1993 240 Sedan.

When the engine is cold, having sat overnight, and then being driven about 1 mile, the engine idle is inconsistent when brakes are applied, once the car stops.

(vrrrrm..(almost stall and engine shakes)..vrmmmm..(low idle)..vrrmmm..vrrmm).

It seems to be directly related to how hard I push on the brakes. If I throw it into park quickly at a stop, and release the brake, idle evens out quickly. Otherwise it will sometimes even itself out after 6-7 rev..hesitate...rev cycles.

As the car warms (time, not necessarily temp guage to the "dot") the idle with brakes evens out, there is still a very faint "vrrmm...vrrmm" but it seems normal.

Finally, when the car is warm, if you stand on the brake hard the car idle goes back to inconsistant as long as heavy pressure is applied. There is also a "hissing" noise directly behind the dash, which is the master cylinder I know. If you lighten the pressure, but do not release, the hissing stops and the idle "flattens".

So, experts and gurus, what do ya think? I don't know brake systems...yet. Or even if the two are related. All ideas welcome!

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Pete

clearly a vacuum leak to or from the brake booster. Check the hoses and replace the valve.

Big metal shere in the engine bay on the firewall right in front of the driver and with the fragile white check valve that might be your problem.

So let me try and understand.

The brake system needs vacuum to operate. It uses vacuum to assist the hydraulic mechanism, I'm guessing, push fluid around to multiply my foot pressure into needed break pad pressure, and then to "push" the pedal back to me (or is that a spring inside the master cylinder).

The reason my engine chunks away when I add pressure is that since the system has a leak somewhere (which is slow and why it takes all night to go ka-ka-koo-koo) there is not enough vacuum to go around. My FI system needs vacuum to operate too, so much as a leak around a spark plug would cause idle problems, so too does an overall lack of vacuum. Every cylinder is effected, engine is starved of something, it chugs...then recovers beacuse something somewhere notices the engine is hungry and feeds it fuel.

As the engine "warms" the system "re-charges" itself enough to overcome the loss from that slow leak.

??? All speculation in my head. Is this even close? If not, can someone give a good explanation of the system. Hey, I'm trying to "learn how to fish" here.

As for that check-valve...can it be swapped out without bleeding the whole "bloody" system.

Thanks,

Where would they be? Easy to locate and swap out?

I'm still a relative newbie, but I know my way around somewhat. Only had her for three weeks and I wasn't a mechanic before then.

:)

Thanks!

I'll third the brake booster vacuum leak theory. On the firewall there's a big round metal part which the master cylinder is attached to. Look for a large vacuum hose going into a white plastic cap type looking thing, this is the check valve. They tend to become brittle with age and when they go it's not pretty. This could very well be your problem. Very easy to replace. Just pop it out, it's probably easiest to cut the hose right behind connection and then pop the new one in if old one is indeed damaged. When mine went the car ran horribly and I had to literally stand on the brake to stop.

There could be a leak in the actual booster but this is rare and usually a gradual thing. You'd probably hear a hissing sound coming from firewall.

Good luck!

Kswan

Well, as to the hissing...

I do hear about 4-5 seconds of hissing when I turn the car off. And when applying very hard brake pressure with the car running. Very faint, but its there.

I hopes it's that valve though.

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Pete