Update 2/26/24:

Turns out I was looking in the wrong place to read the ECU error code: the CEL light won't flash on 91 Integras when jumping the connector, instead, the red light on the ECU under the passenger side carpet will flash with long and short flashes to give you a code. I jumped the connector and the ECU flashed one long flash and two short flashes: code 12. Based on other threads, this is the same EGR system error code.

I started inspecting the EGR valve more thoroughly this morning and did see the pin was sticking pretty bad, worse than yesterday and the day before. I was able to remove the entire EGR valve by unscrewing the two 12mm bolts to check for carbon build up and from what I can tell, it doesn't look so bad that air flow would be restricted and the spring moves pretty easily.

I am wondering how to best clean the pin that is getting sticky. Can I use a small drop of brake clean to lube it up? I've read on other threads that seem to suggest that the EGR can be deleted/disconnected/unplugged and that was going to be my next move, but the connector is really tricky to unplug. Best move might be to replace the EGR valve entirely? Thanks for reading and for any help!

 

Update 3 2/27/24

I am happy to report that my car is back up and running like it used to! Here's what I ended up doing:

I removed the entire EGR valve system as pictured in my previous update and used some brake clean to clean up some of the carbon on the valve itself and the position sensor, which is that little pin connected to the black triangle topper. Reading previous posts, I learned that both this little silver pin can get sticky and not spring back as it's supposed to, as well as the valve that lets exhaust air in can also have difficulty opening and closing due to carbon buildup. In my case, the silver pin was the culprit I believe as it was sticking really bad and so I applied a drop of brake clean and ensured it would spring back easily.

Before putting everything back together, I pulled the 10amp ECU fuse from under the hood to perform an ECU reset. Once I got everything back on, including putting the fuse back, I started the car and let it idle for 15 minutes to reach operating temperature without pressing on the pedal, no electronics on, no AC, and with the doors closed. Once 15 minutes passed, I shut the engine off and then less than a minute later I started it back up and took it for a test drive. Immediately it seemed to be back to normal as the idle felt better, the throttle was responsive, it shifted through all the gears, and best of all no check engine light! I drove it all day today and everything seems back to normal.

Hypothesis is that the silver pin was getting stuck in the up position, leaving the electrical connection open for a while and the ECU was freaking out as a result. I also wonder if it had to do with doing the ECU reset correctly as I normally would just pull the negative cable from the battery and this was the first time I instead pulled the ECU fuse out. The blown ECU fuse is still a mystery but I've read it might have been caused by something grounding out and I wonder if the EGR had something to do with it somehow. Crossing fingers that's not the case and I'll be okay!

Helpful Resources I Used:

How To Fix Your EGR Check Engine Light Code 12 In A 1992-1996 Honda Prelude.

Search results for 'egr' - G2IC - Generation 2 Integra Club

Club devoted to 1990-1993 Acura Integra

forum.g2ic.com

 

So glad that you were able to get this rectified!!

A personal "Thank You" is definitely in order from me. Time & time again people join the site, ask a question, possibly figure it out, and do not update their original post on what they found. This causes an issue for the next person who might have the same issue and unable to find a helpful answer. Your posts were well detailed, and actual punctuation was used that assists with understand your original concern. Lastly, the fact that you actually searched the site and other resources also says a lot. More times than none, people are looking to be spoon-fed answers even with simple questions that have been answered over & over again. I love helping people, but if I know a simple search wasn't conducted, I don't even reply and/or answer.

Kudos!!

 

Update 4 3/19/24

Unfortunately my good luck seems to have run out! My CEL returned on Sunday evening when driving home after an otherwise smooth last three weeks or so. Ran the code as soon as I got home and it is the same notorious code 12. Tried to follow the same procedure as I did last time to no avail, the CEL turns on right when I accelerate and go over ~2500 RPMs. At this point, from reading other threads, it seems like this will be a perpetual issue, but I do want to try replacing the entire EGR valve system, even though those are hard to come by for these cars.

In the meantime, I will have to still be commuting for work during the week and want to minimize further damage to the engine if possible. Threads seem to suggest that since this is primarily an emissions component, it doesn't impact too much apart from some fuel economy during city driving. I took it out for a test run this morning on the highway and I think I was experiencing "limp mode" where it limits speed and revs as I couldn't go faster than ~60mph. I am curious what others might recommend when driving with a CEL and things to avoid. Thanks in advance for any help!

 

Update 5 3/20/24

Writing an update to describe the symptoms I am experiencing with this issue after driving around with the CEL on for a few days now.

First thing is that the CEL does not come on immediately but only once the car begins moving and is at operating temperature. Based on other threads, this makes sense as the computer checks for the signal only when the car is moving and driving at a certain RPM range.

