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GMC: 2000 gmc savanna 3500 5.7 Backfires and almost stalls

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2000 gmc savanna 3500 5.7 Backfires...
2000 gmc savanna 3500 5.7
Backfires and almost stalls when in reverse or drive with little or no throttle input.
New fuel pump
New filter
New cap and rotor
Replaced wires
Had exhaust checked
Not throwing a code anymore
Did throw a p0174 a few days ago but not for days
Have a live data scanner showing LT FLTRM2 at or near 25
Checked the one vacuum line I could find and vacuum was fine.
Fuel pressure about 62 burst holds at about 55 no drop when I turn the key off.
Cleaned MAF AND THROTTLE BODY.
Ran without MAF same problem
Also took it to a shop who said they tested it and couldn't figure it out. They also gave it back with the MAF installed backwards, we did it by accident but they never checked it. When I got it back I noticed it when I pulled the air box off.
Not sure what to do next.
Submitted: 2 years ago.Category: GMC
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Answered in 40 minutes by:
6/7/2015
GMC Mechanic: GM Bob, ASE Master Technician replied 2 years ago
GM Bob
GM Bob, ASE Master Technician
Category: GMC
Satisfied Customers: 1,152
Experience: 30 plus years experience as GM technician, ASE Master Technician and GM Certified
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Hello my name is ***** ***** I can help.
Can you do this test?
Start the engine.
2. Allow the engine to idle at normal operating temperature.
3. Open the throttle quickly.
4. Monitor the fuel pressure. The fuel pressure should approach the top of the specified range.
Does the fuel pressure rise quickly? 60-66 psi
Let me know
Thank you
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Customer reply replied 2 years ago
Fuel pressure is fine and I upgraded the spider injectors.
Customer reply replied 2 years ago
Now it has very high idle almost 1500.
Still doing the same thing in reverse
Customer reply replied 2 years ago
No codes
Loss of power under load, and backfires when used a load (R or D) with no throttle input.
At a red light or backing up.
In park or neutral it sounds good, other than the high idle right now, and revs up without hesitation.
Customer reply replied 2 years ago
Any chance it is a transmission issue?
Customer reply replied 2 years ago
Just replaced it 3 weeks ago
GMC Mechanic: GM Bob, ASE Master Technician replied 2 years ago
The trans will not cause the engine to backfire. No power yes.
Have you done anything with mass air flow sensor?
It runs ok down the road?
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Customer reply replied 2 years ago
MAF unhooked and nothing changed.
It is really struggling now just to get up to speed. Especially going up any kind of hill.
Customer reply replied 2 years ago
Idle is at 1400-1500
Customer reply replied 2 years ago
When it is struggling up a small incline I can hear a metal tap noise near the passenger floor board, maybe under the seat.
Raspy and random but fairly often while inclining. Once I get up the hill in pulls better and no more noise. I say better still not good.
Customer reply replied 2 years ago
I really need to address the scanner reading 25 LT FLTRM2
I have triple checks everything including with the meter and can not find a leak anywhere.
What else could cause that reading.
All O2 sensors passed
GMC Mechanic: GM Bob, ASE Master Technician replied 2 years ago
Have you replaced bank 2 o2 sensor or swapped with the other side see if the lean moves?
P0174 is bank 2 running lean
What is bank 1 fuel trim reading?
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Customer reply replied 2 years ago
I did test the O2 sensors with carb cleaner in the intake.
Went up to .8 when I spray then back down when I stop.
Accurate every time.
Customer reply replied 2 years ago
It only threw that code once and that was Thursday I think.
No codes
Customer reply replied 2 years ago
I am really leaning towards exhaust
Customer reply replied 2 years ago
Was going to take it to the muffler shop and get a test pipe put on.
Customer reply replied 2 years ago
Back to the high idle real quick.
With everything cleaned and with upgrading to the MPI spider injectors, should I hit a dealer and get the computer reprogramed?
Customer reply replied 2 years ago
Also other than the high idle she revs up quick and sounds strong in park.
GMC Mechanic: GM Bob, ASE Master Technician replied 2 years ago
I think you have a vacuum leak causing lean. Would cause high idle.
If you pull the PCV valve out and the breather tube out. With engine running plug both holes in valve cover. One with your hand. It should build pressure. If you have vacuum than you have a intake leaking internally
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Customer reply replied 2 years ago
Checked the exhaust by the plugs with an IR heat gun.
The middle exhaust on bank 2 is reading low under 200. All others are around 3 up to 450ish.
Customer reply replied 2 years ago
Muffler shop also noticed a strong smell of un burnt fuel.
It was so strong while on the lift your eyes almost water.
Customer reply replied 2 years ago
I am going to pull the plugs and see what I find.
Customer reply replied 2 years ago
BTW both cats are empty and the muffler has all the pieces in the bottom.
Can't replace until I fix the engine issue. But it is not what is causing the fuel smell.
GMC Mechanic: GM Bob, ASE Master Technician replied 2 years ago
I would do a compression test. When the plugs are out
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Customer reply replied 2 years ago
I have rechecked and the exhaust closest to the front on the passenger side is between 150-180ish degrees.Much lower than all the others.This must be the cylinder that is not firing.
GMC Mechanic: GM Bob, ASE Master Technician replied 2 years ago
Check make sure you have good spark on the wire.
Need to pull the spark plug. check the plug and compression test
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GMC Mechanic: GM Bob, ASE Master Technician replied 2 years ago
Also make sure you wires are on right. Correct firing order.
I have seen new distributor caps go bad. Causing arcing to another cylinder. When you check the spark compare with the other cylinders
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GM Bob
GM Bob
GM Bob, ASE Master Technician
Category: GMC
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