Code 56

[mmkkpro]

Hello guys, I have been working on a 1990 ZR-1, got her running and driving. Working on my last code a 56. My problem is a vacuum leak. I have had the plenum off 3 times and can't seem to find the leak. With the plenum off and drivers side vacuum bose blocked there is no leak, soon as I put the plenum back on and connect the driver side vacuum hose to center of plenum my pump runs on and on, I can pull the hose back off put my finger over it and pump stops. It's almost like the plenum itself is leaking. I'm almost sure this is what's causing the 56 code. I'm thinking when I put the plenum back on its pushing on a connection or line, is there any place on the plenum that can leak besides the few vacuum ports of which I've checked over and over, I'm missing something, any help or ideas would be greatly appreciated, I'm to the point of considering the SPT delete. Thanks

[Hib Halverson]

If you have the Factory Service Manual for a '90, you'll find an in-depth diagnostic charge for Code 56 on an LT5. There are a number of places in the SPT system where leakage can set that code.

At one time, I owned a '95 and had that code set. Following the FSM and using a scan tester and a test light, I was able to solve the problem.