broken from age... Should be a cheap, easy repair if its broken.

-Jay

I would love to convert my 1980 Buick to TBI, but the more I learn, the more I think the project is beyond my ability and budget. Apparently it can be done pretty reasonably with junkyard TBI parts from a Chevy Astro. However, I think it requires a pressurized fuel system, return lines, etc...

I would like to do it to my 81 Regal as well, but to do it right I think I would have to start with a decent parts car.

-Jay

Take the TBI stuff off of an 88-93 Chevy or GMC truck. My 89 Chevy was TBI. I believe they ran it until 93, give or take a year. Pretty basic, and fuel pressure is not very much, like 15 tops I think. Taking everything you need off of one of those trucks from the intake manifold up. Take the wiring harness and computer also.

Since the 3.8 V6 was not used in a truck that I know of (except for an El Camino maybe), I would have to use an 84-87 Century/Regal as a parts car. I'd need the fuel tank too, as the fuel injected Regals had a special anti slosh tank underneath to deal with the electric fuel pump.

-Jay

No, I do not believe the 3.8 was ever used in a truck. You would probably be looking at mid 80's and up for a parts car to pull from. I've seen TBI on 4 cylinder motors like the 82 Chevy Citation I used to have right up to big V-8's in the trucks. My 89 was a 4.3 V-6, damn good motor.

The Astro's TBI seems to be the most popular donor for TBI conversions, apparently it works great on a variety of engines.

The Astro shares the same 4.3L found in the full size pick ups and the smaller S-10/15 pickups, and some cars also. Finding full size pickups, around here anyway, to pull parts from is about as daunting as finding a fossil. There are just so many still on the road that even when you find one in the junkyard, there isn't much left to pull. Perhaps the Astros are just a little easier to come by?

Of the various forms of fuel injection, you will find the GM TBI set up pretty easy to work on and adapt to. It is a low pressure system, 15 psi tops I think. And for the most part, much of what you need is sitting right on the intake manifold. You've got your throttle body, which includes the IAC, TPS fuel pressure regulator, injector(s), the MAP is usually mounted to a bracket on the intake, the EGR and solenoid, the CTS mounted at the front of the intake. Virtually everything you need is located right on the intake manifold, with the exception of the knock sensor and O2 sensor, and of course the pcm. Even the distributor, and ignition module have to be removed to get the intake off, so take that also. And I almost forgot the ignition coil is also mounted to the intake manifold.

froggy81500, what about the pressurized fuel system? Is 15psi low enough that you can just put an external electric pump between the tank and the throttle body?

Yes I believe so. There were some early forms of FI that Ford used. CFI was one that comes to mind. My parents had an 84 LTD that had two fuel pumps. One low pressure one in the tank and one high pressure one on the fuel line under the car. I kid you not. I looked this up in the repair manual way back when they still had the car. Why 2 pumps I could not tell you. I think you shouldn't have an issue with an external pump on a TBI. I will try to track down my old repair manual for my Chevy with TBI to confirm that fuel pressure but I know it is not a lot. Also, there is a way to adjust the regulator on them. They are non-adjustable but have an adjusting screw that is soldered in place. Also, the spring for the regulator are known to rot out (ask me how I know this!) There is a slot in the side of the regulator housing and you can almost see the spring in there. since it is open like it is, moisture and crap get in there and rot the spring to pieces and leaving the engine will very minimum fuel pressure. Its not a hard fix and some places used to sell adjustable regulators, but I fould you could break the solder on the replacements and still adjust them.

You will need a return line though on these. With low pressure like it is, I can't see why regular double walled (with the braid inside) fuel line can't be used. Unlike port injection that runs upwards of 50 psi, TBI is just a little bit higher than a carb.

https://www.autozone.com/shopping/rep...00c1528008f1ea

That is the repair info for the full size GM trucks, which is what I had. The fuel pressure testing procedure calls for 9-13 psi test pressure with the engine running.

