dimented24x7

iTrader: (2)

I have a performer RPM airgap and Ive been having some issues with running lean at low RPMs. The fuel system is a holley 670 TBI with mass air flow, so it works in a similar manner to a carb.

This mainly comes up because I have a TKO 500, and I cant easily get into first unless Im at a dead stop, so alot of rolling take offs are in 2nd gear, and it seems like the engine is lean under moderate load at low speeds.

Basically, the problem is that if you put it under moderate loads or floor it at below 2000 RPMs, the engine leans out. As a test (bad for the motor, I know...) I floored the car at 1500 RPMs and in fourth gear and held it there with the brakes. On the wideband oxygen sensor, the AFRs are around 16:1 and dont recover even though Im commanding 12.5:1, so its not a pumpshot issue. The PCM is reporting an airflow of around 75-80 grams per second from the MAF sensor here, which equates to a volumetric efficiency of 90% or so and the injectors appear to be delivering the proper ammount of fuel. This all seems to make sense, so it doesnt seem like an issue with the fuel system. As the RPMs build, the AFR on the wideband approaches 12.5:1. After 2000 RPM or so at WOT, the actual AFR is consistently 12.1-12.8:1 the whole way up untill redline.

The easy solution is not to run it at high load and low RPMs, but Im curious as to why does this? Is it an air distribution issue in the manifold? The car has an edelcrap performer plus cam with only like 204 degrees of duration at the intake lobe, so it doesnt seem like its overlap throwing the reported AFR off, but who knows. It does seems like its running lean, though, and its sort of annoying when the car is in 5th at low RPMs on the highway. At 60 MPH, I contantly have to shift between 4th and 5th because it feels (and looks) like its leaning out under moderate load in 5th.

onebad89RS

To me everything sounds correct with the exception of your MAF input. 75-80 grams per second sounds WAY to high for 1500rpm wot. Now i'm not 100% sure about TBI maf's cuz i never played with one, but in school i got to play with an LTI camaro with MAF input and when we pulled it on the dyno from 1500 maf was only reading around 40Gm/S. but this is also kind of backwards because if the pcm sees high airflow it should deliver more fuel.

dimented24x7

iTrader: (2)

This is a late model fuel injection retrofit. Its a TBI truck PCM along with an LT1/LS1 MAF. Originally, it had speed density (barf...)

Thats the thing thats weird about it. Like you said, it moves alot of air down low then it should. For all I know, the cylinders may not even be running lean, it may just be lots of unburned charge making its way out into the exhaust. The PCM is delivering the correct injector firing time for the incoming airflow, so it is giving it more fuel.
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I guess one way to prove it conclusivly would be to adjust the AFR to be really rich there. If it runs way better and the wideband shows the correct AFR, then its lean. If it stumbles or completly pukes, then its a false lean condition.