Rough idle
 

My 1989 civic hatchback idles perfectly with nothing turned on. But as soon as I turn the lights on or hit the brake it starts to idle rough. About 20% of the time it will idle perfectly and seems to compensate for the load, the rest of the time it cant

Any ideas?

On my 90 I took off the air box & sprayed it with carb cleaner & that helped but the real help came when I ran a few tanks of fuel with lucas injector cleaner.

Hello,
Have you checked for a vac. leak ,to do this spray carb. cleaner on vac. lines and listen for a advance in idle, then you found a leak. It could also be that the iacv is bad or needs to be cleaned. Hope you find the problem. Keep us updated on the progress. Thanks, :cool:2010cicivlx

Already cleaned the iacv. Thats what fixed the idle in the first place. I was mostly sure and now am 100% positive its not a vac leak. Turning the lights on caused the same idle drop as braking.

I went out and bought a bottle of lucas at wally world for 11 bucks. Have gone about 40 miles on the tank so far and it seems to have worked. Braking causes no idle change, lights cause no idle change. Doing both causes a slight drop but no where near as bad as it was. But then again ive only gone 40 miles. Thanks for the fix.

If you have rpm drop with application of brake, you may have bad/leaky booster.

Im pretty sure its actually a leaky master cylinder. Since when I apply the brakes it slowly sinks.

Sinking brake pedal is a problem in hydraulic system. Apply pressure to the pedal and see if it really sinks all the way to the end of travel.

Engine rpm drop as brake is applied is due to faulty booster diaphragm. Vacuum leak. I had changed one on my friend's Accord, in 1980 something. No, may be in the 90s. I don't remember. Too, many years ago. To see if this is what you have, apply pressure while idling as if you are in emergency stop. In some cases, engine will die.

Alternator is kicking in as the lights on and brake being applied due to current draw. You may be noticing that rpm drop and may be worrying. How is battery condition? New? Old? Load tested lately?

Ive never let it get to the floor. But stopped on a steep hill for 5 to 10 mins it will sink the the point where I have to let go and roll a bit for the pedal to return. It probably could get to the floor if I let it.

It used to drop rpm, it never killed the car. But would drop to like 300 rpm *probably no tach*. It doesnt drop at all now. Ive pushed really hard on the brakes now and no rpm drop.

The rpms would drop to a very rough idle, like shaking the car rough. Battery should be good. Brake place I took it to told me to get a new one, but I think that was more of a sales thing than need.

Yeah its on my to do list. Which I probably will get around to someday. The idle problem was much higher priority as it made driving at night really suck.