Posted to Technical Tips Forum on 7/28/2017
33 Replies
A 2011 Corvette came in yesterday. It had been to another
dealer, a Corvette specialty shop, and a general repair shop
to repair a continuing SES complaint. The customer stated
that the SES light came on every day with no driveability
complaints. The other shops have already replaced the spark
plugs and wires, the fuel injectors, fuel pump and crank and
cam sensors without repairing the complaint.
The SES light was on when I drove the car in the shop.
Scanning the control module system, I saw p0300 set as
current in the PCM and no other codes set in any module.
Looking at the history misfire counters, each was reading in
the thousands without too much variance between cylinders.
The car idled normally and no misfires were set yet the
active counters kept increasing.
I cleared the counters and let the car idle in park for two
minutes and recorded this.....
[2011 Chevrolet Corvette, ECM/Inputs/Outputs Scan Data]
During this test, the vehicle idled normally and no misfires
were felt. I have always called this condition where misfire
counters increment but no actual misfires occur as "ghost
misfires." Next, I cleared all the counters again and put
the car in drive and idled for 2 minutes and saw this...
[2011 Chevrolet Corvette, ECM/Inputs/Outputs Scan Data]
Any ideas? This is a seldom seen failure on Corvettes.
Inside the torque tube where they cannot be seen without
disassembly, front and rear phenolic type couplers are used
on the actual driveshaft. These couplers can wear and
develop play or actually start to come apart. In park or
neutral when just the drive shaft and torque converter are
spinning, back and forth movement can occur that the PCM
sees with the crank sensor and can interpret as misfire on
all cylinders. Putting the vehicle in drive or reverse loads
the driveshaft and stops this motion and the ghost misfires
cease. If you look at the misfire graph, you can almost see
the cycling occurring.
If you remove the torque tube and tear it down, the couplers
can be visualized. In this case, the vehicle had a GMPP
extended warranty so the entire torque tube was replaced and
after idling in park for 20 minutes showed no abnormal
counters incrementing.
Something to remember in case you run into this scenario as
many shops have their shorts handed to them when faced with
this type of failure.
James from Florida
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