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Fixed misfire problem. Still experiencing poor performance/mpg.

2K views 13 replies 5 participants last post by  M_Hoff 
#1 ·
My car (07 sts 3.6) has been suffering from poor engine performance/acceleration, rough idle, poor mpg and flashing CEL misfires. Initially I thought this was due to low compression in cylinder 1 which gave readings of both 120psi and 70psi on two separate compression tests. However, a more recent compression test showed all cylinders within 145-150psi (tested multiple times to be sure). Scanner revealed codes P0122, P0222, P2119, P0300, P0301, P2229 and P2008 and mechanic replaced IM tune valve solenoid, TB connector pigtail and spark plugs which were bad. so far there has not been a single misfire recorded and no CEL or codes but the poor performance and poor mpg is still there. The car drives and shifts smooth like normal but feels like a crappy 4 cyl rather than a 6 cyl like the power/torque has been cut in half. Occasionally (10% of the time) I will start the car and it will run perfectly like there is nothing wrong with it plenty of low end torque and very responsive as if it were brand new and then the next day its weaker than my minivan again. Since no codes have come back so far my first thought is to have the exhaust inspected for damage to the cat & other components
due to the misfires it endured.
Could theP2229 code have been relevant here? Also one of my oxygen sensors constantly reads 0 volts which is exhaust related no? I would be greatfull for some opinions as to what could be causing low performance/mpg considering the prior codes. I think it's exhaust related but I don't want to miss anything. Any advise is apreciated. Car has approx 140k miles filters and coils are in good shape.
Thanks!
 
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#4 ·
I went through something similar. I needed an oil change (though the system hadn't recognized it yet). I let the problem persist too long because I didn't know about the oil issue. By the time I did learn about it and change the oil, the plugs were toast. I ended up changing them. No more problems. I make sure to change my oil BEFORE the system tells me to do it.
 
#5 ·
Oil in car is freshly changed with mobile 1 spark plugs are ac delco. I am aware that this engine Needs to have oil constantly checked especially at this mileage.
My car was taken to a shop that is very Savvy to these newer cadillacs so I assumed that they knew the correct gap length but I will verify. I am going to have ignition coils replaced next becsuse it's been a while. I am baffled that the car has such low performance/mpg and there are no lights or codes. You would think that the "high-tech" computer would detect something, what could be slipping under its radar???
 
#6 ·
So there seems to be an issue with my 02 sensors. On both banks 1 and 2, 02 sensor 1 constantly reads 0 volts which could explain why my car is having issues. However when I changed the one of the sensors (bank 1 sensor 1) it still reads 0 volts at all times. Checked the fuse which was in good shape so thinking its probably a wiring issue but why are there no codes? Bad 02 sensors/wiring should definitely throw a code could my computer be faulty?
 
#8 ·
There are no codes or lights. I am monitoring the sensors (sensor 1 on both banks) and they read 0 volts. Replaced one of the sensors and it is still at 0. Both Sensor 2 work fine. Even if it is a wiring problem the computer should detect it and throw a code leading me to believe something is up with the computer. Thoughts? Car is running badly.
 
#11 ·
I think a key point is that 1st post had "one of my 1st O2 sensors is reading 0". When it occasionally performs well, before celebrating the day, get a reading to see what is different. My personal theory is wiring to the "bad" sensor which occasionally makes good contact.
 
#12 ·
It fixes itself pretty randomly could be anywhere from a couple days to a few weeks before it decides to run good again but the next time it happens I will see if there's any difference. This is very frustrating. I'm going to get the sensors voltage checked and have the wiring inspected and if that's all good I'm going to get my ecu checked/replaced because idk what else to do my car is running horrible and the computer should be throwing a code but it's not which leads me to believe that it's defective.
 
#13 ·
I'm assuming you have a code reader that shows real time,Watch the vacuum numbers and the long term trim levels. Also I am wondering if the MAF sensor is good. Try unplugging the sensor and see if it runs any better. The car will go to default fuel numbers if it is unplugged.
 
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