1992 Chevy 1500 no injector pulse

fire-fighterJune 9, 2010

Hi, Im back again. Truck was running fine til the other day. Ran fine and the next day it wouldnt start. Found out the injectors werent pulsing sending fuel into intake. Has a new fuel pump, sending unit and tank. Tested and it has around 14 lbs which is right for this vehicle. Spark is great, new plugs and wires along with cap and rotor button. tested coil and it was ok, tested ign module under cap, tested fine, tested pickup coil and that also was fine. Swapped out computer with known good one, still no pulse from injectors. put a noid light on them and they have juice. This truck is killing me, already spent over 2000 to get it up and running before this happened. Need help before it becomes fire department burn test truck lol

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jemdandy

You had me going when you used the word, injectors, indicating several injectors. However, I reviewd you older posts and this is what I believe you have:

1992 Chevy 1500
4.3 L engine
TBI (throttle body injector)

You have one injector per venturi (barrel). I don't know if this is a single or two barrel setup.

In you original post, the engine was running too rich.

You will need a good shop manual and diagonsitc tools to sort this out. I think it would pay you to have a good set of diagnostics run. A good techican can tease out some surprising information. For example, I had one tech miss my problem and the recommended fix had no effect. He was puzzled, pulled off the valve cover and discovered the problem tight away. He apolgized for missing it the first time around. It was a trashed cam shaft and lifters. The valves were not opening all the way. That caused low vacuum and messed with the engine controller.

I don't have any manuals for your engine so can only give a little of what I remember about GM and TBIs from the 1980s. If you have the unheated O2 sensor, it is ignored on start up and the engine controller assumes a default setting for the injector. This setting is derived by averaging the past three starts and can be fooled when the weather takes a drastic change since the last shut dowm, for example, on a winter night, the temperature goes from 40 F to 15 F.

After the coolant warms to about 140 F, the engine controller uses the O2 sensor output for adjusting the fuel mix. Before this point, the controller may not give full ignition advance to aid in a faster warmup.

This temperature sensor may be separate from the one that controls the electric fan. If this sensor is faulty, or the thermostat is missing or stuck open and the coolant does not attain 140 F, the controller will never switch to the O2 sensor and likely, the fuel mix will be rich. Spark plugs may begin to foul with continued operation.

If the controller senses that the output from the temperature sensor is out of bounds, it may ignore it, but should set a trouble code.

Have you read the trouble codes? These can give clues.

Since this is a pre 1996, it likely has the OBD1 system and requires a GM type code reader for that year. If you are lucky, there may exist a procedure with the ignition key to retrieve the codes.

    Bookmark   June 10, 2010 at 2:54AM
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john_g

When a shop posts a help request similar to what is above, the normal reply is to tell them to substitute a known good technician.

VBG...

Test this and change that is not how to diagnose and repair a problem like this, well, by now you probably realize that because it simply doesn't work. Now sure there are times that this method does work, and occasionally you don't even spend a fortune doing it. But as a pro, you'd fail so often you would not last as a technician.

A "noid" light is not an accurate test of the systems ability to turn the injectors on. The injectors initially start to draw current at such a level that if they were allowed to stay turned on long enough they would each draw twenty amps+ of current. To prevent overheating the injectors the computer uses what is called a peak and hold system. How it works is one transistor (also referred to as a "driver") turns on and starts to open the injector. Once a sufficient current flow is reached, a second transistor turns on that has a resistor in series with it, the first transistor turns off and that limits the current flow to around 1.8 amps. The "peak" part of the voltage waveform causes the injector to open, and the hold portion allows the injector to remain open long enough to dispense the right amount of fuel. A noid light will flash with as little as 1ma, or .001 amps. This type of testing has tricked just about everyone who relies on it at least once. The right way is to measure the current flow with a low amps probe an an oscilloscope. You can also use the scope to monitor the ground side of the injector to watch for both drivers signatures, and their inductive kick on turn off which is an indicator of the strength of the magnetic field that was created when the computer turned the injector on.

Now just because current is flowing that does not mean that the injectors physically opened, nor does having fuel pressure guarantee there is fuel at the injectors under sufficient pressure to be injected into the engine. There are screens on the injectors themselves that serve as a last defense filters, as well as there is usually an inlet screen to the throttle body just inside of the fuel inlet nut.

Once you prove that electrically the injectors should be opening, by measuring the current flow with them connected, and/or measuring their inductive kick. Plus you re-confirm that there is fuel pressure, AND volume. Then you have proven that problem is inside the throttle body, and you will need to take it apart to find out what is going on.

    Bookmark   June 12, 2010 at 11:53AM
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fire-fighter

Well, Truck sat at friends garage for almost 2 months.(told him not to rush it) Was there for a few hours and probing around some wires and tested a fusible link on firewall. Looked OK but it wasn't, replaced it and truck runs great! thanks for all the help!!!!

    Bookmark   July 16, 2010 at 9:50AM
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