I have a 1990 Iroc-Z that has the speed density system. The PROM is stock from the factory. At idle, my Block Learn is around 108 and my Integrator is around 111... and I idle in Cell 4, so I assume I'm running rich. While I have the cruise control set at 50 mph, both numbers are in the low 90's. I've been researching the whole thirdgen message board archives for help, but I just want to get a little clarification. Will a vacuum leak cause this condition? I also surge at idle after the engine has warmed up, another symptom of a vacuum leak (from what I've researched.) This is the first fuel injected car I've owned, so I'm a little green to the concept. I'm used to turning a screw to fix this condition. =)

Thanks.

 

What mods? Any recent mods? Had the intake off recently to make you think of the possibility of a vac. leak? Is it that low of blms at all loads/rpms? Put bigger inj. in? If not, I'd think it was leaking inj. Try checking the fuel press-does it drop right away after turning it off.

 

The only mods it has is a cat-back exhaust. The only thing I've done recently is change out the distributor. The low BLMs is at all loads/rpms, usually around 90 under load, and 110 at idle.

I got the vac leak idea after reading people's posts with similar problems to mine.

If the fuel pressure dropped right away after turning it off, I assume that means a leaking injector, right? Is there a test to find which injector?

 

I pull the rail so I can see the injectors.Then turn the key forward and check for drops.I think leaky injectors are your problem"IF" you have a stock prom/memcal.

 

I've attached a fuel pressure gauge to the schrader valve on the fuel rail. It read 42 pounds pump on, and 37 pounds engine at idle with vac connected. I shut the engine off about 30 minutes ago and I still have 30 pounds of pressure in the line. I'm not familiar with this test (I got it from another forum on this site), so I'm not sure how much pressure drop is normal. I've seen one example of having 35 pounds of pressure after 20 minutes being normal, so I assume I'm not leaking an injector, or I'm not leaking one really bad.

I tried resetting the computer and the BLMs/Integrator went right to 108/110 within a few minutes. Idle still goes up and down after it goes into closed loop operation.

Could I possibly have an O2 sensor starting to go? I've tried to locate a vacuum leak with some carb spray while idling but I didn't find any significant surge in RPM speed.

A solution I can personally think of off the top of my head is to adjust the Injector Constant with a new PROM, but I'd figure that to not fix the actual problem, since the stock PROM should be correct, right?

Thanks for any assistance.

 

Well, I replaced the O2 sensor and I still have the same problem. I'm completely baffled by what this could be.