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single cylinder misfire

  • trsvan
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26 Mar 2020 22:36 #38687 by trsvan
single cylinder misfire was created by trsvan
Good evening guys I need a little help. I have a 2007 Chevrolet Tahoe with a 5.3ltr engine. It has a single cylinder misfire on the #5 cylinder. The spark plugs, coil, fuel injector and AFM system lifters have been checked. Power and ground circuit, intake manifold (for leaks) have been checked also. Not sure where to go next. Any ideas?

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27 Mar 2020 06:01 #38691 by Monde
Replied by Monde on topic single cylinder misfire
If you have spark and fuel, you need need to check compression.

Never assume, diagnose!
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27 Mar 2020 07:06 #38692 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic single cylinder misfire
Compression issues tend to give a compeletly dead cylinder at idle but smooth out at higher RPM
Ignition coil pack and spark plug issues are often most notieable at wide throttle openings at low or medium RPM in high gears.

Compression test or cylinder leak down test would also be my next step,

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



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28 Mar 2020 08:55 #38719 by John Curtis
Replied by John Curtis on topic single cylinder misfire
Is it a dead miss or random single cylinder miss?

Scope the control (I’ve highlighted blue)
And scope the low ref (I’ve highlighted red)

Checking to see if the computer driver may be dropping out here.

Post a capture here.

If that checkouts out relative compression is the way to go, but if it’s not a dead miss make sure to do an extended crank time. Compression gauges can miss low compression in some instances. Relative compression will show if you have an intermittent compression issue which is why I’d recommend a long crank time.

Thinking out loud always helps me in the process.
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28 Mar 2020 15:33 #38729 by Tutti57
Replied by Tutti57 on topic single cylinder misfire
Just to gather a little data, what are your LTFT and STFT at idle and while holding 3k RPM?

Nissan Technician

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28 Mar 2020 15:58 #38732 by trsvan
Replied by trsvan on topic single cylinder misfire
They were verry close to zero. Probably a 5 to 8 % spread. Now the vehicle has the #1,4,7 fuel injectors opening when the key is turned on without attempting to start. The injectors are not stuck open because i switched the harness connector to one of the ones not opening and it opened immediately. Trying to narrow that down now.

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28 Mar 2020 16:27 #38734 by Cheryl
Replied by Cheryl on topic single cylinder misfire
So your filling those cylinders up with fuel?? Are you comfortable using an ohm meter to find a short to ground? Or bad pcm driver?

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28 Mar 2020 16:34 #38736 by John Curtis
Replied by John Curtis on topic single cylinder misfire
Does the computer think you’re getting an RPM signal when they engine is off for some reason?

Are you sure you don’t have a harness issue. Maybe the injector control wires rubbed through and are finding a path to ground.


Try test light on B+ and backprobe the control wire then wiggle the harness and see if your test light goes out. If you don’t find anything then unplug the ecm and see if connector and see if the test light comes out. It should shut off. If it doesn’t it’s a harness problem. Tap test on ecm to see if it’ll stop providing constant ground to injectors?

Thinking out loud always helps me in the process.
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28 Mar 2020 21:13 #38743 by trsvan
Replied by trsvan on topic single cylinder misfire
I used the test light to positive and probed one of the injector harnesses and the light came on. I then unplugged the ECM and the light went out. I purchased another ECM and programmed it and now the injectors are fine. Engine runs ok but still has the cylinder 5 misfire along with Bank 1 lean code P0171 and a bank 2 rich code P0175. I really dont understand that one. I guess all the fuel that was dumped into the exhaust either clogged up the O2 sensors or catalytic converter. I will look at that tomorrow.

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28 Mar 2020 21:53 #38744 by John Curtis
Replied by John Curtis on topic single cylinder misfire
Lean on the same bank as misfire only when cold or at all times? What are your fuel trims at idle? I see you posted them at 3000 rpm. Bank specific makes me think intake manifold gasket.

If you’re concerned about a clogged cat, aside from a back pressure test you can go WOT while monitoring calc load. It should go above 90% if volumetric efficiency is fine.

Thinking out loud always helps me in the process.

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28 Mar 2020 23:07 #38747 by trsvan
Replied by trsvan on topic single cylinder misfire
oh, this is really crazy. I just did another scan and im getting a lean code P0171 and a Rich P0172 code on the same bank. which happens to be the bank with the misfire. I must be jinxed. time to try another scanner. Need to clear my head and start from scratch.

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29 Mar 2020 08:24 #38758 by Cheryl
Replied by Cheryl on topic single cylinder misfire
The po172 might be due to oil contamination. With the injectors leaking fuel probably got passed the rings and got in the crankcase. I’d change it reset trims and go from there.

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