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post #1 of (permalink) Old 01-15-2002, 10:53 AM
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Engine dies when brakes applied

I've got an 85 CJ-7 with the 258. Thanks to California's emission laws, engine is stock. It came equipped with power brakes.

When I'm driving and apply the brakes, my rpm's drop and the engine will actually stall if I don't do some fancy footwork and actually rev the engine as I'm slowing down. The problem seems to be worse if I brake suddenly as opposed to lightly braking and coasting to a stop. If I'm sitting at a stop and apply the brakes, the RPM's are unaffected.

I've come up with 2 theories? The change in momentum of the vehicle slowing down is somehow altering the gas flow into the carb???


I believe the brake booster somehow utilizes vacuum pressure from the engine. How it works, I don't know. Could there be a problem with the brake booster or vacuum system, that would cause a vacuum leak and therefore cause the engine RPM's to drop???

Anyone that's experienced the same thing or has any advice would be greatly appreciated.

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post #2 of (permalink) Old 01-15-2002, 11:22 AM
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Re: Engine dies when brakes applied

Both ideas are possible. Start with the brake booster. Does appply the brakes while the vehicle is idling affect idle quality? If there is a defect in teh booster (which runs off vacuum ) it will show up then.

If it'snot the booster, then you carb could have sediment in it or other blockage which needs cleaning out. Read up on the carter carb cleaning idle air screws, or better yet just get ambitious, buy a rebuild kit, tear it down and clean everything thoroughly..


84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster
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post #3 of (permalink) Old 01-15-2002, 11:55 AM
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Re: Engine dies when brakes applied

Ditto what Tim said, but since you stated that when you're stopped and apply the brakes the RPMs are unaffected, I would check the carb first. Probably a worn needle and seat or sediment. The rebuild kits are pretty inexpensive and the project isn't hard at all. Don't forget a new fuel filter, too.


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post #4 of (permalink) Old 01-15-2002, 12:08 PM
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Re: Engine dies when brakes applied

I'd suspect that it's not a vacuum problem... but a problem with the entact emissions stuff and BBD Carb.

When you lift your foot and start braking the Pruge Signal is sent via the comptuer and tells the engine to suck up the fumes building up in the gas tank and route them to the carb to be burned. Over time, and because of circumstances (Like off camber Jeeping, and over-filling the tank) the charcoal emissions canister gets full of gas. So now when you lift you foot and start to break the purge line is not sucking fumes and mixing it with the intake air. The carb's now sucking raw gas from the driver's side emissions canister and flooding the engine.

For a possible cure, and a better desciption see:

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post #5 of (permalink) Old 01-15-2002, 12:13 PM
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Re: Engine dies when brakes applied

Rather than spend the money on a $65 charcoal canister now, chekc your vacuum lines first. My buddy's Jeep had the same symptoms, it was a bad line.

85 CJ-7 258/T176/D300 Scout D44s F&R with Lock-Rites and 4.56 gears. 4" springs with lift shackles 36" SS TSL Radials Hella Rallye 4000s for guiding small planes into the woods
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post #6 of (permalink) Old 01-15-2002, 12:24 PM
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Re: Engine dies when brakes applied

Leve, who's page is that? that index has lots and lots of info. I think I've seen parts of that other places, but not sure. Never mind, I found it. thanks for the link

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post #7 of (permalink) Old 01-16-2002, 12:45 AM
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Re: Engine dies when brakes applied

Thanks guys. You've given me some great ideas. I got tired of always being told every problem was due to my vacuum lines, so I replaced all of them not too long ago, but it's always a possibility so I'll check them over. I never would have suspected the charcoal cannister. That's one of the few parts that hasn't been replaced or repaired in the 16+ years of the Jeeps existence so there's a good chance it's the culprit.

post #8 of (permalink) Old 01-16-2002, 04:40 PM
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Re: Engine dies when brakes applied

What about the carb float? The gas level in the carb bowl could be to high. I'm stock also and running "perfect". I have a new carb and new brake booster. If I apply the brakes the idle will change slightly. If I panic stop and lock up the brakes it sometimes will die. The nature of the beast? I dunno.

post #9 of (permalink) Old 01-16-2002, 07:40 PM
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Re: Engine dies when brakes applied

Just one other thought and is very unlikely since this sounds like it has been happening for a while. But on my old Commanche, when the alternator would start to die, applying the brakes was just enough draw to stall the engine. I doubt this is your problem, but I thought I would offer it anyway.

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