Code 56 fixed

[petefias]

Couple weeks ago at Summit Point my car came up with an SES light and limited my RPM to approximately 3.5K. It was interesting to complete the lap in 5th gear so I won't hit the rev limiter.

Needless to say I didn't have my books with me and even though I could read the code (DTC56) I didn't know what it was. A fellow driver hooked up a Tech 2 to read it (he said once the code comes up, the text explanation of it also displays), but we were unable to get it. MY 95 has the OBD II connector, but it is still an OBD I system. I don't think the Tech 2 can read the Z codes correctly. Perhaps I already cleared the code before he hooked up the Tech 2, I can't remember.

My second plan was to call my wife on the phone and have her find the code in my books at home. That was a real interesting scene. She is an english teacher and knows nothing about "secondary air inlet valve actuator vacuum sensor circuit" to say the least. She found at least three DTC56 in the books, but none of them were the real thing. I knew she didn't find the real code (one was for security, but the engine was running), so I just reset the codes and hoped for the best. Of course after a couple of laps the code came right back.

Since it was Sunday afternoon already I figured I'll investigate this problem once I got home. It didn't take me very long to find the DTC56 description at home and hooking up my vacuum pump told me instantly that the problem was under the plenum.

I didn't get the opportunity to work on the beast for a couple of days, but once I pulled the plenum and I got past the frustration of "admiring the beautiful engineering work that went into the secondary throttle design" it became obvious where the leak was.

Most of the vacuum line connections were good, but the rubber connector on top of the vacuum reservoir was leaking. Luckily the major components all checked out OK. Vacuum activators - check. Vacuum solenoid - check. Vacuum sensor - check (reference voltage was very close to the table in the book). One way valve - check. Vacuum reservoir - check.

The rubber connector looked good externally, but its holes seemed way too big for the size of the vacuum hoses. Over time, from the heat and vibration the rubber relaxed and there was no way to correct it.

I took the rubber connector to my friendly GM parts counter and the guys had a good laugh at me, you want what? The guys had a box of rubber connectors with all sizes and shapes as general replacements. Getting two L-shape connectors came as an instant fix for my dilemma of what to use.

For a little extra protection I used a little lock-tite and tie wraps on them and it went back the plenum.

I measured my coil and ignition wire resistances while I had the plenum off and they were all within spec.

With all the "fixes" in place my vacuum pump does not run at all (once they stop) with the key on. For the last couple of years my pump would cycle every 15 seconds and finally every 3 seconds (when it got so bad that it threw DTC56.) We had a warm day on Sunday and my oil temp was around 230F. I didn't push the car excessively and it was a one lap at a time time trial (NCCC high speed autocross).

The best part, the guys at the parts counter said: no charge today. Who says the Z is expensive to fix?

A word of advice: without a vacuum pump it would have been very hard to discover where the vacuum leak was. I highly recommend getting one for your tool box.

Sorry for the long post, but it could be the same reason your vacuum pump is cycling with the key on.

I'm convinced however, if I do a 415, those secondary throttle gizmos are coming out for sure!

Pete '95 #442