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Bad brake booster & poor idling..........are they related?


NWmonte71

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Hey everyone!

 

My 71 monte carlo has been my daily driver to work for the last 3 days. Previously it has been sitting in the garage all weekend. The first day of driving is ran great with no issues, on day 2 the engine's idle started becoming erractic and dropped rpms. Day 3 (Today) the car's idle is so low, it barely runs and lopes bad. You can bring the rpms up and it runs silky smooth. I also noticed over the last 2 days that the brakes would occasionally be harder to push when slowly idling around on streets. Finally, today I drove the car around in my driveway and the pedal has turned completely hard and no longer has a "powered" feel to it. I also noticed the last 3 days that a strange vibrating/humming noise is intermittently coming from within the engine compartment. 

 

What I would like to know is if the failing power brake booster is causing the erratic super low idle and hard brake pedal feel? The booster visually looks old and crusty. The main vacuum hose from the booster to the engine is brand new and not leaking. One thing I tried was to press the brake pedal with the engine off and then started it, the brake pedal only moved maybe an inch once the engine started. Didn't matter, running or not the brake pedal feel was rock hard (non-powered feeling). What do you guys think? Does all arrows point to the booster with the bad idle & sudden hard brake pedal?  

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Easy enough to test disconnect the vacuum line to the booster and cap it off and see if your idle returns to normal, also check all other areas for vacuum leaks if you still have an issue, I use starting fulid but WD40 is safer.

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Also if the engine isn't running well enough at idle then you won't have enough vacuum to make the booster work correctly. I would start by fixing your running issue first (off the top of my head I would check carburetor ). Usually if the booster diaphragm fails your idle will be higher from the vacuum leak. But you can verify that from the test suggested above  

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Tonight I went out and started the car and warmed the engine up to full operating temperature. The engine seemed to idle smooth in both Drive and Park. I then decided to block off the vacuum line to the booster and check for any differences in how it runs. After blocking off the vacuum line, the engine rpm stayed the same for Drive and Park, just like before the line was capped off. Welp, that busted the theory of a bad brake booster. Also at no time was the pedal hard (no powered feeling). Everything seemed to be working/running correctly. The only thing that happened was the car sitting overnight from the time it ran poorly to this evening of starting it up. One side note was that the fuel tank level was below 1/4 tank when the car started acting up real bad. I guess it could be possible that running the gas tank down that low caused me to start picking up condensation (water at the bottom of the tank) and small pieces of settled crud into the carburetor? I did fill up the tank to approximately 1/2 tank and let it sit over night. Perhaps the tiny pieces of crud settled back down to the bottom of the tank, along with any water? I'm trying hard to justify why one day is ran horrible and now it runs perfect. Any thoughts or theories?

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There are a couple of things that could be going on here. You could have had a piece of crud between your needle and seat in the carb that got flushed out eventually ( although I wouldn't discount the water theory that you had too. A can or two of heet wouldn't be a bad idea ). The other is that your fuel pump could be on it's way out and the extra fuel that you added is pushing the fuel out the tank easier and helping you with low fuel pressure. I would lean towards the crud. A way to verify this is to change your fuel filter and see what's inside. If your pump is weak you will experience the same running problem as the level goes down. Don't crutch this by keeping the level in the tank up, the pump is only going to get worse. I lean towards the crud and it would be a good idea to change out the filter in any case. Good luck, keep us posted 

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Sounds like you might have a vacuum leak and as the car is warm or cold the vacuum leak is coming and going. I would check all vacuum lines for any breaks and then spray a little carb cleaner around  the base of the carb to see if the idle changes. If it does, try snugging up the carb bolts and then test again with the carb cleaner. If the idle still fluctuates, replace the carb gasket and test again. 

 

You can also spray the carb cleaner around any of the vacuum lines to see if the idle changes to find a possible leak.

 

Other possibilities...check the PCV that it isn't getting stuck (should rattle when you shake it but is cheap enough to just replace) or it could have been bad gas, I'd add a can of dry gas. Also, when was the last time you replaced your fuel filter?

 

Steve 

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