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BOBBYSS, Master Technician
Category: Chevy
Satisfied Customers: 17342
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I have a 2004 Silverado 5.3 4wd that I am working on. My

Customer Question

I have a 2004 Silverado 5.3 4wd that I am working on. My buddy brought it to me to fix complaining of a lack of power, excessive white smoke/ soot from the tailpipe, an odor to the exhaust and a flashing check engine light. I put my code reader on it and
found a bank 2 sensor 1 o2 sensor code, misfire code, and a bank 2 lean code. I put in a new oxygen sensor at that location and noted that the sensor I pulled out was caked in soot. After clearing the codes and driving the truck it now has a lack of top end
power, misfire at high RPM. When I give the truck heavy throttle and wind the engine up to say 4000rpm and up the check engine light will flash and I can hear detonation. My code reader says that there is a P0101 and P0300 code pending. I replace the MAF sensor,
but it made no difference. I suspect that the CAT is majorly clogged on the bank 2 side that had the bad O2 sensor. I put a pressure gauge in the upstream bung to measure backpressure and am getting about .5 psi at idle and 2.5 psi at 3500 RPM. What is the
spec for backpressure spec / procedure for measuring it on this truck? I am going to rerun that test and do it on Bank 1 as well to compare. I also want to check the fuel pressure to rule that out.
Submitted: 2 years ago.
Category: Chevy
Expert:  BOBBYSS replied 2 years ago.
Hello, my name is Bobby. Thanks for the question. Can you tell me if there is still smoke coming from the exhaust? Did they say that this started happening gradually or all of a sudden? Has the truck been going through coolant at all?
Customer: replied 2 years ago.
No coolant usage. No smoke now that the O2 sensor that was throwing a code was changed. Just a high rpm detonation, lack of power at high RPM, and a multiple random cylinder misfire code and a MAF sensor range/performance problem code.
Expert:  BOBBYSS replied 2 years ago.
Thank you. It sounds to me like you have a fuel system issue. If you have a fuel pressure gauge see if the pressure is holding above 55 psi when driving. If not monitor the fuel trim readings through the computer scanner. If there is a high request for fuel then the pressure may be low.
Customer: replied 2 years ago.
I checked the fuel pressure just now. I was getting 58 at idle. Did a couple full throttle stabs. It did go below 57 or so. I'll hook up the gauge and go for a drive though.
Expert:  BOBBYSS replied 2 years ago.
Lets see what it does while driving. Also let me know what the fuel trim readings are.
Customer: replied 2 years ago.
I took the truck for a drive and noted that the Short trim fuel trims jumped around at first tending to go high when you accelerated (28-30 ish) and then would dip a little below zero when at idle or decelerating (-2 to -3 ish). Long term fuel trims would cycle between high and zero it seamed (28 on bank 1, 17 on bank 2) and then drop down to zero for a little bit and then go high again (28 and 17).Also, fuel pressure stayed rock steady at around 58 even at full throttle/high rpm acceleration.I sprayed carb cleaner around the intake at each runner and injector and could not find any vacuum leaks (no change in idle/no jump in O2 sensor voltage looking at the scanner).I did also check the backpressure at bank 1 to compare to 2. I was only getting a little above 1 at 3500rpm on bank one compared to 2.5 on bank 2. I've got a test adapter that screws into the upstream bung now with a barb for a pressure/vacuum tester. I may redo this on bank 2 since I trust the test adaptor I have now moreso than the rubber one I used originally.
Customer: replied 2 years ago.
Could i request that backpressure spec/ test procedure along with any other suggestions you had in mind. I strongly suspect that cat is heavily plugged based on how much soot was caked on the o2 sensor from that bank/ how much soot and rich fuel smell was coming from the exhaust before I changed that sensor.
Expert:  BOBBYSS replied 2 years ago.
The back pressure should be less than 3 psi. If it is over that than it may be the issue. I would also do a vacuum test to see if you possibly have a timing issue.
Customer: replied 2 years ago.
could you expand on that? 3 psi at what rpm? Also what procedure would you like me to follow for the vacuum test?
Expert:  BOBBYSS replied 2 years ago.
Held at 2500 rpm once warm. As for the vacuum test let me know what it is at idle and when holding 2500 rpm. While holding that rpm let me know if it falls of or holds steady.
Customer: replied 2 years ago.
16 in HG at idle. Jumps to 19 at 2500rpm and holds steady. I feel like I can hear a vacuum leak now. I'll try to track that down.
Expert:  BOBBYSS replied 2 years ago.
Those readings are a little low. It may be a vacuum leak or it can be from a restricted exhaust.
Customer: replied 2 years ago.
The hissing i was hearing was the crankcase vent. Blocking that off stopped the hissing sound but did not raise my vacuum. I'll rerun that backpressure test.
Expert:  BOBBYSS replied 2 years ago.
Try that again and let me know.
Customer: replied 2 years ago.
1.5 psi at 2500 rpm
Customer: replied 2 years ago.
Sounds like that is not likely the issue? I'm not going to be able to work on the truck any more til thursday, so there will be a delay in my responses for a while.
Expert:  BOBBYSS replied 2 years ago.
Will the fuel pressure get above 60 psi with the vacuum hose pulled from the regulator?