codes say lean.....i say rich???

**jmatt07** · 08-17-2009, 06:24 PM

ok so 98 ss cam/heads/intake/42# injectors on mail order tune for now......the car has been studdering for a while and most ppl say its prolly running really rich and fouling out the o2's. which makes sense bc the gas mileage is extremely terrible and you can smell tons of gas in the exhaust....anyway i got the codes read and its P0131 and P0151 low voltage on both o2's....and autozone told me it says low voltage when the car is running lean, but it seems the car is running very rich not lean. i took out a few of the plugs and they dont look real bad, a lil rich but the exhaust is coated in black soot. so why would the codes say lean when its running rich?

**35th-ANV-SS** · 08-17-2009, 06:44 PM

The PCM could be detecting a lean condition that it is trying to satisfy by adding more fuel. However, the condition is never being satisfied. Thus, it just keeps adding fuel. This is why you smell gas.

I would suggest checking the O2 sensors or the wiring to make sure it didn't melt to the headers. The best way to check the o2's are with a scanner.
The o2's should be toggling up and down from 980mv to 50mv.
If they are not toggling up and down, and are stuck between 400mv and 600mv, they are usually bad.

Could also be low fuel pressure or a vacuum leak/exhaust leak.

**Frost** · 08-17-2009, 08:19 PM

This kind of thing is typically misfiring... Misfires push fresh unburned O2 out into the exhaust. The PCM thinks it is lean and over fuels trying to 'correct' the problem. It cannot correct it though, and the O2s will always see a low voltage from reading the fresh air and it will really over fuel the remaining cylinders.

I would begin by inspecting (replacing if needed) plugs and wires.

**5abivt** · 08-17-2009, 08:36 PM

Originally Posted by Frost

This kind of thing is typically misfiring... Misfires push fresh unburned O2 out into the exhaust. The PCM thinks it is lean and over fuels trying to 'correct' the problem. It cannot correct it though, and the O2s will always see a low voltage from reading the fresh air and it will really over fuel the remaining cylinders.

I would begin by inspecting (replacing if needed) plugs and wires.

Definitely have to agree here. I ran into a similar scenario tuning my vette. one wire was bad and it caused it to over compensate by dumping fuel.

it could be a few things. Check your Injectors, Wires and plugs and for exhaust leaks near the o2 sensor

**jmatt07** · 08-19-2009, 09:00 AM

if it was misfiring wouldnt it be throwing a code for that? it has new tr6 plugs and msd wires on it and this is the second time ive replace the plugs just to be sure it wasnt them. could one of the wires b bad even tho they are new? ill try throwin the old ones back on there and see if it changes anything. Thanks for the tips guys. hopefully i get this figured out soon.

**Frost** · 08-19-2009, 11:08 AM

Originally Posted by jmatt07

if it was misfiring wouldnt it be throwing a code for that? it has new tr6 plugs and msd wires on it and this is the second time ive replace the plugs just to be sure it wasnt them. could one of the wires b bad even tho they are new? ill try throwin the old ones back on there and see if it changes anything. Thanks for the tips guys. hopefully i get this figured out soon.

You'd have to log misfire counts/cyl to know, not wait for a CEL, and you'd also need to understand that misfire detection is often wrong or even totally useless depending on mods made to the car (cam or stall then forget it for the most part).