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Misfire

  • Cavemanhands
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06 Apr 2019 11:31 #28468 by Cavemanhands
Misfire was created by Cavemanhands
2003 Chevy C1500 2wd 5.3 engine. Misfire on cylinders 2 and 7. I did a tuneup on this truck several months ago and all was well. Now it is here with a misfire that I am having trouble figuring out. I am using the Verus Pro scanner, do not have amp probes so I am doing voltage and ohms testing.
Cylinder 7 has a good spark, replaced the coil after detecting a no spark condition. Cylinder also has a new injector only a few months old.
The injector + has system voltage (14.4 running) .
The only thing I can come up with is a bad ground issue, I am getting 2Mohms between block and injector - terminal. Using the same injector - to battery + nets me a max of 8.8 volts..Injector disable does not change engine performance. Where is this ground located?
Ok, so I also have the same thing with cylinder 2, but it seems to come alive above 1300rpm. After holding RPM level to 2800 rpm a few seconds, cylinder 2 seems ok, now cylinder 4 is acting up.
As is right now, cylinders 7,4 and 2 are misfiring every revolution.
Compression is good, no combustion gas in antifreeze. Used upper engine clean. 2 weeks ago.
Fuel trim indicates a near 0 ST but the LT is nearly maxed out positive before resetting it.
Any ideas, tips, suggestions?
Thank you!

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06 Apr 2019 15:02 #28469 by guafa
Replied by guafa on topic Misfire
Hi cavemanhands.

Injector grounds are aplied from injector drives through PCM. The 2 megaohms are usefull just to compare between them.

8.8 volts is an average, which depends of duty cycle aplied for PCM.

LTFT is telling you that PCM is trying to add fuel. Seems a common issue for both banks (2,4 & 7). Do you remember LT for every bank?

Can you read voltage from both upstream O2 sensors?

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06 Apr 2019 17:40 #28472 by Cavemanhands
Replied by Cavemanhands on topic Misfire
I don't have the scanner at the moment, going from memory i believe the ltft was similar on both banks, maybe one around 19 and one 18. O2 voltage was high, steady around 700 - 800 mv. I can get the readings again.
The following user(s) said Thank You: guafa

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06 Apr 2019 18:37 #28473 by guafa
Replied by guafa on topic Misfire
Yes, you better get readings again.

If O2 readings steady around 700 - 800, PCM subtracts fuel and LTFT should be maxed out negative.

At this point LT & O2 voltage seems to be contrary

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  • Tyler
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06 Apr 2019 20:59 #28482 by Tyler
Replied by Tyler on topic Misfire
Pegged positive fuel trims, and a miss that goes away off idle? I'd start looking for leaking intake gaskets. Classic failure on that engine. Cylinder #2 and #7 are on the ends of the intake, which usually leak worse than cylinders in the middle of the intake. If you had trouble codes P0171 and P0174, that'd be even better.

The SD water test will work perfectly B). Or a smoke machine if you have one available.

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07 Apr 2019 11:10 #28489 by Cavemanhands
Replied by Cavemanhands on topic Misfire
I was mistaken, the data is as follows. Those changed some, but are indicative.
Lt B1 16%, LtB2 5%
Upstream O2S are normal and change from near 50mv to 900mv
Downstream are the high ones, pretty steady 600mv to 800.
DTC p0200 and p0300. Interior circuit and misfire.
Number 7 is steady dead, the other 2 come alive with engine speed.

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07 Apr 2019 11:53 #28490 by Tyler
Replied by Tyler on topic Misfire

Cavemanhands wrote: DTC p0200 and p0300. Interior circuit and misfire.


Does P0200 reset after clearing the code and restarting the engine? If so, stay on the injector path.

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08 Apr 2019 18:37 #28522 by Cavemanhands
Replied by Cavemanhands on topic Misfire
Yes, it returns immediately. I think I'll get the wiring diagram printed out at work and start checking wires. This one is tricky but i like the challenge and learning from it. Thanks for the input!

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