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GM Bob
GM Bob, ASE Master Technician
Category: GMC
Satisfied Customers: 963
Experience:  30 plus years experience as GM technician, ASE Master Technician and GM Certified
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GMC: 2000 gmc savanna 3500 5.7 Backfires and almost stalls

Customer Question

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: 1 year ago.
Category: GMC
Expert:  GM Bob replied 1 year ago.
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
Customer: replied 1 year ago.
Fuel pressure is fine and I upgraded the spider injectors.
Customer: replied 1 year ago.
Now it has very high idle almost 1500.
Still doing the same thing in reverse
Customer: replied 1 year 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: replied 1 year ago.
Any chance it is a transmission issue?
Customer: replied 1 year ago.
Just replaced it 3 weeks ago
Expert:  GM Bob replied 1 year 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?
Customer: replied 1 year 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: replied 1 year ago.
Idle is at 1400-1500
Customer: replied 1 year 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: replied 1 year 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
Expert:  GM Bob replied 1 year 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?
Customer: replied 1 year 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: replied 1 year ago.
It only threw that code once and that was Thursday I think.
No codes
Customer: replied 1 year ago.
I am really leaning towards exhaust
Customer: replied 1 year ago.
Was going to take it to the muffler shop and get a test pipe put on.
Customer: replied 1 year 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: replied 1 year ago.
Also other than the high idle she revs up quick and sounds strong in park.
Expert:  GM Bob replied 1 year 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
Customer: replied 1 year 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: replied 1 year 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: replied 1 year ago.
I am going to pull the plugs and see what I find.
Customer: replied 1 year 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.
Expert:  GM Bob replied 1 year ago.
I would do a compression test. When the plugs are out
Customer: replied 1 year 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.
Expert:  GM Bob replied 1 year ago.
Check make sure you have good spark on the wire.
Need to pull the spark plug. check the plug and compression test
Expert:  GM Bob replied 1 year 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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