V-6 and V-8's used two fuel injectors, while the 4 cylinder motors had just one.

Drat, that's the other issue I forgot. How would one install a return line? I suspect you'd have to drill and weld it into the gas tank, no?

What is the return line for, anyway? Really, why does it need to go back? Why can't it just pull the amount of fuel it needs and not let any more come forward?

Like another person posted above, the plumbing is the intimidating part of the whole thing...

Its the way the regulation is done. The regulator bleeds off extra fuel pressure thru the return line. I imagine if you are working with a fuel system that has a vapor line between the tank and engine compartment, that might suffice. The gas tank on my truck had 3 lines coming off the sending unit. 1 was the pressurized fuel line, 2 was the return line, 3 was a vapor line.

On a lot of the newer fuel systems, Dodge in particular, they run just one fuel line to the motor and have the fuel pressure regulator located as part of the sending unit/fuel pump unit. That was it vents extra pressure right back into the tank instead of having to run a second line.

I would think you would need to replace the tank as well. I know GM used special anti slosh tanks on fuel injected vehicles. Carbs don't care because they ran off of the fuel in the bowl of the carb. If there was a momentary interruption in the fuel flow due to sloshing in the tank the engine kept running. With fuel injection the vehicle stalls. I remember my dad had an early model Chevy Lumina minivan that did not have a good anti-slosh tank in it. He said the van would start sputtering at 1/4 tank.

-Jay

I had the tank down once and didn't notice anything special about it from other tanks. The sock is mounted to the bottom of the fuel pump and sits on the bottom of the tank. So it is always picking up fuel from the bottom. On carb equipped cars, its been so long since I had one open that I can't remember where the pickup sits in the tank.

Bingo.

You don't NEED to replace the tank, but it is a good idea if financially possible (a custom-made FI tank for an older car can run about $1000 - https://www.rockvalleyantiqueautoparts.com/catalog.htm). But for me that was never an option, so I use my factory tank and just make sure the car never gets below 100%.

My car has never stalled due to lack of fuel, but it has ruined several fuel pumps from overheating when fuel wasn't available to keep the pump cool (see below).

The new tanks will accommodate internal pumps (which are best for these scenarios, although I have always used external), and will have the internal baffling to minimize slosh away from the pickup.

It works a bit differently than that. On a carbed car, the pump only pumps what is required by the engine. On an FI car, the pump is always pumping 100% of its rated flow, and the leftovers go back to the tank. So if your pump can do 100 gallons/hour, it will pump 100 gallons/hour and the regulator will maintain pressure but return 96 gallons/hour back to the tank. This is to keep the pump cool, by always letting it flow 100% of volume, these pumps are very sensitive to overheating and the fuel is what cools the pump.

-Bob C.

I think the cheapest option is to find a suitable tank from a FI car and hang that underneath. For example. On my 1981 Buick Regal - the exact same car was made with TBI from 1984 - 1987. I would have to find a fuel injected GM A body car, which would not be hard. Just about any 1984 - 1987 Malibu, Monte Carlo, Regal, Grand Prix, Cutlass Supreme, or Century woud be able to donate a suitable tank. If converting a full size car then a tank from a GM full size fuel injected car up to 1991 should be almost a bolt-in replacement. If converting a 70's or 80's model GM truck get a tank from an 87 C-10. It will fit with slight modifications. 87 was the last year of that body, and came stock with fuel injection.

Otherwise as you said before, don't let the fuel tank get low.

-Jay

why not try modifying the stock fuel pick up in the tank of a non-FI car? Use an external electric pump and modify the fuel pick up.

So, with an external pump retrofitted, I could skip the return line for my TBI as long as the pump can maintain 15psi? That is, assuming the pump is rated for non-continuous flow (or I hack up a cooler for it)...

Maybe like Jay said I'd just get a tank from a newer model, that hadn't occured to me.

