The computer was half right right

We got back to the idle surge on the 2002 Civic today.

The surge is common on them if there is a vacuum leak, or issue with one of the idle valves. It operates different than most fuel injected cars, where they will run very fast if there is an intake leak. The Honda will shut off the fuel injectors if the idle goes above about 2000, then turn them back on when below 1000, so it surges on and off. It is really wacky how they handle fast idle, there is a bypass that has a mechanical plunger in it that closes off the large bypass when the coolant that flows through it warms it. This valve is mounted right on the throttle body. If the large bypass does not close when computer thinks it should, the computer then starts the surging.

I closed off the intake with my palm, and the engine died, so that told me that there was no intake leak, even though the computer said PVC leaking, that is where the half wrong comes in.

I then restarted the engine and allowed it to surge, I pinched off the IAC bypass and could control the idle speed by changing how much it was pinched off. I got it to idle real smooth by me controlling the air bypass. I am going to replace the valve, think that will solve the issues.

FYI, there are two small coolant hoses that go through the idle valve, make sure they are warm and not air bound, I did this first.

For the half right part, the computer did also say bad IAC valve.

I don't remember a Civic issue like that but an Accord YES...

I remember they had a screen on'em that would get stopped up that could be cleaned... I have played with that thermostat and fixed it BUT the issue would always come back... The thermostat was the issue...

I am gonna give ya a name use it with key words to nail down your issue...

Dan Marinucci

I googled Dan Marinucci honda idle system

I got quite a few hits on articles he wrote...

https://www.motor.com/magazine-summary/foreign-service-june-2000/

My Mitsubishi play car had a similar idle speed control system originally. Attached to the throttle body was a cast housing with the idle speed control stepper motor and a thermal type air bypass. The thermal bypass like many thermostat type devices failed after many years and well over 100k miles. Of course it failed open so the idle speed control motor would run to full closed position and there was still too much air to keep the idle below the fuel cut RPM so it surged. Since it was in the evolution to a hot rod play car I did not really want the throttle body being heated by the coolant anyway. I made a new body for the idle speed control motor without the additional thermo air passage. Looped the coolant lines together.

Starts fine in what we get for cold weather. Rare that it gets much below 25. Plus it parks in a garage that never gets below ~50. In really cold weather the lack of the thermo air bypass might be a problem though.

My 2011 Silverado 4.8 V8 liked to rev up a few hundred rpms when I would inch it along or be moving in and out of N like when trying to get lined up on a trailer. Problem was dirt had accumulated around the air intake butterfly that the computer controls in determining engine speed for a given set of circumstances. Seems small dust particles....apparently getting through the air filter, and EGR engine oil vapor for a "binder" accumulating on the sealing surface of the butterfly made the engine run lean so the computer adjusted the fuel flow to keep it at 12% or whatever the design number.

Cleaning around the baffle and where it seals against the air plenum stopped the problem.....alcohol on a long, wooden stem, Q-tip type cotton swab and my finger....to hold the baffle open did the trick.