my 91 accord was running fine a couple of months ago. I decided to pour an entire can of seafoam into my tank (my stupidity)just to keep things clean. Now car has slight studder when under slight acceleration but comes out of it if you step on it. Tried more cleaners thru it. Same thing. Car runs great when cold, but after warm up starts hesitating and still missing out. I notice more spark knock. I've checked the vacuum lines, all look fine so i started researching and came upon clogged egr ports? Same symptoms etc... Could this be the culprit? I did a tune up, plugs, rotor, wires, cap, fuel filter etc. Could i have broken some crud loose in the tank and now its stuck in the injectors? I'm just at a total loss and the missing out is driving me INSANE!!! Someone please help! Thanks!

Since it runs fine until warmed up and goes bad in closed loop you are right to assume it's something feedback controlled.

But while lack of EGR can cause pinging when given lots of throttle under load (climbing hill, passing in high gear, etc) it shouldn't cause any noticeable hesitation or misfire, especially on light throttle application. And if the problem was a lean condition caused by an EGR sticking open slightly, it wouldn't idle smooth.

Still I'm not saying it wouldn't be worth a look in the EGR pipe if it's not hard to get at. I might also pull the o2 sensor(s) and check for additive fouling...that's another item that doesn't give feedback until closed loop begins...

It sounds like a similar problem I was having with my '96 Accord Ex. I was getting a misfire code and had a EGR improper flow code as well. After a lot of searching I found this article: http://autorepair.about.com/library/faqs/bl053c.htm
For your car it may be the same set up. For my car I did not have the brass plugs to worry about. All I had to do was pull some intake parts (3 nuts, 5 bolts) and found all the carbon build up. Took me about 1.5 hours and cost me 1 can of carburator cleaner. If you do need the plugs you can go to your honda dealer and get them for about $0.65 a piece, part number 17199-PT3-AHM. A coat hanger works great for scraping the carbon off and out of the ports. I am not a mechanic or anything and I found this fairly easy.

It sounds like a similar problem I was having with my '96 Accord Ex. I was getting a misfire code and had a EGR improper flow code as well. After a lot of searching I found this article: http://autorepair.about.com/library/faqs/bl053c.htm
For your car it may be the same set up. For my car I did not have the brass plugs to worry about. All I had to do was pull some intake parts (3 nuts, 5 bolts) and found all the carbon build up. Took me about 1.5 hours and cost me 1 can of carburator cleaner. If you do need the plugs you can go to your honda dealer and get them for about $0.65 a piece, part number 17199-PT3-AHM. A coat hanger works great for scraping the carbon off and out of the ports. I am not a mechanic or anything and I found this fairly easy.
I had the same issues. Ports were clogged. I also had a check engine light come on - code 22 I think. I tried everything to clear that EGR code, and in the end what did it was I went to a junk yard and for $20 I replaced that little black box with vacuume lines running through it that sits in the front, up near the drives side light and the ABS pumps. Good luck

my 91 accord was running fine a couple of months ago. I decided to pour an entire can of seafoam into my tank (my stupidity)just to keep things clean. Now car has slight studder when under slight acceleration but comes out of it if you step on it. Tried more cleaners thru it. Same thing. Car runs great when cold, but after warm up starts hesitating and still missing out. I notice more spark knock. I've checked the vacuum lines, all look fine so i started researching and came upon clogged egr ports? Same symptoms etc... Could this be the culprit? I did a tune up, plugs, rotor, wires, cap, fuel filter etc. Could i have broken some crud loose in the tank and now its stuck in the injectors? I'm just at a total loss and the missing out is driving me INSANE!!! Someone please help! Thanks!

if you added seafoam to the tank idk how it could have caused your egr port to get clogged up. maybe adding the seafoam cleaned something small enough to pass the fuel filter but not the filter in the injector causing it to be a small restriction