Vacuum pump not coming on?

[1963korvette]

Bought car 3 months ago. It has been running very rough. Idle seems high at about 2000 rpm's. bogs down thru 3000 to 5000 rpm's. Tested the vacuum pump testerday. Turned on ignition key and did not hear pump come on. Don't know where to go from here. Appreciate any help or ideas.

[QB93Z]

You have done valuable troubleshooting already. 2000 rpm idle probably means an air leak in one of the vacuum lines attached to the plenum.

The bogging in the 3000 to 5000 rpm may be due to the seconday system being commanded on and then commanded off again when the vacuum switch senses that the secondaries did not open.

Try running the car with the Power Key off. If the bogging down goes away you have confirmed that problem is the secondaries.

My 1994 ZR-1 had similar problems after I bought it last May. The vacuum pump was seized and rusted, probably from running all the time trying to keep up with a vacuum leak. In my case it was a failed check valve in the secondary vacuum system.

You may have a disconnected vacuum line, 2000 rpm idle is a large air source to the plenum.

Check the Manifold Air Pressure Sensor (back of the plenum) hose and make sure it is connected to the plenum. It is under the MAP sensor.

It is possible to test the vacuum pump using jumper wires from a 12 volt source connected to the terminals, but I would guess 95% probability that the pump is bad. I haven't checked on the availability of the 1990 style vacuum pumps lately but there have been some recent posts about replacing the pump using the later model pump by changing the connector.

Have you ever pulled the plenum on an LT5?

If you need help, I might be able to find someone near you that can assist.

TomTom are you listening?

Good luck,

Jim

[secondchance]

Originally Posted by QB93Z

You have done valuable troubleshooting already. 2000 rpm idle probably means an air leak in one of the vacuum lines attached to the plenum.

The bogging in the 3000 to 5000 rpm may be due to the seconday system being commanded on and then commanded off again when the vacuum switch senses that the secondaries did not open.

Try running the car with the Power Key off. If the bogging down goes away you have confirmed that problem is the secondaries.

My 1994 ZR-1 had similar problems after I bought it last May. The vacuum pump was seized and rusted, probably from running all the time trying to keep up with a vacuum leak. In my case it was a failed check valve in the secondary vacuum system.

You may have a disconnected vacuum line, 2000 rpm idle is a large air source to the plenum.

Check the Manifold Air Pressure Sensor (back of the plenum) hose and make sure it is connected to the plenum. It is under the MAP sensor.

It is possible to test the vacuum pump using jumper wires from a 12 volt source connected to the terminals, but I would guess 95% probability that the pump is bad. I haven't checked on the availability of the 1990 style vacuum pumps lately but there have been some recent posts about replacing the pump using the later model pump by changing the connector.

Have you ever pulled the plenum on an LT5?

If you need help, I might be able to find someone near you that can assist.

TomTom are you listening?

Good luck,

Jim

Well said!
Only exception possibly is my personal experience with blown vacuum to MAP resulted in the motor huffing and puffing and barely could hold idle.

[tccrab]

First things first, find the vacuum leak.
You've got a big one.
The vacuum pump only assists the secondary system when the engine is at high rpms for an extended time.
Don't worry about him right now, but know that you'll have to come back to him later.
First thing is to check the MAP sensor hose.
It's behind the plenum on the drivers side.
Use a flashlight and look for a pipe that is disconnected.
It's a common thing, this hose pops off when the engine backfires during a startup when the key is not held in the start position until the engine is running.
The LT5 takes longer to start than a regular Chevy engine for a couple of reasons, but primarily it's because there's no distributor. All the sensors have to polled by the ECM before the actual signal to fire the coils is sent.

Here's all you need to know about your secondary system:
http://www.corvetteactioncenter.com/...diagnosis.html

After you get your leak fixed, then it's time to work on the vacuum pump.
Let us know when you get there. There are a few options.

TomC
"Crabs"

[1963korvette]

Thanks for the replys guys. When I first noticed the bogging it seemed to be only with full power. However, now it feels the same either way.

[tomtom72]

Reporting as ordered, Sir!

Have tools & will travel........Road Trip! NJ, what exit??? Now I really have to go get another EZ-pass module for the Z!

'63 PM me if you need help. Or thomastom72@aol.com is my e-mail addy. I have a scanner & software for a 90, plus a ton of spare parts.

As others have said, vacuum leak(s) will give an alternate air route to cause a higher idle. Or, the MAP hose is blown off, I'd check that first as it's the easiest thing to check. If MAP is okay, then start looking at all the vacuum lines after plugging the hose from the pump that goes under the plenum. You could throw a mity-vac pump on that one to check if the under plenum stuff is okay. Also, on a 90, the fuse for the pump is over on the driver's side bolted to the rear inner wheel house panel. It's a small rectangle usually near the s/s brake lines. That fuse could be blown, probe the connector @ the pump for 12v w/KOEO & see what's up.

Bogging, does it knock when bogging? Could be a lean bog, bad injectors or a big vac leak? At idle, no missing, right? You checked for good spark? Condition of the plugs, PCV's, Fuel filter are all okay? Or, do you mean "it's breaking up" or "chugging" when you put your foot into it? Bad MAP or disconned MAP hose.


Tom

I will check my software to see what yrs are covered, I have a 90 so that's where I stopped reading.

[cward]

Originally Posted by tccrab

The vacuum pump only assists the secondary system when the engine is at high rpms for an extended time.

TomC
"Crabs"

I am late getting this on but I was told by Graham B many years ago that the Yanks made them put in the vacuum pump for the very specific reason mentioned by Crabs. I did not have a vacuum pump on #364 and it ran just fine even at high rpms for an extended time. That said, I know that most Z owners will want the pump anyway. Just thought I would mention this in case you did not want to spend the money.

Curtis

[1963korvette]

Ok, fixed vacuum pump. When car warms up idle is about 1400 rpm, then every 20 sec drops down to about 800 rpms and it sounds like you are shaking a can with rocks in it. Can't see any other leaks I assume will have to pull pellum, but want to find out what I will be replacing first. Any thoughts?

[1963korvette]

No I did not locate the map sensor yet. I did not really look for it yet. I was so happy just to find the PVC laek I forgot to look for that. I will look tonight.
Thanks

[QB93Z]

Just for reference, here is a picture showing the location of the MAP Sensor. This is a 1993 LT5, and there are some differences from your 1990, but the MAP is the same.



If the MAP hose is softened, it will collapse under high vacuum in the plenum. High vacuum occurs at idle and low throttle operation.

Good luck,

Jim