Vacuum Leak? Hesitation Upon Acceleration

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Jivermo
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Vacuum Leak? Hesitation Upon Acceleration

Post by Jivermo » Wed Feb 12, 2014 3:52 pm

My '78 bus, once again, has symptoms of a vacuum leak, upon acceleration. Inspecting the usual suspects offers no clues. Should I track the plastic line under the bus towards the booster? Is there any possibility of leakage along there disrupting the engine. When I do the rich/ lean pedal pumping test, I don't detect any fault with the brakes or apparent vacuum. Will valve cover leaks have that effect? I did the spray test, and nothing shows up that way. This damn problem keeps coming back to visit. I can drive hundreds of miles fine, and then one day...here we go. Colin, come on down after Thomasville! I got a new coffee machine.

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Amskeptic
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Re: Vacuum Leak? Hesitation Upon Acceleration

Post by Amskeptic » Wed Feb 12, 2014 8:45 pm

Jivermo wrote:My '78 bus, once again, has symptoms of a vacuum leak, upon acceleration.
Ahhh gwasshoppa, we do no to have "symptoms of a vacuum leak upon acceleration".
We have only "symptoms upon accelerating".
Do not describe your symptoms with diagnostic terms. Just tell us what the actual symptoms are.
Do you have:
low power?
mild bucking?
initial hesitation, smooth follow-through?
smooth initial acceleration followed by more protest further up?
lousy rear tires?

"When I do the rich/ lean pedal pumping test, I don't detect any fault with the brakes or apparent vacuum."
. . . because this test does not ask about faults with the the brakes or "apparent vacuum". This test only tells YOU if the engine is lean (rpms drop noticeably), just right (rpms drop almost imperceptably), or rich (idle goes up). So, is the engine lean, just right, or rich, as you tell us more about this acceleration symptom?
Colin
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 94,615 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,990 miles

Jivermo
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Re: Vacuum Leak? Hesitation Upon Acceleration

Post by Jivermo » Thu Feb 13, 2014 5:04 am

Ah, David Carradine once more appears. Brake pump test shows engine "just right". Engine starts right up, idles well. When I rev it, I get some light backfiring as it decelerates. On the road, I start out. Upon shifting into second, if I hit the gas, it will begin to forge ahead, but then it will die, as though starved for gas. The engine does not quit. If I ease it along, it will drive OK; only when a quick acceleration is given does it "starve".

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Amskeptic
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Re: Vacuum Leak? Hesitation Upon Acceleration

Post by Amskeptic » Thu Feb 13, 2014 9:57 am

Jivermo wrote:Ah, David Carradine once more appears. Brake pump test shows engine "just right". Engine starts right up, idles well. When I rev it, I get some light backfiring as it decelerates. On the road, I start out. Upon shifting into second, if I hit the gas, it will begin to forge ahead, but then it will die, as though starved for gas. The engine does not quit. If I ease it along, it will drive OK; only when a quick acceleration is given does it "starve".
Thank-you. Now then, can you distinguish between "starve" and "bog"?
Colin
starve - lean
bog - rich

experiment - move black cog 6 teeth COUNTER-clockwise does "starve" diminish, or does "bog" worsen?
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 94,615 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,990 miles

Jivermo
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Re: Vacuum Leak? Hesitation Upon Acceleration

Post by Jivermo » Sat Mar 01, 2014 10:44 am

Finally had some time to get back to this irritating deal. I had my son drive around with the timing light hooked up to the center coil wire, and we have an almost constant light stream, even when the motor hesitates. It can be described as initial smooth acceleration, then, almost when the revs reach a gear change rpm, the engine loses most power. Then it will surge forward again. I just rechecked all grounds again. I have a spare fuel pump I'm going to throw on and see what that does. This is fun.

Jivermo
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Re: Vacuum Leak? Hesitation Upon Acceleration

Post by Jivermo » Sat Mar 01, 2014 2:20 pm

Spare fuel pump installed, new filter, and away we go. End of problem...for the moment.

And...10 more miles and it's running great. I cut open the filter, and it was clean; only about 500 miles on it. Since all the contacts were tight, I think the fuel pump may have been giving up the ghost.

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Amskeptic
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Re: Vacuum Leak? Hesitation Upon Acceleration

Post by Amskeptic » Sun Mar 02, 2014 7:07 pm

Jivermo wrote:Spare fuel pump installed, new filter, and away we go. End of problem...for the moment.

And...10 more miles and it's running great. I cut open the filter, and it was clean; only about 500 miles on it. Since all the contacts were tight, I think the fuel pump may have been giving up the ghost.
Good for you!

So, the hesitation was most apparent at higher rpms under load? In retrospect, does it seem like the symptoms actually reflected what we'd expect of a failing fuel pump?
Colin
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 94,615 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,990 miles

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