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2000 Chevy Silverado 5.3 [ P0300] random misfire


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I recently got used engine swap for my 2000 chevy silverado. After drving it for 30 minutes, the check engine light came on. I took out my Obd2 scanner, p0300 came out.

- Spark plugs are good

- Air filter brand new

- There is Spark in each coil pack

- I found 2 fuel injectors that were working at all. I also notice a small vacuum leak on the intake manifold as a i sprayed carb cleaner. The engine started to shake. 

Can a Small vacuum Leak turn off only 2 fuel injectors?

Or is it just that the 2 fuel injectors are bad? 

3 Answers

A vacuum leak can’t turn off fuel injectors but it can cause misfire. Fix the leak and replace the bad injectors (you better off changing all injectors), clear the code and see if it comes back. 

Thank you so much for the information. I change them all and fix the leak.


You determined that 2 injectors aren't working.  You didn't say how and that would have been a help. (unlikely a vacuum leak could do that)

In your shoes, if I didn't have an HEI ignition spark tester, the 1st thing I'd do is swap the coils from the "bad" cylinders with coils from "good" cylinders.

See if the injector issue follows the coils.

"Spark" isn't always "good enough spark".

An HEI ignition spark tester forces the ignition coil to produce its maximum output. You need to know if it's doing that.

You seem to be wondering if the computer has shutdown those 2 injectors because of severe misfires in those 2 cylinders (to prevent damage to the catalytic converter),  or if the injectors themselves are damaged, or if the wiring harness, either from the Ignition battery+ to the injectors, or if the PCM injector drivers to those injectors are fried.

Even though you'd test this part last, let's discuss it 1st.

The computer shutting off "ground pulse" to the injectors (because of severe misfires) in those 2 cylinders.

The computer will shutoff injector ground pulse to a cylinder if the percentage of misfires in a cylinder exceeds a value (number) that's pre-programed into the computer.

It does this to prevent damage to the catalytic converter from incomplete (or no) combustion in a cylinder.

By shutting down fuel to the offending cylinder, the (misfiring) cylinder is now only pumping air (which won't damage the catalytic converter).

One more thing should be noted. The computer doesn't hold a grudge.

If it shuts down injector ground pulse to an injector, it doesn't automatically "remember" to shut down injector ground pulse to that injector the next time you start the truck. Each "drive cycle" the percentage of misfire counts in a specific cylinder has to be reached before the injector ground pulse is shutdown.

The reason this should be "noted" is because nobody likes working on a hot engine. Contacting flesh with hot engine parts hurts.

But if you unplug your injector connector and stick a noid lite on it and then do a cold start, you may see the noid lite flashing and assume, "I have a good ground pulse signal from the computer".

See how you could be fooled? You see a good ground pulse, you plug the injector's connector back in and later the computer  determines the misfire count has exceeded the predetermined acceptable limit and shuts down the injector pulse.


One of Four things is happening here.

1. Faulty wiring to the injectors.

2. Faulty injectors. (electronic or mechanical issue)

3. PCM shutting down those 2 injectors because of misfires

4, PCM injector drivers fried (for those 2 injectors)

First, determine that you have Battery voltage (12-14 volts) going to one wire on each of those 2 injectors' connectors.

Use a test light. (Plenty of youtubes how to do that).

Got battery voltage there? Then move on.

So even though I cautioned about the computer not holding a grudge, you can use that to your advantage on a cold start.

If you connect a noid lite to one of the "in question" (disconnected) injector connectors and at Cold Start you see the noid lite flashing you can eliminate two of the four possible causes. Numbers 1 and 4.

Now determine if (#3) is happening. You tested for Ground Pulse at a Cold Start.  Let's assume you had it.  You stated that the 1st time it took about 30 minutes for the CEL light to come on.

So clear the code. Drive the truck until the CEL light comes on again, and then WITHOUT SHUTTING THE ENGINE OFF, unplug one of the injectors in question and connect a noid lite to the connector.

Does the noid lite still flash? If it does the computer isn't shutting down the Ground Pulse to the injector in response to the misfires.

That leaves #2. Faulty injectors. Do a resistance test on them using a multimeter. (Plenty of youtubes on that).

What if it's mechanical? What if those injectors are stuck/sticking closed or slightly open or not opening at all and THAT'S causing the misfires?

Here you'd connect a fuel pressure gauge to the fuel rail. Turn the KEY to ON to pressurize the fuel rail and then Turn to KEY to OFF.

Watch the pressure gauge. Is fuel pressure "bleeding off" quickly? It shouldn't and with your symptoms we'd expect those injectors aren't closing all the way. (Leaking)

What if they aren't opening?  You have the fuel pressure gauge connected.  Disconnect the connector to one of the questionable injectors. Do the same thing KEY ON pressurize the fuel rail and then switch the KEY to OFF.

This time apply Battery voltage (from the Positive terminal of your battery) to one terminal on the injector, (connector off) and Ground to the other injector terminal.

If the fuel pressure "bleeds off" the injector is opening.

Don't play around with this. Do the test once for each of the questionable cylinders and just as soon as you see the fuel pressure decreasing slightly, remove your jumper wires. You don't want to flood a cylinder with gasoline and cause hydrolock. When you go to start it. It could damage the engine.

Hydrolock occurs when a volume of liquid (in this case gasoline) greater than the volume of the cylinder at its minimum (end of the piston's stroke) enters the cylinder. It can't be compressed and you could damage a connecting rod.

If you get nervous that you did it too long, remove the spark plug and wait a while, then crank the engine with the spark plug removed to clear any liquid gasoline.

One last thing.

What if you aren't getting a Ground Pulse to those injectors at Cold Start (or any other time)?

Is the problem in the wiring from the PCM to those injectors or with the (internal) PCM injector drivers?

You sure don't want to tear apart that wiring harness looking for damage.

You would determine that battery voltage (12 - 14 volts)  (KEY ON)" is being provided to the injector at its connector.

Then you'd go to that specific injector's driver connection (injector connection plugged in) at the PCM and determine if that same voltage is present there (KEY ON)

If in the KEY ON position you have battery voltage TO the injector AND at the PCM driver terminal for that individual injector then you know the problem isn't in the wiring.

Lastly you'd test for Ground Pulse at that PCM injector driver terminal.

Hook up your test light to the battery + post and then touch the probe to the PCM driver terminal (back probe the connector). If you're getting a Ground Pulse to the injector the test light will flash. PCM is good.

Let's look at your fuel injector wiring circuit. It has the pinout numbers on the ECM which is useful. You didn't say which ones are giving you a problem so for "example" let's say #1 is the problem.

But you're having issues with 2 injectors so don't ignore the 2 injectors that share the same Battery voltage connectors on the diagrams. 1&3, 5&7, 2&4, 6&8. That could be a clue.

Here's a link to both banks of your injectors wiring without my scribbling on them.

I'll "Light it up". Red is Battery voltage, Green is Ground.

.....and that's all I have to say about that












The problem is in bank 2, i switch coils with bank 1 and still the same problem. I even switch the 2 fuel injectors and still the same. i was check the intake manifold gasket and spray carb cleaner and nothing happen. I think has to do something with the valves or or lifters .
This motor was from a junkyard and installed by someone else. He gave me 3 month warranty. I am going to let him check it out.