2014 Silverado 5.3 #3 cyl dead miss at idle only.

[LeBaron]

Replying to KARNUT.  What are you talking about?

[KARNUT]

40 minutes ago, LeBaron said:

Replying to KARNUT.  What are you talking about?

Ok I’ll simplify. It has a few miles on it. Drive it. If it gets worse rebuild it. 

[asilverblazer]

36 minutes ago, KARNUT said:

Ok I’ll simplify. It has a few miles on it. Drive it. If it gets worse rebuild it. 

I posted something similar in another thread, if you can't readily determine the problem, run it till you can.

[LeBaron]

Got it.  Thanks for clarifying.  That's what we are doing so far, hoping it becomes worse so we can find the source of the problem.  Thanks.

[superdave160]

I'm wondering how the ECM determines that there is a miss on a single cylinder?  If we knew this then it might help determine the cause of the issue.  Does it somehow determine that the coil did not fire?  Does it detect unburned gas in the exhaust?  Knowing this may help in determining the cause.  I would think that if it were a coil/wire/plug issue, then it would present itself under load, not at idle.

[asilverblazer]

On 11/9/2024 at 10:09 AM, superdave160 said:

I'm wondering how the ECM determines that there is a miss on a single cylinder?  If we knew this then it might help determine the cause of the issue.  Does it somehow determine that the coil did not fire?  Does it detect unburned gas in the exhaust?  Knowing this may help in determining the cause.  I would think that if it were a coil/wire/plug issue, then it would present itself under load, not at idle.

The truck can detect a miss fire by anomalies in the crankshaft rotation speed. Each cylinder fire introduces a pulse to the crank shaft rotation, these pulses each correspond to a cylinder known to the PCM by the firing order. A missing pulse means that cylinder didn't fire. 

 

The PCM doesn't know why it didn't fire. Other clues OBDII codes might suggest other anomalies if they are strong enough to trigger. A lean condition might indicate lack of fuel from a failing injector or too much air from a manifold leak. A rich condition may exist from a lack of ignition of the injected fuel.

[superdave160]

On 11/11/2024 at 10:20 AM, asilverblazer said:

The truck can detect a miss fire by anomalies in the crankshaft rotation speed. Each cylinder fire introduces a pulse to the crank shaft rotation, these pulses each correspond to a cylinder known to the PCM by the firing order. A missing pulse means that cylinder didn't fire. 

 

The PCM doesn't know why it didn't fire. Other clues OBDII codes might suggest other anomalies if they are strong enough to trigger. A lean condition might indicate lack of fuel from a failing injector or too much air from a manifold leak. A rich condition may exist from a lack of ignition of the injected fuel.

Thanks for the information.  Based upon this and other information provided, I believe the most likely cause of the idle misfire is likely a vacuum leak.  Vacuum is highest at idle, so a vacuum leak (unmetered air entering the combustion chamber) would be much less noticeable away from idle.  I'd say check for leaks around the intake manifold.

[LeBaron]

Superdave160, I thought so too. However I replaced the intake gaskets when I replaced the injectors. I have also sprayed around the intake while the engine was running. I don’t think a Vacuum leak is not the problem. Any other suggestions?

[asilverblazer]

I am still curious about the miss only occurring after the engine has warmed up. Have you confirmed whether it begins at switching to closed loop operation or just when things start getting hot?

[LeBaron]

Silver blazer,

 

I don’t know how to check that. How can I find out if it’s switched to closed loop or not?

[asilverblazer]

If you have a scan tool with live data, you could probably watch for it.