Low idle/vibe when brake pedal pressed

I’ve noticed a new quirk with my 90 ls. When I’m sitting at a light, with the brake pedal pushed, the idle dips and I get some nasty vibration. When I let off the brake (still just sitting) the idle comes up about 200rpm and the vibration is less (but still there). Anyone have a similar experience?

Also, I’ve noticed my MPG drop from 31 to 27. Maybe its just the winter gas, but I’m wondering if the two simptoms are connected some how. Also I ran some seafoam through the intake last oil change hoping that might help the rough idle, but it didn’t do much. In fact, I’m wonding if it made things worse!

With 155k I’m not expecting much performance from the engine but I need my Teg to last another year! Fuel filter, air filter, plugs, wires, cap and rotor are only 3k old. Should I look at the O2 sensor? New injectors?



check your ground cables maybe. lighting the brake lights should cause a slight dip in engine speed, then the computer should adjust the airflow to compensate, and it should come back up.

Also could be a vacuum leak in the brake booster, though I would expect idle to jump up then. You could try removing the line from the brake booster, plug it temporarily, and see what happens when you hit the brakes.

I’ve got the same problem in my car. I’m going to look into this…

So this is weird…

When my headlights are on and I hit the brakes, the idle will go down significantly and stay there until I let off the brakes.

When the headlights are off and I hit the brakes, the idle will remain about the same, but it will surge up temporarily when I let off the brakes.

Doesn’t seem normal to me. Anyone had this problem and know a fix?

:bump: Any ideas? Anyone?

I have the reverse problem. My teg idles about 200rpm too low and when I push the brake pedal hard, the revs come up. I also have just replaced the ignition coil, distributer cap and the ignitor (ignition module) but that was because when driving, the car decided it was tired of running and would just stall out and die, then after a few minutes would fire back up and go for a bit, then die again. Any guesses on that??

I have the exact same problem as xep. Where my car shakes and vibrates a litle all at an idle of about 700 rpms or so. I would really like to fix this aswell.

The idle drop is from the vacuum booster on the brakes not able to maintain a good vacuum due to your master cylinder seals being gone, and air getting past them.

This has happened on every Honda I have owned. Replace your brake master cylinder and vacuum booster. Most likely you will also find your brake pedal won’t slam to the floor anymore before the brakes have to engage.

Well actually my brakes are extremely responsive. I was thinking it might be my brake booster since I just got a brand new master brake cylinder some 30k miles ago. Can anyone confirm this?

Changing the brake booster is a relatively major job that requires a brake fluid flush.

Ummm well i just had the MC replaced like two summers ago and my brakes don’t do anything like they used too. Anyone else know what it could be?

I have the same exact problem and my brakes feel solid as well but I was told by a Honda Mechanic (been with Honda for 30 years) that my booster is bad; so that explanation of the cause of the shaking makes perfect sense, but it could be something else as well but everything else checks out ok.

could be the alternator trying to overcompensate for a dieing battery or dieing alternator. Also make sure there is break fluid in the reservoir.

My alternator has never given me any problems and my battery is pretty new, though it’s a crappy DieHard. As for brake fluid, it’s at the Max level.

if you change the brake booster u do not have to flush the system, lol, it actually has nothing to do with the fluid besides pushing the master pistons in. then the master takes care of the fluid. if you hit the brakes like someone stated and the rpms drop your booster is bad, small vaccumm leak in the booster. if you want to check connect a vacumm gauge to the booster hit the brakes look for drop in vacumm, if u leave it alone eventually u will get a very stiff pedal… change the booster

Well it’s probably about time I have the fluid flushed anyway. It’s been almost 50k miles.

Well i will check out the brake booster then. Does anyoneknow the approx cost to getting a new one? Could if lead to a low idle that makes the car virbate when its warm?

Between $140-170 for a remanufactured one. About $350 for a brand-new one from Honda.

Ok thanks man. Damn sorta expensive, oh well.

my car shakes at idle, when i turn the accesories on, headlights, brake lights, ac blower, etc… what should i look at?

My car always runs shitty on cold winter mornings.But really good once it warms up a bit.Maybe you should raise your idle.