Well, with a GM TBI unit, you'd either need to run a second line from the open side of the regulator. Or cap it off and figure a different way to regulate fuel pressure and flow. I have not looked at external pumps for some time so I do not know if you can find one that self regulates. Yes, there are some hassles with trying to retrofit this but I think you would find the GM TBI about as easy to adapt as anything.

GM made cars on the same body/frame as your 1980 Buick until 1991. The easiest to find would be either a 1990 or 1991 Caprice tank should be pretty much a direct replacement.

-Jay

I'm not aware of any electric external fuel pump that can work without a return system for full flow. A mechanical pump will not produce good results with a FI system.

A non "return-style" regulator will prevent the pump from getting sufficient flow to stay cool.

"Maybe" the Holly Blue pump (a rotary vane pump, instead of an inline pump) might work, but it is really designed to work with carbs and not with FI.

-Bob C.

I doubt a mechanical pump will work at all on FI. Mechanical pumps deliver fuel in pulses. The bowl on the carb evens this out. I think if you could even start an engine with FI and a mechanical pump it would sputter and miss horribly, and probably would not be very driveable.

-Jay

Thanks for highlighting the pitfalls. Didn't think about the tank, pump, lines or intake design changes.
It's a 76 Chevy C30 with a 1970 400 small block usually hauling 3 tons. Figuring per ton mileage isn't bad. The odometer's not working so mileage is a guess anyway.
If I go TBI, I'll need to buy a mid 80's roached donor truck.:rolleyes:

I don't think they ever fuel injected the C30 on that body. I know when I was the vehicle officer at the rescue squad we had an old 1989 Chevrolet K30 ambulance with a 454 in it for driving in the winter, because so few of our roads were actually plowed. That truck had a carb in it, no fuel injection. Because of that I don't think they switched to FI on the 1 tons until they switched the 1 ton to the new body (90 or 91 I think) I'm not sure how a FI system designed for a 350 and a 1/2 ton truck will do with a 400 and a 1 ton chassis/load. I'd do more research on it first, and maybe do one of the Holley FI retrofit systems designed to replace carbs, at least you will be able to tweak the settings with a laptop so it will run right if necessary.

-Jay

I drive a 1988 model CBS (73-87 pickup body style) 3/4 ton Suburban with a 454 TBI. The model 700 TBI unit in that is the same body as the 350 (5.7) and 305 (5.0) V8's, and even the 4.3 and 2.8 V6's. The 2.8 has the smallest bore, 4.3/5.0/5.7 have the medium size, and the 454 is the largest. Injectors match bore size for each. So a 4.3 TBI unit is the same as a 5.7 TBI unit. That means Astros, S-10 Blazers, pickups, vans, and Suburbans all have the same unit and are interchangeable. TPS sensors changed mid-year in 1989 so get the harness from the truck you pull it from to make sure it's right. All this info comes from here:

https://s-series.org/component/option...,26/task,item/

The directions to change a small TBI for a large one will be similar to changing out carb for TBI. Of course you'll want the throttle cable, all the sensors and vacuum hoses, and the computer and wiring harness, unless you go aftermarket (MegaSquirt, others).

As far as C30 CBS trucks getting TBI, I know the 1987 models were still carbureted. Look at the 8th digit of the VIN code. R means it's a 350 TBI. K, L, or M means a 350 4bbl Q-jet. F and G are a 305 with a carb. H is a 305 no matter what, but can be carb or TBI. Z is the 4.3L V6. They made CBS trucks through 1989 for sure. CBS Suburbans through 1991 but Suburbans were TBI from 1987 on (for the most part).

IIRC Suburban fuel tanks fit real nice where the spare is on a 73-87 pickup. 40 more gallons of range. (OR 26 or 34 depending on the tank you grab, most were 40 though)

TMI yet?

believe it or not, occupant, the 4.3L and 5.7L have the same bore and stroke.