Stumblin Bumblin Z

[XfireZ51]

If the motor is stock, then 3000rpm is the magic number for secondary issues. Opening on 2nd or 3rd pull isn't exactly what you would define as high performance.
Typical secondary issues:

1. Pump Key On, does it keep running?
2. Is system holding vacuum?
3. Solenoid operating?
4. Cannisters binding?

[QB93Z]

Is your secondary vacuum pump operating properly?

Turn the ignition key to ON with engine not running. You should hear a brief period of operation of the vacuum pump located below the passenger-side headlight. If it doesn't run, repair it or replace it.

If it runs continuously, there is a vacuum leak or a leaking check valve in the secondary vacuum system. Post up if this is the symptom and we can do further trobleshooting.

If the vacuum pump runs for short periods (2-3 seconds) and stops and then repeats, there is a slight vacuum leak in the secondary vacuum system.

There is a pressure switch in the vacuum system that tells the ECM if the secondary vacuum system is not operating properly. What year is your Z? Recently, I have seen two ZR-1's with bad pressure switches with symptoms similar to yours.

I recommend some more analysis before you do the plenum pull.

Jim

[1963korvette]

Yes, the vacuum pump is working, although it is squelling. I also checked the rest of the system and it holds vacuum nicely.

[1963korvette]

Oh! 1990

[QB93Z]

On a 1990, the secondary system pressure switch is located on the bracket under the ECM. You can test it by disconnecting the vacuum line, connecting a vacuum pump and then use a ohm meter to verify that it operates at the correct pressure setting, (which I don't remember but it is in the Service Manual.)

If you have access to a Tech 1, there are very good tools available to verify the operation of the secondary system. I see that you are not too far away from WAZOO. If you could get over here, we would be glad to help troubleshoot your Z.

If you are convinced that the secondary system is holding vacuum and that you can see the secondary actuators move in the open direction, I would move to troubleshooting the secondary injector relays and the secondary injectors wiring.

Jim
jim@gizmosart.com

[1963korvette]

I will check the presure switch when I get home tonight. As far as the actuators go the one bank seemed a little lithargic, but it did open. i do not have a tech 1, got a snap on. I will also look into the relays and wiring you mentioned. thanks

[secondchance]

Last time I had a similar problem my Z was also throwing a code 56 (I think...). Referenced code indicated Pressure Differential Sensor (Jim, perhaps you remember the terminology?). In essence this is a sensor that looks a lot like the MAP sensor but compares differential pressure and allows for activation of 2ndary system. For 94 model this sensor is mounted under side of the plenum.
I will go through my manual tonight and clarify further.

[tomtom72]

Originally Posted by 1963korvette

Yes, the vacuum pump is working, although it is squelling. I also checked the rest of the system and it holds vacuum nicely.

The motor bearings may be dry in the vac pump. Not to be a wise guy, but when you say the system holds vacuum nicely did you do individual checks while you were under the plenum doing your injector work? Or are you using the connection from the vac pump at the right bank? I'm asking because sometimes you have to rotate the actuator canisters so that the arms move freely when vacuum is applied. They should be fully open between 6" & 8" Hg, and their movement should be smooth and linear, not choppy.

Some of the symptoms mirror a DTC 61: secondary vacuum failure. Is the motor still resisting that 3k rpm barrier? That is what happens with a DTC 61. The cause could be a leak, a bad secondary MAP or it's hose, bad tank, bad check valve ( doubtful ), leak in the plastic tubing or at either of the actuator canisters, leaking 2* solenoid.


Tom

[1963korvette]

I used a mity vac to check the vacuum. I did a visual from the top of plemum to see canisters opening when i grounded the pink wire in the ecm.

[scottfab]

Ok, after pouring over this thread I picked up on something.
You say you have 42 lb of fuel pressure and it goes to 50 when you rev up. You should have 50lb or so without reving.
Could be that your secondary fuel pump is intermittent.
If you do some hard accelerations up to the stumble a few times see if a lean code gets set. Then its pretty certain.
I had something very similar but at 4500 rpm. It never set a code.
When I replaced the secondary fuel pump it went away.
My fuel pressure with both pumps whet to 44lb when I just turned the key on. Then it would leak down pretty fast.
When I replaced the pumps it went to 52 and came down very little over time.
You may need to connect a fuel pressure gauge and go for a drive up to the stumble.