P0401 1998 camaro 3.8
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Gas cap
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If you've already replaced the EGR, and are getting a PO401 code then the next most likely issue (which should have been checked 1st before the valve was installed, unless there were other code(s) prior) would be flow restricted by carbon (or ???) through the system to intake.
It's fairly easy to check - with engine OFF, remove the EGR from it's base and set aside, with ports now visible look for obvious carbon deposits in the holes, if none seen then start the engine and listen to idle - is idle 'normal'? if the idle is normal then the port from EGR to intake manifold is plugged somewhere along it path. If idle is 'odd' (very rough, high RPM or other) then most likely the path from the exhaust manifold up to intake is blocked somewhere.
Fix is about the same for either;
If it's in the path to intake use Carb Cleaner Spray (OX2 Sensor safe type) and some type of wire (drain router spring wire or even a coat-hanger) and the spray to clean the passage as best as possible until you can blow through the passage easily.
If it's in the path to exhaust do the same procedure, but I try to vaccum out as much of the plugging 'junk' as possible as I go to prevent dropping a bunch into the Cat path and causing other issues ...
And :yes: a loose or 'bad' cap can cause some of these MIL code(s) - most often they are pending or present codes that will reset by themselves after a few start-drive-stop cycles, during which time you've either found the cap loose and/or re-tightened the cap when you refueled.
Most often a loose or bad cap will cause only intermittent MIL indication - it is always something that should be checked as part of good diagnosis before you start replacing parts.
Hope this helps some
It's fairly easy to check - with engine OFF, remove the EGR from it's base and set aside, with ports now visible look for obvious carbon deposits in the holes, if none seen then start the engine and listen to idle - is idle 'normal'? if the idle is normal then the port from EGR to intake manifold is plugged somewhere along it path. If idle is 'odd' (very rough, high RPM or other) then most likely the path from the exhaust manifold up to intake is blocked somewhere.
Fix is about the same for either;
If it's in the path to intake use Carb Cleaner Spray (OX2 Sensor safe type) and some type of wire (drain router spring wire or even a coat-hanger) and the spray to clean the passage as best as possible until you can blow through the passage easily.
If it's in the path to exhaust do the same procedure, but I try to vaccum out as much of the plugging 'junk' as possible as I go to prevent dropping a bunch into the Cat path and causing other issues ...
And :yes: a loose or 'bad' cap can cause some of these MIL code(s) - most often they are pending or present codes that will reset by themselves after a few start-drive-stop cycles, during which time you've either found the cap loose and/or re-tightened the cap when you refueled.
Most often a loose or bad cap will cause only intermittent MIL indication - it is always something that should be checked as part of good diagnosis before you start replacing parts.
Hope this helps some
Had this problem on my sons '98 Malibu. Go to a bicycle shop and see if they will give you some old brake/gear cable. Cut a piece around 18" and insert into your drill. Run it back and forth through the port (take off the egr) into the throttle body area. There should be one end of the cable that is not frayed on the end. Run that end through the ports. Good luck!
Okay I just took of the hose and throttle body and cleaned inside the throttle and the manifold. There was a little carbon that looked like dust.. but it didnt look near enough to clog it up. I also sprayed some cleaner in the exhaust manifold port. I drove the camaro for about 20 miles and the CEL came back on again. However, this time the codes were P0300 and P0113. P0300 is random misfires and P0113 is "Intake air temperature circuit high input." When I took off throttle body, some antifreeze was leaking out of a hole. My uncle was there and unscrewed the radiator cap to release the pressure and the leaking stopped. It spilled on one of the sensors I remember. I never wiped it off and later plugged in the sensor. The sensor was right below the throttle body. Could that be why the code came up? (P0113) And should I go get a new sensor?
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Took off the Throttle Body and 'cleaned in manifold' ... why ... 
The problem was with the EGR circuit - not the manifold or TB ...
Don't get this at all given the information you were given to check/address to rectify the specific problem you posted about.
Don't know what to tell you now ...
The sensor most likely is contaminated or just wet from coolant causing a 'short' or low impedance that is giving the PO113 code.
The PO300 is most likely from the coolant getting in intake stream and causing firing issues.
The root to both is the 'leak' in intake of coolant - don't have a clue what you did to cause the leak while in there or how to repair, you'll have to retrace your work and find issue ...
Then go back and fix EGR or the previous code we were addressing will just reset
The problem was with the EGR circuit - not the manifold or TB ...
Don't get this at all given the information you were given to check/address to rectify the specific problem you posted about.
Don't know what to tell you now ...
The sensor most likely is contaminated or just wet from coolant causing a 'short' or low impedance that is giving the PO113 code.
The PO300 is most likely from the coolant getting in intake stream and causing firing issues.
The root to both is the 'leak' in intake of coolant - don't have a clue what you did to cause the leak while in there or how to repair, you'll have to retrace your work and find issue ...
Then go back and fix EGR or the previous code we were addressing will just reset
yikes....check out this article related to the plastic manifolds Chevy used back then:
http://bernardsblog.blogspot.com/2008/12/gm-38l-plastic-composite-intake.html
http://bernardsblog.blogspot.com/2008/12/gm-38l-plastic-composite-intake.html
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I had kinda forgotten about the intake failures, most our cars are/were west coast and we didn't see them fail as often as I've heard others state ...
I did replace one on my daughter's Olds se got as a pass me down from her Grandmother - daughter was driving it from Bay Area to LA every other week for a year or so and I found some water-in-oil signs a few months into her trekking down there. Turned out to be the intake failing ...
Replaced with a Dorman unit that had just come out, it was actually less than the unit I had checked on from a Stockton late-model GM wrecking house
But, the issue there was a faint coolant 'smell' after driving and some slight signs of coolant in oil (moisture in oil fill and under cap) - not a P0401, or other, code(s)
Might still be good info for OP :thumbsup:
I did replace one on my daughter's Olds se got as a pass me down from her Grandmother - daughter was driving it from Bay Area to LA every other week for a year or so and I found some water-in-oil signs a few months into her trekking down there. Turned out to be the intake failing ...
Replaced with a Dorman unit that had just come out, it was actually less than the unit I had checked on from a Stockton late-model GM wrecking house
But, the issue there was a faint coolant 'smell' after driving and some slight signs of coolant in oil (moisture in oil fill and under cap) - not a P0401, or other, code(s)
Might still be good info for OP :thumbsup:
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