Diagnostic question

Tiny
BILLAK
  • MEMBER
  • 2005 CHEVROLET TAHOE
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 150,100 MILES
Permanent misfire in number three cylinder. New plugs, wires, injectors. Cylinder pressure is 140 PSI. Leak down test is less than five percent. A colleague advised to complete a running compression check. Flooring the accelerator with no substantial increase in cylinder pressure would indicate a lower engine failure such as a sticking push rod or rounded cam lobe. Is this correct? I have never heard of a running compression check. Thanks for taking the time!
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Friday, March 31st, 2017 AT 10:23 AM

4 Replies

Tiny
HMAC300
  • EXPERT
There is no such thing as a running compression check. Normally to save money have a mechanic check this out as it can be a bad or weak sensor or a basic engine problem you have already spent over $100.00 for parts that are probably good. The mechanic can check sensor voltage and a few other things with his scanner that you cannot to diagnose the problem. The engine in your car does not fail camshafts that often like rarely as they are steel billet a lifter can collapse, push rod break or valve spring break. If you want pull the valve cover off and spin engine over see if you can see any of this. But I always advise to go to a mechanic to save money strange as it seems especially with misfires as they are very hard to diagnose now a days.
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Friday, March 31st, 2017 AT 12:18 PM
Tiny
BILLAK
  • MEMBER
Thanks for the information. Leak down test was less than five percent. I performed a "running compression check" and verified the cylinder pressure decreased to 30 PSI at open throttle instead of increasing beyond the 140 PSI measured with normal compression check. I interpret this as a lower end issue and will remove valve cover to check springs and rods first. I believe if springs and rods are fine I will not remove the head to check lifters. This may be a candidate for engine replacement at that point. Which sensor are you concerned about that may cause a misfire? Thanks again for taking the time HMAC300!
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Friday, March 31st, 2017 AT 2:08 PM
Tiny
HMAC300
  • EXPERT
It will no't be a lower end issue if you are thinking a rod bearing etc. More than likely push rod/valve spring something like that.
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Friday, March 31st, 2017 AT 3:17 PM
Tiny
BILLAK
  • MEMBER
Lord knows, I hope you are right. Stay tuned and have a really great weekend!
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Friday, March 31st, 2017 AT 3:41 PM

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