1999 Silverado No Power

William Mechanic Tennessee Posted   Latest  
Resolved
Driveability
1999 Chevrolet Silverado 1500 5.3L (T LM7) 4-spd (4L60-E)
Lack Of Power At Wot

Truck is a 99 silverado with the 5.3, 160k miles. Truck has a problem where if you floor it it just bogs down and backfires a little. ½ to ¾ throttle it runs great.

Both o2s on both banks show lean (50-150 mv) under wide open throttle. At idle or normal driving half throttle o2s go up/down as normal

Thought it was fuel pressure, tested it and showed around 40 at idle. replaced pump/filter, still same issue. 

checked fuel injector balance test, all injectors flowed around 17-19 psi. Now fuel pressure is at 60 psi, 55 when running.

Why are both o2s going lean at wot?

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Jason Owner/Technician
California
Jason
 

Restricted exhaust?

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William Mechanic
Tennessee
William
 

Truck is catless, both cats have been replaced with straight pipes but good thought, I was sort of thinking a plugged cat too.

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Glenn Owner/Technician
Texas
Glenn
 

Hello William, You might need to confirm the timing chain has not jumped a tooth. Place cylinder #1 on TDC at the compression stroke. Remove the cap and carefully check the rotor position. The flat cap can fool you so make sure of where #1 plug wire lines up inside the cap traces. The other issue that is very common with the 5.3 engines is the intake leaking. While the engine is running try…

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William Mechanic
Tennessee
William
 

Hi, this engine doesnt use a cap/rotor ignitions system. It has ignition coils, maybe you are thinking of the 5.7 which this truck did have an option for.

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Glenn Owner/Technician
Texas
Glenn
 

Hi William, Thanks for clarifying that. There were a lot of transition going on back then.

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Rusty Owner/Technician
Massachusetts
Rusty
 

An intake air leak will have zero impact on A/F metering at WOT 🙄 On a MAF system it causes lean codes at or near idle.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Rusty, Good point.

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Adam Owner/Technician
South Dakota
Adam
 

What's VE? Maf grams at wot?

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Glenn Owner/Technician
Texas
Glenn
 

Hi William, I forgot this truck should have an inline fuel filter in it. It also needs to be replaced.

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Stuart Mobile Technician
Illinois
Stuart
 

See what the #s are for the MAF sensor and it might need cleaning or if you need to replace it do it with either a Genuine GM or a Delphi or else you will have further problems

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David Owner/Technician
Alberta
David
 

You would maybe want to do a scan and check for codes to start with? MAF would be a good thing to look at and VE as mentioned by Adam. I expect you checked the air filter etc. What is fuel pressure at WOT? Is the fuel quality known? Any idea if this problem started suddenly or got progressively worse?

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William Mechanic
Tennessee
William
 

I talked to the customer and it appears that he was driving around 70mph when the truck began to lose power slowly and it stalled on him, so he had towed it here at the shop .However, he said he's never really floored it , just maintain the speed limit.

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Obie Technician
Washington
Obie
 

You have fuel pressure at idle. What about WOT? And with pressure is flow. Has that been checked?

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Steven Owner/Technician
Illinois
Steven
 

You didn’t mention a code P0300. Otherwise I’d say try to do a crank relearn. The last few I’ve had that ran that way also had the random misfire code and needed the relearn.

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Rusty Owner/Technician
Massachusetts
Rusty
 

I agree with Obie. Make sure FP holds under your fault conditions. A simple test of the MAF on that truck is to unplug the MAF, let it stall, and restart it. It’ll run fine in SD mode. If your symptom is gone it confirms a mass air issue. If the air intake system is unrestricted and not altered, a new sensor would be called for. Good Luck

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Helpful
Cody Diagnostician
Illinois
Cody
 

MAF sensor is suspect. what is calculated load% at WOT? could do a WOT snapshot of data stream and use the numbers to put into VE calulator. i'd take the fuel pressure gauge with you on a test drive to rule out loss of fuel pressure. could be a restriction in the fuel line or the fuel pump may be getting low voltage or weak ground.

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Glenn Owner/Technician
Texas
Glenn
 

Hi William, Was the fuel inspected in the tank to make sure there was not a big blob of water/ foreign liquid floating around in the bottom? If water is in the tank, it will surround the nylon pick up sock and starve the fuel supply to the engine. I have had to totally empty the tank of several vehicles in recent months due to excess water and or mystery contamination.

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Rusty Owner/Technician
Massachusetts
Rusty
 

One more thought: Being “catless“ might suggest an aftermarket ECM calibration and if so, most fuel management system rules are iffy. If it doesn’t lose fuel pressure under load and it’s not a MAF fault, I’d be interested in what your trims are at (St & LT) at idle, 2000 cruise, and WOT

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Timothy Owner/Technician
Wisconsin
Timothy
 

At WOT, or fuel-enrichment mode, there isn't any trim correction. You'll see nothing but zeros when you do it. If it does not get into enrichment mode, then trims will be observable. At times STFT will show fuel addition, but not always. The MAF disconnect is still valuable as a tool for checking whether the system can deliver though, as you've stated.

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Rusty Owner/Technician
Massachusetts
Rusty
 

I knew that . thanks

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Jesse Owner/Technician
Pennsylvania
Jesse
 

The fuel pressure looks low to me , plus scan tool shows lean, this truck should have vacum controlled fuel pressure regulator, disconnect and see if fuel press goes up, also if vacuum line comes out of the top of regulator the regulator is adjustable on oem build, small torx with security hole in middle, a normal rump on throttle causes the computer to play catchup, by adjusting the fuel…

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Jesse Engineer
Missouri
Jesse
 

Around this timeframe, they had flex fuel trucks. The fuel filter on the frame was the fuel pressure regulator. It was internal and had a return line on the side that went back to the tank. Normal filter $20, this filter $100. Had one fail and it took the fuel pump at the same time. Wouldn' t rev over 2000 rpm in park. With it slowly dropping speed I'd say you're losing fuel volume. Fuel…

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William Mechanic
Tennessee
William Resolution
 

Also forgot to close this question, sorry guys. MAF sensor was reading skewed, went ahead and cleaned it real good with maf cleaner, test drove it and what a difference. Almost feels like it has a 6.0 in it the way it gets up and goes. O2s stay rich 800-900mv under load going down the highway, everything is good.

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Glenn Owner/Technician
Texas
Glenn
 

Hi William, Has the owner been using a K&N filter that required a light mist of oil on it? That has caused a lot of MAF problems over the years.

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