Smog problems with lt1 egr
05-06-2014, 01:54 AM
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Smog problems with lt1 egr 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
05-06-2014, 11:49 PM
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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
05-07-2014, 05:54 AM
#4
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.
05-11-2014, 10:06 PM
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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
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11-07-2018, 11:26 PM
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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?
11-08-2018, 03:56 AM
#9
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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?
11-10-2018, 01:59 PM
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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?
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?
The egr will hold vacuum when the car is off but at idle will not. There's not enough back pressure to keep the egr open. When I pump with the mityvac with the car running, the egr vacuum will go up but come back down immediately.. unable to hold.
The egr valve will hold when car is not running though. It's a new egr valve with the right part number from AC Delco.
I tested the egr solenoid and vacuum hose from the intake manifold to the solenoid.. all working. So the prob is somewhere from the exhaust lines.
Just installed a brand new catalytic converter thinking it'll help, made no difference on nox numbers. But my HC and other numbers are really low and clean. Smog tech is convinced it's an egr issue.
I am in the process of testing for any leaks in that little pipe coming from the stock exhaust manifold to the intake manifold. Also gonna pull the egr valve off to see if there's an issue with as blockage there.
I can't think of anything else I don't have headers and have a stock cat. It's gotta be a blockage or a leak somewhere in the exhaust lines or passages that don't allow for proper back pressure to keep the egr continually open
But OP, wouldn't blocking the bleed holes in your egr valve cause it to never close on it's own? It's there so when exhaust back pressure is low it closes the egr valve right?
How exactly did you clog it?
11-10-2018, 10:03 PM
#11
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.
11-12-2018, 11:24 AM
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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.
I ran a datalog and plugging the bleed holes like op suggested and it shows changes in the rpm at idle it runs much rougher when egr is activated.
Also without the bleed holes plugged the egr valve doesn't open all the way since the negative pressure from the intake manifold was sucking the shaft and pintle back down which is why when I applied my mityvac it would show some vacuum but not about to hold.
Anyway the biggest test would be an actual smog test so I'll update my results.
11-12-2018, 04:25 PM
#13
"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.
Verbatim from the GM manual. And there's no such thing as "negative pressure". It's vacuum, or low pressure.
11-12-2018, 10:52 PM
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"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.
Verbatim from the GM manual. And there's no such thing as "negative pressure". It's vacuum, or low pressure.
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.
Last edited by ridiqls; 11-12-2018 at 10:57 PM.
