I have spent a lot of time working on this car, and have done extensive searching on essentially everything you can think of. If I could have found the problem, I wouldn't be here asking. That said, I'm going to be as detailed as possible.

I drive a 2000 Accord with the F23A1 (2.3 Vtec). Just shy of 300k miles.

As of the past few months, I have started having a bad hesitation problem under low throttle. It's mostly noticeable if you're coasting almost to a stop and then touch the throttle. It'll hesitate and jerk for about a second or two, and then everything will be fine. Other than the low speed hesitation, it runs perfectly fine. The other symptoms I have noticed is the idle is very inconsistent. Mind you, this is all when it's warm and has been running for a while, sometimes it will idle around 750 RPM, and other times it will be just below 1000. The IACV was just replaced and is not suspect. The problem that I have noticed the most however is a dramatic loss in fuel economy. I have gone from being able to get 550 miles out of a tank to around 400 miles.

I suspect it's possibly a vacuum leak, however I have tried the carb cleaner trick to no avail. My next thought is there could be a valve stuck in the vacuum system, however I'm not sure where to start there.

Because it is nearly to 300k miles, and I have had the funds to throw money at it lately, I have done a lot of work on it in the past year. Here's a list:

-Replaced IACV
-Map Sensor is new
-Cleaned EGR valve and resealed with new gasket
-Cleaned throttle body including throwing a can of the aerosol seafoam down the intake.
-Entire ignition system is new including the distributor
-All fluids are relatively new, including transmission fluid
-Alternator and battery checked out good
-PCV Valve is brand new

 

disconnect the egr valve connector after the car has completely warmed up....when the check engine light comes on, see if the issue goes away

 

I would suspect the throttle position sensor. Watch Scotty Kilmer's YouTube video "Is your car hesitating? Might need a new throttle position sensor"

 

Yesterday, I decided to dismantle the throttle body and intake in an effort to fix the problem. I deep cleaned the egr valve along with the ports, and replaced the throttle position sensor. Scraped out as much of the gunk in the intake as I could, and re sealed everything. It seems quite a bit better, and the car actually sounds better now, but it's still doing it slightly.However, when I took the plenum off I realized it was quite wet inside the EGR side of the intake. What would cause that?

 

Depends on what it is/smells like.
Oil it could be excess oil leak from the engine(rings/valve seals).
Gas it could be a problem with the fuel system.
Or if it has just never been cleaned it could simply be build up.

 

This may be a dumb question, but could an internal leak in the brake booster cause a bad draw on the engine? It's my only idea at this point, and it does show signs on being on its way out.

 

If the booster is leaking vacuum pressure then yes, it would act as a vacuum leak on the engine.

 

I can't believe nobody else has said anything yet, but It sounds awfully like worn out spark plugs. I would try checking to see if those need replacing.

 

Watch Scotty Kilmer's YouTube video "Finding engine vacuum leaks with a cigar" and, if that doesn't solve it I would consider getting a BlueDriver (made by Lemur) and analyzing the fuel trims. Watch Schrodinger's box YouTube video "Secret to engine problem diagnosis - Fuel Trims" and also his video "TPS (Throttle Position Sensor) Diagnosis and Understanding"

 

The spark plugs are iridium and only have about 20k miles on them. I'll be pretty pissed if they're bad. Lol