Dies at Idle

[Dead Parrot]

One possible vacuum leak is the power brake booster if you have power brakes. I had a booster on my C10 that leaked enough to cause idle issues but still worked well enough the problem wasn't obvious. Another often missed place is the small bit of hose on the modulator on automatic transmissions.

If you have an old style vacuum gauge, they often included a fuel pressure gauge as well.

[Classics Fan]

Originally Posted by Dead Parrot

One possible vacuum leak is the power brake booster if you have power brakes. I had a booster on my C10 that leaked enough to cause idle issues but still worked well enough the problem wasn't obvious. Another often missed place is the small bit of hose on the modulator on automatic transmissions.

If you have an old style vacuum gauge, they often included a fuel pressure gauge as well.

I checked the power brake booster hose assembly but totally missed the transmission modulator. Good tip. My vacuum gauge doubles as a pressure gauge, I will see if I can make that work. Thanks!
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[Dead Parrot]

Originally Posted by Classics Fan

I checked the power brake booster hose assembly but totally missed the transmission modulator. Good tip. My vacuum gauge doubles as a pressure gauge, I will see if I can make that work. Thanks!
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Its not just the booster hose. The booster proper can develop an internal leak bad enough to cause idle issues. At the same time, it can still work well enough that you assume it is good. Quick test: With the motor running, push the brakes a couple of times to get the feel. Turn motor off. Leave for a couple of minutes. Then with the motor still off, press and release the brakes just like you did with the motor running. The feel should be the same and you should hear a hiss from under the dash. A harder feel and/or no hiss indicate a possible booster problem. A 2nd test but more work is pull the booster hose and cap the carb/manifold fitting. If your idle gets better, probably a leaky booster.

But with your initial symptom list, a fuel flow problem was one of the things I thought of first.

[Classics Fan]

Originally Posted by Dead Parrot

Its not just the booster hose. The booster proper can develop an internal leak bad enough to cause idle issues. At the same time, it can still work well enough that you assume it is good. Quick test: With the motor running, push the brakes a couple of times to get the feel. Turn motor off. Leave for a couple of minutes. Then with the motor still off, press and release the brakes just like you did with the motor running. The feel should be the same and you should hear a hiss from under the dash. A harder feel and/or no hiss indicate a possible booster problem. A 2nd test but more work is pull the booster hose and cap the carb/manifold fitting. If your idle gets better, probably a leaky booster.

But with your initial symptom list, a fuel flow problem was one of the things I thought of first.

That’s what I like about forums like these: I always learn something new. The challenge is remembering what I learned later when I need it again. Sometimes for me it’s use it or lose it especially as “maturity” creeps up. I’ll check to booster can to eliminate that possibility. Thanks for the education.
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