idle surge:
It happens when ever it feels like doing it, its not consistent and triggers no check engine light. I dont recall it ever happening with a cold engine, but it may have. the only thing that i can say that is associated with it is electrical load. its most pronounced at night when i drive with my lights, heat and radio on. as i turn my heat and lights off while pulling into a parking spot (coasting in neutral w/ engine idling) the surge will begin immediately after killing the lights and/or heat.

It will occasionally surge when i'm turning on the off ramp and coasting in neutral. I can make it stop by turning the heater on a higher setting and adding the rear defroster (increasing electrical load)

attempts at a fix: I previously thought that it was a leak that i had in the exhaust. I fixed that, surge still there. Then thought it had to do with my failing alternator. replaced that this weekend and the surge is still there. ground wire for engine and trans checked ok. today i cleaned the iacv and throttle body. no change. ECU reset done after each one of these attempts to fix...



Any ideas?

94 civic dx
manual trans
208xxx

 

Warm up engine, turn all lights and accessories off, transmission in neutral, unplug IACV wire, turn air screw on top of throttle body to get 450 rpm. If you can't do that (it insists on running too fast), there is an air leak somewhere.

 

I have no tach so I'm based solely off the sound of engine.. which way would i turn the air screw? if i screw it clockwise vs counterclockwise what should my engine response be?

just to add to the description of the surging. since i have no tach, i have no idea what range its going to and starting from and what its baseline tach is. when it starts to surge, it first sounds like it drops in rpm then jumps to life rising in rpms just to drop back down and repeat. if i turn lights back on, idle is normal.

 

450 rpm is quite low, it will shake some and act like it's going to stall. The screw will vary the rpm, turn it in clockwise to go slower. When you reconnect the IACV it should speed back up to normal idle of 750.

If this just started look around for broken or disconnected hoses, check the gromets around the fuel injectors, etc.

 

its been doing it for a while, but i always had some maintenance that needed to be done that i could blame for the surge. I disconnected the iacv wire. idle dropped, but not to the point where it thought about stalling. CEL light came on. Plugged back in rpm went back up. CEL cleared with ecu reset.

I didnt see any visable damage to hoses or injectors, but its possible there are smaller cracks.

I'm going to mess with the screw later. W/o a tach is there a limit that i should turn the screw w/ the iacv disconnected? as low as it will allow with out stalling?

 

update: the throttle airscrew seems to have fixed it (for the time being at least).