Inconsistent Idle, after warmed up, input would be appreciated....

Just got a "new" '88 Civic Hatchback has 1.5L -- 131K mi.

Car has an inconsistent idle once it is warmed up. When idling it jumps very consistently from a normal idle to almost nothing and back again. Repeats this very rythmically. Doesn't happen until car is warmed up.

vrmmmm.. putt putt.. vrmmmm.. putt putt.. vrmmmm.. putt putt etc.

Is this some type of sensor malfunction?
Previous owner said it has done this for a long time. He took it to two mechanics but they couldn't figure anything out. He has driven the car this way for 60,000mi. Can live with it but would like to fix it if possible.

Any input would be appreciated.

okay a couple of questions for you....

is the check engine light on?
is anything disconnected under the hood?
does the car have any of the following symptoms:
-overheating
-loss of power
-smoking
-using/burning oil/water

have you done a tuneup? (distributor cap, rotor, spark plugs, plug wires, oil & filter change, air filter, fuel filter)

Have you checked the ignition timing?
have you checked engine compression?

10 bucks says its the sensor with the two coolant lines going to it on the back of the intake manifold. the IACV i think its called?

Just unplugged the IACV green connection and nothing significant changed. It did alter the rythm of the change in idle a bit.

But after that I unplugged the TAS (Throttle Angle Sensor) and the sporadic idle stopped. But the check engine light did come on. Which I guess could be expected if I was disconnecting parts. Haven't driven it for a long ways yet, but I'll have to see. I'll update when I think It's fixed.

Any ideas as to what would account for this scenario?

just unplugging the IACV wont fix it. i had the same problem on my 90 si.

Just unplugged the IACV green connection and nothing significant changed. It did alter the rythm of the change in idle a bit.

But after that I unplugged the TAS (Throttle Angle Sensor) and the sporadic idle stopped. But the check engine light did come on. Which I guess could be expected if I was disconnecting parts. Haven't driven it for a long ways yet, but I'll have to see. I'll update when I think It's fixed.

Any ideas as to what would account for this scenario?

Yep. Idle air control valve The computer uses this when it senses a load like ac, and alt charging. First I would try to clean it out with a little carb cleaner. Just unbolt it from the back of the im. you can do this without disconnecting the coolant lines. Hold the openings downward and keep the motor and the connector up, try not to get much spray on the motor from the inside.

One other thing have you cleaned out your throttle body? Grime buildup will make your idle do funny things too...

Good luck
SH

Problems back. Definitely was skeptical about the problem going away if I simply unplugged the iacv and tas. After poking around on this forum, which is an awesome resource by the way, I found a few posts about lifting up the accelerator.

This works in my scenario. When I put my foot under the accelerator and lift it up, the idle goes down and becomes steady. Sounds just like it should. It's late tonight however, but I will probably look at the throttle cable tommorrow.

I have it worst now
I have 2 91 civic's and now both of them are doing giving me idle problems.
but try the throttle cable,while having some idle issues try to drop the idle a bit more and see if it levels out.it fixed it for a while for me but now I think its time to get one from the junkyard or napa for cheap, also the Iacv is very easy to take off I think 2-12mm bolts and just spray some carb cleaner in there.I had a pound of black shit come out but still no luck.
sorry for long post

So I recently loosened the throttle cable a couple of turns, and my idle is back to normal. I've driven about 500 miles since and that seems to have been the problem.

My car always idles stupid. 750, sometimes it goes up to 1200ish, back down after a bit. It used to surge from like 1000 to 3500 and I fixed that, but from now on I give up.