Rod Bender

After driving my 96 Z28 about 20 minutes it develops an on again off again misfire. The SES comes on. Then it seems if I get on it and wind it out a few times the misfire goes away. In normal driving I do not lug the engine and try to keep it above 1500 rpm . I have replaced both oxygen sensors because according to Auto Zone the SES was for oxygen sensor. The car has headers but no cat and there is a sensor in each header. I also removed two plugs on the left bank. They were carboned up, but the electrodes were still clean on the tip. The plugs are iridium and were new about 5,000 miles ago. The plug wires are MSD and I checked the resistance on one and it showed .1 resistance or the same as when I touch both probes together. I then checked one plug on the right bank and it was clean. When I pull the battery fuse the SES light goes out and does not come on again unless the misfire gets pronounced.

Injuneer

Get the exact numerical code. There are multiple codes for each O2 sensor, and not all of them necessarily point to replacing the sensor. A simple OBD-II code reader is so cheap, you really need to get one.

Of course the SES light goes out when you pull the battery power... that's how you clear the codes.

Sounds like a heat soak problem... Opti cam position sensor, ICM, sticking injector, fuel pump. Code should point you in the correct direction.

Can't seem to conceive a circumstance where the total resistance of a plug wire could be 0.1 ohms. Even MSD quotes 40-50 ohms/foot for the 8.5mm Super Conductors. Are you sure the meter scale wasn't set to Kohms?

http://static.speedwaymotors.com/pdf/54732343-3.pdf

A misfire causes that bank of the engine to run rich. If you have a severe misfire, the SES should be flashing, not steady. General misfire code is P0300, no way that could be mistaken for an O2 sensor code.

Rod Bender

Thanks Injuneer for the information. The misfire gets more extreme if I push the engine and then it will cause the SES light to flash and then stay on. You have given me some things to check. Thank you.

Injuneer

When you open the throttle, the harder you open it, the higher the combustion chamber pressure. The higher the combustion chamber pressure, the more voltage you need at the plug tip to fire it. If there is a weakness in a wire, that's when it's going to show up... spark finds something closer to jump to, through the insulation (or a crack or burn). Or a weakness anywhere in the ignition system is going to show up with the engine under heavy load.

Rod Bender

Injuneer, thanks for the explanation. Then I believe my problem is not plug wire or plug, because when I get on it, the misfire goes away. I did have the SES read again tonight. The AutoZone code reader gave "PO131 Ho2S-11 (Bank1 Sensor!) Circuit Low Input" Under "Suggested Parts" it said "Fuel Pump". When I asked the Auto Zone tech about the fuel pump he basicly said, "The engine has 150,000 miles and it is getting worn out."Fuel Pump" does not mean anything." So that is where it is at. A low rpm miss can be expected. For what it is worth, the Security light now sometimes comes on. However I had experienced problems with the security system about a year ago so I quit using the key fob and never engage the power locks.