4.3 Ltr Runs Great when cold

fire-fighterNovember 28, 2009

I have a 1992 Chevy 1500 with a 4.3 Starts up and runs great when its cold. About 5 minutes after started the idle races for 3 seconds then bogs down and runs real rich and smells flooded and black smoke. Replaced every sensor on and near throttle body along with o2 sensor(PAIN IN THE ASS). Im stumped need vehicle sick of walking everywhere

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jemdandy

You have replaced several parts, but without the desired effect. Have you read the trouble codes to see what the system is complaining about? Mind you, this is only the start of the diagnostics. The trouble codes are clues, not necessarily the final diagnostic. For example, a bad connection in the wiring harness can cause off kilter trouble codes.

Your problem is an interesting one, and at least, it has been reproducible which is a good start. The fuel mixture goes overly rich a few minutes after startup.

After the engine stalls, try this work-around. Disconnect the electrical lead to the TBI and crank the engine a few revolutions. It may fire up momentairly and run the excess fuel out of the intake manifold. (No more fuel should pass out of the injector with its lead disconnected. If it does, the injector may have dirt lodged in its valve seat or it may need replacing.) After the engine stops, reconnect the TBI lead and restart the engine. If the engine continues to run, drive the truck to your favorite shop for diagnostics and repair.

There is the danger that after the truck leaves your driveawy, it could foul up again and require a tow.

    Bookmark   November 29, 2009 at 1:55AM
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john_g

The O2 sensor was a PITA? You should try doing diagnostics and repairs on a vehicle that has already had a grocery basket load of parts thrown at it without any attempt at real diagnostics being performed.

The computer uses inputs from the MAP sensor, and engine speed to calculate the initial fuel injector pulse width. From there it uses the coolant and intake air temperature sensor inputs, battery (system voltage if running) as well as the movement of the TPS to modify the base pulse width calculation. Then the computer looks into its learned memory, and applies the long term fuel trim, known as block learn on your truck. Finally when in closed loop the computer then uses the O2 sensor(s) and modifies the fuel pulse (integrator) to achieve good fuel trim.

That being said the computer does not know what the actual fuel pressure is, and it does not know if something is making the engine not breathe correctly, such as a clogged catalyst. Those are things the technician diagnosing the vehicle must test and be sure about. The computer does not know the actual flow rate of the injector, if that flow rate has changed from its initial design. Lastly, while most vacuum leaks on these engines simply result in a faster idle, a specific leak that drops the vacuum signal to the MAP sensor would simply look like a partial engine load to the computer and the computer would start with a larger base pulse width than the engine really needed, and you would see fuel trim once in closed loop having to try and take that fuel back away. Fuel trim is monitored in scan data, and again on your truck its block learn for the long term trim and integrator for the short term trim. The ideal numbers, no correction from the initial modified pulse width is 128 on both of them. When the computer subtracts fuel from the pulse width, that is reported as a number lower than 128. When the computer adds fuel to the base pulse width, that is displayed as numbers above 128.

There was one more thing that would occasionally occur with these systems. Noisy distributor grounds can cause the PCM to think the engine is running faster than it really is. That causes additional fuel pulses to be delivered.

So whether the fuel pressure is incorrect, (restriction inside the throttle body or return line) the injectors sloppy, the MAP inaccurate, a vacuum leak for the MAP signal (the throttle body base gasket is a common cause for that), the distributor ground is noisy, the exhaust is clogged, corrosion in the ECT harness altering that signal causing the engine to look too cold, or something spiking the TPS signal, or the pcm is detecting low system voltage, or the pcm itself is faulty this should be an easy diagnosis for an experienced technician. Provided you haven't added any problems as you swapped parts.

    Bookmark   November 29, 2009 at 7:47AM
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fire-fighter

Yes the O2 sensor was a PITA!!! Tired to unloosen and it wouldn't budge, got it red hot and still nothing, finally ripped away from exhaust pipe, threaded part and all. after I got it out put the torch to it again and got it cherry red again still wouldn't budge. Finally got a small piece of pipe drilled a hole in it cut it lengthwise and slipped it over the old hole with a few clamps. worked awesome but still engine would run great for a few minutes then like a flip of the switch idle jumped for 2 seconds then dogged out and ran real rich and black smoke started coming out.

    Bookmark   November 30, 2009 at 9:50AM
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fire-fighter

FIXED!!!! Replaced the Temp sensor in front of the TBI and runs fine. Theres 2 sensors on these vehicles, 1 for gauge and one to tell ECM how warm the engines running. Replaced that one and POW runs good. But do have another issue...

    Bookmark   December 1, 2009 at 11:56AM
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john_g

FIXED!!!!

NOT!!!!

    Bookmark   December 2, 2009 at 9:33AM
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