ClarenceT

..if I just pull the vacuum line off of it - wouldn't this close the valve if it happened to be open? How can I be sure?

ZylaRace

Sure it will close, if it was open. It is suppose to be open only during part throttle crusing conditions.

If is sitting there idleing, it should be closed. I don't recommend running along side while you try and unplug it, but you are welcome to try.

VAHROOM

I test mine with the motor off first. Apply vaccuum to the EGR by pumping the line down. (Or just attach a line and suck on it.) Make sure it holds vaccuum until you release it. I had one that you could pull open and it leaked closed and set codes.

CentralCoaster

At idle, exhaust backpressure bleeds of the vacuum to it and keeps it shut.

CFI-EFI

I had one that was opening too far, too soon. The return spring was weak, but still worked. It caused a lean misfire, at cruise. This isn't as easy to test for. The vacuum pump will only tell you if it opens and closes.

RACE ON!!!

CFI-EFI

At idle, the ECM shuts off the vacuum supply via the solenoid.

RACE ON!!!

CentralCoaster

yes, but it doesn't open it to atmosphere. so the line will still hold vacuuum and hold the egr open. that's what the bleed valve is for.

See, we're both right.

65Z01

On my site there is a procedure to check the EGR system using engine vacuum at idle.

CFI-EFI

What does that have to do with back pressure?

I am not familiar with the "bleed off valve". Most diaphragms don't hold vacuum, well. The ECM shuts off the supply of vacuum and the spring closes the valve. Exhaust back pressure, if anything, would try to act against the spring and open the valve when it isn't supposed to be open.

RACE ON!!!

tempest

i don't know if this is the same with the L98 (again) but the LT1's valve diaphram has a tiny hole to bleed off the vacuum. mayhaps that's what CentralCoaster's talking about.

back to the original question i agree with CFI that a vacuum pump will show you a faulty valve (at least if it holds vacuum) pretty easily.

hey CFI, how did you figure out the lean misfire problem? the valve still held vacuum, right?

87NassauBlue

You can either use a little mityvac to check it or just put a piece of vacuume line on it and suck on it with your mouth and see if it holds vacuume. You just want to make sure your diaphram isn't busted or leaking.

CFI-EFI

Yes. The valve opened and closed, just like it was supposed to. The problem was, due to an old, worn, spring, it was opening, further, earlier, than it should, and caused a lean mixture. It failed a smog test at 2500 rpms. It was fine at idle. After trying everything *I* could think of, I finally had them scope it. THAT revealed the lean A/F mixture at cruise. Thankfully, they installed a vacuum bleed, just before the EGR valve to weaken the signal to the valve. Otherwise, I'd have had to pay them to replace the EGR. As it was, I had to pay their $60.00 minimum to have it scoped. *I* would have never found the problem on my own.

RACE ON!!!