My motor is a 383LT1 using an Accel gen 6 DFI. It has TEA fully ported LT1 heads, the cam is basically the GM 847 it just has Crane's name on it. 234/242 .574/.595. I'm pretty sure Crane makes most of the GM performance Parts. The DFI was tuned by John Sealock of Woodbridge Dynotech and it ran pretty well at first with a best of 11.34@118.43 After about six or eight passes performance started to drop and that's when we started troubleshooting.

We pulled all the plugs on Saturday and as we went down the line from front to back it got richer and richer. My friend says he remembers reading somewhere that this can happen if you are getting too much air past the throttle blades and not enough air through the IAC at idle. Something about the front cylinders taking more of the air at lower RPM. Is this true? Can you think of any other reasons why an engine would run richer in the back than in the front. BTW, it's pretty much even from side to side.

I am using a Edelbrock 58MM throttle body. I removed it and noticed that it doesn't seal very well around the IAC port in the intake manifold and seems like it would allow quite a bit of the air that comes through the IAC into the upper plenum.

I have read LJ's website about modifying aftermarket throttle bodies and I'm probably going to do that modification while I have it off. However, his website only talks about split BLM's, it says nothing about running richer from front to back.

 

While I can't think of a reason you are running ricker int he back I can tell you it has nothing to do with your IAC circut. The ECM moves the valve to different positions depending on throttle position. Beyond about 1/3 throttle it is wide open.

If you are running a DFI, why don't you just retune the cylinders that are running rich.

 

Most people may not believe me when i say this, because i have gotten shot down in the past. My 383 was doing the same thing. It was running really rich, on the back four cylinders. I had a 58mm BBK throttle body. I made the modifications to it and it fixed my problem. With the BBK it has no IAC port hole drilled through. But even if it did on the back side the IAC Port is milled out to the big ports, which means you do not have an Idle air control at all.

 

My throttle body has an IAC port drilled, but as you said the back side is open to the throttle blades allowing most of or at least a lot of the air into the upper plenum.


I have the throttle body off and have purchased the stuff I need to do LJ's mod. so I'm gonna give it a try.

 

You should try fixing that machined opening. I did and it fixed my problems. I had a buddy of mine machine a piece that press fit in. It filled the missing walls and fixed my problems.