Stumblin Bumblin Z

[1963korvette]

Need help. In the last 6 months, at different times, I've replaced the Coils, Injectors and fuel pumps. Now, when I'm in Full Power mode it gets to 3,000 rmp's and goes no where. It gurgils and sputters along. I have checked the vacuum, no leaks. I've checked the fuel pressure 42 steady revs it and it goes to 50. Checked the secondary injectors they read 14.8 across the board. Looked at the secondary through the pellum and they seem to be working although the right side seemed a bite slow on the first try, than got better on 2 & 3 pulls. I'm thinking that it might be the secondary system in general. Could the linkage for secondary be fubard. Would like to narrow it down before I pull the pellum. Oh there are no codes. Any thoughts.
Rich :(

[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

[tomtom72]

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.

:cheers:
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.

[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.

[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]

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. Its probably 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]

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

[Paul Workman]

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.

I agree with you - far as a static test goes. Mine does that too. However, under dynamic load (pressure gauge taped to the windshield), with just the primary pump, pressure is in the 44-46 range (depeding on load). But, as soon as the throttle/rpm reaches the threshold where the secondary pump activates, pressure pops to about 52 ± a bit and holds steady.

For what its worth, when the secondary pump was not working, FP dropped to 38# at WOT, and the AFR went very lean!

[scottfab]

I agree with you - far as a static test goes. Mine does that too. However, under dynamic load (pressure gauge taped to the windshield), with just the primary pump, pressure is in the 44-46 range (depeding on load). But, as soon as the throttle/rpm reaches the threshold where the secondary pump activates, pressure pops to about 52 ± a bit and holds steady.

For what its worth, when the secondary pump was not working, FP dropped to 38# at WOT, and the AFR went very lean!

If the secondary is not working at all it does not surprise me to see <40lb.
However, if you get one failing like mine mid 40s is not good enough.
Mine Now starts with 52lb and stays there until sustained WOT then drops to about 49lb then shoots back to 52lb when the secondary pump kicks in.
At this point I think he needs to rule out the basic stuff before looking at secondary actuators. It's hard enough chasing one's tail but if your chasing the wrong tail? She better be worth it [-X
By what he's describing the secondary fuel pump could very well be seized up. Best rule that out.

[1963korvette]

If one or two of the secondary injector were not firing, wouldn't that act as more a miss. I would think the other 6 would pull it along. This feels like either no fuel or no air mixuture. The primeries are driving the engine. I know I called it a stumble but really its just a no power feel. right up to 7,000 rpm's.

I can feel another plemun pull is in my future. At which time the vacuum system is history.

[FU]

It's a 4 1/2 hour drive for me to Westminster from the southeast shore on L.I.

[1963korvette]

Ok, so I will tape my fuel gauge to my windshield tonight. It would be an easy fix if the pump is the problem. Seems I should have some kind of error code for this problem.

[John Boothby]

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?

FSM 6E3-A-46 & 47

You say that you do not have any codes? The DIC will show SES light if the secondaries do not open, so it probably is not your secondary vac sys. So this narrows it down to fuel or ignition. Is it blowing smoke when it starts to bumble and gurgle? Have you checked your plugs to see if they are wet? Color?

When the VSS (Vehicle Speed Sensor) goes bad, it limits the rpms to 3k and does some weird things from what I know. It also may or may not throw a code 24. It could also be an intermittant code 24. It will affect other things also like the Ride Control;Radio; or Speedometer.

Anyhow, this is just an idea to check.

Forgot to ask does it do this when the power key is turned off? If so, it probably is not the VSS.

[1963korvette]

With valet mode car runs fine rev's up to 7,00 with no problems. There are no codes. I have not seen any smoke out back but than again I'm in car alone. Will take wife with me tonight. Will pull plugs after ride and report findings. Yesterday my radio antenna was very squerrilly wouldn't go up , than worked, than wouldn't, than would. The car goes over 3,000 just doesn't run right.

[Kevin]

i didn't think you could wind the car to 7,000 without the key on

[John Boothby]

With valet mode car runs fine rev's up to 7,00 with no problems. There are no codes. I have not seen any smoke out back but than again I'm in car alone. Will take wife with me tonight. Will pull plugs after ride and report findings. Yesterday my radio antenna was very squerrilly wouldn't go up , than worked, than wouldn't, than would. The car goes over 3,000 just doesn't run right.

The VSS does not effect the radio antenna, it just effects the way the radio automatically increases volumn with speed.

If it is running ok in "Normal" mode, it probably is not the VSS.

I am leaning toward secondary fuel problem.

[scottfab]

i didn't think you could wind the car to 7,000 without the key on

I believe it does go on up to 7k but with less vigor :)

About the VSS probability of failure.....
Another test for it is cruise control. It will not work.

The only thing that bothers me about all this is that the stumbling
starts at 3k. Seems like it should set a lean code if held at the 3k point.

[1963korvette]

Do believe I found problem. As I said earlier" I replaced the fuel pumps". Well apparently the secondary pump had left the building first. One night when I went to start the car it would not start. Assuming that the fuel pumps had both died, at the same time?, I replaced both. Last night before pulling the fuel pumps out to check if I had done something wrong installing them I decided to test them with my meter. So, using the method decribed in the "solutions area". I checked the pumps and got only 5.5. I think the secondary pump is either defective or I have a blow fuse. Which gets me down to a new question where is this HUSH PANEL? Don't see it, manual doesn't show it. Please help get this car running right!!!!

[Paul in VA]

I believe the hush panel is the piece of carpeting/panel in the footwell (which would be right above your knees/ankles if you are sitting normally). You may be looking for the fuses under the passenger side hush panel. Note: I have not seen these, just read about them.

[1963korvette]

Ok, found fuse for secondary pump. It was blown. I replaced with new fuse. Useed the meter again still only got 5.5 amps. Guess will pull pumps out tonight, unless someone else has another idea.

[1963korvette]

Hey! Just for s--ts and giggles decided to take it for a ride. Well she's back started out a little stumbley. The furher I went the better she got. I assume from not having full power for so long things go gummy. She's running now tho. When I got back used my meter again 10.5. I want to thank everyone for there help.
Rich

[scottfab]

Glad to see progress. :dancing
You just replaced the fuse? I'd me weary of another failure. If the secondary pump is seizing it will do it again. Most probably once the fuel level gets low enough for it to suck air once.
Just based on the pain of getting in there I'd replace both again.
One of the big no-nos is "testing" a pump outside the tank.
This dry-air testing damages the pump.
They do not just have one failure mode.
They can simply stop
They can fail to hold pressure
They can run at low psi
and worst of all
They can work intermittently :*(

[1963korvette]

No, I do believe that the secondary pump was already out. Then the primery pump went. At the time I had a SES light so I could get secondaries to come on. Not know that the secondary was blown I replaced both. Never had secondary pump working because of blown fuse.

[John Boothby]

Good for you!! I know it gets frustrating at times, but usually it is something simple. AND, once you get the car running good, it will be worth all the work.