Apply brakes to shift and truck dies

Tiny
TONKA TOMMY
  • MEMBER
  • 2001 GMC SONOMA
  • 2.2L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 125,000 MILES
Changed fuel pump, fuel filter drove fine for two days then I flushed radiator the 2nd night drove for 2 hours and parked it overnight. Started truck next morning ran rough stepped on brake to put it in drive and it dies out. It starts fine put pedal to the floor and it cuts out and decelerates back to idle. Check engine light on with code p0171b fuel mix too lean.I can't locate any vacuum leak. O2 sensor 1 has 0.9 volts and #2 has 0.1 at idle. Manifold vacuum at idle is 4.5. Schroeder valve pressure when I turn key on is 55psi and drops to 30psi when at idle speed. My dads been a mechanic all his life and he thinks it might be brake booster problem Any help appreciated
Wednesday, November 5th, 2014 AT 11:16 PM

10 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,176 POSTS
The intake vacuum is very low. You must have a vacuum leak at some point. You mentioned that it was at 4.5. It should be around 17.
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Thursday, November 6th, 2014 AT 8:49 AM
Tiny
TONKA TOMMY
  • MEMBER
  • 6 POSTS
I think I found the cause. There is sufficient vacuum from manifold to brake booster but when brake pedal is applied, the pedal hisses abnormally loud which makes me think its the booster diaphram not holding vacuum properly. Never heard that noise before this problem. Pedal sinks to floor fast when I hold my foot and start engine. It would explain vacuum loss in manifold. Does that theory make sense?
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Friday, November 7th, 2014 AT 2:00 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,176 POSTS
Yes, that makes sense. Disconnect the brake booster and block the vacuum supply to the intake and then see if it stalls. If it doesn't you found the problem.
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Friday, November 7th, 2014 AT 6:09 AM
Tiny
TONKA TOMMY
  • MEMBER
  • 6 POSTS
It still dies when I take off manifold to booster vacuum hose and apply the brake. I have no idea what could be the problem
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Saturday, November 8th, 2014 AT 4:06 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,176 POSTS
Did you plug the vacuum supply from the engine? Have you checked for other vacuum leaks?
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Saturday, November 8th, 2014 AT 6:50 PM
Tiny
TONKA TOMMY
  • MEMBER
  • 6 POSTS
Plugged vacuum coming from engine to booster no change still dies when brake pedal pushed. Could it be transmission related?
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Sunday, November 9th, 2014 AT 10:40 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,176 POSTS
Just pushing the brake shouldn't kill the engine. If the torque converter clutch (TCC) isn't releasing, that could kill it, but the check engine light should be on indicating a problem. However, your engine dies when you hit the brakes. I have to be honest, that doesn't make sense.
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Monday, November 10th, 2014 AT 1:39 PM
Tiny
TONKA TOMMY
  • MEMBER
  • 6 POSTS
I appreciate all the help kind sir but I think I messed my truck up worse. I finally got it back to normal kinda by unhooking the battery, the computer under the hood, and taking fuel filter off and back on and letting system reset. Ran normal for 5 min then started idling real rough so finally I unhooked everything again, pulled 2 spark plugs and re gapped them, cleamed up the two pos. Cables with a mega fuse runnin between them under battery. Put it all back and go to start it and I hear just a click I think comin from relay under hood where fuses are. And now its like 10 degrees out so I have to fix it while freezing.
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Monday, November 10th, 2014 AT 6:22 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,176 POSTS
Have you checked to make sure the battery is properly reconnected? Could the clicking be coming from the starter?
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Tuesday, November 11th, 2014 AT 7:34 PM
Tiny
TONKA TOMMY
  • MEMBER
  • 6 POSTS
Got it runnin again. It was loose wire to starter. Still dies when I shift or put on brakes. Weird thing is that I can start it in neutral and now reverse lights stay on all the time now. Engine still idles rough and o2 sensor 1 voltage is never steady, its all over the place but sensor 2 is always at 0.4 to 0.6 always pretty steady. Ran new vaccuum lines but there's no vacuum from engine to charcoal canister. Don't know if their should be tho. Thank you for your help
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Sunday, November 16th, 2014 AT 2:30 PM

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