Years ago, I swapped TBI units because I was suspicious of partially-plugged injectors. In the process, I discovered some casting differences between the original '88 TBI, and the replacement which came from a shelf in some salvage yard.

5.7L TBI idle air passage and gasket differences:




Recently, I bought a pair of 7.4L TBI units. There's similar differences between the older casting and the newer casting. I have no idea what model year or vehicles these are from--they were laying on the floor of the salvage yard shed.



The older unit has a flat, horizontal 3-pin TPS sensor. Note semi-open, swirly Idle Air passages. There's a vacuum nipple, and round vacuum port just above the left (in the photo) throttle blade.



Newer unit has round, vertical three-pin TPS sensor. Note divided Idle Air passages. This TBI does not have the vacuum nipple or the vacuum passage above the left (in the photo) throttle blade. The casting can be drilled and a nipple pressed-in to provide a vacuum source, but GM didn't bother for this application.



"Open" gasket, Fel-Pro p/n 60786



Enormous base gasket needed for later TBI. Gasket is Fel-Pro 60954



Don't use the early gasket on the later TBI. You'll have a HUGE, sucking vacuum leak.



The difference between the older, flat-horizontal TPS sensor, and the newer round-vertical TPS sensor. 5.7L (and others) similar.

This is not exhaustive--there may be other casting variations and gasket styles not pictured here. BE SURE OF WHAT YOU HAVE when matching-up gaskets or other replacement parts!

 

rganks for taking to time to post this