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15 Posts
Discussion Starter · #1 ·
I have made four posts regarding mu 97 Z28 with codes PO 300 ( random multi-cylinder misfires) and 132 ( high voltage O1S1 ). The problem appears to be centered on the right bank with a high O2 Sensor reading with NO WAVE FORM stuck at .8 . Negative fuel trim Bank1 as high as -36% after warm up.

Things done so far:
New plugs and wires bank 1 ( visually inspected plugs for oil found none) wet with fuel but not oil
New Coil
New Injectors
New O2 Sensor Bank 1
Check for continuity of wires running from ECU to O2 B1S1 test showed continuity
New Opti Spark
Check for vacuum and exhaust leaks and cracked manifold (all good)
Pulled Bank One valve cover off and found no broken springs or other anomalies like sticking lifters
Used scan tool to check for check other sensors e.g. MAF, Coolant Temp all working fine
Took manifold vacuum test from brake booster line got 21 inches
Checked fuel pressure 45lb
Pulled the left side bank 1 cat off and checked to see if what plugged no problem

Finally yesterday while scanning for the 100th time it threw Code PO 303 , misfire cylinder 3, while the engine was Luke warm I did a compression test on cylinder I got 160 lbs. So with adequate compression, a new plug wire, plug, injector and distributor why would a cylinder misfire?

Nine out of 10 times I will just get PO 300 (multi-cylinder random misfire) but very rarely I have gotten 303, 305 and 307 ( all Bank 1)

All the elements are there fuel, compression and ignition the only thing I can think of is ignition timing?

I have sketched the problem out to be that the misfires are causing too much unburnt fuel to be dumped into the exhaust manifold which is driving up the high Bank1 O2 Sensor 1 reading to a steady .8. that in turn causes the ECU to drastically limit fuel to that bank. All that is causing it run poorly and roughly, it sometimes bucks like a bad MAF will cause.

My question is what is causing these misfires solely on Bank1 only?

15 Posts
Discussion Starter · #3 ·
I actually pulled the left (bank 1) Y pipe and cat to examine it looked fine. Plus, I ran the car with the cat disconnected and it did the same thing so that ruled out the cat being plugged.

FYI, removing that pipe and the cat to get at plugs 5 & 7 and/or the O2 sensor is a HUGE time saver.

Were back to something causing PO 300 multi-cylinder misfires only on Bank1
The misfires are sending unburnt fuel into the manifold that's tricking the O2 into thinking the motor is rich thereby ultimately resulting in negative fuel trim and even worse misfires.

These things have been done:
New Opti
New Plugs and Wires ( inspected removed plugs for oil accumulation, sign of bad rings of leaky valve guides, found lots of fuel but no oil)
New Coil
New injectors
New O2 Sensor B1S1 ( yes I did swap B1S1 sensor with B2S1 and no difference)
Yes, I did check for continuity of the 02 wires from ECM to B1S1 O2 Senor (OK)
Compression test of cylinders on Bank 1 got at least 120 with a cold motor
Manifold vacuum check got 21
Inspected for exhaust manifold leaks with Seafoam ( only leak was way down stream behind B1S2
Fuel pressure test 45 psi
Used scan tool to verify the operation MAF, MAP and Coolant Temp Sensors (OK)
Checked for vacuum leaks (didn't find any)
Pulled Bank1 valve cover off to check for broken valve springs and observed valve train with the motor running didn't see anything out of the ordinary.
Pulled and inspected Bank1 Cat (ok) and ran car without cat didn't make any difference
When I pulled the ECU unit for the O2 wire test, I didn't observe any corroded jacks or bent pins. I also opened the unit and visually inspected all four surfaces of the two circuit boards. ( didn't see any obvious flaws like broken solder joints of burnt capacitors. The visual inspection may not guarantee the ECU is good but it's reduces the probability that its' bad.
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