I was fighting a false lean condition about 6 months ago and I finally fixed it after changing a blown driver's side intake manifold gasket. I now have the same problem. My vacuum with the blown intake gasket was about 16in at idle and it is currently between 16.5in and 17.5in at idle. Having changed all the intake gaskets three months ago could I rule out a vacuum leak? I have sprayed all around the TPI and have found nothing. I have checked my fuel pressure also and it is good, 50psi at idle with the vacuum line unplugged. I have also checked the pressure under load by taping the gauge to my windshield while driving. I made several WOT and the fuel pressure read right at 50psi. The problem is only under throttle with load, my idle o2 and BLM are good, even if I rev it. Under almost an loaded throttle my o2 drop down very low and WOT drops down to 0volts. Does it sound like a fuel starvation problem? Could I have another vacuum leak? Thanks for any help.

BTW I installed new fuel injectors in one year/7k miles ago for a unrelated problem. Here is a link to my log file: http://www.fbody.com/members/Formula355/log.CSV

EDIT: I forgot to add why I believe it is a false lean. My emissions are through the roof, 6x on HC and 4x on CO. My buddy was behind me and he said my car smelled very rich. My gas mileage has dropped to about 12mpg. This may be a curve ball, but my idle also smells very rich even though my numbers say its running good.

 

Yeah, buy a new 02 sensor. Here's an easy way to test the 02 sensor. First step is to make sure the car is at operating temperature (without that you're f*cked). Note the 02mV's at idle on the scanner, then pull the vaccum line on the fuel pressure regulator and cap it off with an eraser or your finger. If the 02mV's don't rise at least over 850mV's under those conditions it's fried and you need a new one.


Brian P
89 IROC-Z 355

 

I agree to test the O2 sensor, but with a bit more detail. You will need to get e DVOM and backprobe the O2 sensor at the plug for it (you can use the scan tool, but it may not update fast enough, and even the DVOM may be marginal on the refresh. These days in the shop we are testing O2 senaor with Oscilloscopes!). With the engine at normal operating temp (run for about 4 or 5 minutes minimum) observe the O2 voltage. Should be switching hi to low, with high being 800Mv and low being 200Mv. Next, give the throttle a good snap. The voltage HAS to get over 900 Mv. You may also need to put the engine under a load, but just snapping the throttle should be sufficient. As soon as you let the throttle snap back closed, the voltage should go to nearly 0Mv... Now the O2 not going high enough isn't absolutely bad, it just means that you are not getting enough fuel.

Now, in your case, it sounds as though you are getting enough fuel. At this point, I have to ask if you have an exhaust leak. If so, on the negative exhaust pulses, it is pulling extra air into the exhaust stream and making the computer think it is lean.

Now, to look at it from a different angle, it could possibly not be burning all the intake charge (Air and fuel both) such as in a misfire condition. The O2 would read lean, because there is indeed excess air, along with excess raw fuel that is probably getting burned in the cat.

If you come up with more details after all this, post more, and we will all do what we can to help!

 

Moates software can update 20 times a second, prob more than my DVOM, Ill just make a log and you guys can tell me how it looks. I tried a test that Vader suggested, exposing the o2 to a propane torch while monitoring the voltage and it went up to .75 or there abouts. I do have an exhaust leak, but it is on a band clamp where my cat should be, well after the o2, but it has been there before this problem started back up. When I had this problem earlier I changed the o2 and it didn't help. Yesterday I put that o2 back in and it reads the same. Check out the pic of the o2 I pulled out, it doesn't look normal to me, I would think it should be black from carbon. I should get another log this evening.

 

Here's the log file. I highlighted the test I did in orange, it is at the very bottom of the log. Also another thing to look at, do my MAF reading look good or do they look high for RPM/LV8 they are at?
Log: http://www.fbody.com/members/Formula355/log2.xls

 

ttt

 

I bought a new o2 just to rule that out and I have the same problem, I reset the ECM also before I ran it with the new o2. I tried unplugging the MAF and it went even richer as it should have. Any help?