The idle is the most noticeable symptom. Generally, the RPMs tend to be higher than normal when at a stop. Additionally, the RPMs seem to fall slowly from 2k down to 1k. When slowing down and RPMs reach 1k, there is a slight jump in RPMs before going back down again to 1k or a bit below it.

Last thing is that it almost sounds like I can hear air rushing in or going through the system in a way I did not notice before, particularly when slowing down to a stop. It's like a very gentle whooshing sound like a literal home vacuum shutting off. I am thinking I have an air leak somewhere or that the EGR valve is actually getting stuck open and so air continues to be pushed into the intake. If I am conceptualizing this all correctly, then this would explain the high RPM and the slow fall of RPMs when coming to a stop since the added exhaust gas acts like a ****ty turbo and adds extra polluted air when it's not necessary. I am curious if the symptoms I am describing may also have something to do with the idle air control valve (IACV), which other threads have said may deserve a cleaning too. However, with the CEL continuing to throw the same code, I think my plan is to do a more thorough inspection of the vacuum lines, replace the gasket for the EGR system, and relube the position sensor. I also want to check the connection points for the EGR hookup, but I have been having trouble carefully disconnecting it. Any tips or thoughts are much appreciated!

 

Update 6 3/23/24

I fixed it again! Here's what I did this time:

Given the symptoms I was experiencing while driving, my assumption going into fixing this the second time around was that the EGR actually was not operating correctly and this isn't a case of a bad ECU or wiring connections. I read up on a bunch of forum posts and watched videos about different EGR types and learned that mine is a vacuum operated EGR valve:

(1)

So for the valve to operate correctly, a vacuum needs to exist in the upper part to pull the valve open against the spring. So here's my EGR, complete with the valve position sensor on top of it which the ECU uses to check for the appropriate voltage (higher voltage means valve is more open) at different driving conditions (ex: idle versus driving at 2k rpms). I noticed before that the topper wasn't exactly on there particularly tight, I could nudge it around with little effort and so I thought maybe air was escaping there and not allowing for a proper vacuum to exist, resulting in the valve not opening as much (if at all) as it was supposed to, resulting in decrease performance and for the ECU to throw a code basically saying "low voltage, EGR isn't working properly." I grabbed a straw and just tried blowing into the vacuum inlet (the little L shaped thing sticking out of the middle section below the black sensor). Sure enough, I could hear air and feel it coming out from in between the sensor and the diaphragm housing below it. In the far right image, I tried to capture the loose fitting of the topper, which had a big enough gap that I could see light on the other side. When I pressed down on the sensor to tighten the seal more, I no longer could blow into the inlet, suggesting that it was air tight in there and would hold vacuum. Seemed like I found my problem!

One user on a Prelude forum said that he fixed this by replacing the triangle gasket on the sensor with an o-ring that he managed to squeeze into it to create a better seal to hold the vacuum (2). My plan was to try the same, but my AutoZone didn't have o-ring sets so I instead went with a different solution of just figuring out how to fix the sensor more firmly on top of the diaphragm housing. You can see that by default, just those two clips on either side of the sensor topper push down on the sensor. My thinking was that if I could tighten that seal by adding force downward on top of the sensor, the same way I did with my hand when blowing into the inlet, then the vacuum would function as it should without any leaks.

You'll see that the two silver clips that hold the sensor down are attached to a single metal plate, which goes on top of a rubber cover which encases the plastic sensor housing. In between the rubber cover and the metal topper, I added a very small flat piece of metal to add a bit of height, creating more downforce and tightening up the seal between the sensor and the diaphragm housing when the clips are locked into place. Please be kind about my creative vacuum tester in the middle image, which shows that the diaphragm is now air tight as the plastic squeeze bottle doesn't fill up with air. The far right image shows that the gaps that were there before are no longer there so air should not be able to escape from there anymore.

I put everything back together, reset the ECU with the same procedure as before (disconnect battery + pull ECU fuse, wait for a few minutes, reconnect, wait for a few minutes, start and idle for 15 min til at operating temp, stop engine), and then took it for a drive and did not get a CEL! Moreover, the car was driving with good power, shifted through all the gears, and I was not hearing any air whoosing when coming to a stop. Crossing my fingers that this fix will last!

References and Useful Links:
(1)

(2) https://www.preludepower.com/threads/code-12-egr-solved.182429/
(3)

(4) Egr valve probelms
(5) New To The Tuner's I Need Help!!!!
(6) 18710-PR4-305 Genuine Acura Valve Assembly, Egr
(7)

(8)

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(10) Egr Valve: What Is It And Why Does Your Car Need One?