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  1. #1
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    Default Plenum Removal - '90 Vette

    I am going to check to see if I am having problems with my EGR valve. While looking at another post awhile back, there was a good link for removing the plenum, but I didn't bookmark it - was just reading it casually. I have to tried to find that thread/link for the past couple of days w/o sucess now that I actually have to remove the plenum. I saw the one Rob wrote for an LT-5, but there seems to be a lot of difference between mine and the ones on the LT-5. I'm hoping someone is familiar with the link and can point me in the right direction. Mine is a '90 coupe.

    Thanks in advance for your help.

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    Plenum Removal - '90 Vette Vettefan87's Avatar
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    I did a quick search and found this thread http://www.corvetteactioncenter.com/...ting-soon.html
    I am not sure if it is the one you originally seen, but it does list the bolt sizes. I have had my plenum off and it is an easy procedure to do.

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    By chance a "pinging", "check engine light" and a code 32? Yah, EGR probably has carbon build up. I replaced an EGR in a '90 recently. I found an easier way to do this. I just loosened the torx head bolts at the bottom side of the plenum (drivers side.) I then removed the torx head bolts on the top of the plenum, both sides. I was then able to pull the plenum body up and out, leaving the feeder tubes in position. There are a couple of vacuum hoses that need to be marked for re-installation. I then replaced the EGR AND GASKET!!!! While the plenum body was off, I noticed the build up inside.....black as coal. I cleaned this build up with carb cleaner. I then replaced the MARKED vacuum hoses, set the plenum body back, installed the top plenum torx bolts and then tightened the driver side bottom torx bolts. Done....in about 30 minutes.

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    Plenum Removal - '90 Vette Vettefan87's Avatar
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    I did not remove the runners either when I pulled my plenum. I found the hardest thing to do was the vacuum line to the brake booster.

  5. #5
    Moderator KANE's Avatar
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    Quote Originally Posted by RonnieH View Post
    I am going to check to see if I am having problems with my EGR valve. While looking at another post awhile back, there was a good link for removing the plenum, but I didn't bookmark it - was just reading it casually. I have to tried to find that thread/link for the past couple of days w/o sucess now that I actually have to remove the plenum. I saw the one Rob wrote for an LT-5, but there seems to be a lot of difference between mine and the ones on the LT-5. I'm hoping someone is familiar with the link and can point me in the right direction. Mine is a '90 coupe.

    Thanks in advance for your help.
    Perhaps this write-up will help...
    TPI Fuel Rail Removal (Fuel Injector Replacement)

    I took my TPI down to the baseplate to swap out injectors. You won't need to go that far- but the process is the same.
    1982 Dark Blue Corvette
    Crossfire Injection | .496"/.495" Cam | AFR Eliminator Heads | EBL | 3.73
    0-60mph in four and a half seconds



    1995 Trans Am
    1 of 1 Trans Am | 100% documented | RPOs 48U, MN6, and GU6
    1982 Chevrolet Corvette , 350 2k RPM Stall Converter + Updgraded Transmission + Governor Set For 5,450rpm Shifts 3.73 Code 42 Dark Mettalic Blue with a twist

  6. #6
    Plenum Removal - '90 Vette TedC's Avatar
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    RonnieH, Why do you think you have an EGR issue? I just replaced my EGR valve, and have first hand experience on testing the EGR system. You can run some basic tests without removing the EGR. Here are a few suggestions.

    1. Attach vacuum pump directly attached to the EGR and us a mirror to see if the EGR valve moves. Does it build vacuum, does it hold vacuum?
    2. Attach vacuum pump to EGR valve vacuum feed line using a long hose (4') and drive car. In closed loop, you should see vacuum when off idle. Taking your foot off the pedal, vacuum should drop quickly to zero.
    3. Idle car in park, attach vacuum pump to EGR valve. What happens as you add vacuum and the valve opens? Does the idle quality change? If the port is plugged, the idle will remain the same.

    The L98 uses a negative pressure EGR. This design actually has two valves. Primary valve is opened by vacuum applied to the external EGR port under ECM control. This is the one you see move when using a mirror. The internal valve is controlled by chamber back pressure (not the ECM), and when enabled, it forces the EGR to close by allowing air in which forces the primary side to close. On my 1989, vacuum was not always holding because of a minor leak in the secondary valve. This is common failure on this type of EGR. If you end up removing the EGR, you can unscrew the EGR valve base and clean out the carbon buildup. Mine was pretty clean for 58000 miles, and not the cause of my EGR issue. I also discovered that NAPA had the best price on the plenum gasket kit. Removing the EGR bolts can be a royal pain if the intake runners are still attached to the base because of limited space to work. Solution was to grind down a short socket that allowed me to attach a claw foot.

