Is This Even Possible? Vaccum Related

[TX '90 ZR1]

I started up the car (1990) this morning to do a fuel pressure test. (I will share my results of this in a separate thread).
At one point I had a backfire. My first thought was "I bet that blew a hose off!"
Shut it down and let it sit a minute. Turned the key back on and sure enough the vaccum pump ran & ran. Walked away for a few minutes a little disgusted. Have plans to drive the car in a couple of days and didn't have time to tear into it before then.
Decided I might as well check it out. Checked the MAP sensor hose and the fuel evap hose thinking I might get lucky. Both obviously connected. OK. I turned the key back on thinking I might be able to hear the leak and have an idea of what I was getting into. By the time I could get from the drivers door to the engine the vaccum pump cut off. What the hell? Guess the pump is out now!
Key back off. Disconnected the hose from pump and there was vaccum in the system??? Turn key on. Pump runs. Put finger over hose and the pump goes off. Pulled 12 in.Hg on the vaccum system. Took it 15 minutes to leak down to 9 in.Hg. Obviously not leaking much.
Drove car. Secondaries actuate. Idle normal. Runs like it should.
I really didn't dream this up. It really happened. Anybody ever hear of something like this before?

[DRM500RUBYZR-1]

I have observed the Vacuum tank "emptying" ( filling?).
This happens often on C-3's with the large storage tank / crash bar.
Takes a few minutes to get back to where it needs to be, longer if little leaks, then fine.
In life, I have learned to not look a gift horse in the mouth.
Drive it and enjoy it.
If the problem returns, we will deal with it.
p.s
Keep the key in the start position a bit longer than other cars, even up to 10-12 seconds.
Most backfires occur right as you release the key and the engine was beginning to start.
These LT-5's have a few idiosyncrasies.

Marty

[TX '90 ZR1]

Marty,
I did feel a rush of relief when everything went back to normal. Did not really want to deal with a plenum pull on short notice.
Going to mark this down as one of those "Don't look at a gift horse in the mouth" moments !!!
The backfire was not at start up. I am well aware of the extended crank time, but thanks for mentioning it.
I think the backfire may have been induced by some other checking I am doing. After a bit more troubleshooting I will probably be posting for help with what I find. (It is fuel pump/pressure related).
I really appreciate you chiming in when questions arise.

[DRM500RUBYZR-1]

Kenny,
If the backfire occurred other than at start up, kindly provide the background and circumstances of what happened, and when.
Also describe the mods to the car, especially the cats, O2 sensors and exhaust.
Momentary fuel starvation followed by an overreaction either computer induced from bad input, or fuel pump pressure related issues would be some of the usual suspects.
I would start with an evaluation of the pumps and validation of fuel quality.

Marty

[TX '90 ZR1]

Originally Posted by DRM500RUBYZR-1

Kenny,
If the backfire occurred other than at start up, kindly provide the background and circumstances of what happened, and when.
Also describe the mods to the car, especially the cats, O2 sensors and exhaust.
Momentary fuel starvation followed by an overreaction either computer induced from bad input, or fuel pump pressure related issues would be some of the usual suspects.
I would start with an evaluation of the pumps and validation of fuel quality.

Marty

Marty,
I was not going to get in detail about the possible fuel issue 'till I have chased all the rabbits down the hole. Presently I am suspecting weak pump. Will post when I get all the checks done.

[BigIke]

Mine did that and blew the map and main under plenum vacuum. Popped the top and connected back up, found a bad vacuum hose behind the TB and cars running better than ever.

Always hold the key for a long time to ensure it doesn't kick back...

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