was wondering if anyone could help. Been trying to get my 95 camaro that i picked up a few months ago to pass ca smog . Everything passed on first test except my egr valve . smog tech cant pull vacuum while engine is at idle . every test i gave it passed and i have 2 of them one brand new and both test the same . while engine is off they both hold vacuum . just cant get vacuum while engine is running . it almost like the intake port on the engine side of the egr is pulling more vacuum then i can or smog guy can which is causing the erg to stay closed . No codes and engine is running good . i can push it with my fingers and make it open which causes the engine to die , just cant do it with vacuum

 

Does the car have headers? If so that changes the back pressure. Here is a link to info and testing
http://shbox.com/1/4th_gen_EGR.html

 

the car does have short tube headers and i noticed there is alot of vacuum on the egr from the intake side of it where it mounts to the engine . im just wondering if theres to much vacuum from the engine since the rpms are high at idle after its warm .Its around 900-950 which i i think is a bit high

 

Read through that link to understand how the valve works. Because the egr tube on the header is on a single primary there is not enough back pressure for the valve to operate properly. If the smog guy will let you reach in and physically open the valve during the dyno part of the test.

 

so i plugged the tail pipes and egr opened right up just got to fig where to go from here

 

for anyone else running into this problem i was able to find a solution . The major problem was the back pressure there just wasnt enough . I was able to plug the lil bleeder holes on the egr and it works like a charm . Thanks flame for the feed back

 

Glad to help!

 

Hey I have this exact same problem, failed smog twice for high nox. EGR valve will hold vacuum when the car is off. What are the little bleeder holes you're talking about?

 

Not the same problem. You failed for high NOx. OP failed for not being able to pull vac @ idle.

High NOx means your combustion temps are too high (usually). Check your EGR valve for overall functionality. Get the car up to temp and push in on the valve (its hot). If the car dies or wants to die, its working fine. In that case, cooler plugs are a good first step. What are the other readings?

 

 

Back pressure changes are not going to affect EGR operation. I've passed the CA sniffer with longtubes and a cam before, with all the factory EGR stuff in place. It has no idea what the exhaust backpressure is anyway, EGR is controlled by the PCM and is commanded under certain load conditions. So either your solenoid is not working, or the EGR valve isn't. It's not a complicated setup.

 

 

"Vacuum to the Exhaust Gas Recirculation (EGR) valve is controlled by the EGR vacuum control signal valve. The vacuum signal to the EGR valve is regulated by varying the duty cycle (on-time) of the solenoid valve. The duty cycle is calculated by the PCM based on data from the ECT sensor, the MAF sensor and RPM. The PCM uses engine coolant temperature (ECT) and intake air temperature (IAT) to determine when to enable the EGR system."

Verbatim from the GM manual. And there's no such thing as "negative pressure". It's vacuum, or low pressure.

 

No it's not, these things are not mutually exclusive. What you're talking about is the EGR system. That part is correct. I'm talking about the design of the EGR valve itself

The negative pressure is the type of EGR valve. There are lots of EGR valves, 2 being positive and negative. The LT1 uses a negative pressure EGR valve that has a secondary spring that closes the lower diaphragm to the upper diaphragm in its default state. When the negative pressure of the intake manifold exceeds the positive pressure of the exhaust backpressure, it pulls on the hollow shaft and pulls the EGR closed. Which is why OP was able to bypass this by blocking the bleed holes.

http://shbox.com/1/4th_gen_EGR.html

"The 4th Gen F-body uses a negative backpressure EGR valve. The amount of exhaust gas is varied, depending on the amount of manifold vacuum and exhaust backpressure. This is why it is typical to get an EGR diagnostic code when the exhaust system is altered. Adding headers or removing the catalytic converter can create changes in backpressure. OBD-II has higher sensitivity to this and will "throw a code" more often than OBD-I will.

The diaphragm on the EGR valve has an internal vacuum bleed hole which is held closed by a small spring when there is no exhaust backpressure. The PCM driven EGR solenoid controls vacuum to the valve.

Engine vacuum opens the EGR valve against the pressure of a large spring. When vacuum combines with negative exhaust pressure, the vacuum bleed hole opens and the EGR valve closes."


This articles explains it further details http://www.tomco-inc.com/Tech_Tips/ttt27.pdf



When I don't plug the bleed holes, I can see the EGR valve open and then close rapidly unable to maintain vacuum because the intake manifold is pulling it closed due to lack of exhaust backpressure.

If you plug the bleed holes, it overrides this control unit inside the valve, essentially making it a regular vacuum EGR like you said.


The reason why positive/negative EGR valves were designed were because exhaust backpressure was a good way to measure engine load so that the EGR system doesn't cause the car to run rough or stumble. But I'm finding that plugging the bleed holes and just allowing only the EGR solenoid to engage the EGR valve is sufficient enough.

But I found a few other threads that talk about this. If you lack exhaust backpressure, the EGR valve cannot remain open due to the intake manifold continually pulling it open.

 

Any kind of updates on this? Been having kind of the same issue with my egr, can feel the vacuum from the hose but not sure if it's actually working correctly.