O2 codes when under load

benboozled · August 16, 2019

I recently purchased my first full size truck, a 2003 Silverado 2500HD with the 6.0L gas engine. I got a great price on it, and I'm really loving the truck so far.

There is 1 major issue I'm trying to solve. The previous owner told me, and I have since seen for myself, that when the truck is towing or under a big load, the check engine light comes on. The codes that are thrown are:
P0131
P0137
P0151
P0157

I'm trying to diagnose this issue and I'd appreciate some advice. From my research I was seeing these types of codes could be caused from lack of fuel. The previous owner said he had just replaced the fuel pump, and from the look of the fuel filter it hasn't been on there very long. The PO also said he replaced all the O2 sensors. Is there anything else I should be checking here? From the fact that it only comes on under load and on both sensor banks, I'm thinking it's probably not a wiring issue. Any help is greatly appreciated! Thanks!

carkhz316 · August 16, 2019

Check fuel pressure. I wonder if the PO redid the harness connector when replacing the fuel pump. I've seen updated connectors with the supplied heat shrink butt connectors turn crusty from moisture ingress and need to be replaced again.

benboozled · August 16, 2019

Quote

Check fuel pressure. I wonder if the PO redid the harness connector when replacing the fuel pump. I've seen updated connectors with the supplied heat shrink butt connectors turn crusty from moisture ingress and need to be replaced again.

Thanks. I'll check it this weekend.

benboozled · August 27, 2019

Finally had the time to get a fuel gauge and check the pressure. The truck had been sitting for 3-4 days, and when I hooked the gauge up initially, there was 0 PSI.

Here are the readings I found:

key turned, cold start: 0

idle ~ 900 RPM: 50 PSI

2k RPM: 47 PSI

3k+ RPM: 45 PSI

From my research, it seems like this is about 5 PSI low across the board. I replaced the fuel filter and these results were the same. Not sure what else to check.

davester · August 27, 2019

It's about 10 psi low, should be right around 60 psi +- a couple. Just turning the key to "on" just get it up to full pressure.

It is possible the fuel pressure regulator, located on the fuel rail under the hood, has failed, and it's certainly cheaper and easier to replace than the fuel pump, so you could try replacing that, but I wouldn't be surprised if the fuel pump needs to be replaced (again, as you indicated the P.O. said they had replaced the pump recently).

Maybe pull the vacuum line off the fuel pressure regulator and see if fuel comes out of that port, as that's a common sign that the FPR has failed.

benboozled · August 27, 2019

I checked the FPR, and it looks new. I'm thinking the PO replaced it along with the fuel pump.

I figured I'd check local parts stores and replace it anyway. In the process I noticed something interesting. The FPR that the parts stores say will fit my truck have a note in the specs sheet that say:

> Fuel Pressure Regulator Pressure Rating 50 PSI

Could it be that somebody replaced the FPR with one that is letting in too little fuel pressure? Would there be any way to test this short of buying a new FPR with a higher rating?

riverbanks · August 27, 2019

I am hoping the one you have on truck is the same,50psi, minimum is a common rating,but you never know

benboozled · August 28, 2019

I did some more testing with the fuel pressure gauge, specifically with regards to the FPR. Engine off, key turned is 50 PSI. Same at idle, 50 PSI. What I discovered is that pulling the vacuum line of the FPR with the engine running is the pressure jumps up to ~60PSI. I ordered a new pressure regulator. Hopefully that will solve my problem once it's installed.

benboozled · August 30, 2019

The FPR with the highest rating I could find is a Delphi brand unit with a rating of 55PSI. After install, I think it's fair to say that the FPR was the cause of the low pressure. With the new FPR, fuel pressure is 55 PSI with the key turned and at idle. With the vacuum line removed it jumps up to 60 PSI. Hopefully this is the issue. I'll do some hauling this weekend to determine for sure. Thanks for all the help while trying to figure this out.