Brake Booster/Idle ?-Searched
 

Hey all,

I have been having idle issues for a while and noticed somthing interesting. When the car is idling, if I pump the brakes, the rpms will increase 300-400 rpms. Is this normal?

Also, the idle trouble I was having was the car would be hard to start and would not hold an idle below 1800 (it used to idle at 1400). And sometimes the car would be idling and all of a sudden just stop and die for no reason.

Well, when it started to die today, I pumped the brakes and the rpms went up almost 500 rpms to recover the idle

what do you guys think?

Bad brake booster valve?

Thanks!

Ryan

bump, feedback would be appreciated!

Sounds to me like you have a massive vacuum leak somewhere. A vacuum leak will cause a high idle, and when you use the brake then the booster will take even more vacuum away which will increase the idle even more (I think).

You are probably missing something obvious right under your nose, or at least under the intake manifold or carb, and you'll kick yourself when you find it.

Start from scratch, and look over the entire engine like you would if someone you didn't know had just finished major work on it and you're not sure they knew what they were doing. Maybe pretend you suspect that the Mafia planted an explosive device? Anyway, you need to look through a fresh set of eyes I think.

Good luck man! :)

LOL! Yes I agree with you. The odd thing is, all this happened just one day when I started the car up. Everything was GREAT, until one day, I started the car and it almost sounded like it was firing off of only 3 spark plugs or that the plugs were fowled. Changed them , and same result. Didn't do much.

I ordered new oem spark splug wires, so we'll see what that does (haven't changed them since I bought the car 2 years ago).

IDK, I will keep you updated though!

Thanks!

Ryan

Ryan,

Try this: When the car is running, pinch the booster vacuum hose and see if it smoothes out. The brake booster can be a major source for a vacuum leak. Let me know what you find.

Mike

Hmmm...that's a good idea. You mean the hose coming off of the intake manifold going to the brake booster, right?

Ryan

Yeah, that's what he means. Sounds like a good idea too.

I had similiar but opposite issues with a booster. Heres my thread about it:

https://www.rx7club.com/1st-generation-specific-1979-1985-18/bad-brake-booster-679357/

This port reminded me to update my old thread, thanks.

Patience grasshopper. :-)

The idle will go up if you pump the brakes.
If your brake booster was leaking or the check valve was bad then your brake pedal would be hard. Therefore noticeing a loss of the power assist when applying the brakes.
To check the operation of the check valve. pull the hose off at the booster. Blow into the hose, air should be allowed to go back to the manifold. Then suck on the hose, there should be no air allowed to come to you.

ok, where is the check valve located then, on the fitting that screws into the manifold??

But, thank you! I will try both of these tests and see what I come up with.

Ryan

Ryan,

Check valve is located in the hose where it attaches to the booster.

Ok guys, so here's what I found:

I took off the hose going to the brake booster and checked the valve. When I blew through the hose, I was able to push air pass the vavle into the manifold, but is was very hard to, I had to blow pretty damn hard. Also, the air that came out from me blowing, it was making a weried noise (that I think I have been hearing when I step on the brakes) and it was not 'even', as if I were blowing through the hose without the check valve in the hose (almost sounded like bubbles coming out of the valve).

Also, when I tried to suck air in from the manifold, I could not, so that worked.

What do you guys think?

Ryan

Sounds like the check valve checks out. Look elsewhere for your vacuum leak...

Ok,
So today I took off the hose coming off of the intake manifold that connects to the hard pipe going to the brake booster. I plugged off the hose so the manifold was not pulling any air from that hose.

The first 2-4 minutes the car idled perfectly (with the hose plugged off). It seemed like as soon as the car started to warm up, the idle started to bog and fall. So I slightly removed my finger just a little, allowing for the motor to pull in some air on the hose going into the intake manifold and the idle began to climb to its original idle. If I kept my finger on the hose, plugging it off, the car would die.

My question is this: should the car idle fine with the hose going into the intake manifold plugged off? As in, should the car run fine with no power brakes. Or is it setup to be able to pull air from the brake booster. I am just wondering if my check valve it bad or not.

Any input?

Thanks!

Ryan

Assuming you have a 12A with emissions components still intact. Check the bottom of the hose (red line below) that runs from the #2 anti-afterburn valve down to the rear of the intake manifold. The bottom can melt from the heat of the exhaust just below it. The hole can seal when cold and open up when it warms up.

http://intertron.com/ron/images/carb/c6.jpg

i second that idea. i've been there enough times. 9 times out of 10, you'll start to question your own existence before you find it, but once it gets you once or twice, you'll check it first when stuff like this happens.

Thanks for the reply guys. I actually have no emissions and have a Racing Beat intake manifold.....sooooo all of this won't help. I should have mentioned that early.

But again, thanks for your input and time!!

Ryan

It should stumble and die if you take your finger away and run fine with you finger on it. Weird!

Really???!?!? Hmmm...well when I took my finger away, the engine reved up, which makes sense b/c with my finger not plugging the hose, the intake manifold essentialy has another small input or way of sucking air in other than the carb, making the a/f ratio leaner...

Anyone else?? Pleaseee!! :)

Thanks!

Ryan

Also, where can I buy a new check valve for the brake booster??

Ryan

It sounds like you idle mixture might be too rich. When the engine warms up it requires less fuel which would explain why it begins to stall and why it runs better with the vacuum leak.

I had thought this at first, but when the idle drops, it is not consistent. As in the car could be idling fine, even when it is warmed up, and then for noooo reason, I think when I get on the brakes (or turn to the right?!?!) the idle just dies, not matter how high I have it.

I'll check the float bowl levels, but they should be fine. I don't think it is anything fuel related, but I'll check.

For the check valve, just shoot it up with some PB Blaster or something similar and clean it out. No need for a new one, this has always worked for me...

Ok, thanks! I will def. try that and see what happens. B/c it should be easy to blow past/into the check valve, right? B/c when I blew into the hose past the check valve, It was really hard! I had to blow as hard as I could to get air past it, into the intake mani.

Yeah, some times they get gummed up...