car shakes then stalls when holding down brake pedal in drive. (88 Gran Fury 2bbl)

[SonOfaTomP]

This happened to me last Saturday. took the car out to the weekly cars and coffee and noticed an issue. first the car stalled when turning onto I-95, ok no biggie. then i noticed the gas mileage got terrible. upon hitting surface streets, i noticed that when sitting at a red light and depressing the brake pedal, the car would begin shaking violently. it has done this before but never to this degree. was previously an intermittent slight shake. this shake was most noticeable when starting the car while the parking brake was active, would go away when it is released but you would sometimes notice it when you had to brake hard for a running out yellow light.

now it will shake extremely violently when the brake peddle is depressed and will lead to a stall. symptoms clear as soon as you hit the gas and the car will coast down the road like nothing is wrong, minus the new shit gas mileage

NO, the shake has nothing to do with rotors or anything. it will not do this when depressing the brake while the car is in park. only when the car is in drive and will do this when stationary. i tried to diagnose a vacuum leak on the side of the road and couldn't find anything out of the ordinary. can of starter fluid wasn't finding anything either. Facebook said it is likely the Brake Booster diaphragm

 

[Mikes5thAve]

If it was because of the booster it should also have an effect when its in park and you said it doesn't.
Stopped in gear it's idling lower then in park so with the other issues you've been having it could be there's still something not adjusted right that's showing up at that low idle.

You can disconnect and plug the vacuum port going to the power brakes to see if that makes a difference. If you had a vacuum pump you could also use that to see if the booster holds vacuum or not.

 

[Camtron]

What’s your timing at, have you put a vacuum gauge on it to see what it’s pulling? Think it should be 18-21” of vacuum depending on timing and how healthy the engine is.

 

[toolmanmike]

Power brake booster has a leak causing a big vacuum leak.

 

[SonOfaTomP]

If it was because of the booster it should also have an effect when its in park and you said it doesn't.
Stopped in gear it's idling lower then in park so with the other issues you've been having it could be there's still something not adjusted right that's showing up at that low idle.

You can disconnect and plug the vacuum port going to the power brakes to see if that makes a difference. If you had a vacuum pump you could also use that to see if the booster holds vacuum or not.

what i'm not understanding is, with all the issues i have, they always appear out of the blue after the car ran perfect the day before. perhaps my carb has a loose screw that's slowly moving and "screwing" things up? its a cheap Chinese BBD clone so it'd make sense. i have gotten it back from the mechanic before and the idle screw was in all the way.

What’s your timing at, have you put a vacuum gauge on it to see what it’s pulling? Think it should be 18-21” of vacuum depending on timing and how healthy the engine is.

i don't know much about timing so i couldn't tell you what it's at, all i can say is, it ran fine the day prior minus the intermittent minor shake. I'll go get a vacuum gauge. where do you test it? port or manifold? i appreciate the benchmark 18-21.

could this possibly be the vapor canister going up again? i've had it shoot crap into the carb before. can it be safely plugged to see what it'll do? i hear some people put a ball bearing in it's line to prevent this.

 

[AMC Diplomat]

Power brake booster has a leak causing a big vacuum leak.

We've told him this on Facebook and apparently he still doesn't believe us

 

[Camtron]

Oh, oh yea. I remember your post from FB.
You’ll want to check manifold vacuum for the diaphragm leak. If you get a digital timing light, vacuum gauge, and learn how to use them, you’ll be well on your way to solving your issues. Set timing, idle speed, vacuum, see if you have a giant vacuum leak, and go from there.

 

[Mikes5thAve]

Start by disconnecting and plugging the port for the brake booster like I said earlier and see what happens.

 

[SonOfaTomP]

appreciate all the tips. its not that i don't believe y'all, just want multiple opinions before i try anything. a mechanic is trying to tell me its a leak from the throttle shaft and i don't believe him.... hell it's probably both...

 

[AMC Diplomat]

appreciate all the tips. its not that i don't believe y'all, just want multiple opinions before i try anything. a mechanic is trying to tell me its a leak from the throttle shaft and i don't believe him.... hell it's probably both...

If someone actually took the time to diagnose it and that's what he came up with... a cheap Amazon carburetor swap is 10x easier than a brake booster

 

[69-]

Carb swap from A? You might be lucky with reman carbs. Well, might. I was not.

 

[Mikes5thAve]

Spray the throttle shaft with starting fluid or something else flammable and see what happens.
If you're still using the Chinese carb you could try changing it. You get what you pay for...
There are easy ways to diagnose these different things so you need to at least follow through doing that otherwise asking opinions about whats wrong is pointless. You need to start ruling things out.

 

[SonOfaTomP]

yep it revs up around there when sprayed with some berryman. also around the pull off valve too. it was the $70 Chinese BBD so you're right. i wouldn't be surprised if it just fell apart, but heck it lasted 4 years so that's kind of a win? well the 6280 lasted 25..... i'll track down a real BBD. shouldn't be hard to find since they're popular with Jeep guys. getting a booster too.

 

[MoparDan]

yep it revs up around there when sprayed with some berryman. also around the pull off valve too. it was the $70 Chinese BBD so you're right. i wouldn't be surprised if it just fell apart, but heck it lasted 4 years so that's kind of a win? well the 6280 lasted 25..... i'll track down a real BBD. shouldn't be hard to find since they're popular with Jeep guys. getting a booster too.

I put remanufactured (probably Chinese) carburetors on both Fifth Avenue and Diplomat, neither ever ran right again, I had the carb on the Diplomat gone through, they found the jets were wrong (and were different sizes!) it had also been machined incorrectly, they did what they could but would still stall in gear

 

[LSM360]

Whenever I had vacuum leaks at booster I could feel it in brakes....hard pedal as if no power brakes.

 

[SonOfaTomP]

Whenever I had vacuum leaks at booster I could feel it in brakes....hard pedal as if no power brakes.

my brakes were always kind of squishy when in drive, but super stiff when the car was off. only heard a hisss sometimes though this might have been only when pressing the brake in park or when the car is off. i think that's normal. always kept a long follow distance from any car as the old girl would need like 3 car lengths to stop. i drove a friends Gran Fury and that thing stops on a dime. i think his brakes were locking up though.

anyways we're swapping out the carb now. old ebay BBD had quite the leak. should have known, carb always hissed but i thought that was normal and it ran fine for the most part. until now...

 

[Camtron]

my brakes were always kind of squishy when in drive, but super stiff when the car was off. only heard a hisss sometimes though this might have been only when pressing the brake in park or when the car is off. i think that's normal. always kept a long follow distance from any car as the old girl would need like 3 car lengths to stop. i drove a friends Gran Fury and that thing stops on a dime. i think his brakes were locking up though.

anyways we're swapping out the carb now. old ebay BBD had quite the leak. should have known, carb always hissed but i thought that was normal and it ran fine for the most part. until now...

Should have one good/soft supported brake after the engine is shut off. If you turn the car off and the pedal is immediately stiff, you gotta a booster leak.car shaking and trembling while you’re braking at a red light, you gotta booster leak. Glad you’re getting it all sorted.

 

[SonOfaTomP]

it's all been sorted out now. thanks for all the advice