craftman555

Hello all,
I have a 99 Z28 LS1 that started missing intermittently on me.

The symptoms:
It Will start and run fine for about two minutes and then start missing like it's lost a cylinder. If I accelerate hard, after the RPMS get up, it will run smooth again but when i let off, it's back to missing. If I stop, turn the car off and start it again, I will run good for about two minutes and start missing again.

The CEL is on and it has a random misfire code but has had that for a while and has been running ok just haven't been able to find the problem.

What I've done so far:
I changed plugs and wires.

Any Ideas?
Thanks

FormulaZR

iTrader: (7)

It doesn't have a misfire code for a specific cylinder, does it?

Exeodus

iTrader: (1)

A bad O2 sensor can cause this, even if it doesn't trip a trouble code.

RockinWs6

Very possible bad O2 sensor.

LS6427

iTrader: (2)

The 02 sensors activate and basically run the engine after about 2-3 minutes after it gets warmed up....IIRC, after coolant temp reaches 122*F, not sure. Until then the MAF is what runs the engine.

Bad 02 sensors cause you to run rich no that's what causes the missing.......when you hit the pedal hard the engine eats up that extra fuel and allows a ton of air in, which leans it out so it feels like its running strong. Then when you remove your foot from the pedal, cutting off all that extra air, you're rich again and it misses.

Get it scanned to see how the front 02 sensors are switching. Rear 02 sensors mean absolutely nothing, they cannot change the way the engine runs.

.

craftman555

The only code that comes up is P0300. I was also leaning towards the front o2's, I'll try to get it scanned soon and report back.

A little more detail:
Regardless of the temp, it does it after about two minutes of running. I can shut it off, restart and it will run great for about two minutes then looses a boat load of power. When I get into it, at right about 2300 rpm, it's like someone flipped a switch and BAM it comes alive. Below 2300, horrible......

Do you think if i unplug the sensors one at a time that it will run in a sufficient open loop condition to diagnose if this is the problem? Not sure if the banks work independently but will at least tell me that it's an o2 problem maybe?

Exidous

iTrader: (11)

For sure the front O2's or something related. When you are in cold startup or in PE (power enrichment) the O2's are ignored. Everywhere else they are referenced for fueling and can change the trims by over +/-20%.

If you can look at your PE table that controls TPS vs RPM to enable. I bet you'd see 2300 RPM in there for a given TPS input.

The banks do operate independantly. You can unplug them. It will force open loop. Meaning inactive fuel trims. However, it is possible that the last known state is used continuing the improper fueling. A reset of the trims is needed.

craftman555

It turned out to be a bad o2 sensor. The random misfire code I've been trying to track down is also gone Thanks to everyone for their help