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throttle body inj swap out??
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gizzardgutz
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May 18, 2009, 5:23 PM
Post #1 of 11
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throttle body inj swap out??
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My youngest daughter recently bought a 1994 ¾ ton Chevy Silverado with a 3” lift and the big 33 inch tires against my recommendations. This is a high mileage truck that has been reasonably well taken care of mechanically if not cosmetically. To say it is a gas hog is to put it mildly! 8 or 9 mpg on a good day. The truck has an automatic transmission and the 350 cu in V8. It also has a worn out throttle body injection system I’m wanting to change out for a 4 bbl carb. I used to run a 454 Chevy 1 ton welding truck with lots of weight on it (exceeded 1 ton) that had the thermo quad carb and got 12 mpg with it. If I can get 10 to 12 out of this 4X4 with a 4 bbl, that’s what I need to do. The parts houses want anywhere from $7 to $12 hundred dollars to replace the throttle body with a new one and I can get it done much cheaper on eBay with a carb and intake. I’m 59 years old and I have fallen way behind on technology on these trucks. I’m told I will have to use an EGR manifold or exchange the heads for non-EGR heads in order to place a 4 bbl on this motor. I’m confused…can’t I just replace the throttle body with a 4 bbl carburetor without changing the manifold? Does the carburetor itself have be designated as EGR? What will need to be done for the “computer” to accept these modifications? I’m going to need help for sure to get this done. If I can get these questions answered that will at least give me a starting place. Thanks for any help provided in advance!
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Jeff Norfolk
Enthusiast
May 18, 2009, 5:38 PM
Post #2 of 11
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Re: throttle body inj swap out??
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If you are anywhere were emissions testing is required, this swap will be a tall tail. In SE Texas even if you manage to get the gasses out of the tail pipe to line up where it should, the engine lite will still be a problem. Here if it is on during an ASM test it is an automatic failure. May be stuck at the mileage that you are at with this one.
Jeff
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Loren Champlain Sr
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May 18, 2009, 6:06 PM
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Re: throttle body inj swap out??
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>> has a worn out throttle body<< What do you mean that it's 'worn out'? About the only thing to 'wear out' is the bushings on the throttle shaft that would cause a vacuum leak. A vacuum leak will be seen by the PCM as a lean condition, and will add fuel to compensate. And, the bushings can be replaced by anyone that does carb overhauling.
As far as fuel economy, a 4bbl won't give her any better. You are fighting the big tires and rear end gear ratio. Thermo-Quads were used on Chrysler products, Quadra-Jets on GMs. The computer in the vehicle is not adaptable for carburetion and will be opening a bucket of worms if you try to 'change it over'. Have you had the PCM scanned for trouble codes? Tire pressures? Tune parts? Injection service?
Loren
SW Washington
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Jeff Norfolk
Enthusiast
May 18, 2009, 6:07 PM
Post #5 of 11
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Re: throttle body inj swap out??
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If that's the case I would say that all you would need is a carb type intake and the carb. The old TBI trucks ran fairly low fuel pressure so the factory pump should work fine. As far as the computer. No real issue if you are not worried about emmisions. The o2 sensors, injector ciurcuts, MAP sensor will all flag a codes causing the engine light. I may be missing something as far a the engine light but agian in your states case, who cares. None of it should affect the performance.
Jeff
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Jeff Norfolk
Enthusiast
May 18, 2009, 6:10 PM
Post #6 of 11
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Re: throttle body inj swap out??
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Just a side not on my earlier post. This is not a s easy as it sounds. Consider this seriously before you try it.
Jeff
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gizzardgutz
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May 18, 2009, 6:25 PM
Post #7 of 11
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Re: throttle body inj swap out??
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>> has a worn out throttle body<< What do you mean that it's 'worn out'? About the only thing to 'wear out' is the bushings on the throttle shaft that would cause a vacuum leak. A vacuum leak will be seen by the PCM as a lean condition, and will add fuel to compensate. And, the bushings can be replaced by anyone that does carb overhauling.
As far as fuel economy, a 4bbl won't give her any better. You are fighting the big tires and rear end gear ratio. Thermo-Quads were used on Chrysler products, Quadra-Jets on GMs. The computer in the vehicle is not adaptable for carburetion and will be opening a bucket of worms if you try to 'change it over'. Have you had the PCM scanned for trouble codes? Tire pressures? Tune parts? Injection service?
I'm assuming worn out. This is a high mileage vehicle and, while I'm not real familiar with throttle body inj. systems, my brother had problems with his Dodge TBI and it had to be completely replaced due to "worn out," $1200.00 worth. I'm not familiar with PCM either? The truck has loud pipes and sounds good, but is sluggish and getting worse. I'm reasonbly sure a tune-up would make some difference, but the performance difference between what this TBI sys has and a 4 bbl carbuetor is worlds apart, as is. I'll look into the bushings at the throttle shaft though. If that's ALL that wears out on this system, it sounds like a simple fix. Thanks.
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Loren Champlain Sr
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May 19, 2009, 1:11 PM
Post #8 of 11
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Re: throttle body inj swap out??
