Stumblin Bumblin Z

[1963korvette]

Got home last night and was able to find pressure sensor easy enough. unfortunetly I spent the better part of 3 hours pouring over the shop manual trying to find out how to check to see if it was working right. can someone tell me. The book did say it would throw a code 56 I believe. I have no codes. Very fustrating.

[QB93Z]

Originally Posted by 1963korvette

Got home last night and was able to find pressure sensor easy enough. unfortunetly I spent the better part of 3 hours pouring over the shop manual trying to find out how to check to see if it was working right. can someone tell me. The book did say it would throw a code 56 I believe. I have no codes. Very fustrating.

You are right, I looked this morning and I could not find a way to test the Differential Vacuum Sensor. I remembered wrong, I thought I had tested it back when I was having the problem you are having.

When I was having a problem with similar symptoms to what you describe, the issue turned out to be multiple failures. The vacuum pump had failed and the check valve in the secondary vacuum lines had failed. I replaced the check valve and the pump. The replacement check valve failled immediately, so I had to do all the troubleshooting over again.

Get back to basics. With the Power Key off, does the car drive properly at full throttle (understanding that it will have less power)?

Next to the alternator, there should be a vacuum line that runs from the vacuum pump to under the plenum. There should be a disconnect fitting in that line. With the engine off, disconnect the line and connect a vacuum gage to the pump side and run the pump. Do you get a strong vacuum that holds when the pump stops?

Connect the MightyVac to the system side of the disconnect fitting and pull a vacuum. Does the vacuum hold?

Jim

[1963korvette]

The car does run fine in valet mode. I have checked the vacuum with mity vac. It hold perfect vacuum.

[Demps]

The WAZOO guys can fix your car if you can get to them. There could be several issues here. If part of it is the check valve under the plenum here's a simple diagnostic/temp fix:

With key on-->Pull vacuum line from passenger-side plenum fitting between 4 & 6. Does vacuum pump run continuously?

Yes=check valve under plenum bad. If so, go to autoparts and buy a vacuum check valve and a piece of small hose (to fit nipple of valve you bought and plenum) to form a patch between valve and plenum. The check valve will fit into the plenum runner gap. This will serve as a temp fix until you can get under the plenum.

Ted

[1963korvette]

Unfortunately the WAZOO guys are not an option for me. I live in Wash. Township Bergen County. Itsprobably a 6 hour ride. I will try your test when I get home.

[John Boothby]

Does it do this when the engine is cold? Does it do this if you just ease the throttle to above 3K? If it does, it may not be the secondary vac sys. Do you get a "Service Engine Soon" light when you step on it?

When my actuator went bad I got a "Service Engine Soon" light intermittently. It would go out after about 10 sec after I let off the throttle. This light will come on it the secondaries do not open.

Just a guess, I havn't heard anyone mention this, but could it be the speed sensor? This failure will limit the engine to 3000 rpm limp home mode.

[1963korvette]

I usually let car warm up before testing it. It does it when you get into it with full power switch on. When you ease it up to above 3,000 it starts guggling. I did have SES light on all the time but that was MAP sensor hose. Since I fixed that The SES light is off all the time now.

Now, how would I check the speed sensor?

[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.

[QB93Z]

If you are sure that the vacuum pump is working correctly (you said it was making noise), I would next check the secondary actuactor valve to make sure it is fully opening. It is possible to have the solenoid valve port vacuum to the actuactors, but also have a leakage path so the system will lose vacuum until the ECM turns the Secondaries OFF.

Another possibility is that you have one or more secondary injector(s) that is (are) clogged. I don't know any way to check for that without pulling the plenum.

We recently had an LT5 that only one side of the secondaries actually operated. Both actuators were moving, but the linkage to the operating levers was disconnected on the passenger side because a retainer clip had been knocked off during a starter motor replacement. By careful examination with a boroscope, we located the problem.

Jim

[Demps]

Originally Posted by Demps

The WAZOO guys can fix your car if you can get to them. There could be several issues here. If part of it is the check valve under the plenum here's a simple diagnostic/temp fix:

With key on-->Pull vacuum line from passenger-side plenum fitting between 4 & 6. Does vacuum pump run continuously?

Yes=check valve under plenum bad. If so, go to autoparts and buy a vacuum check valve and a piece of small hose (to fit nipple of valve you bought and plenum) to form a patch between valve and plenum. The check valve will fit into the plenum runner gap. This will serve as a temp fix until you can get under the plenum.

Ted

You know, I've lost my mind. This will help only if the pump were to run continuously because the check valve has failed.

Ted