Replacing fuel injector throttle body with carburetor

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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

Re: Replacing fuel injector throttle body with carburetor


On Fri, 14 May 2010 00:08:14 -0600, rfloyd@none.com wrote:

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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.


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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?



Re: Replacing fuel injector throttle body with carburetor



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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


Re: Replacing fuel injector throttle body with carburetor


On Fri, 14 May 2010 08:36:20 -0500, rfloyd@none.com wrote:

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BS... Totally completely utterly BS.

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30 seconds to five minutes typically.

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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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Re: Replacing fuel injector throttle body with carburetor


 
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.

Re: Replacing fuel injector throttle body with carburetor


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.



Re: Replacing fuel injector throttle body with carburetor


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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