Replacing fuel injector throttle body with carburetor

I have a 91 GMC pickup with a 350 engine, with throttle body fuel injector. The truck is basically in good condition and engine runs, but only in good weather. I cant figure this one out at all. In the
summer when the weather is warm and sunny, the truck runs like a champ. The minute the weather gets cool, the truck runs like total crap. It stalls, kills, runs rough, lacks power, and does all sorts of things except runs properly. Sometimes it will run good for a while, then seems to go into a state of chaos, then will run good again minutes or an hour later.
I've checked out numerous things, changed the plugs, fuel filter, air filter, made sure all vacuum hoses are ok, and all wires are plugged in tightly. Everytime I get it running right, (or seems to be), the next time i drive it, it's likely to begin running terrible again. Of course, if the weather is in the 60s or above and sunny, it runs perfectly every time.
I took it to a mechanic. He carefully looked it over and said everything looks to be working properly, but said there's no way to know for sure until it's connected to a computer, and said taht since my problem is intermittant, it may not show up on the computer at the time of the test. He told me it would cost me $200 to $300 to hook it to the computer. This does NOT include the cost of repairs or parts.
Well, I'm not going to stick $300 into just a damn test, then possibly pay hundreds more, when I only paid $1000 for the truck in the first place. I have fixed all my own cars since the 1960's and done so with basic tools and a few testers like a timing light, dwell meter, and electrical meter.
What really gets me, is that I have a 1989 Chevy car with a 307 engine and a carburetor. The car gets almost twice the gas milage, and it's nearly the same weight (empty) as the truck. It's not the extra 43 cu inches, it's those damn injectors causing the extreme over use of gas. (and thats when it runs good).
Before I even think of spending $300 just to test the damn thing, I'd rather replace the throttle body with a carburetor. I have an old 350 engine with a carb, so that wont cost a cent. I assume I'll have to change the intake manifold to make it fit. (of course I got that too on the other engine). Then I know the in-tank fuel pump needs to go, and be replaced by an electric fuel pump on the firewall. (I have that too). I wont miss that damn in the tank fuel pump either, after having had to replace the pump once, then replace the sending unit which sprung a leak. (no more tank removal sounds great).
My reason to post this is to find out about the ignition system. I heard that I need to modify the coil and other ignition parts, but what parts and how? Does anyone know?
What I want to do is turn this into a basic, simple engine, without all the junk added, and particularly without that damn fuel injection. I'll never buy another F.I. vehicle. Where I live, we do not have emissions testing, so I dont have to keep it stock.
Has anyone done any of this? What else do I need to know?
Thanks
Ralph
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On Fri, 14 May 2010 00:08:14 -0600, snipped-for-privacy@none.com wrote:

He's telling you something important: HE DOESN'T WANT TO WORK ON YOUR TRUCK! It doesn't cost anything to scan the truck, it uses a relatively standardized GM ODBI interface, there are thousands of scan tools available, both PC based, and standalone versions. eBay will have many listed. I've got several myself. This is not uncommon, nor a difficult concept, except when you have a mechanic who doesn't want to do the work. Find a new mechanic.

And don't... Why? You can buy the tool for less than that and run tests until the cows come home.
What transmission does this truck have? Automatic or manual, if auto, what model?
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Thanks for the response. According to him, these tests require thousands of dollars of computerized equipment, and he said it would l take 3 to 4 hours to run all the tests, at a cost of $80 per hour.
I think you're right, he dont want to work on it. I really could not understand how it could take 3 to 4 hours just to run tests.
The tranny is an automatic. How do I find out what model it is? It's a 3 speed with overdrive. Thats all I can tell you without knowing what to look for.
How do I know what kind of scan tool, or whatever you call it, to get for this truck? I'd like to see what they cost on ebay.
Thanks
Ralph
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On Fri, 14 May 2010 08:36:20 -0500, snipped-for-privacy@none.com wrote:

BS... Totally completely utterly BS.

30 seconds to five minutes typically.

If you can find one, a GM Tech I tool would work just fine. Do a web search for PC based scan tools (ODBI, not ODBII) as well. You want one which will display live data, in addition to DTCs.

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In the summer when the weather is warm and sunny, it runs like a champ. The minute the weather gets cool, it runs like total crap. It stalls, kills, runs rough, lacks power, and does all sorts of things except runs properly. Sometimes it will run good for a while, then go into a state of chaos, then run good again minutes or an hour later. I've changed the plugs, fuel filter, airfilter, made sure vacuum hoses are ok, and all wires are plugged in tightly. Every time I get it running right, (or seems to be), the next time i drive it, it's likely to begin running terrible again. Of course, if the weather is in the 60s or above and sunny, it runs perfectly every time. ___________________________________________________________________
The symptoms suggest a sticking Idle Air Control valve, although the temperature contribution is puzzling.
Old plug wires can cause problems if moisture is associated with the low temperatures
Less likely is an intermittent failure in the ignition module. In my 1992 350 TBI, it is a small package mounted inside the distributor.
Another possibility; fuel flow variation or obstruction.
My experience with these engines is that they start and run better with the TBI that they ever did with a carburetor.
Good luck,
Rodan.
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Go to alldata.com , buy a subscription for your truck and you'll have all the shop manual data right there on your computer. Your gonna have to get used to dealing with injected motors and since you seem to know your way around under the hood now's the time to learn on a easy to work on vehicle. A code reader, fuel pressure gauge are now standard tools in the mechanics tool box along with internet resources to figure out each brand of vehicle.
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I am not sure where you live, but in my area, some auto parts stores, like Autozone, will read the codes for free. Why not try that?
Invest in a good shop manual, and I'll bet you can figure it out. Autozone has some information related to the engine controls available for free online. Go to autozone.com and register. Then you can pull up a partial repair guide for your truck. The information you need is most likely in the Carbureted Electronic Engine Controls section. The most likely cause of you problem is a bad temperature sensor. Here is the next from that section related to the coolant temperature sensor (you'll need to go to the website to get the pictures):
Coolant Temperature Sensor (CTS)
OPERATION TESTING REMOVAL & INSTALLATION
OPERATION
See Figure 1
The coolant temperature sensor is a thermistor (a resistor which changes value based on temperature). Low coolant temperatures produce high resistance (100,000 ohms at -40F/-40C) while low temperatures causes low resistance (70 ohms at 266F/130C). The sensor is mounted in the coolant stream and the ECM supplies a 5 volt signal to the sensor through a resistor in the ECM and measures the voltage. The voltage will be high when the engine is cold, and low when the engine is hot. By measuring the voltage, the ECM knows the engine coolant temperature.
Fig. Fig. 1: Coolant temperature sensor location
TESTING
See Figures 2, 3 and 4 Remove the sensor from the vehicle.
Immerse the tip of the sensor in container of water.
Connect a digital ohmmeter to the two terminals of the sensor.
Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance illustration.
Repeat the test at two other temperature points, heating or cooling the water as necessary.
If the sensor does not meet specification, it must be replaced.
This might not be your problem, but it is worth checking.
Fig. Fig. 2: Submerge the end of the coolant temperature sensor in cold or hot water and check the resistance
Fig. Fig. 3: Coolant temperature sensor wiring diagram
Fig. Fig. 4: Coolant temperature sensor temperature vs. resistance values REMOVAL & INSTALLATION
Disconnect the negative battery cable.
Drain the cooling system below the level of the sensor and disengage the sensor electrical connection.
Remove the coolant sensor.
To install:
Install the sensor and engage the electrical connector.
Refill the cooling system and connect the negative battery cable.
Ed

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