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Heres a new one, brakes make my 3.0 idle weird

Old 04-14-2008, 09:44 AM
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Heres a new one, brakes make my 3.0 idle weird

Some people are probably gonna think i'm nuts, but i had to pinch myself too, once i realized what was happening.

so once my 3.0 warms up to opp. temp, the idle will stay constant, UNTIL, i apply the brakes, then the idle drops off about 400-500 RPM and surges up and down at a consistant rate. take my foot off the brake, and it idles fine.

the only thing i can think is that the brake MC is conected with some vacuum lines that are up my idle, but it don't think th MC has any vacuum lines to it.

any help would be greatly appreciated, this really boggles my mind. thanks
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Old 04-14-2008, 10:31 AM
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Not the MC, but the booster it's bolted to had a LARGE vacuum hose running to it straight from the intake manifold; check it for leaks. If the hose (and the check-valve on it) is fine and there's no leaks, the booster itself may be shot.
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Old 04-14-2008, 11:57 AM
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Definitely sounds like a leaking vacuum booster
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Old 04-14-2008, 12:06 PM
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It's probably the booster. However it would be worth it to check your idle. My idle was set too high on my 22re and it was idling fine, then idling up and down when I pressed the brake.
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Old 04-14-2008, 12:13 PM
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Spray the vacuum line going from the brake booster with starting fluid or carb cleaner and see if the idle surges up. That will tell you if the hose itself is leaking, but since it only happens when you apply the brakes I bet there's a tear in the booster diaphragm.
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Old 04-14-2008, 03:24 PM
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Originally Posted by fishon1213 View Post
It's probably the booster. However it would be worth it to check your idle. My idle was set too high on my 22re and it was idling fine, then idling up and down when I pressed the brake.
Had this same problem. Of course I didn't realize it until I'd checked the vac lines, booster, valve, AFM, TPS, injectors, cap, rotor, wires, plugs, O2 sensor and finally replaced the intake gasket to make it idle smooth. Then we found out it was idling too high to compensate for that busted gasket.


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Old 04-15-2008, 12:05 PM
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Originally Posted by CC_yota View Post
Spray the vacuum line going from the brake booster with starting fluid or carb cleaner and see if the idle surges up. That will tell you if the hose itself is leaking, but since it only happens when you apply the brakes I bet there's a tear in the booster diaphragm.
i'll check it out, but since i don't know jack about brake boosters, i'm kinda curious what cause the diaphragm to to tear
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Old 04-15-2008, 12:51 PM
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they are thin and become brittle and brake down like anything else. If sometime before you owned it someone could have hooked up a fuel line and got gas in there but that is kinda unlikely. They so occasionally go bad.
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Old 01-24-2019, 02:08 PM
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Sigh.

No resolution to this?
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Old 01-24-2019, 02:34 PM
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Originally Posted by JToyota4x4 View Post
No resolution to this?
After 11 years they all probably moved on, if you have the same problem try either starting fluid or a propane bottle to squirt around under the hood looking for vacuum leaks. if you don't find any it is probably the brake booster, harder than usual braking would also back up that diagnosis.
also post your own question as a new thread and include your make/model/year
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