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Vaccum signal to unseat EGR valve

Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
I have been testing the EGR valve, and I noticed that with the engine warm and a 15 Hg In at the EGR valve, I could feel a slight movement (or vibration) on the valve diaphragm, but it did not rise noticeably. The test calls for a minimum of 5 Hg In rise in vacuum at the valve, but the valve did not unseat even with 15 Hg In Gage at the valve.

Using a vacuum pump, I had to apply approximately 20 Hg In of vacuum, and it lost 5 Hg In of vacuum in about 33 seconds.

I am suspecting that it should not take a very strong signal to unseat the valve, and so perhaps my EGR valve is bad.

Does this make sense? ;shrug ;help
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
The short answer is that I think that the valve was ok.

The reason I was testing the EGR valve was rooted in the fact that the valve had a ton of carbon build-up from years of use since factory install. And when I did the test, I was suspecting that the carbon I tried to remove from the valve (I simply partially dipped the valve base in carburetor cleaner) was not completely removed and that perhaps the needle seat was defective due to carbon build-up.

I recall that the EGR valve on the L81 is designed to scavenge those EPA harmful gases at cruising speeds. The EGR valve (in the L81) will not apply vacuum (which then the spring in the valve overcomes and allows scavenging) during idle or at WOT conditions. The idle part makes sense, for it is starting from cold, so you would not want to vent vacuum at that time, and the WOT makes sense as well, for you don't want to loose vacuum signal during high loads conditions. The computer commands when the right time to scavenge is.

If I am not mistaken, the negative back pressure EGR valve does not have a vacuum connector to the top of the valve. The ported on the L81 allows the use of the solenoid so that the computer can decide when to apply vacuum to the valve.
 
Joined
Oct 7, 2007
Messages
719
Location
Amherst, NY
GerryLP, My 1989 L98 seems to have a similar set up to your L81 using a negative pressure EGR. When testing it with a vacuum pump it took several quick pumps before the EGR would start to move. At first I thought the EGR was defective. Once it started to move, either partially or completely, it showed no signs of leak down. I found your post interesting because the FSM doesn't go into much detail on testing the EGR itself. The EGR does help on the emission front, but more importantly it flows exhaust gas back into the intake (not at idle or WOT) to help control combustion chamber temperatures. Sounds odd, but plugging the EGR ports will cause engine ping. I just ordered a new EGR and will compare operations.
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
GerryLP, My 1989 L98 seems to have a similar set up to your L81 using a negative pressure EGR. When testing it with a vacuum pump it took several quick pumps before the EGR would start to move. At first I thought the EGR was defective. Once it started to move, either partially or completely, it showed no signs of leak down. I found your post interesting because the FSM doesn't go into much detail on testing the EGR itself. The EGR does help on the emission front, but more importantly it flows exhaust gas back into the intake (not at idle or WOT) to help control combustion chamber temperatures. Sounds odd, but plugging the EGR ports will cause engine ping. I just ordered a new EGR and will compare operations.

In the L81 manual, it says that a minimum of 5 Hg-In (mercury -inches of vacuum) is required to open the valve, and when mine took nearly 20 Hg-In of vacuum, that was telling me that it is operating, but it should not require that much vacuum signal, so my stock valve was working, but only marginally so, so therefore, I replaced it. The same with the bleed-down. The manual considers a bleed-down faster than 15 seconds a bad valve, but mine was taking 20 seconds to bleed down, so it was again performing only marginally, and it was soon to fail.

In your case, a no bleed-down seems to be showing a good valve. But how much vacuum signal it took to open the valve?

GerryLP:cool
 
Joined
Oct 7, 2007
Messages
719
Location
Amherst, NY
I don't remember the exact vacuum required, but it was much higher than 5 Hg-In. I'll test it again, along with the new one when it arrives, and post the results. On the L98, EGR replacement is a pain due to the upper intake and runners covering it.
 
Joined
Oct 7, 2007
Messages
719
Location
Amherst, NY
GerryLP, I received a new AC Delco EGR today. When testing it, it started to open between 2-3 Hg-In. The 19 year old EGR on my 1989 L98 was not as responsive. I also noticed full open occurs at a much lower vacuum with the new EGR. These negative pressure EGRs have two diaphragms and two springs. The lower EGR diaphragm and spring play a critical role is sealing a vent hole that the upper diaphragm depends on. So my guess is that the lower spring had weakened with age, or the seal was failing, or both. I experimented by adding the vacuum pump in-line with the EGR vacuum line, manually forcing EGR operation (open/close) while driving. If I forced the EGR to stay closed, after a few minutes I could hear engine ping under light acceleration with a fully warmed up engine. Really appreciate you sharing your L81 EGR findings which helped me determine that my EGR required my attention.
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
TedC,

Thank you for your feedback and valuable insight in the solution of this issue. Your corroboration of my limited findings and partial results will prove useful to future CAC members researching EGR problems.

To me at least, ever since I replaced the EGR valve in "Gail", she gained a little over 1/2 Hg-In of vacuum at idle and she seemed more responsive. And as I recall, the EGR valve was not that expensive at all.

As far as the "pinging" that you experienced by manually closing the EGR valve in light acceleration, I theorize that the computer is scheduling a predetermined advance timing at that point, and since you were by-passing the control of the EGR, the resultant was the pinging (the air-fuel mixture was supposed to be diluted by the exhaust gases if the EGR was open, but instead, there was "wrong" advanced timing for the relatively-rich fuel mixture charge). Furthermore, given the fact that our Vette engines lack a knock sensor, the computer could not detect the pinging when it happened (which on a knock sensor system, once detected, the ignition timing gets retarded until it goes away). ;shrug

I am trying to keep it straight in my head, but if a relatively rich fuel mixture (only so because the exhaust gases are not being added) requires more time to burn, then by preventing the exhaust gases from mixing with the fuel charge, the timing that the computer had scheduled was not enough??? :confused;shrug And if so, wouldn't that mean that if the scheduled timing by the computer was not enough, then more timing would have been required, and thus the "pinging" shouldn't have been present to begin with? But you heard the pinging, so the advanced timing perhaps was too much even at that point where the relatively rich fuel mixture charge was being employed (in other words, the predetermined advanced by the computer was too much still)? So, could it be that since you were closing the valve manually with the vacuum pump, that the engine was not employing or loosing vacuum signal to open the EGR valve, and perhaps given the higher vacuum signal, the pre-determined timing by the computer was too much still, and thus the pinging your heard?

Wow, talk about deep...too deep for not having fresh alcohol in my system right now...:L;LOL

We may need someone with thorough understanding of this process to help us explain why you got the pinging when you closed the EGR valve in light acceleration. ;help

GerryLP:cool
 
Joined
Oct 7, 2007
Messages
719
Location
Amherst, NY
According to the FSM, the EGR is really designed to help control (reduce) combustion temps. This is accomplished by the EGR sending a small amount of exhaust back into the heads, diluting the mixture. When I manually disabled the EGR, the combustion temps must have gone up. It took a few minutes to take effect. According to the FSM, EGR operation is on the top of the list to check if you have engine pinging. I manually disabled EGR just to see if the FSM was correct. My 1989 L98 does have an anti-knock sensor which I know works from prior testing.
 

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