Like any good mystery, you have to have build up the characters and develop the plot:

1994 Integra GSR Sedan
143k miles
changed timing belt at 90k
changed clutch master cylinder at 100k (stock unit leaking)
changed clutch disc and pp at 110k due to stock unit spring failure
changed radiator at 120k (stock unit plastic endtanks cracking and leaking) - noticed higher idle than normal and slow to warm up immediately after installation.

Ran three quarts low for a few k miles - did not get low oil indication
During this time had trips to red line and noticed erratic VTEC engagement which caused me to check the oil level. It was low.

Surging idle noticed around this time 142k miles and noticed stumble on shutdown
Also, when I let up on the gas pedal and coast, the throttle will seem to thump causing the car to jolt. This will happen no matter how lightly or slowly I let up on the gas pedal.

Went to the Acura dealer and spent $80 for them to tell me what I had already diagnosed - bad fast idle thermo valve. They said that was causing BOTH the surging idle AND the stumble on shut down.

Is this true? could it be the FITV causing the surging AND stumble on shutdown?
I thought the FITV would only effect the surgin idle. Dealer said $250 to replace FITV.

My valves have not been adjusted in about 60k miles - timing has not been adjusted either. Dealer said $140 to adjust valves.

What do you guys think the problem really is?

 

make sure you are completely full with coolant - when you changed radiator, did you bleed the system to get rid of all air? - low coolant level will cause erratic idle.

 

well, I was pretty sure I bled the coolant.

I did it again and got some bubbles initially. Actually seemed like the coolant was overfilled.

Anyone know what that fitting is that looks like a schraeder valve with a green cap that is at the L junction of the line coming out of the water pump?

Is that another bleed fitting?
I can't find any mention or picture of it in the Manual.

 

Bring it's to a different shop and get it check up If they all say the same thing or similer than that It's probably true.