  7. #7
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    Default Great Responses

    Thanks for all the GREAT HELP AND SUGGESTIONS!!!!! I have been chasing this idling problem for a couple of months and I am reasonably sure the EGR is my problem - either faulty EGR or carbon contamination. The car idles great until up to idling temp, then idles terribly. Accelarates great and runs at speed with no miss. I only have 35,000 miles on the car. I got in the garage day and blocked off the EGR and the idle seemed to smooth right out. Of course, out on the road it didn't run too good. Once I get the plenum off, can do some better troubleshooting as you all have suggested. Will let you know how I make out.

    Thanks again for all the great help. Folks on this forum are the best!!!

    I thought I read someplace the electronic EGR's for the '90 are hard to find- anyone have a good source/distributor if I do in fact have to replace mine?

  8. #8
    Plenum Removal - '90 Vette TedC's Avatar
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    RonnieH, You are correct, the 1990 L98 does not use the same EGR as 1986-89, but it does look similar. I'm guessing it is still a negative pressure valve. I checked the Eckler's catalog and they did not list a 1990 replacement EGR. Autozone listed a 1990 EGR that matches OEM numbers #17090106 or 17112240 for $50.

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    TedC

    Thanks very much for taking the time to check this out. I was actually going to call AutoZone and NAPA today and check GM Parts Direct. I looked in the Mid-America catalog last night and they do not carry them.

    Thanks again. Hopefully I can return the favor some day.

  10. #10
    tcramstrek
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    My 91 L98 is experiancing the same condition. I just picked up an EGR valve at NAPA (GM p/n 17113456 as instructed by the Dealer for my VIN) and it is clearly a OEM Delco Negative EGR. ($65.35 and $13.16 for the Plenum Gasket kit. Use your AAA discount at all NAPA stores and save)....

  11. #11
    New Member keither's Avatar
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    Quote Originally Posted by TedC View Post
    RonnieH, You are correct, the 1990 L98 does not use the same EGR as 1986-89, but it does look similar. I'm guessing it is still a negative pressure valve. I checked the Eckler's catalog and they did not list a 1990 replacement EGR. Autozone listed a 1990 EGR that matches OEM numbers #17090106 or 17112240 for $50.
    Maybe I was misinformed, but I just plugged the vacuum line to my EGR valve to deactivate it, as I thought it is un-neccessary for running, may even rob power, and cause heat. I know it's an emissions thing, but there are no emissions tests here in south Texas. IJ also removed the air pump and all its plumbing and valves, and installed an air pump eliminator pully kit. 25 pounds off the engine! The car runs great---maybe a little idle surging. Does killing vacuum to the EGR valve hurt anything? No codes.

  12. #12
    Plenum Removal - '90 Vette TedC's Avatar
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    I'm surprised your ECM doesn't trap a code. My engine will ping if I plug the EGR. The ECM expects a diluted combustion mixture off idle when running in closed loop mode. EGR is closed at WOT or at idle. The ECM uses the engine knock sensor, and under knock (ping) it will retard timing which is not a good thing.

  13. #13
    eddie20875
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    Quote Originally Posted by TedC View Post
    RonnieH, You are correct, the 1990 L98 does not use the same EGR as 1986-89, but it does look similar. I'm guessing it is still a negative pressure valve. I checked the Eckler's catalog and they did not list a 1990 replacement EGR. Autozone listed a 1990 EGR that matches OEM numbers #17090106 or 17112240 for $50.
    ecklers does list the egr, part number 36923, sold for 74.99

    also from another post here, the original egr solenoid is not available but ecklers has a replacent that you can substitute for it, part 36929, for 72.99.

    any egr valve you buy must be a negative pressure valve. i went crazy on my 90 with the 32 code coming up and idling good but running crappy at driving speed. if you start car and let it idle hook up any hose that is drawing vacuum to it. if there is no change to idle either the valve is bad or the intake manifold is clogged.

    after going crazy with mine for quite a long time, it appeared to be that i had a vacuum leak at the intake manifold. i had checked for leaks but never could find any but upon removing my intake as i thought it was clogged but wasn't, when i reinstalled it low and behold my problem cleared up plus i dont get the dreaded 32 code showing up.

    good luck with it.

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