No check engine light but scanner reads misfire on multiple cylinders?

Tiny
JJROBIE
  • MEMBER
  • 2017 CHEVROLET SILVERADO
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 145,000 MILES
I had my transmission rebuilt and the transmission place took it for a test drive with scanner on it. No check engine light but scanner red number 7 misfire. They told me I had to get it fixed or it will void warranty. Did new spark plugs in wires and air filter. Put scanner on it and took it for a test drive and now I have multiple misfires on multiple cylinders, but you cannot feel anything, the only thing you can feel is some high revs and a couple of hard shifts. I do not want to avoid my warranty on my rebuilt transmission need to get fixed went to three different mechanics in they are dumbfounded.
Thursday, June 22nd, 2023 AT 2:37 AM

5 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,101 POSTS
Hi,

The one thing that can cause a misfire and not set a code is a failing crankshaft position sensor. When driving, have you ever noticed the tachometer jumping?

Also, by chance do you have a live data scan tool?

If you can, let me know the codes that are being found. I suspect it will be a P0300 and a P0307. Please confirm that for me and let me know if there are other codes.

Take care,

joe
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Thursday, June 22nd, 2023 AT 9:47 PM
Tiny
JJROBIE
  • MEMBER
  • 3 POSTS
No codes are popping on scan tool but reads misfires while driving with scan tool on! There are some high revs while trying to switch to next year!
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Friday, June 23rd, 2023 AT 6:06 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,101 POSTS
When it is reading misfires, can you tell which cylinder or cylinders are missing? Also, when it happens, does the check engine light flash? Also, when you first turn the key from off to the run position, does the check engine lamp turn on? If it is showing there are misfires, there should be codes.

Let me know.

Joe
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Friday, June 23rd, 2023 AT 8:15 PM
Tiny
JJROBIE
  • MEMBER
  • 3 POSTS
Yes, check engine light lights up when I turn the key on! And no check engine light does not flash at all! And yes, it shows on the scanner when you're driving what cylinders are misfiring, they were pretty much all except for cylinder number 5!
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Saturday, June 24th, 2023 AT 2:53 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,101 POSTS
Hi,

As I mentioned, I suspect the crankshaft position sensor is failing. A crankshaft sensor can fail and not set a code. That is the only thing that makes sense unless the PCM is failing.

I wish I had a clearer answer, but this is an odd one. BTW, when you have the scan tool connected and it's running, watch the RPM signal to see if it glitches. If it does, then the sensor is my first suspect.

Joe
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Saturday, June 24th, 2023 AT 8:25 PM

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