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Sounds that some diagnostics need to be done before anything is done. The TBI may be fine. Could be the gasket between the TBI and intake manifold thats leaking...if it even has a vacuum leak. That hasn't been established, yet. The throttle body is a very simple device. It has an IAC (idle air control) motor attached to it that is controlled by the PCM (computer) which opens and closes at a very high rate of speed to contol air flow into the intake. On fuel injected vehicles, the fuel supply and pressure is constant. You add air to increase speed. Should be called an air pedal instead of a gas pedal. The two fuel injectors at the top of the TBI are supposed to supply a very fine spray of fuel. They are no more than electrical soleniods opening and closing depending on engine load. The amount of fuel required is controlled, again, by the PCM via engine sensors that give it the needed information. A properly working fuel injection system is by far superior to any carburetor in performance and fuel economy. The fuel injection system does require maintenance, just like any other mechanical item. It should be cleaned at 30-40K mile intervals.
If you have a vacuum leak, the MAP (manifold absolute pressure) sensor could be seeing a loss of vacuum, which it would see as the engine being under load and adding fuel. Of course, this would affect performance and fuel economy.
The CTS (coolant temp sensor) could be reading wrong and telling the PCM that you are in Fairbanks in January. It would tell the PCM to add fuel. The O2 (oxygen) sensor could also be faulty, giving the PCM incorrect info. Soooooooo,
have this diagnosed before you do anything. The diagnostic charge will more than likely save you money in the long run and will definitely control hair loss.
Loren
SW Washington
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gizzardgutz
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May 19, 2009, 1:30 PM
Post #9 of 11
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Re: throttle body inj swap out??
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Sounds that some diagnostics need to be done before anything is done. The TBI may be fine. Could be the gasket between the TBI and intake manifold thats leaking...if it even has a vacuum leak. That hasn't been established, yet. The throttle body is a very simple device. It has an IAC (idle air control) motor attached to it that is controlled by the PCM (computer) which opens and closes at a very high rate of speed to contol air flow into the intake. On fuel injected vehicles, the fuel supply and pressure is constant. You add air to increase speed. Should be called an air pedal instead of a gas pedal. The two fuel injectors at the top of the TBI are supposed to supply a very fine spray of fuel. They are no more than electrical soleniods opening and closing depending on engine load. The amount of fuel required is controlled, again, by the PCM via engine sensors that give it the needed information. A properly working fuel injection system is by far superior to any carburetor in performance and fuel economy. The fuel injection system does require maintenance, just like any other mechanical item. It should be cleaned at 30-40K mile intervals.
If you have a vacuum leak, the MAP (manifold absolute pressure) sensor could be seeing a loss of vacuum, which it would see as the engine being under load and adding fuel. Of course, this would affect performance and fuel economy.
The CTS (coolant temp sensor) could be reading wrong and telling the PCM that you are in Fairbanks in January. It would tell the PCM to add fuel. The O2 (oxygen) sensor could also be faulty, giving the PCM incorrect info. Soooooooo,
have this diagnosed before you do anything. The diagnostic charge will more than likely save you money in the long run and will definitely control hair loss.
Thanks for your input. Looks like the days the days of Johnny fixin' his vehicles is over. I guess I'll have to find a mechanic to run a diagnoses on this truck. I honestly have never heard anything good about TBI. Everyone calls it it a "Detroit abortion" and worse. I don't know any mechanics other than shade tree, and we are a thing of the past. I appreciate the time you took to explain this to me, makes a world of difference from where I'm standing. Thanks! 
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Loren Champlain Sr
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May 19, 2009, 1:52 PM
Post #10 of 11
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Re: throttle body inj swap out??
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Hey, don't feel bad. You and I are only a year apart. It's really tough for us old pharts to try and keep up.
I'm not especially fond of TBI, either. Multi-port is sooooo much better! And, keeps improving. It's unbelievable to get 400hp out of a 350cu in engine and still get 20mpg. No chokes, no warm up in the mornings, ect....
But, there may be hope for me! I just bought an iPod. Downloaded all my 50's and 60's music to it. My wife said I'd ruin it by putting that stuff on it! Wrong.
Loren
SW Washington
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gizzardgutz
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May 19, 2009, 8:22 PM
Post #11 of 11
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Re: throttle body inj swap out??
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Hey, don't feel bad. You and I are only a year apart. It's really tough for us old pharts to try and keep up. I'm not especially fond of TBI, either. Multi-port is sooooo much better! And, keeps improving. It's unbelievable to get 400hp out of a 350cu in engine and still get 20mpg. No chokes, no warm up in the mornings, ect.... But, there may be hope for me! I just bought an iPod. Downloaded all my 50's and 60's music to it. My wife said I'd ruin it by putting that stuff on it! Wrong.
Converting the truck to multi-port is out of the question, unless I win the lottery! If I won the lottery tho, this truck would go in the junk pile for another one in MUCH better shape  or new!! I have my music on an MP3 player...it's amazing how well it works under a welding hood. Wish I'd had one of these back in the day. Of course there would have been safety issues working in the oilfield or the shops I worked either one. Nice to be retired and working for myself as a hobby welder! Money's not as good, but benefits outweigh the rest! Coffee time is when I feel like it and the boss (me) is a helluva cook!! 
(This post was edited by gizzardgutz on May 19, 2009, 8:28 PM)
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